Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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LOGGING DEVICE WITH DOWN-HOLE TRANSCEIVER
FOR OPERATION IN EXTREME TEMPERATURES
FIELD OF THE INVENTION
[0001] The present device relates to logging radar devices. More particularly,
the
present invention relates to radar systems that use electromagnetic wave
propagation to
locate and identify changes in electromagnetic waves in the ground, wherein at
least a
portion of the operation occurs in a borehole (also known as "down-hole" as
referred to in
the art).
BACKGROUND OF THE INVENTION
[0002] Oil exploration and developing oil wells often pose great financial
risks
because the costs are substantial. To mitigate some of the financial risks, '
logging has
become essential in nearly every phase of exploration as well as drilling,
completing and
producing the well. Logging techniques provide information on the depth of
formations,
the presence of oil, the bottom-hole or formation temperature as well as data
associated to
the success of completion techniques, initial formation/reservoir pressures
and various
data related to stimulation treatments that are often applied to increase
production rates.
[0003] Often the key to attaining an acceptable production rate and its
associated
financial results lies in the well's response to stimulation techniques (in
particular
hydraulic fracturing). The technique referred to as hydraulic fracturing
describes a process
in which a fluid (either thin or viscous) is pumped into the targeted
formation at a rate in
excess of what can be dissipated through the natural permeability of the
formation rock.
This results in a pressure build up until such pressure exceeds the strength
of the formation
rock. When this occurs, the formation rock fails and a so-called "fracture" is
initiated.
With continued pumping, the fracture grows in length, width and height. At a
predetermined time in the pumping process, solid particulate is added to the
fluid that is
being pumped. This particulate is carried down the well, out of the wellbore
and deposited
in the created fracture. It is the purpose of this specially designed
particulate to keep the
fracture from "healing" to its initial position (after pumping has ceased).
The particulate is
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said to be propping open the fracture and is therefore designated as
"proppant". The
fracture, which is generated by the application of this stimulation technique,
creates a
conductive path to the wellbore for the hydrocarbon. Critical to the process
of optimizing
the design of a hydraulic fracturing treatment, is the determination of the
created fracture
geometry (in particular fracture length).
[0004] Currently there are logging techniques that give limited information on
fracture height, but virtually no technique that gives any reliable data
connected to
propped fracture length.
[0005] The lack of an accurate assessment of propped fracture length is due to
a
combination of factors. First and foremost is the fact that propped fractures
can extend for
hundreds of feet away from the wellbore. Prior to the development of the
technique of the
present invention, which utilizes penetrating radar waves, there was no proven
technology
available that could determine this substantial length aspect (with a
reasonable degree of
accuracy). Secondly, the down-hole conditions (in particular temperature and
pressure)
encountered by logging equipment limited the electronic equipment that could
be used,
types of signals that could be generated and the type of data gathered by this
type
equipment. It is not uncommon for logging equipment to be subjected to
temperatures in
excess of 200 C and pressures up to and exceeding 10,000 psi.
[0006] Thus, via logging and other technologies such as pressure analysis and
production history matching, the potential productivity of a given well can be
more
accurately evaluated. However, current logging devices do not address all
critical data
requirements and more sophisticated equipment may not stand up well to the
environmental conditions of a borehole. For example, temperatures may exceed
200 C
down-hole, and this type of heat limits the electronic sensors and circuits
that can be used
in a logging device.
[0007] Fig. 1 shows an example of a typical wellbore that is reinforced with a
metal casing 100. Perforations 105 are created in the metal casing at pre-
determined
depths in the wellbore to enable hydrocarbon (oil or gas) to flow into the
casing. A
fracturing fluid (either with or without propping agents) is pumped at high
pressures
through the perforations to create a fracture and to transport the proppant to
the designed
fracture length. This propping agent (also called proppant) prevents the
fracture from
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closing once pumping has ceased. The predominant fracture configuration is in
the form of
two wedge-like shapes oriented approximately 180 degrees from each other and
extending
out from the wellbore. Such a configuration would be characterized by
dimensions of
width "W", height "H" and length "L". The propped fracture provides a highly
conductive
conduit for the hydrocarbon to travel from the reservoir into the wellbore.
[00081 Ideally, fracture location and orientation, and its dimensions width,
height,
and length would be known values. However, as mentioned previously, there is
limited
data available on fracture height and virtually no method available to
accurately measure
an extended propped fracture length. Therefore, there has been a long-felt
need in the art /
industry for a logging device that can be used to generate this critical
element of fracture
geometry while being subjected to the elevated values of temperature and
pressure (for
example about 200 C, or greater, and 10,000 p.s.i.) associated with down-hole
wellbore
conditions. There is also a need in the art for a system that can be arranged
to operate with
existing wellbores that have already been perforated and fracture stimulated
and newly
drilled wells that may be completed according to the present invention to
simplify the
measurement process or to enhance its ability to describe the propped geometry
generated
from a fracturing treatment.
SUMMARY OF THE INVENTION
100091 The invention provides a radar logging device/tool, system and method
for
determination of propped fracture length, height and azimuth (direction from
the
wellbore). The present invention addresses the industry need for accurate
measurement of
these important aspects of fracture geometry. The invention accomplishes this
goal using
a design expressly suited to operate under adverse conditions associated with
a wellbore,
as it penetrates the producing formations and its associated elevated
formation
temperatures and pressures.
[00101 The present invention provides a radar logging system, apparatus, and
method that includes above ground instrumentation and a down-hole hybrid
transceiver.
Microwave signals, which are generated above ground, are used to drive an
intensity
modulated (IM) laser. The laser output travels along a fiber optic cable down
into the
wellbore.
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[0011] The radar logging system includes a transceiver that may include a
photodiode
or has a photodiode separately connected thereto. The radar signal will locate
holes/perforations
in the casing and determine which holes/perforations are connected to a
fracture of some propped
length. Once the signal has access to the propped fracture it may also
generate data that can be
used to describe the propped fracture length and height. This technology can
be used to describe
all fractures connected to the wellbore and differentiate between the
dimensions of the two
vertical wings of a propped fracture. Commonly accepted theory says that most
fractures are
orientated in the vertical direction (the width of the propped fracture is
largest at the wellbore)
and that vertical fractures have two wings orientated approximately 180 from
each other. In
most instances (due to the inability to accurately measure fracture length)
the two propped
fracture wings are assumed to have similar propped geometry (length, width and
height). This
technology will allow actual measuring of the geometry of both wings.
[0012] Inside the wellbore, the transceiver, containing passive components
that can
withstand the high temperatures, such as a photodiode, converts the IM laser
signal back to a
microwave signal. The signal is split between an antenna and a mixer, where
the output from the
antenna is transmitted out into the fracture. The fracture containing proppant
serves as a wave
guide for the radar signal and inconsistencies in the fracture, including the
fracture termination
reflect the radar signal to form a reflected wave. This reflected wave is
mixed to generate a beat
frequency used to determine the dimensions (e.g., length) of the fracture.
Advantageously, the
transceiver is operable at low down-hole temperatures or high (e.g., about 200
C, 220 C or
300 C) down-hole temperatures without any cooling apparatus.
[0012.1] In accordance with one aspect of the present invention, there is
provided a
logging radar system for measuring propped fractures and down-hole formation
conditions in a
subterranean formation by a logging device, comprising: at least one radar
source for generation
of at least one source radar signal; at least one optical source; at least one
optical modulator in
communication with at least one said radar source and at least one said
optical source for
modulating at least one optical signal according to at least one said source
radar signal having at
least one predetermined frequency; at least one photodiode for converting the
modulated optical
signal output from at least one said optical modulator to at least one said
source radar signal; at
least one transmitter and receiver unit comprising: (a) at least one antenna
communicating with at
least one said photodiode, for receiving at least one said source radar signal
from at least one said
photodiode and transmitting at least one said source radar signal into the
subterranean formation
and receiving a reflected radar signal; and (b) a mixer, in communication with
at least one said
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photodiode and at least one said antenna, for mixing the reflected radar
signal with at least one
said source radar signal to provide an output.
[0012.21 In accordance with another aspect of the present invention, there is
provided a
logging radar system for measuring propped fractures and down-hole formation
conditions in a
subterranean formation by a logging device, comprising: a first radar source
for generation of a
first source radar signal; an optical source; an optical modulator in
communication with the first
radar source and the optical source for modulating an optical signal according
to the first source
radar signal having a predetermined frequency; a transmitter photodiode, for
arrangement down-
hole in a wellbore, for converting the modulated optical signal output from
the optical modulator
to the source radar signal; a transmitter adapted for arrangement down-hole in
the wellbore, the
transmitter comprising: a transmitter antenna communicating with the
photodiode, for receiving
the source radar signal from the photodiode and transmitting the source radar
signal into the
subterranean formation; and a receiver photodiode for arrangement down-hole in
the wellbore; a
receiver adapted for arrangement down-hole in the wellbore, the receiver
comprising a receiver
antenna for receiving a reflected radar signal, a mixer, in communication with
the receiver
photodiode and the receiver antenna, for mixing the reflected radar signal
with a second source
radar signal to provide an output based on a difference in the second source
radar signal and the
reflected radar signal; wherein the receiver photodiode, mixer and receiver
antenna are coupled,
wherein the transmitter antenna and the receiver antenna are the same antenna
or different
antennas, wherein the receiver photodiode and the transmitter photodiode are
the same
photodiode or different photodiodes, wherein the first source radar signal and
the second source
radar signal are the same source radar signal or different source radar
signals.
[0012.31 In accordance with yet another aspect of the present invention, there
is provided
an apparatus for arrangement down-hole in a wellbore of a subterranean
formation, comprising:
(a) at least one photodiode for converting at least one modulated optical
signal output from at
least one optical modulator to at least one source radar signal; (b) at least
one antenna
communicating with at least one said photodiode for receiving at least one
said source radar
signal for transmission into the subterranean formation and receiving a
reflected radar signal; and
(c) a diode mixer in communication with at least one said photodiode and at
least one said
antenna for mixing the reflected radar signal with at least one said source
radar signal to provide
an audio frequency output.
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[0012.41 In accordance with still another aspect of the present invention,
there is provided
a method for radar logging down-hole in a wellbore of a subterranean formation
without any
down-hole cooling devices, the method comprising the steps of providing at
least one radar
source having at least one predetermined frequency; providing at least one
optical source having
a predetermined wavelength; modulating at least one optical signal with at
least one optical
modulator in communication with at least one said radar source and at least
one said optical
source, at least one said optical signal being modulated according to at least
one said radar source
having the predetermined frequency; providing at least one said modulated
optical signal to a
transmitter and receiver unit, wherein the transmitter and receiver unit
comprises: (a) at least one
photodiode for converting at least one said modulated optical signal output
from at least one said
optical modulator to at least one radio frequency source radar signal; (b) at
least one antenna in
communication with at least one said photodiode; and (c) a mixer in
communication with at least
one said photodiode and at least one said antenna; and at least one said
antenna receiving at least
one said radio frequency source radar signal from at least one said
photodiode, at least one said
antenna transmitting at least one said radio frequency source radar signal
into the formation, and
at least one said antenna receiving a reflected radar signal; and the mixer
mixing the reflected
radar signal with at least one said radio frequency source radar signal to
provide an output, then
utilizing the output to identify facture geometry.
[0012.51 In accordance with yet still another aspect of the present invention,
there is
provided a method for logging in a wellbore of a subterranean formation,
without any down-hole
cooling devices, comprising the steps of. (I) locating perforations in a
wellbore casing and
determining perforations where fractures have originated by: (a) lowering a
logging device
comprising a gyroscope and a transmitter and a receiver unit for transmitting
at least one first
signal in the range of about 7 to 12 GHz into a well at a predetermined depth,
wherein a series of
perforated intervals are arranged in the wellbore casing; (b) anchoring the
logging device in
place; (c) scanning the wellbore casing for perforations with the at least one
first signal;
(d) transmitting data from a reflected signal of the at least one first signal
used to scan for
perforations; (e) determining which perforations are connected to a fracture
of at least
predetermined minimal length based on the data transmitted in step (d); and
(f) cutting a
10-15 cm narrow slot to dissect a perforation identified as connecting to the
fracture, and
(II) determining aspects of fracture geometry by: (a) generating a microwave
radar signal;
(b) coupling the microwave radar signal to an IM laser to form a modulated
laser signal and
transmitting the modulated laser signal down-hole; (c) utilizing a down-hole
transceiver
comprising a photodiode to convert the modulated laser signal to a radio
frequency signal;
(d) splitting the radio frequency signal by sending a first Rf portion to an
antenna and a second
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Rf portion to a diode mixer; (e) transmitting an output from the antenna,
through the perforations
and into the fracture, and receiving back a reflected wave; (f) mixing the
reflected wave with the
second Rf portion and generating an audio frequency signal having a beat
frequency;
(g) transmitting the audio frequency signal, without amplification, to a
receiver; and
(h) determining the fracture geometry aspects from the beat frequency.
[0012.61 In accordance with a further aspect of the present invention, there
is provided a
method of determining portions of a wellbore casing in contact with fractures
of a subterranean
formation, comprising the steps of (a) transmitting a signal of a
predetermined frequency into
the subterranean formation through at least a portion of the casing adjacent
to a fracture in the
subterranean formation, the portion of casing being substantially transparent
to the predetermined
frequency; (b) receiving a reflected signal reflected back from the
subterranean formation
through the portion of casing; and (c) locating a position of the fracture
based on a reflected
signal.
[0012.71 In accordance with yet a further aspect of the present invention,
there is provided
a method for logging a wellbore to determine aspects of fracture geometry, in
the absence of
down-hole cooling devices, comprising the steps of. (a) generating a microwave
radar signal;
(b) coupling the microwave radar signal to an IM laser to form a modulated
laser signal and
transmitting the modulated laser signal down-hole; (c) utilizing a down-hole
transceiver
comprising a photodiode to convert the modulated laser signal to a radio
frequency signal;
(d) splitting the radio frequency signal by sending a first Rf portion to an
antenna and a second
Rf portion to a diode mixer; (e) transmitting an output from the antenna into
a fracture and
receiving back a reflected wave; (f) mixing the reflected wave with the second
Rf portion and
generating an audio frequency signal having a beat frequency; (g) transmitting
the audio
frequency signal, without amplification, to a receiver; and (h) determining
the fracture geometry
aspects from the beat frequency.
[00131 In an embodiment, the invention permits accurate radar logging
measurements
using only passive components down-hole (no amplification down-hole). The more
temperature
sensitive active components are above-ground and away from the high
temperatures, pressures
and potentially corrosive environment often associated with down-hole well
conditions.
[00141 Also, the invention advantageously achieves very low loss signal
transport
mechanisms. For example, an embodiment has a fiber microwave feed exhibiting
only a 1.2 dB
RF/electrical loss per kilometer. Also, audio frequency output signal can be
transported for
kilometers over a twisted pair of wires with minimal loss.
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[0015] A notch or slot antenna may be employed to radiate the signal outside
the
casing. The vertical notch is created in the wellbore casing by a high-speed
saw/cutting
device (or cutting laser) that can be a part of the radar logging device. The
notch is
created in the location that is determined by the tool as oriented toward the
fracture. This
notch location is determined by probing the casing perforations with a RF
signal (whose
wavelength is short enough to pass through a single perforation) to locate
those
perforations shown to be in communication with the propped fracture.
Typically, the notch
is approximately 5 mm to 20 mm in length. The tool orients its vertical slot
antenna with
the casing notch. Contact of the antenna with the casing is not required. The
slot antenna
of the tool and casing notch create a strong electrical coupling allowing the
tool's
electromagnetic signal to enter and penetrate the propped fracture.
[0016] The logging device may be positioned down-hole in conjunction with a
gyroscope. The gyroscope has a dual purpose of being necessary to pinpoint the
location
of perforations that are found to be in communication with the propped
fracture and it also
provides useful information that contributes to the invention's ability to
determine the
azimuth/direction of the propped fracture as it leaves the proximity of the
wellbore.
[0017] The gyroscope may also be used to position the above-mentioned notch
that
is used as an exit point for the device's RF signal.
[0018] While the notches in the casing can be made down-hole with a cutting
tool
or saw/cutting device, it is also within the spirit and scope of the invention
that, in an
embodiment, wellbore casings are manufactured with prefabricated notches that
are
selected/customized according to the specifics of the wellbore.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] For purposes of illustration and not intended to limit the scope of the
invention in any way, the aforementioned and other characteristics of the
invention will be
clear from the following description of a preferred form of the embodiments,
given as non-
restrictive examples, with reference to the attached drawings wherein:
[0020] Fig. 1 is a drawing of a wellbore that is known in the art;
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[0021] Fig. 2 is a drawing of a first embodiment of a radar logging device
having a
passive transceiver arranged down-hole in the casing;
[0022] Fig. 3A is a block diagram of an embodiment of the present invention;
[0023] Fig. 3B is a block diagram of another embodiment of the present
invention;
[0024] Fig. 3C shows the cutting tool and a wellbore with existing
perforations;
[0025] Fig. 3D illustrates a wellbore according to an embodiment of the
invention -that
includes composite pups;
[0026] Figs. 3E-3K illustrate one way that a logging device according to the
present
invention would be lowered into a wellbore, having articulated arms locking
the device in place
and taking readings at both 10 GHz and 1 GHz;
[0027] Fig. 4 is an illustration of the communication cables used in the
embodiment
disclosed in Figs. 2 and 3A, 3B; photodiode bias wires normally included in
this embodiment are
not shown to simplify the figure;
[0028] Fig. 5 is a schematic of an embodiment of the photodiode and
transceiver portion
of the radar logging device of Fig. 3 constructed for operation in a high-
temperature down-hole
environment;
[0029] Fig. 6 is a schematic of an embodiment of the mixer that is part of the
transceiver
shown in Fig. 5;
[0030] Fig. 7 is a drawing of a laser driver board that includes the laser
diode, driver,
and modulator;
[0031] Fig. 8 is a drawing of an embodiment of the modulator of the embodiment
of
Fig. 3;
[0032] Fig. 9 is a schematic of a wideband bowtie antenna for an arrangement
down-
hole; [0033] Fig. 10 is a graphical illustration of the antenna return loss in
dB;
[0034] Figs. 1 IA-I ID illustrates sandstone test equipment used to evaluate
propagation;
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[0035] Fig. 12 is a drawing of the transceiver board including the hybrid
coupler
(unpopulated);
[0036] Fig. 13 is a drawing of the setup for an example of the present
invention being
tested for high temperature performance in an oven;
[0037] Fig. 14 is a graphical illustration of the output signal received
versus the
temperature in an experiment;
[0038] Fig. 15 is a graphical illustration of the Doppler shifted return
signal frequency
from a first experiment utilizing a moving target in lieu of a fracture in a
wellbore; and [0039]
Fig. 16 is an illustration of a propagation test setup for a second experiment
in which a short PVC
tube with proppant is used as a sample.
DETAILED DESCRIPTION OF THE INVENTION
[0040] It is understood by a person of ordinary skill in the art that the
drawings are
presented for purposes of illustration and not for limitation. The embodiments
shown and
described herein do not encompass all possible variations of the arrangement
of structure, and an
artisan appreciates that many modifications can be made within the spirit of
the invention and the
scope of the appended claims. In the following description, well-known
functions or
constructions are not described in detail since so as not to obscure the
description of the
invention. For example, power sources, bias voltages, and their respective
connections are not
shown in the drawings so that the subject matter emphasized in the description
is not obscured
with unnecessary detail. However, an artisan understands and appreciates that
any such items not
shown may be advantageous and/or required for operability.
[0041] There are a number of ways that the present invention may be practiced.
For
illustrative purposes, we will discuss at least two methods that adapt to the
respective type of
casing. While newer casings can be designed with the present invention in
mind, older
casings/wellbores can be modified so that an antenna located down-hole will be
able to transmit
and receive a signal through a series of notches that are cut into the casing.
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[0042] Figs. 2 and 3 A illustrate one embodiment of the present invention,
with Fig. 3A
providing a block diagram of the components shown in Fig. 2. The entire system
can be
arranged-in-part both above and below ground 200. With regard to Fig. 2, a
wellbore is lined
with a casing 100. Down-hole the temperature can be in the vicinity of 200 C,
and this exceeds
the temperature permissible for viable operation of typical active electronic
components, which
at best would require the assistance of special cooling devices in an attempt
to have the device
operate correctly.
[0043] Referring to Fig. 2, there is depicted a driver and instrumentation 222
connected
above ground to a source 215 of cable 217. Typically, the source 215 is a roll
of cable. The cable
217 is fed down-hole and has an outer rigid sheath 216 connected to a down-
hole signal sending
and receiving apparatus 202. Thus, the signal sending and receiving apparatus
202 only has
passive components capable of operation under such conditions and is arranged
down-hole and
along/within a lower portion of the casing 205 of the wellbore. [0044] Fig. 3A
shows that the
above ground instrumentation 222 of Fig. 2 includes a microwave signal source
(microwave
frequency generator) 221, a laser driver (laser transmitter) 225, a modulator
226, an audio
amplifier/filter 230, mixer 250, microwave frequency generator 240 and RF
spectrum analyzer
260. The microwave signal source 221, laser driver 225 and modulator 226
generate a
microwave radar signal, and couple the signal to an IM laser to modulate laser
light to be sent
down-hole. For example, microwave radar signals are generated above ground and
intensity
modulated (IM) onto a 1550 nm laser signal. An audio amplifier/filter 230,
mixer 250,
microwave frequency generator 240 and RF spectrum analyzer 260 are also
situated above
ground and act as an audio frequency receiver and signal processor so as to
receive from down-
hole a beat frequency. The microwave generator is typically configured to
generate two signals
with different frequencies such that a beat frequency will be generated in the
transceiver's mixer.
Likewise, the microwave generator may be chirped in order to generate a beat
frequency in the
transceiver's mixer. The beat frequency characterizes certain fracture
geometry properties, in
particular fracture length, to provide logging data consistent with the
propped fracture system
that has been generated.
[0045] Fig. 3 A also shows a cable roll 215 (above ground) and a signal
sending and
receiving apparatus 202. The signal sending and receiving apparatus 202
includes a
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photodiode 505 and a transceiver 510. In particular, the photodiode 505
converts the IM
laser light back into a microwave signal, with the output of the microwave
signal source
221 being in the range of about 1GHz to 10 GHz but typically about lGHz (e.g.
0.5 to 2
GHz or 0.7 to 1.3 GHz). It should be noted that the examples, photos, and
descriptions are
all for the 10GHz realizations and are provided for illustrative purposes and
not intended
to limit the scope of the invention. The modulator 226 is connected to the
photodiode 505
by optic fiber 300, typically polyimide coated optic fiber. The transceiver
510 is connected
to the audio amplifier/filter 230 by a pair of twisted wires 302. The twisted
pair 302 may
also carry the DC bias for the photodiode 505. (In an alternative embodiment
(not shown)
the DC bias for the photodiode can be carried on a separate pair of wires.)
The photodiode
505 can be connected to the transceiver 510, for example, by a coaxial cable.
However, the
photodiode may be included as part of the transceiver assembly by mounting the
photodiode directly onto the transceiver (not shown).
[0046] Microwave signals, which are generated above ground by the microwave
signal source 221 (e.g. radar signal source), are used to drive a laser
intensity modulated
(IM) by intensity modulator 226. The laser output travels along the fiber
optic cable 300
down into the wellbore.
[0047] Inside the wellbore, a transceiver 510 containing passive components
that
can withstand the high temperatures, such as a photodiode 505, converts the IM
laser
signal back to a microwave signal (e.g. radar signal). The radar signal is
split between an
antenna 512 (Fig. 5) and a mixer 520 (Fig. 5), wherein the output from the
antenna 512 is
transmitted out into the fracture. The fracture, which contains proppant,
serves as a wave
guide for the radar signal. Inconsistencies in the propped fracture, including
the propped
fracture's change in direction or termination, reflect the radar signal to
form a reflected
wave. This reflected wave is mixed in the diode mixer 520 to generate a audio
frequency,
such as a beat frequency, to determine the dimensions (e.g., length) of the
fracture.
[0048] It should be noted that while there could be various implementations of
the
components shown in Fig. 3A, the functionality would be similar. For example,
while it is
preferred that the invention mixes the reflected wave with the source wave to
generate a
beat frequency, an artisan appreciates that a signal other than a beat
frequency (based on a
mixing of the waves) can be generated in order to calculate the size of the
fracture. Also,
in Fig. 3A the transceiver 510 functions as a transmitter and receiver unit.
However, in
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embodiments, not shown, the transmitter and receiver unit could be a separate
transmitter
and receiver each having their own antenna and in communication with different
photodiodes. Also, rather than sending a single signal from a single radar
signal source,
multiple signals from multiple radar signal sources can be employed. For
example, a
transmitter could employ one source radar signal and a receiver could employ a
second
source radar signal or a transceiver could be fed two source radar signals,
one to operate
with transmitter components of the transceiver and another to operate with
receiver
components of the transceiver.
[0049] Fig. 3B illustrates another embodiment of the invention including a
second
radio system. The second radio system is similar to the one shown in Fig. 3A,
however it
generates a different frequency. The use of two separate radios facilitates
employing two
different microwave frequencies. The first frequency (e.g. about 7-12 GHz,
typically about
10 GHz) is used to orient the tool and detect perforations (that are in
communication with
the propped fracture). The second frequency (e.g. 0.5 to 2 GHz, typically
about 1 GHz)
about is used for transmission via a notch or slot antenna to determine the
fracture
dimensions, (e.g. fracture length, width, and/or height). The reference
numerals shown in
Fig. 3B are the ones used in Fig. 3A except that an additional digit has been
added at the
end to show each component in the second radio has a reference numeral ten
times that of
a similar component in Fig. 3A. It should be noted that the radar source,
optical source and
optical modulator are duplicated. In an alternative embodiment (not shown) the
radar
source, optical source and optical modulator are not duplicated.
[0050] Still referring to Fig. 3B, the first and second radio systems, which
are
typically located above ground except for the signal sending and receiving
apparatus 202,
2020, use the same twisted pair 302 to carry the audio signal generated down-
hole by the
signal sending and receiving apparatus 202, 2020 which receives the source
radar signal
from each transceiver 510, 5100 and each respective reflected wave. There can
be a simple
switch 229 or any other type of coupling connection that functions to allow
the same
twisted pair 302 to communicate via wires 3020 and 302A with both the 1 GHz
system
and the 10 GHz system, respectively.
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Installation Phase
[0051] In addition, the logging device can be arranged (at least partly) down-
hole
in wellbores that would require modification of existing casings, or
customized casings
can be constructed to permit an antenna to transmit the signal toward the
fracture. Some of
the installation steps and the modifications to the structure of the logging
device that assist
in this process are described below.
[0052] As shown in Fig. 3B and discussed above, there are two radios, each
with
its own fiber optic cable 300, 3000. In this case, a common twisted pair 302
carries an
audio signal. However, if desired the common twisted pair could be replace by
two twisted
pairs. Depending on preference, an artisan appreciates that the same twisted
pair can be
used for both transceivers, or a separate twisted pair can be used for each
transceivers.
Additionally, it is also preferable (but not required) that there are two
complete
transceivers down-hole.
[0053] Fig. 3C illustrates the items schematically other than the transceivers
510,
5100 shown in Fig. 3B, which are used to orient and install the logging device
in a casing
already installed down-hole. These items include a gyroscope 430, a
retractable cutting
device 410 (e.g. high speed saw/cutting device or laser) capable of accurately
generating a
narrow slit/slot (of a prescribed length) in the casing, attachable hardware
such as
retractable anchors (two shown) to hold the tool stationary, and the ability
to create and
store data concerning the position of existing perforations and to position
accurately the
cutting device to create/cut a slot intersecting a given perforation.
Notch Created Down-hole using Perforations and Gyroscope
[0054] Fig. 3C shows a further embodiment of the device 405 which includes the
signal sending and receiving apparatus 202, as well as a gyroscope 430 and a
cutting tool
410. It should be noted that the cutting tool could be anything from a saw to
a laser. In the
case of wellbores with existing perforations, the process includes creating a
vertical notch
in the casing to facilitate transmission of the signal for determining the
dimensions of the
fracture.
[0055] To determine the location for placing the notch in the casing, the
device
405 transmits a signal to locate casing perforations connected to a proppant
packed
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fracture (of at least several meters). Once such perforations are found, the
saw or cutting
device 410 (which may comprise a laser) cuts a vertical slot or notch 99 in
the casing
(dissecting the perforation). The slot is designed to be sufficient in size to
allow a lower
frequency signal (suitable to make the trip down the fracture and back to the
transmitter/receiver) to travel from within the casing to the propped
fracture. During the
cutting, the gyroscope 430 assists in positioning the cutting device 410 (e.g.
high-speed
saw or laser) for cutting the slot or notch.
[0056] In accordance with Figs. 3E-3K, one way that the present invention can
be
lowered and situated down-hole for operation is as follows:
[0057] (a) As shown in Fig. 3E, the tool is lowered into the well to the depth
to be
examined, which is the where series of perforated intervals are arranged. The
tool ends up
at the depth shown in FIG. 3F.
[0058] (b) Fig. 3G shows the tool is then anchored in place by attachable
hardware
such as retractable arms, or an electromagnet.
[0059] (c) As shown in Fig. 3H, using the gyroscope and a 10 GHz signal from
the
10 GHz radio, the tool scans the casing for perforations.
[0060] (d) As perforations are located, data from the return of an approximate
10
GHz signal is gathered and analyzed (data being transmitted to the well
surface through
the twisted pair). From the aforementioned data, it is determined which
perforations are
connected to a propped fracture of at least minimal length.
[0061] (e) As shown in Fig. 31, once each of the perforations has been
examined,
then the cutting part of the tool is positioned so that a 10-15 cm narrow slot
can be made to
dissect the perforation (identified as connecting to a propped fracture).
[0062] (f) As shown in Fig. 3J, once the narrow slots are in place, a 1 GHz
signal
is pulsed or otherwise sent through the narrow slot and out into the
connecting propped
fracture. Data concerning the returning signal waves is transmitted back to
the surface for
analysis. By repeating steps (e) and (f), the geometry of all propped
fractures intersecting
the wellbore and in communication with a perforation can be examined and a
fracture
configuration can be developed.
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[0063] (g) After the fracture geometry is measured, the tool can then be
raised out
of the wellbore, as shown in Fig. 3K.
[0064] Using a similar procedure to steps (e) and (f) above, the fracture
height can
also be established. In this case, the saw/cutting device 410 would be
positioned to cut
narrow slots above and/or below the first or last perforation to be identified
as being in
communication with the propped fracture connected to the fracture. These
narrow slots
(above or below the communicating perforation) can be used to establish if the
propped
fracture extends past the perforation in question.
[0065] In addition, the use of this technology/logging device is simplified
when all
or at least portions of the casing are made of composite material which is
relatively
transparent to the signals. This procedure, in which a signal can be
transmitted through
composite casing material, is a variation upon the embodiment discussed above
and would
be particularly applicable to new wells. Old wells are typically built .
without these
composite sections. However, a customer building a new well can decide to
include
composite casing material to facilitate employing the present invention to
determine the
created fracture geometry of the well.
[0066] For example, new casings being arranged down-hole can be made to
include portions, referred to as pup joints or "pups" 402 made of composite
material. The
composite pup joints 402 comprise lengths of casing referred to in the art as
"subs"
arranged along the length of the casing and adjacent to the producing
formation. The subs
are prefabricated for use with such new casings.
[0067] These composite pup joints (made of subs) would be substantially
transparent to the RF signal being directed at the fracture area and could
simplify locating
the position of the fracture. The composite pup joints 402 permit the signal
from the
logging device 405 to penetrate through the composite pup joints 402 and pass
into the
propped fracture about the wellbore casing without the need for a slit or
other opening in
the casing. Thus, a cutting saw/cutting device 410 (or similar device) is not
needed to see
through the composite pups. Typically, at least 80%, preferably at least 90%,
of the RF
signal passes through the composite material.
[0068] An advantage of arranging the composite pup joints in the casing is
that the
composite material makes it easier to position the logging device.
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[0069] Also, an advantage of arranging the composite pup joints in the casing
is
that the logging device could be fully functional for sending a signal through
the
composite pups with only a source having an approximately 1 GHz signal, e.g.,
1 GHz +
0.1 GHz. Thus, the approximately 10 GHz signal, e.g., 10 GHz + 1 GHz, and the
cutting
saw/cutting device 410 would not be used when transmitting a signal through
the
composite pups. However, a transceiver for the approximately 10 GHz signal and
the
cutting saw/cutting device 410 may be included, even if being used with a
casing
including composite pups, in the event the logging device is also to be used
to send a
signal through parts of the wellbore casing not made of composite pups.
[0070] In an embodiment of the invention, for maximum definition of the
fracture
height and length, the production interval can be completed using only
composite pipe
(instead of the combination of composite subs and conventional casing). In
such an
example it would be possible to scan the entire production interval without
interruption.
[0071] Typical composites are fiber glass reinforced-cured epoxy resins. To
maximize the geometry data obtained through the use of this invention, all or
a substantial
portion of the casing, positioned adjacent to the producing interval, may be
made of
composite material. An example of such composite material is RED BOX
fiberglass
reinforced aromatic amine cured epoxy resin casing and tubing available from
Future Pipe
Industries. Such casing and tubing is designed for downhole service of medium
to high
pressure at depths as great as 13,000 feet.
[0072] Additionally, the use of composite pup joints or joints permits one to
check above and below the zone of interest to see if the top and bottom of the
propped
fracture have been located, to provide accurate height determination.
[0073] In contrast, employing the method of the present invention on casings
made without the composite pup joints, such as existing casings already
installed down-
hole that are cemented across the treating interval, normally includes an
additional step of
using the cutting saw/cutting device 410 to enlarge the perforations 105
already in the
casing 100 (as shown in Fig. 1) to a desired size for transmitting a signal
therethrough.
[0074] Fig. 4 shows the cable 215 of Fig. 2 has within its outer rigid sheath
216
both a fiber optical cable 300 designed for high temperature use, and
typically, a twisted
pair of cables 302 to return an audio signal above ground. The twisted pair is
used as a
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return for the audio frequency (in this case beat frequency) created by the
difference of the
original signal and the signal reflected off the fracture. The fiber optical
cable 300 is
typically a polyimide coated fiber cable on which the modulated optical fiber
is sent
down-hole to the transceiver. RF power losses are on the order of 1.2dB/km of
high
temperature tolerant fiber. The twisted pair typically has TEFLON coated
wires. The
photodiodes also are provided with DC bias voltage (not shown), which could be
sent to
the photodiodes over another twisted pair of wires (not shown).
[0075] Although an artisan will understand and appreciate that other values
can be
substituted, the invention has been designed using as many "off the shelf'
components as
possible to aid in manufacturing ease and reduce costs. For example, the
optical fiber
selected was a high temperature polyimide coated mode 1550 nm fiber and can
withstand
the 2100C and above temperatures. In addition, intensity modulators that
operate in the
1550 nm wavelength range are available. There are also many lasers that
operate with
sufficient power at the 1550 nm wavelength range. Moreover, there are Erbium-
doped
fiber amplifiers available that can provide further optical amplification of
1550 nm signals
should more optical power be required.
[0076] With regard to Fig. 3A, a typical modulator 226 may be a Mach Zehnder
modulator that is the industry standard modulator for microwave frequencies.
An example
of a suitable Mach Zehnder modulator is a JDS Uniphase Lithium Nobate electro-
optic
modulator that converts optical phase modulation into intensity modulation via
an optical
interferometer (a Mach Zehnder interferometer). The typical insertion loss
into a Mach
Zehnder modulator is just over 3dB (optical). Thus, if the loss in the fiber
is 0.6dB/km,
then in a setup with 1 km of fiber, the loss will be 0.6 dB.
[0077] It should be noted that virtually any type of signal propagation can be
compatible with the present invention. For example, a pulsed wave sent in
bursts, or a
continuous-wave (such as in a Doppler system), and/or the signal may comprise
a chirp
that increases or decreases in frequency, linearly or geometrically. An
artisan appreciates
the use of these and/or other known systems as being within the spirit of the
invention.
[0078] TABLE 1 is an overview of the operation of an embodiment of the
invention along with a discussion of the methods steps. In particular, TABLE 1
identifies
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as steps some of the functions that are performed by the logging system, and
notes
whether these functions occur above ground or down-hole.
TABLE 1
Operation and Method Steps Where Steps Are
Performed
STEP A Generate Radar Signal Above Ground
STEP B Modulate IM LASER With Radar Signal Above Ground
STEP C Transmit Modulated Optical Signal along a fiber cable to Above Ground
a Photodiode Down-hole
STEP D Optical Signal Converted to Radio frequency signal Down-Hole
STEP E Frequency split, part to RF Antenna to reflect off fracture, Down-Hole
other part sent to diode mixer
STEP F Diode Mixer mixes the two signals, and generates an Audio Down-Hole
Frequency Signal having a beat frequency
STEP G Audio Frequency Signal transmitted without Amplification Down-Hole
over a twisted pair of wires to above ground receiver
STEP H Compilation of the recorded reflections, e.g., Conversion Above-Ground
of beat frequency into logging data regarding
measurements of the fracture
[00791 Fig. 5 is a schematic of the photodiode 505 and transceiver portion 510
of
the radar logging device that is constructed for operation in a high-
temperature down-hole
environment. The photodiode 505 converts the IM laser signal into a microwave
signal.
The microwave signal output 504 from the photodiode 505 is then split between
an
antenna 512 and a diode mixer 520. The portion of the signal that is sent to
the antenna
512 propagates out through the propped fracture and is reflected from
inconsistencies in
the fracture, including a change in direction or fracture termination. The
reflected signal
then returns to the antenna and via the hybrid coupler 515 is sent to the
mixer 520 for
mixing with the original microwave signal. A beat frequency is generated which
is used to
determine the range. If the microwave signal is chirped, the beat frequency
will
correspond to the length being measured. The casing has a series of
perforations/slots
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large enough to permit a radar transmission from the antenna. There can be a
series of
perforations in the casing, and it is possible to use an antenna array, with
each antenna in
the array transmitting via a perforation.
[00801 The use of the beat frequency to determine the range is a principle
similar
to the principle of many modern police radars, and even garage door opening
systems. The
greatest recorded range with a significant return corresponds to the length of
the propped
fracture being measured.
[00811 An artisan will appreciate that other methods of coding the microwave
signal, such as direct sequence coding, can be employed within both the spirit
of the
invention and the scope of the appended claims. For example, in the case of
direct
sequence coding, the outgoing signal is modulated by a digital code with
tightly controlled
auto-correlation properties. After the signal is propagated out to a target
and is reflected
back, the mixer will correlate the original signal with the delayed received
signal. The
source signal can then be controlled to give a correlation peak at only one
particular range
(delay). Thus, range gated radar measurements can be made with direct coding
in lieu of
using a chirp. The aforementioned are two of several ways to measure the
fracture.
[00821 Fig. 6 shows a schematic of the diode mixer 520 that is part of the
transceiver 510 shown in Fig. 5. The I and Q ports are each populated with
zero bias RF
mixing diodes to form a single balanced mixer configuration. Such detail has
been
provided merely for illustrative purposes and not to limit the structure of
the mixer 520 to
the components shown, or equivalents thereof.
[00831 An example of typical diodes D1A, D1B for use in the mixer 520 include
Skyworks Semiconductor SMS7630-006 diodes which are low bias diodes and
provide
high conversion efficiency (12 dB conversion loss) when operating these diodes
D 1 A,
D 1 B with zero DC bias. TI and T2 are used to provide a DC ground to the
diodes D 1 A,
D1B, and R1, T3 and Cl provide a match to the mixer output port. Still
referring to Fig. 6,
the output of the diodes D 1 A, D 1 B is an audio frequency difference signal
that is sent to
the surface through the twisted pair of wires 302. The frequency is typically
on the order
of MHz whereas the microwave signal input is typically between less than 1 GHZ
to
1 0GHz.
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[0084] Circuit topology could change if the frequency is changed. For example,
the transmission line that could be employed with a 10 GHz system could be
replaced with
direct inductors in a 1 GHz system. Also, the hybrid coupler employed in a 10
GHz
system could be replaced by having the mixer itself couple part of the signal
to the
antenna. In other words, while operation at different frequencies is
substantially the same,
different components may be used at different frequencies that are better
suited to the
particular frequencies.
Typical Materials Useful For Determining Fracture Geometry
[0085] The invention measures aspects of propped fracture geometry. Propped
fractures provide a conductive pathway for the flow of hydrocarbon and they
are designed
to be stable in their environment.
[0086] Typical suitable proppants include sand, ceramics and resin coated sand
and ceramics to prop fractures.
[0087] It is possible to mix additive particles/filler with proppant and a
variety of
additives and/or fillers can be used for determining the geometry of the
fracture. The
additives and/or fillers (hereinafter additives and/or fillers will be termed
"particles") can
be electrically conducting, semi-conducting or electrically non-conducting.
However, the
particle size of the additive particles / filler should be selected to not be
significantly
smaller in particle size than the proppant. The use of an overly fine particle
(as part of a
mixture with a standard proppant) may result in a loss in fracture
conductivity.
[0088] Electrically conducting particles can be used for reflecting the
electromagnetic radiation signals. Semi-conducting and non-conducting
particles can be
used to absorb the electromagnetic radiation signals or to propagate them
during radar
operations and/or imaging operations. The particles and/or proppants can be
either
electrically conducting, semi-conducting or non-conducting if desired. In an
exemplary
embodiment, the particles and/or proppants are electrically conducting and can
reflect the
electromagnetic radiation that it incident upon them. The electrically
conducting particles
facilitate the transmission of incident and reflected electromagnetic
radiation. In another
exemplary embodiment, the particles have a high dielectric constant and can
facilitate the
wave-guiding of the radiation signal.
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[0089] In one embodiment, the semi-conducting and/or non-conducting particles
are transparent to the electromagnetic radiation signals, i.e., they permit
the
electromagnetic radiation signals to pass through without any substantial
attenuation. In
another embodiment, the semi-conducting and/or non-conducting particles are
opaque to
the electromagnetic radiation signals, i.e., they completely absorb the
electromagnetic
radiation signals.
[0090] In one embodiment, a combination of semi-conducting, conducting and
non-conducting particles and/or proppants may be introduced into the fracture
to facilitate
the process of developing an image of the fracture. Combinations of different
types of
particles and/or proppants can be used to improve imaging capabilities of the
process. For
example, it may be desirable to screen certain sections of the fracture from
the
electromagnetic radiation signals to facilitate imaging of other sections.
Different types of
particles and/or proppants can be introduced into the fracture either
sequentially or
simultaneously.
[0091] Examples of electrically conducting particles are metallic particles,
non-
conducting particles with metallic coatings, carbonaceous particles,
electrically conducting
metal oxides, electrically conducting polymer particles, or the like, or a
combination
comprising at least one of the foregoing particles.
[0092] Examples of non-conducting particles that can be coated with metals (in
order to render them electrically conducting) are polymers such as
thermoplastic
polymers, thermosetting polymers, ionomers, dendrimers, or the like, or a
combination
comprising at least one of the foregoing polymers. The polymers are generally
electrically
insulating but can be made electrically conducting by coating them with a
layer of
electrically conducting metals. In an exemplary embodiment, the conducting
particles and
the non-conducting particles with metallic coatings can be magnetic or
magnetizable.
[0093] When non-conducting particles are coated with metals (e.g. by disposing
a
metallic coating upon a polymeric substrate), it is generally desirable for
the coated
particles to have a bulk density of about 0.5 to about 4.0 grams per cubic
centimeter
(g/cm3). It is desirable for the polymeric substrate to withstand down-hole
temperatures.
For example, it is desirable for the polymeric substrate to withstand
temperatures of up to
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2000 C. In an extreme case, proppants such as sintered bauxite or coated
bauxite may be
used at temperatures as high as about 300 C.
[00941 Examples of carbonaceous particles are carbon black, coke, graphitic
particles, fullerenes, carbon nanotubes such as single wall carbon nanotubes,
double wall
carbon nanotubes, multiwall carbon nanotubes, or the like, or a combination
comprising at
least one of the foregoing carbonaceous particles. Various types of conductive
carbon
fibers may also be used to reflect the electromagnetic radiation.
[00951 In one embodiment, the proppants or particles may comprise ceramic
substrates or polymeric substrates that are coated with an electrically
conducting coating
that comprises polymers, carbon nanotubes and/or carbon black. The
electrically
conducting coating generally has a bulk resistivity of less than or equal to
about 105 ohm-
cm. In ' another embodiment, the electrically conducting coating generally has
a bulk
resistivity of less than or equal to about 103 ohm-cm.
[00961 It is desirable for the conducting particles and/or proppants to have
an
electrical resistivity less than or equal to about 1012 ohm-cm. In one
embodiment, the
conducting particles and/or proppants have an electrical resistivity less than
or equal to
about 108 ohm-cm. In another embodiment, the conducting particles and/or
proppants
have an electrical resistivity less than or equal to about 105 ohm-cm. In yet
another
embodiment, the conducting particles and/or proppants have an electrical
resistivity less
than or equal to about 103 ohm-cm.
[00971 The semi-conducting particles can comprise silicon, gallium-arsenide,
cadmium selenide, cadmium sulfide, zinc sulfide, lead sulfide, indium
arsenide, indium
antimonide, or the like, or a combination comprising at least one of the
foregoing
semiconducting particles.
[00981 Non-conducting particles and/or proppants include insulating polymers
such as those listed above. The non-conducting particles and/or proppants and
the semi-
conducting particles and/or proppants referred to herein are all at least
electrically non-
conducting or semi-conducting. Non-conducting particles are also termed
dielectric
particles. Non-conducting particles or also include inorganic oxides,
inorganic carbides,
inorganic nitrides, inorganic hydroxides, inorganic oxides having hydroxide
coatings,
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inorganic carbonitrides, inorganic oxynitrides, inorganic borides, inorganic
borocarbides,
or the like, or a combination comprising at least one of the foregoing
inorganic materials.
[0099] Non-conducting particles and proppants also include electrically
conducting metallic substrates or non-metallic inorganic substrates that are
coated with
electrically non-conducting polymeric coatings or electrically non-conducting
ceramic
coatings.
[0100] One exemplary class of non-conducting particles and/or proppants
includes
high dielectric constant particles and/or proppants. One class of non-
conducting particles
and/or proppants comprises non-conducting polymeric substrates that have
filler dispersed
in the particle. The non-conducting filler can comprise non-metallic inorganic
particles,
naturally occurring organic particles such as ground or crushed nut shells,
ground or
crushed seed shells, ground or crushed fruit pits, processed wood, ground or
crushed
animal bones; synthetically prepared organic particles, or the like, or a
combination
comprising at least one of the naturally occurring particles.
[0101] Another class of non-conducting particles is granules comprising a
porous
glass or ceramics that can absorb electromagnetic radiation incident upon
them. Suitable
granules can comprise a ferrite such as nickel-zinc or barium-ferrite, wherein
the mass of
carbon to ferrite is greater than 0.225. Examples of such materials are
described in
patent/patent application WO 02/13311. These granules have an average particle
diameter
of 0.2 to 4.0 millimeters. The total porosity is about 70 to about 80 volume
percent. The
bulk density if about 0.5 to about 0.8 grams per cubic centimeter.
[0102] The particles can have any desirable geometry and any desirable
particle
size distribution. The particle geometry can be platelet like, spherical,
spheroidal, cuboid,
conical, cylindrical, tubular, polygonal, or the like, or a combination
comprising at least
one of the foregoing geometries. The particles can have aspect ratios of
greater than or
equal to about 1. The aspect ratio as defined herein is the ratio of the
largest dimension to
the smallest dimension of the particle. In one embodiment, it is desirable to
have an
aspect ratio of greater than or equal to about 5. In another embodiment, it is
desirable to
have an aspect ratio of greater than or equal to about 50. In yet another,
embodiment it is
desirable to have an aspect ratio of greater than or equal to about 100.
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[0103] The particles and/or proppants can be modified after being introduced
into
the fracture. For example, electrically non-conducting particles and/or
proppants can be
reacted after introduction into the fracture to form electrically conducting
or semi-
conducting particles and/or proppants. In one embodiment, the electrically non-
conducting particles can be disposed upon a support prior to introduction into
the fracture.
The support can be a proppant, a porous inorganic substrate, an organic
substrate, a fiber,
or the like. In one embodiment, the electrically non-conducting particles can
be coated
onto the support and can exist in the form of a continuous coating upon the
support. In
another embodiment, the electrically non-conducting particles can form
discrete particles
on the support. After introduction into the fracture, the reaction converts
the electrically
non-conducting particles into electrically conducting or semi-conducting
particles.
[0104] Examples of electrically non-conducting particles are metal salts such
as
metal sulfates, metal nitrates, metal chlorides, metal chlorates, metal
fluorides, metal
hydroxides, metal iodides, metal hydroxides, metal carbonates, metal acetates,
metal
bromides, or the like. The electrically non-conducting particles can be
reacted with a
gaseous or liquid reactant to form an electrically conducting particle. The
reactants can be
contained in the fracturing fluid or can be added to the fracture independent
of the fracture
fluid to facilitate the reaction. The fracture temperature, which is about 100
to about
250 C, can facilitate the reaction. Examples of suitable metal salts are
aluminum nitrate,
copper sulfate, copper nitrate, or the like, or a combination comprising at
least one of the
foregoing.
[0105] It is desirable for the smallest dimension of the particle to be on the
order
of 0.1 nanometers or greater. In another embodiment, the smallest dimension of
the
particle can be on the order of 10 nanometers or greater. In yet another
embodiment, the
smallest dimension of the particle can be on the order of 100 nanometers or
greater. In yet
another embodiment, the smallest dimension of the particle can be on the order
of 1000
nanometers or greater.
[0106] If desired, particles having specific predetermined reflecting or
absorbing
characteristics different from other proppant may be restricted to being the
first proppant
pumped. This should ensure their deposition near the tip/end of the propped
fracture (point
of greatest distance from the wellbore). Thus, a first portion of proppant can
be injected
through the wellbore into the subterranean formation and subsequently a second
portion of
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proppant can be injected through the wellbore into the subterranean formation
such that
the first portion of proppant travels to ends of fractures of the subterranean
formation
distal to the wellbore, wherein the first portion of proppant contains
particles which reflect
or absorb the source radar signal and the second portion of proppant has an
absence of
such particles.
[01071 If desired, the logging device could be employed in a method comprising
injecting a first portion of proppant through the wellbore into the
subterranean formation
and subsequently injecting a second portion of proppant into the subterranean
formation
such that the first portion of proppant travels to ends of fractures of the
subterranean
formation distal to the wellbore, wherein the first portion of proppant
contains particles
which are nonlinear and create new frequencies, frequency distortions, or
frequency
disruptions from the source radar signal and the second portion of proppant
has an absence
of such nonlinear particles of the first portion. Use of nonlinear particles
can help
differentiate a reflection off a wall or turn from a signal/reflection which
results from a
signal from the transmitter reaching the end of the propped fracture.
[01081 Typical nonlinear materials function as a rectifier or a piezoelectric
material or create intermodulation. Examples of non-linear components include
lithium
niobate, nickel oxide, iron oxide (ferric oxide or ferrous oxide), or copper
oxide (cuprous
oxide or cupric oxide).
[01091 For example, in detecting the end of a fracture using intermodulation
noise
the end of a fracture may be tagged using materials with a non-linear
relationship between
impedance and the voltage (or current) to which the tagging material is
exposed. Such a
non-linear relationship creates new frequencies that arise from the sum and
difference of
the frequencies that the material is exposed to and is called
"Intermodulation". In
particular, materials that create intermodulation contain a non-linearity
(e.g., a non-linear
bond) having impedance that varies according to the magnitude of the voltage
or current to
which it is exposed. When signals at two different frequencies (fl and f2)
pass through a
non-linearity they create signals at their sum and difference frequencies (fl -
f2 and fl +
f2). These are known as "intermodulation products". When three signals pass
through a
non-linearity they create signals at the sum and difference frequencies of
each pair of
frequencies, plus frequencies corresponding to a number of other sum and
difference
relationships between them to achieve typically 6 intermodulation products in
total.
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[0110] Thus, in an advantageous method of the present invention such a tagging
material with a non-linear relationship between impedance and the voltage (or
current) to
which the tagging material is exposed is sent into a fracture with a first
portion of proppant
to be preferentially sent to fracture tips distal to the wellbore. Subsequent
portions of
proppant sent into the wellbore would not include the selected tagging
material of the first
portion. Then, by simultaneously applying two or more different frequencies
downhole to
these fractures, intermodulation products can be created to create a
distinctive signal or
signals which can help differentiate a reflection off a wall or turn from a
signal/reflection
which results from reaching the end of the propped fracture.
[0111] In an alternative embodiment the tagging material is in all the
proppant sent
into fractures.
[0112] Ferrous metals, ferrite materials, metal salts, intermetallic species
of
copper, aluminum, iron, carbon and silicon are examples of materials which
exhibit such a
non-linear relationship between impedance and applied voltage to create
intermodulation
products.
Classes and causes of Intermodulation:
[0113] 1) Presence of ferrous metals in the region of high RF fields: The
hysteresis
associated with permeable materials and a non-linear V-I curve produce
intermodulation.
Typical materials are steels, nickel alloys, and nickel iron alloys, for
example INVAR
(also called FeNi36, is an alloy of iron (64 wt. %) and nickel (wt. 36%) with
some carbon
and chromium) or variations such as FeNi42. "Super-Invar" (31 wt. % Ni-5 wt. %
Co-
Balance Iron, possibly with some carbon and chromium) or nickel-cobalt ferrous
alloys,
such as KOVAR (29 wt. % nickel, 17 wt, % cobalt, 0.2 wt, % silicon, 0.3 wt. %
manganese, and 53.5 wt. % iron).
[0114] 2) Metals in contact: This can form an inefficient rectifier: Cuprous
oxide is
a p-type semi-conductor. "Tunneling" through a thin oxide layer between
similar metals is
another mechanism.
[0115] 3) Microarcing: Non-touching surfaces in close proximity can microarc
above a certain potential, especially at high temperature and altitude.
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[0116] 4) Electrostriction of dielectrics and magnetostriction of ferrite
material:
The physical stress in the material alters the material's physical dimensions.
[0117] 5) Excitation of spin-wave modes in Below Resonance Ferrite Devices: By
controlling the microscopic properties, the bulk properties are modified.
[0118] 6) Proximity to Ferromagnetic Resonance in Above Resonance Ferrite
Devices: The mechanism which causes intermodulation generation also causes the
non-
reciprocity in the ferrite medium.
EXAMPLE 1: 10 GHz prototype design and testing
Equipment
[0119] An embodiment of the invention was tested with the components that
would be down-hole tested in an oven at temperatures from 20 to 210 C.
[0120] The test apparatus had the components shown in Fig. 3A and was used to
measure the distance of the moving blades of a fan (not shown). Thus, the
embodiment
employed in this example included a microwave signal source (microwave
frequency
generator) 221, a laser driver (laser transmitter) 225, a modulator 226, an
audio
amplifier/filter 230, mixer 250, microwave frequency generator 240 and RF
spectrum
analyzer 260, as well as a photodiode 505 and transceiver 510. The modulator
226 was
connected to the photodiode 505 by 1 kilometer of polyimide coated fiber 300.
The
transceiver 510 was connected to the audio amp/filter 230 by a twisted pair of
wires 302
(shown in Fig. 3A). Also, in the example a DC bias was applied to the
photodiode 505
through a twisted pair (not shown). A DC bias source and some type of wiring,
for
example a twisted pair (not shown), is typically used to supply the photodiode
with DC
voltage.
[0121] The laser transmitter 225 employed in the example encompassed all of
the
components required to generate a laser signal in fiber, and modulate the
signal with an
RF carrier. Since in use at an oil well the laser transmitter would reside
above ground and
would not be subject to any unusual temperatures, in these tests the laser
transmitter was
not in the oven. The laser transmitter included a custom driver and
temperature controller
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board, an external intensity modulator, and a power RF amplifier/driver. An
operation
wavelength of 1550 run was chosen for these tests for the following reasons:
[0122] The availability of high temperature polyimide coated single mode 1550
nm fiber.
[0123] The availability of high-RF bandwidth 1550 nm intensity modulators.
[0124] The availability of lasers with sufficient power at 1550 nm.
[0125] The availability of photodiodes with sufficient bandwidth and good
responsivity at 1550 rim.
[0126] The capability to provide further optical amplification of 1550 nm
signals
with Erbium doped fiber amplifiers.
[0127] The RF power delivered over an intensity-modulated link can be derived
from fundamental principles. The RF signal power, S,f delivered to the
photodiode at peak
AC photocurrent) is given by,
S,)f=I2RL13l2=PorRLI3/2
where I is the peak AC photocurrent, RL is the load impedance, Po is the
average
optical power, il, is the quantum efficiency of the photodiode, and [3 is the
optical
modulation index. Taking 0=1 and it =0.8, results in calculating that an
average optical
power of 7 mW (8.4 dBm) is required at the photodiode in order to obtain a 0
dBm RF
power output into RL= 50 92.
[0128] The typical insertion loss into a Mach Zehnder intensity modulator is
just
over 3 dB (optical). The loss expected in the fiber is 0.6 dB/km. Hence, in
the setup for
these examples, with 1 km of fiber, the loss will be 0.6 dB. Hence the laser
itself must
deliver at least 8.4 dBm +3.6 dB = 12 dBm (16 mW) of optical power.
[0129] In particular the apparatus of these examples included a JDS Uniphase
CQF
935.708 telecommunications laser. This laser is rated for up to 40 mW of
optical power.
This telecommunications laser includes a built in thermoelectric cooler and
thermistor.
[0130] A regulated current source supply and a temperature control system were
also provided to be used with the laser. The thermal control circuit used
Analog Devices
ADN8830, a microchip designed specifically for laser temperature control. This
is a
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pulsed current driver for the thermoelectric cooler with PID feedback
compensation. The
regulated current source uses a pair of Analog Devices ADN2830 chips. This
device is
designed specifically for driving laser diodes.
[0131] The laser diode, driver, and, cooling controller were designed as a
single
board as shown in Fig. 7. The Mach Zehnder modulator employed in these
examples was a
JDS Uniphase Lithium Niobate electro-optic modulator that converts optical
phase
modulation into intensity modulation via a Mach Zehnder optical
interferometer. An RF
drive power of approximately 26 dBm is required in order to obtain a 100%
modulation
index. Hence, the Mach Zehnder modulator was preceded with a Nextec-RF NB00422
10
GHz power amplifier. A photograph of the modulator is shown in Fig. 8.
[0132] The hybrid transceiver 510 included the components shown in above-
discussed Fig. 5, namely, the hybrid coupler 515, the antenna 512, and the
single balanced
diode mixer 520. The lab version had a packaged photodiode connected to the
transceiver
assembly via semi-rigid coaxial cable. A production device could have the
photodiode
mounted on the transceiver assembly.
[0133] The photodiode 505 converted optical power into approximately 0 dBm (1
mW) of RF power at 10 GHz. As shown in Fig. 5, a portion of the 10 GHz signal
was
split in the hybrid coupler 515 and radiated through the antenna 512. The
other portion
was sent to the mixer 520 and used as a local oscillator (LO). The signal that
was radiated
from the antenna 512 was then returned through the same antenna 512 after
propagating to
and from the target (in this example the fan blades). The signal received by
the antenna
512 was routed through the coupler 515 back to the mixer 520, where it was
mixed with
the LO. If the source RF signal was chirped, the return signal was shifted
slightly in
frequency from the LO and an audio beat frequency was produced by the mixer.
[0134] The photodiode 505 of this example was a Discovery Semiconductor DSC
40S photodiode. This device is designed for 10 Gb/s telecommunications
applications, and
has sufficient bandwidth to receive our 10 GHz intensity modulated signal. The
diode is
not rated for high temperature operation but, as seen below, the data
resulting from this
example did not indicate any degradation at our test temperatures of up to 210
C.
[0135] The antenna 512 of this example was a wideband bowtie antenna design.
The bowtie antenna was modeled as a transmission line terminated in a
radiation
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resistance. The characteristic impedance is a function of the bowtie angle and
the
resistance seen at the terminal is a function of both the antenna length and
the bowtie
angle. The antenna was fabricated on Rogers 4350 laminate, due to the high
frequency
characteristics and high temperature thermal stability of this material.
[0136] The antenna 512 was a bowtie antenna with a full width antenna angle of
90 degrees with a characteristic impedance of 198 Ohms as measured by time
domain
reflectometry. The best-measured match to the antenna was at 9.9 GHz where a
feed-point
impedance of 53-2 j Ohms at the bowtie feed-point was measured. Antennas other
than a
bowtie can be used, but the bowtie antenna is preferred for approximately 10
GHz
operation.
[0137] The antenna dimensions are shown in Fig. 9. Fig. 10 is a graphical
illustration of the antenna return loss in dB. It should be noted that the
loss measurements
were approximately -17.8dB (at Marker 4) for 10GHz transmission, and -30.1dB
at 9.9
GHz transmission (at Marker 3).
[0138] The antenna 512 was fed through a one wavelength transmission line
having a characteristic impedance of 147 Ohms. The feed-line was designed to
be one
wavelength long at 10 GHz to avoid transforming the antenna feed-point
impedance. As
will be discussed below, the inventors achieved better results with a signal
on the order of
1 GHz rather than 10 GHz.
[0139] The hybrid coupler of this example was a standard 180 degree circular
coupler, designed to operate at 10 GHz. A photograph of the coupler is shown
in Fig. 12.
RF power coupled into the input is split evenly between the antenna port and
the mixer's
LO port. Power received by the antenna is split between the mixer's RF port
and the IN
port. Power sent to the IN port is not used; this is a byproduct of the
splitting function.
[0140] RF power sent to the LO and RF mixer ports is split between the "I" and
"Q" ports on the (square) 90 degree splitter. The "I" and "Q" ports are each
populated with
zero bias RF mixing diodes, forming a single balanced mixer configuration. The
output of
the diodes is an audio frequency difference signal that is sent to the surface
through
twisted pair. Note that the input port contains a quarter wavelength stub;
this provides a
DC ground for the photodiode. Fig. 12 shows the transceiver board and hybrid
coupler
(unpopulated).
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Procedure
[0141] The following design, prototyping, and testing of each component, the
radar system of this example was tested at down-hole temperatures of up to 210
C. The
experimental setup is shown in Fig 13.
[0142] The continuous 1550 nm laser signal from the transmitter board was
intensity modulated with a 10 GHz CW microwave tone. The modulated laser
signal was
routed into an oven through 1 km of high temperature single mode polyimide
coated fiber.
The optical power loss of the fiber was measured as 0.6dB/km. This corresponds
to only
1.2 dBe/km of electrical/RF loss. The RF power delivered to the photodiode was
up to -2
dBm with a DC photocurrent of 6 mA and an optical power of 7.4 mW.
[0143] The photodiode and custom transceiver were placed inside the oven. The
oven contained a small (18 cm x 28 cm) window, through which the 10 GHz signal
could
be radiated and received. The audio frequency signal output from the
transceiver was fed
back out of the oven through TEFLON coated twisted pair wires. For this
experiment the
photodiode bias was delivered through a separate pair of TEFLON coated twisted
pair
wires, although in the field, this DC bias voltage would be sent down-hole on
the audio
twisted pair.
[0144] Since this example was not performing tests in an anechoic chamber or
the
actual down-hole environment, the tests were performed with a moving test
target to
distinguish the target signature from background clutter such as workbenches,
test
equipment, and building structural components. Moreover, signal processing
development
was not included in this example and the moving target allowed the performance
of the
example without additional signal processing.
[0145] The target was a 30 cm diameter fan with metal blades. The target was
placed just outside of the oven window. The metal cage was removed from the
front of
the fan such that the RF would reflect from the moving fan blades. A picture
of the setup
is shown in Fig. 13. The Doppler shift from the fan produced an -8 10 Hz audio
frequency
return on the twisted pair output lines. The return was amplified and filtered
to remove
background 60 Hz power line hum. In lieu of audio frequency spectrum analysis,
the
audio return signal was mixed up to a center frequency of 10 MHz and viewed on
an RF
spectrum analyzer.
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Results
[0146] The oven temperature was then increased from 20 C to 210 C. A plot of
the measured return signal power vs. oven temperature is shown in Fig. 14. A
plot of the
radar return showing the Doppler shifted audio frequency output is shown in
Fig. 15. Fig.
15 shows the return signal frequency from a first experiment utilizing a
moving target in
lieu of a fracture in a wellbore. As discussed above, in this simulation a
metal blade fan
was set up to be the target. As shown in Fig. 15, a Doppler signal (A) of
approximately -
810 Hz is created by moving the fan. At point B on the graph, the fan is
stopped and thus
the Doppler signal (B) is about zero.
Analysis
[0147] The data in Fig. 14 indicated a reduction in received signal as the
temperature is increased. Near room temperature the slope is -0.05 dB/ C.
Several factors
together may contribute to the observed trend. Small temperature dependent
changes in the
circuit board substrate dielectric constant result in changes to the microwave
matching in
both the antenna and mixer. The carrier density and mobility within the
photodiode and
the mixing diodes are somewhat temperature dependent. The sensitivity of the
photodiode
is slightly temperature dependent. The moisture content of the air in the oven
(and hence
absorption at 10 GHz) is temperature dependent.
[0148] When the oven temperature exceeds 150 C, the slope became steeper.
However, the solder connecting the photodiode to the receiver board was
visibly melting
and the SMA connector (between photodiode and transceiver) was coming out of
the
transceiver board under its own weight. This drop in signal was accompanied by
a
reduction in photocurrent. When cooled back down to room temperature, the
measured
signal remained degraded. After the solder connection was repaired, the signal
level
returned to within 1 dB of its previous level at room temperature. These
results are all
consistent with the melting solder that was observed.
[0149] To further verify that the melting solder near the photodiode was the
only
mode of failure, the photodiode was removed and a 10 GHz signal was injected
directly
into the transceiver port. Individual components including the antenna and
diode mixer
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were individually heated with a 260 C heat pencil and no performance
degradation was
observed.
[0150] The photodiode was then placed in the oven alone, and the RF power from
the photodiode over temperature was measured. Less than 0.5 dB change in power
was
observed over the temperature test range of 22 C to 180 C. These results all
point to
melting solder as the only mode of failure. Thus, this can be remedied by
using a higher
temperature solder.
[0151] It is expected that if the solder used here is replaced with a higher
temperature solder, then the 0.05 dB/ C trend will return and the output
signal will be
reduced by approximately 10 dB as the temperature is increased from 20 C to
210 C.
Increasing the laser drive power and/or employing good signal processing
design in the
receiver can counteract this reduction in power.
EXAMPLE 2: Propagation Experiments
[0152] The system of Example 1 was designed to operate at 10 GHz with a free
space wavelength of 3 cm. This short wavelength was chosen to facilitate a 24
mm form
factor on the down-hole receiver and antenna.
[0153] However, it was desired to learn about the electrical properties of
various
proppant that could be introduced into a subterranean fracture. The real and
imaginary
dielectric constant of such proppants influences the electromagnetic guiding
properties of
the fracture. This information, as well as the size of the casing, assists in
choosing an
optimum operating frequency.
[0154] Thus, after completing the 10 GHz prototype design and testing, the
propagation loss in a few sample proppant materials was measured
(understanding the
optimal frequency may not be exactly 10 GHz).
[0155] The propagation experiment example was set up as follows. Two known
lengths of 5 cm inside diameter Polyvinyl Chloride (PVC) pipe were capped at
both ends
after being filled with proppant. Slits were cut in each end of the pipe,
allowing our
broadband bowtie antenna to be inserted at each end. A photograph of the setup
is shown
in Fig 16.
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Results
[01561 Transmission loss, return loss, and delay between antennas were
measured
between 5 GHz and 15 GHz using a 20 GHz network analyzer. Table II summarizes
the
GHz network analyzer measurements:
5
TABLE II: 10 GHz Propagation Measurement Summary
Proppant Length 10 GHz Transmission 10 GHz Return Delay
Material (m) S21 (dB) Loss, S11 (dB) (ns)
Air 11=0.42 -48 2.3
Sand 11=0.42 -39 3.1
CERAMAX 11=0.42 -45 3.3
proppant
Air 12 =2.47 -53 -13 10.5
Sand 12 =2.47 -44 -5 14.9
CERAMAX 12 =2.47 -76 -7
proppant
Analysis
[01571 Referring to Table II, comparing the results from the air filled PVC to
the
proppant filled PVC, indicates the presence of the proppants has a dramatic
effect on the
10 transmission loss (in dB).
101581 Measurement of transmission loss through the air filled PVC tube shows
a
much lower loss than one would get from purely free space transmission,
indicating that
even the air filled PVC tube provides dielectric guiding. The measured 10 GHz
propagation losses in PVC tubes filled with air, sand, and CERAMAX curable
resin
coated ceramic proppant, available from HEXION Specialty Chemicals, Houston,
Texas
are estimated as follows:
L (dB/m)=[S21(l )-S21(12)]/(12-11)
La;r(dB1m)=[-48 dB-(-53dB)l(2.47 m- 0.42m)]=2.44 dBlm
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Lsand(dBlm)=[-39dB-(-44dB)l(2.47m-0.42m)]=2.44dBlm
Lceamas(dBlm)=[-45dB-(-76 dB)1(2.47m-0.42m)]=15.12dBlm
[0159] The CERAMAX proppant material, with 15dB/m, has far too large an
attenuation to be of practical use in a radar system. The air filled tube and
sand filled tube
both exhibited 2.44 dB/m attenuation. This ratio indicates that the PVC tube
itself can be
a significant source of loss when it is the only guiding dielectric material.
However, note
that sand filled tube produces a much longer propagation delay, indicating
that the sand,
not the PVC, is the dominant guiding material when the tube is sand filled.
[0160] TABLE III summarizes measurements of the propagation delay that were
also made in order to estimate the effective dielectric constant of the
proppant. The
measured delay does include the delay required to radiate and receive the
signal, therefore,
the propagation velocity is best estimated from the longer, 247 cm
measurement. The
delay can be compared to the calculated free space delay to determine the
relative velocity
factor and index of refraction as shown in TABLE III. The air filled PVC tube
alone has a
velocity of 0.8 x c. The addition of sand reduces the velocity to only 0.55 x
c. The loss in
the CERAMAX proppant was too high to make an accurate measurement of the delay
and
velocity factor.
TABLE III: Measured Velocity Factors
Parameter Air Filled PVC tube Sand Filled PVC tube
Velocity Factor 0.8 0.55
Ref Index (nff) 1.3 1.8
Dielectric Const (Er) 1.1 1.3
[0161] Measurements of the antenna return loss were also made to diagnose any
detuning of the antenna due to dielectric loading. In the presence of both
proppants, the
antenna resonance is shifted below 10 GHz and the return loss becomes greater
than our
acceptable value of -15 dB.
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[0162] Table IV summarizes results from measurements made at a frequency of 2
GHz. This experiment was done to evaluate the benefits of operating at lower
frequencies.
The 2 GHz measurements were made with the same antenna designed to operate at
10
GHz; therefore the antenna was electrically short and mismatched at 2 GHz.
Furthermore
the receiving antenna in the short 42 cm tube is not in the far field at 2
GHz. Hence, these
results cannot be used to make a very accurate measurement of attenuation per
unit length.
[0163] Nonetheless, it is noted that the overall transmission loss over 247 cm
is
significantly better than in the 10 GHz case in spite of the large antenna
mismatch. Hence,
the inventors conclude a reduction in frequency of operation to well below 10
GHz will
significantly reduce overall transmission loss. This is consistent with theory
that suggests
approximately a 10 dB reduction in absorptive losses per order of magnitude
reduction in
frequency.
[0164] These results point toward the conclusion that the 10 GHz operational
frequency results in unacceptably high transmission losses, and that the
losses can be
improved by reducing the frequency. The choice of operating frequency will be
based
upon a trade-off between antenna form factor efficiency, and frequency
dependent
transmission loss in the proppant.
[0165] The 10 GHz frequency is not preferred to measure a substantial propped
fracture length (> 20 ft) but has benefits if the use is to identify when a
casing perforation
is connected to a propped fracture.
[0166] TABLE IV shows the success with some of these items:
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TABLE IV: 2 GHz Propagation Measurement Summary
Proppant Length (cm) 2 GHz Transmission S21 (dB)
Material
Air 42 -44
Sand 42 -36
CERAMAX
Proppant 42 -31
Air 247 -49
Sand 247 -34
CERAMAX
Proppant 247 -37
[01671 The new down-hole radar-logging device overcomes two important
challenges. Namely, the radar can operate at high temperatures, and the
microwave signals
can be sent through several kilometers of fiber to reach the down-hole
transceiver with
minimal loss.
[01681 The invention solved the propagation problem by employing a high
temperature low loss (about 1.2 dBe/km) fiber optic signal feed. The high
temperature
problem is solved by using a passive electronic design for the down-hole
transceiver. The
unique design uses passive electronic parts down-hole, including a photodiode
and mixing
diode. Both components performed well at high temperatures in this example.
[01691 Tests were preformed demonstrating the invention at temperatures as
high
as 210 C. This demonstration included all down-hole electronics and optics.
For the first
demonstration we used a moving test target. It was then determined that
additional signal
processing would be required to view a static target in the cluttered lab
environment.
[01701 Temperature tests showed some degradation in signal level at high
temperatures. Reduction of signal above 150 C was found to be due to melting
solder, a
problem that will be easily remedied by use of a higher temperature solder in
the next
design. Additionally, a signal reduction of 0.05dB/ C was observed. This drop
is
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acceptable since it can be countered by increasing the optical drive power.
[0171] There were some propagation measurements performed to determine the
expected propagation loss in some sample proppants. It was determined that at
10 GHz,
signal attenuation is too high to provide a practical range measurement over
more than a
few meters in the CERAMAX proppant. The sand proppant may be usable over a
larger
range at 10 GHz with additional signal processing.
[0172] Preliminary propagation measurements made at 2 GHz showed the
propagation loss is lower (better) than at 10 GHz.
EXAMPLE 3
[0173] This example used a simulated fracture sandstone model to perform
propagation tests with proppants to determine the optimal operational
frequencies to be
used in fracture length detection. The test frequency range used was from 250
MHz to 3
GHz. Sand and CERAMAX proppants were used in the sandstone test.
[0174] FIG. I I A shows the electronics setup for the propagation test.
Antennas
600, 601 are placed in the slots within the braces and surrounded by foam
encapsulated by
the plywood structure. Two antennas 600, 601 were used in the tests.
Transmitting
antennas (601) were placed at different locations as shown in FIG. 11A. The
microwave
network analyzer 620 was used to generate signals ranging from 250 MHz to 3
GHz. As
shown in FIG. 11 A, the attenuation of the signal was measured by comparing
the received
signal level at the receiving antenna (element 600) as the transmitting
antenna (element
601) was changed.
[0175] The network analyzer 620 generates a signal in the frequency range from
250 MHz to 3 GHz. This signal is amplified and sent to the transmitting
antenna (element
601). The received signal is measured from the receiving antenna (element
600). Before
the measurement, the system is calibrated to account for loss in the cables
and any internal
losses in the instrument.
[0176] FIG. 11B shows a setup of the simulated fracture sandstone model 651,
wherein the gap is adjustable and simulates the fracture and will be filled
with coated sand
and CERAMAX proppants. The model 651 was 2 feet high and 24 ft. long. It was 1
inch
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width at the simulated fracture and constructed from '/ in. and 3/ in.
exterior plywood (one
side water resistant). It is mounted on seven 2 inch x 12 inch x 10 ft. planks
for stability.
Sides were supported by glued 2 x 4 boards 652 and 4 x 4 boards (not shown),
attached to
the 2 inch x 12 inch by 10 feet boards 654 at 4 foot intervals. End cap 650
and spacer are
made from 5 mm x %2 in. firing strips, and glued in place. All joints are
sealed with
plumber's putty. In the tests, sandstone was used to "enclose" the proppants.
Twelve
sandstone slabs 660 (six on a side) were used to create the simulated
fracture. The
sandstone slabs 660 are 2 feet x 4 feet x 2 inches thick, six on each side of
the fracture
enclosure. The model had cleats 653 and forms 655.
[0177] The antennas 600 and 601 are mounted parallel to the sandstone sides.
The
antennas are linearly polarized, and the azimuthal radiation pattern is
approximately
ominidirectional around the neck of the antenna.
[0178] The fracture model 651 simulates different environments. With different
kinds of proppants, we have the following testing scenario matrix as listed in
TABLE V:
TABLE V
Medium Medium Condition Sandstone
Condition
Air Dry Moist
Pro ant (sand) Dry Moist
Pro ant (sand) Wet Moist
Pro ant (Ceramax) Dry Moist
Pro ant (Ceramax) Wet Moist
[0179] The testing media "air" connotes a simulated fracture that is not
filled with
any propant. Tests were done with wet and dry proppants, because water may be
present
in the actual porosity of the fracture. The sandstone was saturated with water
for three
days prior to the test in order to simulate water content in the surrounding
formation.
Theory
[0180] The propagation test is to measure signal loss from 250 MHz to 3 GHz in
the proppant media under different conditions. In theory, the power received
by the fixed
receiving antenna can be related to the location of the transmitting antenna
as follows:
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P(z2) [dBm] = P(z1) [dBm] - a[dB/m] x (z2[m]-z1[m]) (1)
where P(z1) is the received power when the separation between transmitting and
receiving antennas is z1, P(z2) is the received power when the separation
between
transmitting and receiving antennas is z2i and where P(z1) is the received
power when the
separation between transmitting and receiving antennas is z1, P(z2) is the
received power
when the separation between transmitting and. receiving antennas is z2, and a
is the
attenuation in Decibels per meter.
[0181] The S parameter measured by the network analyzer measures the power
loss (in dB) between the transmission and reception ports, such that equation
(1) can be
rewritten as follows:
S21(z2) [dB] = S21(z1) [dB] - a[dB/m] x (z2[m]-z1[m]) (2)
[0182] Using equation (2) and measuring the S21 at z1 and z2, the attenuation
per
unit length, a can be determined. Equation (2) is a good estimate only when
the
transmission mode is a guided wave and when z1 and z2 are large enough to
ensure that
measurements are made in the far-field.
[0183] Using the above-described simulated fracture sandstone model, tests
were
performed for a specialized down hole radar. In the field, the radar signal
will be
propagated through a down hole fracture, which will be reinforced/filled with
proppant.
The model was developed to obtain electromagnetic propagation characteristics
in this
environment.
[0184] It was a goal of the test to determine the attenuation characteristics
of the
electromagnetic modes that would be supported in a fracture with proppant.
[0185] As described above, the simulated fracture sandstone model is outfitted
with slots allowing the insertion of wideband antennas 600 and 601 into the
simulated
fracture at discrete locations. Network parameter (S21) measurements were made
at each
of these locations to determine propagation characteristics.
[0186] The measurements were performed under both wet and dry conditions. The
wet condition simulates porosity with irreducible water saturation.
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Practical Considerations
[0187] Propagation through the fracture model will occur in two types of
modes.
First, a portion of the launched power will travel as a guided mode as
described by
equation (2).
[0188] However, a portion of the launched power may also travel as an unguided
mode, expanding in a spherically shaped wave front as it traverses the
fracture. However,
propagation loss of this unguided mode is expected to decrease very quickly
according to
the following equation. Therefore at large distances, the guided mode
described by
equation (3) is more important.
L[dB] = 20 x log(r2/rl) (3)
where L represents the signal path loss in dB between radius r2 and radius r1
when
the transmission antenna is placed at the origin.
Test Procedure
[0189] A receiving antenna 600 was embedded at the origin of the model (as
shown in Fig. 11 A).
[0190] FIG. 11 C shows a photograph of the model and showing a Slot for the
receiving antenna as well as Slots A, B and C (and their distances from the
receiving
antenna slot at the origin.
[0191] The receiving antenna 600 was a wideband high pass antenna (1.1 GHz
3dB roll-off). This antenna 600 was connected to Port 2 (element 640) of the
network
analyzer 620. Two additional transmission antennas 601 were then embedded in
two of
the three available antenna slots. The transmission antennas 601 were
identical wideband
high pass antennas (606 MHz 3-dB roll-off). The active transmission antenna
601 was
connected to Port 1(element 630) of the analyzer 620.
[0192] Two of the following three S parameters were then measured over
frequency: S21-A, S21-B, S21-c. Here S21_A implies excitation of the
transmission antenna
(601) at Slot A and reception by the receiving antenna (600) placed at the
origin.
[0193] Before performing any measurements, the network analyzer 620 and
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antenna feed lines were calibrated by use of a 50 Ohm load, short circuit, and
open circuit
termination. The calibration removes the effects of frequency dependent loss
in the
transmission line and internal to the analyzer.
[0194] After embedding the antennas 600, 601 into the model, the S21
parameters
were measured with an empty fracture (air). The fracture was then filled with
proppant
and the measurements were repeated. The proppant was soaked with water and the
measurements were again repeated. Finally, the foam surrounding the fracture
was soaked
with water and the measurements were repeated.
[0195] Measurements were performed at two sets of antenna slots. In the first
experiment, S21 -A, and S21_B were measured. In the second set of experiments
[0196] S21-B, and S21_C were measured. By comparing the measured levels at
either
pair of locations, attenuation per unit length was estimated according to (2).
[0197] Choice of measurement location involved some trade-offs. In making the
S21 -A measurement, Slot A is only 0.12 meters from the source. This implies
that at our
lowest frequencies (<800 MHz), the antenna at Slot A and the receiving antenna
will
exhibit near field coupling. Therefore, the S21_A measurements are more
meaningful at
higher frequencies. Results for frequencies below 800 MHz were not recorded in
the S21_A
data TABLES VI and VII.
[0198] Similarly, in making the S21_C measurements, the signal loss between
the
origin and Slot C was high enough to attenuate the highest frequencies below
the noise
floor of the analyzer 620. Thus, the S21-C measurements at the lower
frequencies provide
more meaningful data. The measured signal level was at least 15 dB above the
measured
(post calibration) noise floor for the data to be recorded in the tables of
results. For these
reasons the data recorded in the tables for each of the experiments is at
different set of
frequencies.
Results
[0199] The measured data for S21_A and S21-B are presented in Table VI. A
sample
network analyzer screen photograph from this experiment is shown in FIG. 11 D.
[0200] FIG. 11D is a photograph of a screen capture S21-B screen from the
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ACFRAC/ACPAK proppant and VERSAPROP proppant with "Wet Proppant/Wet Foam"
in Table VI.
[0201] The measured data for or S21_B and S21_c are presented in Table VII. As
discussed above, the most meaningful measurements from Table VI are those made
at the
highest frequencies, while the most meaningful measurements in Table VII are
those made
at lower frequencies. The reduction in attenuation with increased frequency in
Table VI is
consistent with near field effects occurring at the lower frequencies.
[0202] The highest frequency measurements from TABLE VI, shows an
attenuation of -j 5 dB/m in the 2-3 GHz range.
[0203] The lowest frequency measurements from TABLE VII for both (dry) air
and PR600 proppant, shows a loss of 4-6 dB/m in the 250 MHz to 1 GHz range.
The
Ceramic proppant is somewhat more lossy with a 6-12 dB/m loss depending on the
frequency and conditions (wet or dry). The air measurement with wet foam also
presents
us with a higher loss of 7-8 dB/meter.
TABLE VI
Propagation measured between the origin and Slot A S21_A and Slot B s21-B .
Proppant Proppant Frequency S21-A S21-B Difference Attenuation
/Foam (MHz) (dB) (dB) dB (dB/m)
Air Dry/Dry 1000 -44 -61 17 6.97
2000 -62 -77 15 6.15
3000 -68 -80 12 4.92
ACFRAC/ACPAK Dry/Dry 1000 -38 -58 20 8.2
and 2000 -58 -72 14 5.74
VERSA PROP 3000 -63 -76 13 5.33
ACFRAC/ACPAK Wet/Dry 1000 -42 -55 13 5.33
and 2000 -65 -79 14 5.74
VERSA PROP 3000 -78 -89 11 4.51
ACFRAC/ACPAK Wet/Wet 1000 -52 -61 9 3.69
and 2000 -65 -78 13 5.33
VERSA PROP
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TABLE VII
Propagation measured between the origin and Slot B (s21_B) and Slot C (S21-c).
* "Dry" foam contained some residual moisture from previous day's
measurements.
Proppant Proppant Frequency S21-A S21-B Difference Attenuation
/Foam (MHz) (dB) (dB) (dB) (dB/m)
Air Dry/Dry 250 -39 -53 14 5.74
1000 -59 -73 14 5.74
1500 -69 -78 9 3.69
PR600 Dry/Dry 1000 -58 -67 9 3.69
1500 -69 -81 12 4.92
PR600 Wet/Dry 250 -40 -53 13 5.33
1000 -58 -67 9 3.69
1500 -72 -81 9 3.69
PR600 Wet/Wet 250 -39 -49 10 4.1
1000 -56 -69 13 5.33
1500 -68 -83 15 6.15
Air Dry/Wet 250 -40 -58 18 7.38
600 -48 -67 19 7.79
Ceramic Dry/Wet 250 -38 -59 21 8.61
600 -40 -70 30 12.3
1000 -60 -75 15 6.15
Ceramic Wet/Wet 250 -41 -56 15 6.15
600 -46 -68 22 9.02
[02041 The present invention provides a significant advantage over attempted
radar
logging devices of the prior art. The high temperature problems associated
with the non-
operation of active components, and/or possible attempts to cool these
components have
been solved by using just passive components down-hole. The transceiver
comprises a
photodiode, a diode mixer, and a hybrid coupler (as well as the antenna). No
amplification
is required of the reflected signal as it will be mixed and travels back along
a twisted pair
as a beat frequency comprising an audio signal that is a fraction of the
original microwave
frequency.
[02051 It is also understood that an artisan will appreciate that another
advantage
of the present invention is that the spectrum analyzation can be performed
above ground
without having the down-hole constraints to overcome.
[02061 It is also clear that, although the invention has been described with
reference to a specific example, a person of skill will certainly be able to
achieve many
other equivalent forms, all of which will come within the field and scope of
the invention.
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[02071 For example, while the generation of the modulated light signal occurs
above the wellbore, it is possible that this signal could also be generated
down-hole. To
elaborate, the laser transmitter 225 and the modulator 226 could be located
down-hole and
the microwave signal generated above the wellbore. Or it is possible the
entire signal
generation occurs down-hole. The placement of any or combinations of the laser
transmitter, modulator or radar source could be below the ground but not at a
depth where
the ambient temperature impacts the operation of the equipment so as to render
it unusable
without cooling devices. In addition, the source radar signal can be encoded
(for example,
encoded other more sophisticated signals including but not limited to direct
sequence
coding) such that the return signal differs from the transmitted signal. The
mixer then
functions as a correlator that cross-correlates the encoded source radar
signal with the
return signal (e.g., reflected radar signal).
[0208] It should be apparent that embodiments other than those specifically
described above come within the spirit and scope of the present invention.
Thus, the
present invention is not limited by the above description but rather is
defined by the claims
appended hereto.
