Note: Descriptions are shown in the official language in which they were submitted.
CA 02524728 2005-10-28
1669P06CA01
SYSTEM FOR MEASURING EARTH FORMATION RESISTIVITY
THROUGH AN ELECTRICALLY CONDUCTIVE WELLBORE CASING
Background of the Invention
Field of the Invention
The invention relates generally to the field of Earth formation electrical
resistivity measuring
devices. More particularly, the invention relates to wellbore instruments for
measuring
formation resistivity from within an electrically conductive pipe or casing.
Background Art
Electrical resistivity measurements of Earth formations are known in the art
for determining
properties of the measured Earth formations. Properties of interest include
the fluid content of
the pore spaces of the Earth formations. Wellbore resistivity measuring
devices known in the art
typically require that the Earth formations be exposed by drilling a wellbore
therethrough, and
that such formations remain exposed to the wellbore so that the measurements
may be made
from within the exposed formations.
When wellbores are completely drilled through the Earth formations of
interest, frequently a
steel pipe or casing is inserted into and cemented in place within the
wellbore to protect the Earth
formations, to prevent hydraulic communication between subsurface Earth
formations, and to
provide mechanical integrity to the wellbore. Steel casing is highly
electrically conductive, and
as a result makes it difficult to use conventional (so called "open hole")
techniques to determine
the resistivity of the various Earth formations from within a steel pipe or
casing.
It is known in the art to make measurements for determining the electrical
resistivity of Earth
formations from within conductive casing or pipe. A number of references
disclose techniques
for making such measurements. A list of references which disclose various
apparatus and
methods for determining resistivity of Earth formations from within conductive
casings includes:
USSR inventor certificate no. 56052, filed by Alpin, L. M. (1939), entitled,
The method for
logging in cased wells; USSR inventor certificate no. 56026, filed by Alpin,
L. M. (1939),
1
CA 02524728 2005-10-28
entitled, Process of the electrical measurement of well casing; U.S. patent
no. 2,459,196, to
Stewart, W. H. (1949), entitled, Electrical logging method and apparatus; U.S.
patent no.
2,729,784 issued to Fearon, R. E. (1956), entitled, Method and apparatus for
electric well
logging; U.S. patent no. 2,891,215 issued to Fearon, R. E. (1959), entitled,
Method and
apparatus for electric well logging; French patent application no. 72.41218,
filed by
Desbrandes, R. and Mengez, P. (1972), entitled, Method &Apparatus for
measuring the
formation electrical resistivity In wells having metal casing; International
Patent Application
Publication no. WO 00/79307 Al, filed by Benimeli, D. (2002), entitled, A
method and
apparatus for determining of a formation surrounding a cased well; U.S. patent
no. 4,796,186
issued to Kaufinan, A. A. (1989), entitled, Conductivity determination in a
formation having a
cased well; U.S. patent no. 4,820,989, issued to Vail, III, W. (1989),
entitled, Methods and
apparatus for measurement of the resistivity ofgeological formation from
within cased
boreholes; U.S. patent no. 4,837,518 issued to Gard et al. (1989), entitled,
Method and
Apparatus for measuring the electrical resistivity of formation through metal
drill pipe or
casing; U.S. patent no. 4,882,542 issued to Vail, III, W. (1989), entitled,
Methods and apparatus
for measurement of electronic properties ofgeological formations through
borehole casing; U.S.
patent no. 5,043,668 issued to Vail, III, W. (1991), entitled, Methods and
apparatus for
measurement of electronic properties ofgeologicalformations through borehole
casing; U.S.
patent no. 5,075,626 issued to Vail, III, W. (1991), entitled, Electronic
measurement apparatus
movable in a cased borehole and compensation for casing resistance
differences; U.S. patent no.
5,223,794 issued to Vail, III, W. (1993), entitled, Methods of apparatus
measuring formation
resistivity from within a cased well having one measurement and two
compensation steps; U.S.
patent no. 5,510,712 issued to Sezginer et al. (1996), entitled, Method and
apparatus for
measuring formation resistivity in cased holes; U.S. patent no. 5,543,715
issued to Singer et al.
(1996), entitled, Method and apparatus for measuring formation resistivity
through casing using
single-conductor electrical logging cable; U.S. patent no. 5,563,514 issued to
Moulin (1996),
entitled, Method and apparatus for determining formation resistivity in a
cased well using three
electrodes arranged in a Wheatstone bridge. U.S. patent no. 5,654,639 issued
to Locatelli et al.
(1997), entitled, Induction measuring device in the presence of metal walls;
U.S. patent no.
5,570,024 issued to Vail, III, W. (1996), entitled, Determining resistivity of
a formation adjacent
2
CA 02524728 2005-10-28
to a borehole having casing using multiple electrodes and resistances being
defined between the
electrodes; U.S. patent no. 5,608,323 issued to Koelman, J. M. V. A. (1997),
entitled,
Arrangement of the electrodes for an electrical logging system for determining
the electrical
resistivity of subsurface formation; U.S. patent no. 5,633,590 issued to Vail,
III, W. (1997),
entitled, Formation resistivity measurements from within a cased well used to
quantitatively
determine the amount of oil and gas present. U.S. patent no. 5,680,049 issued
to Gissler et al.
(1997), entitled, Apparatus for measuring formation resistivity through casing
having a coaxial
tubing inserted therein; U.S. patent no. 5,809,458 issued to Tamarchenko
(1998), entitled,
Method of simulating the response of a through-casing resistivity well logging
instrument and its
application to determining resistivity of earth formations; U.S. patent no.
6,025,721 issued to
Vail, III, W. (2000), entitled, Determining resistivity of a formation
adjacent to a borehole
having casing by generating constant current flow in portion of casing and
using at least two
voltage measurement electrodes; U.S. patent no. 6,157,195 issued to Vail, III,
W. (2000),
entitled, Formation resistivity measurements from within a cased well used to
quantitatively
determine the amount of oil and gas present; U.S. patent no. 6,246,240 BI
issued to Vail, III, W.
(2001), entitled, Determining resistivity offormation adjacent to a borehole
having casing with
an apparatus having all current conducting electrodes within the cased well ;
U.S. patent no.
6,603,314 issued to Kostelnicek et al. (2003),entitled, Simultaneous current
injection for
measurement of formation resistance through casing; and U.S. Patent No.
6,667,621 issued to
Benimelli, entitled, Method and apparatus for determining the resistivity of a
formation
surrounding a cased well.
United States Patent Application Publications which cite relevant art include
no. 2001/0033164
Al, filed by Vinegar et al., entitled, Focused through-casing resistivity
measurement; no.
2001/0038287 Al, filed by Amini, Bijan K., entitled, Logging tool for
measurement of resistivity
through casing using metallic transparencies and magnetic lensing; no.
2002/0105333 Al filed
by Amini, Bijan K., entitled, Measurements of electrical properties through
non magnetically
permeable metals using directed magnetic beams and magnetic lenses. and no.
2003/0042016
Al, filed by Vinegar et al., entitled, Wireless communication using well
casing
The foregoing techniques are summarized briefly below. U.S. patent no.
2,459,196 describes a
method for measuring inside a cased wellbore, whereby electrical current is
caused to flow along
3
CA 02524728 2005-10-28
the conductive casing such that some of the current will "leak" into the
surrounding Earth
formations. The amount of current leakage is related to the electrical
conductivity of the Earth
formations. The `196 patent does not disclose any technique for correcting the
measurements for
electrical inhomogeneities in the casing.
U.S. patent no. 2,729,784 discloses a technique in which three potential
electrodes are used to
create two opposed pairs of electrodes in contact with a wellbore casing.
Electrical current is
caused to flow in two opposing "loops" through two pairs of current electrodes
placed above
and below the potential electrodes such that electrical inhomogeneities in the
casing have their
effect nulled. Voltage drop across the two electrode pairs is related to the
leakage current into
the Earth formations. The disclosure in U.S. patent no. 2,891,215 includes a
current emitter
electrode disposed between the measuring electrodes of the apparatus disclosed
in the `784
patent to provide a technique for fully compensating the leakage current.
U.S. patent no. 4,796,186 discloses the technique most frequently used to
determine resistivity
through conductive casing, and includes measuring leakage current into the
Earth formations,
and discloses measuring current flowing along the same portion of casing in
which the leakage
current is measured so as to compensate the measurements of leakage current
for changes in
resistance along the casing. Other references describe various extensions and
improvements to
the basic techniques of resistivity measurement through casing.
The methods known in the art for measuring resistivity through casing can be
summarized as
follows. An instrument is lowered into the wellbore having at least one
electrode on the
instrument (A) which is placed into contact with the casing at various depths
in the casing. A
casing current return electrode B is disposed at the top of and connected to
the casing. A
formation current return electrode B* is disposed at the Earth's surface at
some distance from the
wellbore. A record is made of the voltage drop and current flowing from
electrode A in the
wellbore at various depths, first to electrode B at the top of the casing and
then to formation
return electrode B*. Current flow and voltage drop through the casing (A-B) is
used to correct
measurements of voltage drop and current flow through the formation (A-B*) for
effects of
inhomogeneity in the casing.
4
CA 02524728 2005-10-28
If the Earth and the casing were both homogeneous, a record with respect to
depth of the voltage
drop along the casing, and the voltage drop through the casing and formation,
would be
substantially linear. As is well known in the art, casing includes
inhomogeneities, even when
new, resulting from construction tolerances, composition tolerances, and even
"collars"
(threaded couplings) used to connect segments of the casing to each other.
Earth formations, of
course, are not at all homogeneous, and more resistive formations are
typically the object of
subsurface investigation, because these Earth formations tend to be associated
with presence of
petroleum, while the more conductive formations tend to be associated with the
presence of all
connate water in the pore spaces. Therefore, it is the perturbations in the
record of voltage drop
with respect to depth that are of interest in determining resistivity of Earth
formations outside
casing using the techniques known in the art.
The conductivity of the Earth formations is related to the amount of current
leaking out of the
casing into the formations. The formation conductivity with respect to depth
is generally related
to the second derivative of the voltage drop along A-B with respect to depth,
when current is
flowing between A and B*. Typically, the second derivative of the voltage drop
is measured
using a minimum of three axially spaced apart electrodes placed in contact
with the casing,
coupled to cascaded differential amplifiers, ultimately coupled to a voltage
measuring circuit.
Improvements to the basic method that have proven useful include systems which
create s small
axial zone along the casing in which substantially no current flows along the
casing itself to
reduce the effects of casing inhomogeneity on the measurements of leakage
current voltage drop.
In practice, instruments and methods known in the art require that the
instrument make its
measurements from a fixed position within the wellbore, which makes measuring
formations of
interest penetrated by a typical wellbore take an extensive amount of time.
Further, the voltage
drops being measured are small, and thus subject to noise limitations of the
electronic systems
used to make the measurements of voltage drop. Still further, systems known in
the art for
providing no-current zones, or known current flow values for measurements of
voltage drop, are
typically analog systems, and thus subject to the accuracy limitations of such
analog systems.
Still further, it is known in the art to use low frequency alternating current
(AC) to induce current
flow along the casing and in the Earth formations. AC is used to avoid error
resulting from
electrical polarization of the casing and the electrodes when continuous
direct current (DC) is
CA 02524728 2005-10-28
used. Typically, the frequency of the AC must be limited to about 0.01 to 20
Hz to avoid error in
the measurements caused by dielectric effects and the skin effect. It is also
known in the art to
use polarity-switched DC to make through casing resistivity measurements,
which avoids the
polarization problem, but may induce transient effect error in the
measurements when the DC
polarity is switched. Transient effects, and low frequency AC errors are not
easily accounted for
using systems known in the art.
Summary of the Invention
One aspect of the invention is an instrument for measuring resistivity of
Earth formations from
within a conductive pipe inside a wellbore drilled through the Earth
formations. The instrument
includes a plurality of housings connected end to end and adapted to traverse
the wellbore. At
least one electrode is disposed on each housing. Each electrode is adapted to
be placed in
electrical contact with the inside of the pipe. The instrument includes a
source of electrical
current, a digital voltage measuring circuit and a switch. The switch is
arranged to connect the
source of electrical current between one of the electrodes and a current
return at a selectable one
of the top of the pipe and a location near the Earth's surface at a selected
distance from the top of
the pipe, and to connect selected pairs of the electrodes to the digital
voltage measuring circuit.
The pairs are selected to make voltage measurements corresponding to selected
axial distances
and selected lateral depths in the Earth formations.
One embodiment of the instrument includes a focusing current source that is
coupled through the
switch to a selected pair of the electrodes to constrain measuring current to
flow in a laterally
outward path proximate the instrument.
One embodiment of the instrument includes a back up arm on one or more of the
housings and a
seismic receiver disposed in the one or more of the housings which includes
the back up arm.
Another aspect of the invention is a method for measuring resistivity of Earth
formations from
within a conductive pipe inside a wellbore drilled through the Earth
formations. A method
according to this aspect of the invention includes inserting a plurality of
housings connected end
to end to a selected depth inside the pipe. At least one electrode on each
housing is placed in
electrical contact with the inside of the pipe. Electrical current from a
measuring current source
6
CA 02524728 2008-07-22
is passed through at least one of the electrodes into the pipe. A return from
the measuring
current source is switched between one of the electrodes and a current return
at a
selectable one of the top of the pipe and a location near the Earth's surface
at a selected
distance from the top of the pipe. Voltages are digitally measured across
selected pairs of
the electrodes. The pairs of electrodes are selected to make voltage
measurements
corresponding to selected axial distances and selected lateral depths in the
Earth
formations.
In a first broad aspect, the present invention seeks to provide an instrument
for measuring
resistivity of Earth formations from within a conductive pipe inside a
wellbore drilled
through the formations, comprising:
a plurality of housings connected end to end, the housings adapted to traverse
the
interior of the pipe; at least one electrode on each housing, each electrode
adapted to be
placed in electrical contact with the interior of the pipe;
a source of electrical measuring current; at least one digital voltage
measuring
circuit;
at least a first switch arranged to selectably connect the source of
electrical
measuring current between one of the electrodes and a current return at a
selectable one
of the top of the pipe and a location near the Earth's surface at a selected
distance from
the top of the pipe;
at least a second switch arranged to connect selectable pairs of the
electrodes to
the digital voltage measuring circuit, the pairs selectable to make voltage
measurements
corresponding to selectable, variable axial distances between the electrodes
and
corresponding lateral depths in the Earth formations;
means for extending at least one of the electrodes laterally outward from the
respective housing associated therewith, a resistance measuring circuit
operatively
coupled between the at least one electrode and the pipe;
an electromagnetic transmitter antenna and an electromagnetic receiver antenna
disposed proximate a contact end of the at least one electrode;
a source of alternating current electrically coupled to the transmitter
antenna; and
7
CA 02524728 2008-07-22
a receiver circuit electrically coupled to the receiver antenna, whereby a
quality of
contact between the electrode and the conductive pipe is determinable by
comparison of a
resistance measured by the resistance measuring circuit to a voltage detected
by the
receiver circuit.
In a second broad aspect, the present invention seeks to provide a method for
measuring
resistivity of Earth formations from within a conductive pipe inside a
wellbore drilled
through the formations, comprising:
inserting a plurality of housings connected end to end to a selected depth
inside
the pipe;
causing at least one electrode on each housing to be placed in electrical
contact
with the inside of the pipe;
passing electrical current from a measuring current source through at least
one of
the electrodes into the pipe;
switching a current return point for the measuring current source between
another
one of the electrodes and a current return point at a selectable one of the
top of the pipe
and a location near the Earth's surface at a selected distance from the top of
the pipe;
digitally measuring voltages across selectable pairs of the electrodes, the
selectable pairs of electrodes to make voltage measurements corresponding to
selectable,
variable axial distances between the electrodes and corresponding lateral
depths in the
Earth formations; and
switching a focusing current source through selected pairs of the electrodes,
and
controlling an output of the focusing current source to constrain current
flowing from the
measuring current source between the one of the electrodes switched thereto
and the
return near the Earth's surface to a path substantially laterally outward from
the wellbore
in the lateral proximity of the wellbore.
Other aspects and advantages of the invention will be apparent from the
following
description and the appended claims.
7a
CA 02524728 2008-07-22
BRIEF DESCRIPTION OF DRAWINGS
Figure 1 shows an example resistivity measurement through casing apparatus
according
to the invention being used in a cased wellbore.
Figure 2 shows a circuit systems of the example apparatus of Figure 1 in more
detail.
Figures 3A through 3C show different examples of current waveform for making
through
casing resistivity measurements according to the invention.
Figure 4 shows an example instrument for measuring resistivity through a
conductive
pipe which includes current focusing systems.
Figure 5 shows an alternative embodiment of an apparatus including a
selectable array of
electrodes on a sonde mandrel.
Figure shows a flow chart of operation of an instrument such as shown in
Figure 4
adapted to automatically optimize control of electrode usage according to a
model based
instrument response.
Figure 7 shows a system for measuring resistivity through conductive pipe
including a
central control unit and a plurality of "satellite" units.
Figure 8 shows an embodiment as in Figure 7 including seismic receivers in one
or more
of the central control unit and the satellite units.
Figure 9 shows one embodiment of an electrode for making electrical contact
with the
interior surface of a conductive casing.
7b
CA 02524728 2005-10-28
Figure 10 shows a cut away view of the electrode shown in Figure 9.
Figure 11 shows a system for estimating quality of contact between an
electrode and the
conductive pipe.
Figure 12 shows an additional portion of one embodiment of an apparatus used
to evaluate the
condition of the interior surface of the pipe.
Detailed Description
One embodiment of a well logging instrument used to measure resistivity of
Earth formations
from within a wellbore, when the wellbore has a conductive pipe or casing
within is shown
schematically in Figure 1. The instrument 10 may include a sonde or similar
mandrel-type
housing 18. The housing 18 is preferably made from an electrically non-
conductive material, or
has such non-conductive material on its exterior surface. The housing 18 is
adapted to be
inserted into and withdrawn from the wellbore 14, by means of any well logging
instrument
conveyance known in the art. In the present example, the conveyance can be an
armored
electrical cable 16 extended and retracted by a winch 28. Other conveyances
known in the art
may be used, including coiled tubing, drill pipe, production tubing, etc.
Accordingly, the
conveyance is not intended to limit the scope of the invention.
The wellbore 14 is drilled through various Earth formations, shown
schematically at 22, 24 and
26. After the wellbore 14 is drilled, a conductive pipe 12 or casing is
inserted into the wellbore
14. If the pipe 12 is a casing, then the casing 12 is typically cemented in
place within the
wellbore 14, although cementing the pipe or casing is not necessary to
operation of the
instrument 10 . While the embodiment shown in Figure 1 is described in terms
of a "casing"
being inserted and cemented into a drilled wellbore, it should be understood
that other types of
electrically conductive pipe, such as drill pipe, coiled tubing, production
tubing and the like may
also be used with an instrument according to the invention. In one particular
example, the pipe
12, rather than being casing, may be drill pipe that has become stuck in the
wellbore 14,
whereupon the instrument 10 is lowered into the stuck drill pipe on an armored
electrical cable
16 to make measurements as will be further explained.
8
CA 02524728 2005-10-28
The armored electrical cable 16 includes one or more insulated electrical
conductors (not shown
separately) and is arranged to conduct electrical power to the instrument 10
disposed in the
wellbore 14. Electrical power can be conducted from, and signals from the
instrument 10 can be
transmitted to, a recording unit 30 disposed at the Earth's surface using the
electrical conductors
on the cable 16. The recording unit 30 may also be used to record and/or
interpret the signals
communicated thereto from the instrument 10 in the wellbore 14. The recording
unit 30 may
include an electrical power supply 32 used to make measurements for
determining resistivity of
the various Earth formations 22, 24, 26. In the present description, any
electrical power supply
used to enable making the measurements corresponding to formation resistivity
will be referred
to as a "measuring current source." The power supply 32 may also be used
merely to provide
electrical power to various measurement and control circuits, shown generally
at 20 in Figure 1,
in the instrument 10. The functions provided by the various circuits in the
instrument will be
further explained below with reference to Figure 2.
Still referring to Figure 1, a measuring current return electrode 34B* is
provided at the Earth's
surface at a selected distance from the wellbore 14. The measuring current
return electrode 34B*
is typically inserted into formations proximate the Earth's surface so as to
provide an electrically
conductive path to the Earth formations 22, 24, 26 penetrated by the wellbore
14. The measuring
current return electrode 34B* provides, in particular, a current path through
the Earth formations
22, 24 26 for electrical measuring current to flow from a source electrode A
on the instrument
10. The current return electrode 34B* may be connected, as shown in Figure 1,
either to circuits
35B* in the recording unit 30, or alternatively may be connected to one of the
electrical
conductors (not shown separately) in the cable 16. A casing current return
electrode 34B, shown
connected to the top of the pipe or casing 12, provides a return path for
electrical measuring
current caused to flow from the current source electrode A on the instrument
10, to the top of the
casing 12. The casing current return electrode 34B may be coupled to circuits
35B in the
recording unit 30, or may be coupled to one of the conductors (not shown) in
the cable 12 for
return to the circuits 20 in the instrument 10.
The instrument 10 includes a plurality of electrodes, shown at A, and PO
through P6 disposed on
the sonde mandrel 18 at axially spaced apart locations. The electrodes A, PO-
P6 are electrically
isolated from each other by the non-conductive material disposed on the
exterior of, or forming,
9
CA 02524728 2005-10-28
the sonde mandrel 18. Each of the electrodes A, PO-P6 is mechanically and
electrically adapted
to make good electrical contact with the casing 12. Various types of casing-
contact electrodes
are known in the art and include brushes, hydraulically actuated "spikes",
spiked wheels and
similar devices. The electrodes A, PO-P6 are each coupled to a selected
portion of the electronic
circuits 20 in the instrument 10.
During operation of the instrument 10 when conveyed by armored cable, the
cable 16 is extended
by the winch 28 so that the instrument 10 is positioned at a selected depth in
the wellbore 14.
Electrical power is passed through the casing 12 and through the Earth
formations 22, 24, 26 by
selective connection between the source electrode A at one end of the current
path, and either the
casing return 34B or formation return 34B*, respectively, at the other end of
the current path.
Measurements are made of the voltage extant between a reference potential
electrode, shown as
electrode PO in Figure 1, and one or more potential measurement electrodes, P1-
P6 in Figure 1.
Depending on the type of electrodes used, for example, brushes or spiked
contact wheels, it may
be possible, in some embodiments, for the instrument 10 to be moved slowly
along the wellbore
14 as the measurements are being made. Other types of electrode, such as
hydraulically actuated
spikes, may require that the instrument 10 remain essentially stationary
during any one
measurement sequence. As the voltage measurements are made, whether the
instrument 10 is
stationary or moving, the instrument 10 is gradually withdrawn from the
wellbore 14, until a
selected portion of the wellbore 14, including formations of interest, 22, 24,
26, have voltage
measurements made corresponding to them, both using the casing current return
34B and the
formation current return 34B*.
One embodiment of the electronic circuits 20 is shown in greater detail in
Figure 2. The present
embodiment of the circuits 20 may include a central processing unit (CPU) 50,
which may be a
preprogrammed microcomputer, or a programmable microcomputer. In the present
embodiment,
the CPU 50 is adapted to detect control commands from within a formatted
telemetry signal sent
by the recording unit (30 in Figure 1) to a telemetry transceiver and power
supply unit 48. The
telemetry transceiver 48 also performs both formatting of data signals
communicated by the CPU
50 for transmission along a cable conductor 16A to the recording unit (30 in
Figure 1) and
reception and conditioning of electrical power sent along the conductor 16A
for use by the
various components of the circuits 20. The CPU 50 may also be reprogrammed by
the command
CA 02524728 2005-10-28
signals when such are detected by the telemetry transceiver 48 and conducted
to the CPU 50.
Reprogramming may include, for example, changing the waveform of the measure
current used
to make the previously explained voltage drop measurements. Reprogramming may
also include
changing the magnitude of the measure current, and may include changing a
sample rate of
voltage drop measurements, among other examples. Still other forms of
reprogramming will be
explained with reference to Figures 4 through 6.
While the embodiment shown in Figure 2 includes an electrical telemetry
transceiver 48, it
should be clearly understood that optical telemetry may be used in some
embodiments, and in
such embodiments the telemetry transceiver 48 would include suitable
photoelectric sensors
and/or transmitting devices known in the art. In such embodiments, the cable
16 should include
at least one optical fiber for conducting such telemetry signals. One
embodiment of an armored
electrical cable including optical fibers therein for signal telemetry is
disclosed in U.S. patent no.
5,495,547 issued to Rafie et al. Other embodiments may use optical fibers to
transmit electrical
operating power to the instrument 10 from the recording unit 30. The cable
disclosed in the
Rafie et al. `547 patent or a similar fiber optic cable may be used in such
other embodiments to
transmit power to the instrument over optical fibers.
The CPU 50 may include in its initial programming (or may be so programmed by
reprogramming telemetry signals) a digital representation of various current
waveforms used to
energize the Earth formations (22, 24 26 in Figure 1) and the casing (12 in
Figure 1) for
determining the resistivity of the Earth formations (22, 24, 26 in Figure 1).
The digital
representation includes information about the frequency content, the shape of
the waveform and
the amplitude of the current to be conducted through the formations and
casing. The digital
representation can be conducted to a digital to analog converter (DAC) 42,
which generates an
analog signal from the digital representation. The analog signal output of the
DAC 42 is then
conducted to the input of a power amplifier 44. The power amplifier 44 output
is connected
between the current source electrode A and a switch 47. The switch 47 is under
control of the
CPU 50. The switch 47 alternates connection of the other output terminal of
the power amplifier
44 between the casing return electrode B and the formation return electrode
B*, or other current
electrodes in other electrode arrangements. Alternatively, the other output
terminal of the power
amplifier 44 may be connected to one of more cable conductors (either 16A or
other electrical
11
CA 02524728 2005-10-28
conductor), and the switching between casing return and formation return may
be performed
within the recording unit (30 in Figure 1). Yet another alternative omits the
DAC 42 and the
power amplifier 44 from the circuits 20, and provides measuring current and
switching features
using the power supply (32 in Figure 1) in the recording unit (30 in Figure 1)
and appropriate
conductors (not shown) in the cable (16 in Figure 1). In the latter example
embodiment,
measuring current may be conducted to the source electrode A using one or more
cable
conductors, such as 16A in Figure 2.
In the present embodiment, voltage measurements can be made between the
potential reference
electrode PO and a selected one of the potential measuring electrodes P1-P6.
The one of the
voltage measuring electrodes from which measurements are made at any moment in
time can be
controlled by a multiplexer (MUX) 40, which itself may be controlled by the
CPU 50. The
output of the MUX 40 is connected to the input of a low noise preamplifier or
amplifier 38. The
output of the preamplifier 38 is coupled to an analog to digital converter
(ADC) 36. The ADC 36
may be a sigma delta converter, successive approximation register, or any
other analog to digital
conversion device known in the art, that preferably can provide at least 24
bit resolution of the
input signal. Digital signals output from the ADC 36 represent the measured
potential between
the reference electrode PO and the MUX-selected one of the voltage measuring
electrodes PI-P6.
One possible advantage of using the MUX 40 and single preamplifier 38 as shown
in Figure 2 is
that the analog portion of the voltage measuring circuitry will be
substantially the same
irrespective of which voltage measuring electrode PI-P6 is being interrogated
to determine
potential drop with respect to electrode P0. As a result, measurement error
caused by differences
in preamplifier 38 response may be reduced or eliminated. Preferably, the ADC
36 is a twenty-
four bit device capable of accurately resolving measurements representing
voltage differences as
small as one nanovolt (W O-9 10-9 volts). Alternatively, each measurement
electrode P l -P6 could be
coupled to one input terminal of a separate preamplifier (not shown in the
Figures) for each
electrode P 1 -P6, thus eliminating the MUX 40 from the analog input
circuitry.
Digital words representing the voltage measurements can be conducted from the
ADC 36 to the
CPU 50 for inclusion in the telemetry to the recording unit (30 in Figure 1).
Alternatively, the
CPU 50 may include its own memory or other storage device (not shown
separately) for storing
the digital words until the instrument (10 in Figure 1) is removed from the
wellbore (14 in Figure
12
CA 02524728 2005-10-28
1). In some embodiments, a sample rate of the ADC 36 is in the range of
several kilohertz (kHz)
both to provide both a very large number of voltage signal samples, preferably
at least one
thousand, per cycle of current waveform, and to be able to sample transient
effects when
switched DC is used as a current source to make resistivity measurements. In
such
embodiments, a switching frequency of the switched DC can be in a range of
about 0.01 to 20
Hz, thus enabling the ADC 36 to make preferably at least one thousand, and as
many as several
thousand, voltage measurement samples within each cycle of the switched DC.
In the present embodiment, the ADC 36 operates substantially continuously, to
provide a
relatively large number of digital signal samples for each cycle of the
current source waveform.
In the present embodiment, such substantially continuous operation of the ADC
36 may provide
the advantage of precise, prompt determination of any DC bias in the voltage
measurements.
Such DC bias must be accounted for in order to precisely determine formation
resistivity from
the voltage measurements. In systems known in the art which do not operate
voltage measuring
devices substantially continuously, it is necessary to determine DC bias by
other means. See, for
example, U.S. patent no. 5,467,018 issued to Rueter et al.
The measuring current waveform, as previously explained, may be generated by
conducting
waveform numerical values from the CPU 50, or other storage device (not shown)
to the DAC
42. Referring now to Figures 3A through 3C, several types of current waveforms
particularly
suited to making through-casing (or through electrically conductive pipe)
resistivity
measurements will be explained. Figure 3A is a graph of current output of the
power amplifier
(44 in Figure 2) with respect to time. The current waveform 60 in Figure 3A is
a low frequency
(0.01 to 20 Hz) square wave, which may be generated using switched DC, or by
conducting
appropriate numbers representing such a waveform to the DAC (42 in Figure 2).
The waveform
60 in Figure 3A is periodic, meaning that the waveform is substantially
constant frequency
within a selected time range, and has 100 percent "duty cycle", meaning that
current is flowing
substantially at all times.
Another possible current waveform is shown at 60 in Figure 3B. The current
waveform in
Figure 3B is a random or pseudo random frequency square wave, also having 100
percent duty
cycle. As with the previous embodiment (Figure 3A), the embodiment of current
waveform
shown in Figure 3B may be generated by conducting appropriate digital words
from the CPU (50
13
CA 02524728 2005-10-28
in Figure 2) to the DAC (42 in Figure 2). Random switching will be
advantageous to avoid
aliasing or other adverse effects related to periodic data sampling.
Another possible waveform is shown at 60 in Figure 3C. The current waveform 60
in Figure 3C
is a periodic square wave having less than 100 percent duty cycle. Less than
100 percent duty
cycle can be inferred from time intervals, shown at 62, in which no current is
flowing. As with
the previous embodiment (Figure 3A), the embodiment of current waveform shown
in Figure 3C
may be generated by conducting appropriate digital words from the CPU (50 in
Figure 2) to the
DAC (42 in Figure 2). Using less than 100 percent duty cycle may be
advantageous to save
electrical power where measured voltage drops are sufficiently large to make
possible a
reduction in the number of voltage samples measured. Using less than 100
percent duty cycle
may also enable determination of some transient effects, by measuring voltage
drops across the
various electrodes (PO b between P 1-P6 in Figure 1) during a short time
interval after the current
is switched off. Such induced potential (IP) effects may be related to fluid
composition within
the pore spaces of the Earth formations (22, 24, 26 in Figure 1). Using less
than 100 percent
duty cycle may also enable better determination of any DC bias, by using the
times with no
current flow 62 as measurement references.
The foregoing examples shown in Figures 3A, 3B and 3C are not the only current
waveforms
that may be generated using the CPU/DAC combination shown in Figure 2. As will
be readily
appreciated by those skilled in the art, substantially any frequency and
waveform type may be
generated, including for example sinusoidal waveforms, by conducting
appropriate digital words
to the DAC (42 in Figure 2). In some embodiments, the digital words may be
stored in the CPU
(50 in Figure 2). In other embodiments, the digital words themselves, or a
command which
activates selected waveform digital words, may be transmitted from the
recording unit (30 in
Figure 1) to the instrument (10 in Figure 1) over the cable (16 in Figure 1).
In other
embodiments, the waveform may be a pseudo random binary sequence (PRBS).
Referring once again to Figure 2, some embodiments may include one or more of
the following
features, either programmed into the CPU 50, or programmed into a surface
computer in the
recording unit (30 in Figure 1). Some embodiments may include automatic
editing of voltage
measurements made across the one or more electrode pairs, PO between any one
of P 1-P6. For
example, if a particular digital voltage sample represents a number outside of
a selected range,
14
CA 02524728 2005-10-28
the sample may be discarded, and an interpolated value may be written to
storage in the CPU 50,
or transmitted to the recording unit (30 in Figure 1) for the outlying sample
value. Alternatively,
if voltage measurements do not increase monotonically as the spacing between
PO and the
various measurement electrodes P1 -P6 is increased, the anomalous voltage
samples may be
discarded; interpolated or otherwise not written directly to storage. Other
embodiments may
include stacking of voltage measurement words corresponding to the same
electrode pair (PO
between any of P1-P6) at substantially the same depth in the wellbore to
improve the signal to
noise ratio of the measurements significantly.
Referring once again to Figure 1, still other embodiments may include
permanent installation of
an array of electrodes, such as shown in Figure 1 at A and PO through P6
inside the casing 16. A
cable or similar device may be used to make electrical connection to the
Earth's surface from
inside the wellbore 14 at a selected depth proximate a petroleum bearing
reservoir, for example,
formation 24 in Figure 1. Measurements may be made at selected times during
the life of the
wellbore 14 to determine movement of a water contact (not shown in Figure 1)
with respect to
time. In such permanent emplacements of electrodes A, PO-P6, the circuits 20
may be disposed
at the Earth's surface, or may themselves be disposed in the wellbore 14, just
as for the cable
conveyed instrument described earlier herein.
Operating the instrument may be performed in a number of different ways, of
which several will
be explained herein. In a regular measurement mode, the instrument 10 may be
moved to a
selected depth in the wellbore 14 at which measurements are to be made. First,
the circuits 20
are operated, either by internal programming of the CPU (50 in Figure 2) or by
command
transmitted from the recording unit (30 in Figure 1) first to enable measuring
voltage drop
caused by current flow entirely along the casing 12. To make casing voltage
drop
measurements, the power amplifier (44 in Figure 2) is connected between the
current source
electrode A on the instrument 10 and casing current return electrode 34B
coupled to the top of
the casing (12 in Figure 1) at the Earth's surface. Voltage measurements
between PO and any
one or more of P1 through P6 are then made. The output of the power amplifier
(44 in Figure 2)
is then switched to return the measuring current at measuring current return
electrode 34B* at the
Earth's surface. Another set of voltage measurements between PO and the same
ones of P1
through P6 are made. The instrument 10 may then be moved a selected axial
distance along the
CA 02524728 2008-07-22
wellbore 14, and the measuring process can be repeated. Values of voltage
difference made between PO
and any one or more of P 1 through P6 can be converted mathematically into a
second derivative, with
respect to depth in the wellbore 14, of the measured voltage drop. The values
of such second derivative
are related to the depth-based current leakage into the Earth formations 22,
24, 26, and are thus related
to the electrical conductivity of each of the formations 22, 24, 26.
Advantageously, an instrument
configured substantially as shown in Figures 1 and 2 does not require
measurement of voltage drop
across cascaded differential amplifiers (all of which would be analog) to
determine the second derivative
of voltage drop with respect to depth.
Performance of an instrument according to the invention may be improved by
providing focusing
current systems to axially constrain the flow of measuring current through the
various Earth formations.
An example instrument which includes focusing current systems is shown
schematically in Figure 4.
The principle of measurement of the example instrument shown in Figure 4 is
described in U.S. Pat. No.
2,729,784 issued to Fearon. The instrument in Figure 4 includes an array of
electrodes disposed at
selected locations along the instrument mandrel or housing (18 in Figure 1).
The electrodes may be
similar in mechanical and electrical configuration to the electrodes described
above with reference to
FIG. 1. The electrodes are adapted to make electrical contact with the pipe or
casing (12 in Figure 1) in
the wellbore (14 in Figure 1).
The electrodes in the embodiment of Figure 4 include two pairs of focusing
current electrodes, shown at
B 1 A, B 1 B and B2A, B2B, approximately equally spaced on either axial side
of a central measuring
current source electrode MO. Reference potential measuring electrodes RIA, RIB
and R2A, R2B are
disposed, respectively, between each focusing current electrode pair B 1 A, B
1 B; B2A, B2B, and the
measuring current source electrode MO. Each focusing current electrode pair B
I A, B 1 B and B2A, B2B
is connected across the output of a corresponding focusing current power
amplifier 44A, 44C,
respectively. In the present embodiment, the focusing current is generated by
driving each power
amplifier 44A, 44C using the output of a corresponding DAC 42A, 42C. Each DAC
42A, 42C can be
connected to a bus or other similar data connection to the CPU 50. As in the
embodiment explained
above with reference to Figure 2, the embodiment shown in Figure 4 may include
digital words stored or
16
CA 02524728 2008-07-22
interpreted by the CPU 50 which. represent the focusing current waveform to be
generated by each
power amplifier 44A,
16a
CA 02524728 2005-10-28
44C and conducted to the casing (12 in Figure 1). Aspects of the waveform
which may be
controlled include amplitude, phase, frequency and duty cycle, among other
aspects.
Each pair of reference potential measuring electrodes R1A, RIB and R2A, R2B is
coupled
across the input terminals of a respective low noise preamplifier 38A, 38B, or
low noise
amplifier, similar to the preamplifier described with reference to Figure 2.
Each low noise
preamplifier 38A, 38B has its output coupled to an ADC 42A, 42B. The ADC 42A,
42B outputs
are coupled to the bus or otherwise to the CPU 50. In the present embodiments,
the ADCs 42A,
42B are preferably 24 bit resolution devices, similar to the ADC described
with reference to
Figure 2. In the present embodiment, potential difference measurements are
made across each
pair of reference potential electrodes R1A, RIB and R2A, R2B, respectively.
The CPU 50
receives digital words representing the measured potential across each
reference electrode pair
R1A, RiB and R2A, R2B, respectively. The magnitude of the focusing current
output by each
power amplifier 44A, 44C can be controlled by the CPU 50 such that the
measured potential
across each pair of reference potential electrodes R1A, RIB and R2A, R2B,
respectively, is
substantially equal to zero. The CPU 50 may cause such adjustments to be made
by, for
example, changing the amplitude or changing the duty cycle of the power
amplifier 44A, 44B
outputs, or both. Changes to amplitude and/or duty cycle may be made to either
or both power
amplifier 44A, 44B. Other methods for changing or adjusting the power output
of each focusing
current power amplifier 44A, 44C will occur to those skilled in the art. The
purpose of making
such focusing current magnitude adjustments so as to maintain substantially
zero potential across
the reference electrodes RIA, RIB and R2A, R2B, respectively, is to assure
that there is a region
within the casing (12 in Figure 1) where substantially no net current flows
along the casing in
either an upward or downward direction.
The embodiment of Figure 4 can include a digitally controlled measuring
current source. The
source consists of, in the present embodiment, a measuring current DAC 42B
coupled to the bus
or otherwise to the CPU 50. Measuring current is generated by conducting
waveform words to
the DAC 42B, which converts the words into a driver signal for a measuring
current power
amplifier 44B coupled at its input to the DAC 42B output. Measuring current
output from the
measuring current power amplifier 44B is coupled to the measuring current
source electrode MO,
and maybe returned at the Earth's surface, at return electrode 34B*, or
alternatively at casing
17
CA 02524728 2005-10-28
current return 34B. Measuring potential electrodes M1A, M1B are disposed on
either side of the
measuring current source electrode MO. Each measuring potential electrode M 1
A, M 1 B, and the
source electrode MO is coupled across the input of a respective measuring
potential low noise
amplifier 38B, 38C. The output of each measuring potential low noise amplifier
38B, 38C is
coupled to a respective ADC 36B, 36C, wherein digital words representing the
value of
measured potential across each respective pair of measure potential electrodes
MIA, MO and
M1B, MO are conducted to the CPU 50 for processing. The measuring potential
ADC 44B is
also preferably a 24 bit resolution device. Resistivity of the Earth
formations outside the casing
is related to the potential across the measuring potential electrodes and the
magnitude of the
measuring current. Waveform, frequency and duty cycle of the measuring current
may be
controlled in a substantially similar manner as explained with reference to
the embodiment of
Figure 2.
Possible advantages of a system as shown in Figure 4 include more accurate
control over
focusing current properties than was previously possible, making measurements
of potential
across the measuring electrodes MIA, M I B more accurate.
Another embodiment of an instrument according to the invention is shown
schematically in
Figure 5. The instrument includes an array of electrodes disposed on the
instrument housing 18
at axially spaced apart locations. The electrodes are designated A, B, P, 0, N
and M. The
electrodes are coupled through a switching system, designated "control unit"
50A (which may be
associated with for form part of a controller similar in design to CPU 50 from
Figure 2). The
control unit 50A selects which electrodes are coupled to which one or selected
circuits. The
circuits include a current source 52. The current source 52 may be a digital
synthesizer, and may
include a DAC and power amplifier (not shown separately). The circuits may
include a voltage
(or potential) measuring circuit 51, which may include a low noise
preamplifier and ADC (not
shown separately) as explained with reference to Figure 2. The circuits may
also include a
voltage feedback unit 53, which may be similar in configuration to the
focusing current source
explained with reference to Figure 4.
To perform various types of measurements, the instrument shown in Figure 5 can
select the
measuring and focusing current sources to be applied to, and voltage
measurements to be made
across, selected ones of the electrodes and selected electrode pairs. Examples
of various modes
18
CA 02524728 2005-10-28
of measurement, and the electrodes used to make measurements in each of the
modes, are
explained in the following table:
Measurement Mode Current source and Potential measured
return electrodes across electrodes
Downhole, completely contained A, B M and N; 0 and P
Deep penetrating resistivity B, current return is at M and N; 0 and P
Earth's surface away
from top of casing
(return 34B*)
Fast measurement M and N A and B; 0 and P
Mixed Mix sources Mix pairs
In the above table, the "Current source and return electrodes" column
represents the electrodes
coupled to the measuring current source 52. Potential measurement is made
across electrode
pairs as indicated in the "Potential measured across electrodes" column.
Various configurations of an instrument according to the invention which
include a suitably
programmed CPU (50 in Figure 2) may provide substantially real-time automatic
control of
selection of the various electrodes for the purposes as explained above with
reference to Figure
4, namely axial spacings of the voltage measuring electrodes, and the spacing
of and amount of
focusing current supplied to various focusing electrodes. A generalized flow
chart showing one
embodiment of a system programmed to perform the foregoing functions is shown
in Figure 6.
At 70, initially configured electrodes, current sources and voltage measuring
circuits emit
measuring current, focusing current and make voltage measurements,
respectively. Initial
configuration may be set by the system operator, or may be preprogrammed.
Preprogrammed or
operator-selected initial configuration may be based on parameters such as
expected thickness of
the various Earth formations and expected resistivities of the various Earth
formations, among
other parameters. At 71, voltages are measured, at least for one pair of
voltage measuring
electrodes. In configurations which include reference potential electrodes,
for example as
explained with reference to Figure 4, such reference potentials may also be
measured. At 72 the
measured voltages are analyzed. Analysis may include determining a magnitude
of voltage drop
along the casing to determine casing resistance, and may include determining
voltage drop of
19
CA 02524728 2005-10-28
leakage current into the formations. Analysis may include determination of
polarization
direction for reference potential measurements which are not substantially
equal to zero. At 75,
the analysis is used to determine if the response obtained represents a stable
set of formation
resistivity calculations. If the response is stable, at 77, the voltage
measurements are used to
determine formation resistivity, typically, as previously explained, by
determining a second
derivative, with respect to depth, of the magnitude of leakage current
corrected for casing
resistance variation in the vicinity of where the measurements are made.
At 73, the voltage measurements may be used to develop a model of the
resistivity distribution
around the outside of the wellbore (14 in Figure 1) proximate the instrument
(10 in Figure 1).
Methods for determining a model of the Earth formations are disclosed, for
example, in U.S.
patent no. 5,809,458 issued to Tamarchenko (1998), entitled, Method of
simulating the response
of a through-casing resistivity well logging instrument and its application to
determining
resistivity of earth formations. At 74, the model is subjected to a
sensitivity analysis. The
model, using appropriate sensitivity analysis, may be used, at 76, to
determine an optimum
arrangement of focusing current electrodes. If the determined optimum focusing
current
electrode arrangement is different from the initial or current configuration,
the configuration is
changed, at 79, and focusing current parameters are changed at 78 to provide
the model with the
optimum sensitivity response.
A different embodiment which may be used to investigate relatively long axial
spans between
electrodes, as well as shorter axial spans, is shown schematically in Figure
7. The embodiment
in Figure 7 includes a plurality of "satellite" or auxiliary instrument units,
shown generally at 62,
coupled to each other axially by cable segments 17. Any number of auxiliary
units 62 may be
used in a particular implementation. Each auxiliary unit 62 may include one or
more electrodes
made as previously explained and adapted to make electrical contact with the
casing (12 in
Figure 1). Each auxiliary unit 62 may include one or more current sources,
configured as
explained with reference to Figure 2, and one or more voltage measuring
circuits, also
configured as explained with reference to Figure 2. The length of the cable
segments 17 is not a
limitation on the scope of the invention, however, it is contemplated that the
length of the cable
segments is typically about 1 to 1.5 meters.
CA 02524728 2005-10-28
The auxiliary units 62 may be disposed axially on either side of, and
electrically connected to, a
central control unit 60. The central control unit 60 may include a central
processor, similar in
configuration to the CPU explained with reference to Figure 2. The control
unit 60 may operate
the various auxiliary units 62 to perform as current source electrodes and/or
current return
electrodes for either or both measuring current or focusing current, these
currents as explained
with reference to Figure 4. The various electrodes on the auxiliary units 62
may also be
configured to make voltage measurements of either or both measuring current
and focusing
current, also as explained with reference to Figure 4. In some embodiments,
the central control
unit 60 may itself include one or more current sources (not shown separately)
and one or more
voltage measuring circuits (not shown separately). The central control unit 60
may also include
a telemetry transceiver, similar in configuration to the transceiver explained
with reference to
Figure 2, and adapted to communicate measurement signals to the Earth's
surface in a selected
telemetry format, and to receive command signals from the Earth's surface,
along the cable 16.
Alternatively, the control unit 60 may include recording devices, as explained
with reference to
Figure 2, to store measurements until the instrument is withdrawn from the
wellbore (14 in
Figure 1).
The embodiment shown in Figure 7 may be electronically configured, in some
instances, to
provide focusing currents across a very long axial span, for example, by
selecting innermost
auxiliary units (those axially closest to the control unit 60) to provide a
focusing current source
electrode, and outermost auxiliary units 62 (those axially most distant from
the central unit 60) to
provide a focusing current return electrode. As will be readily appreciated by
those skilled in the
art, such a long axial span for focusing current may provide a relatively
large radial (lateral)
"depth of investigation" of the measuring current, because such measuring
current is constrained
to flow laterally a larger distance than when the focusing current traverses a
smaller axial span.
A possible advantage of the control unit 60/auxiliary unit 62 arrangement
shown in Figure 7 is
that the various electrodes may be selectively configured and reconfigured
electronically, by the
central control unit 60, to make a wide range of different radial depth and
axial resolution
measurements of Earth formation resistivity outside of a conductive pipe. More
specifically, the
electrical connections between the one or more electrodes on each of the
auxiliary units 62 may
be individually addressable by the circuitry in the central control unit 60.
While the
21
CA 02524728 2008-07-22
configuration shown in Figure 7 could conceivably be adapted to a single,
elongated
instrument housing, it will be readily appreciated by those skilled in the art
that a set of
axially shorter units (60, 62) interconnected by flexible cable segments 17
may be more
readily inserted into and withdrawn from a wellbore, particularly if the
wellbore is not
substantially vertical or includes places of relatively high trajectory
tortuosity ("dog leg
severity").
Another embodiment is shown in Figure 8. The embodiment of Figure 8 includes a
central control unit 60 configured as explained with reference to the
embodiment of FIG.
7, and includes a plurality of auxiliary units 62, also configured as
explained with
reference to the embodiment of Figure 7. The auxiliary units 62 are connected
end to end
to each other and to the central control unit 60 by cable segments 17. The
entire array of
auxiliary units 62 and central control unit 60 can be conveyed into and out of
the
wellbore by the cable 16 or other conveyance known in the art.
In the present embodiment, any one or more of the central control unit 60 and
the
auxiliary units 62 may include a seismic receiver SR disposed within the
housing. The
housing of the one or more units which includes a seismic receiver SR
preferably
includes a selectively extensible back-up arm 63 for urging the respective
housing into
contact with the interior surface of the pipe or casing (12 in Figure 1). The
seismic
receiver may be a single sensor element (not shown separately), or may be a
plurality of
sensor elements arranged along different sensitive axes. The sensor element
may be a
geophone, accelerometer or any other seismic sensing device known in the art.
A suitable
arrangement of actuating mechanism for the back up arm(s) 63, and seismic
sensors is
shown in U.S. Pat. No. 5,438,169 issued to Kennedy et al. The one or more of
the units
having the seismic receiver SR therein preferably includes circuits (not shown
separately)
for converting signals detected by the one or more sensing elements into
appropriately
formatted telemetry for recording in the central unit 60 and/or for
transmission to the
Earth's surface in a selected telemetry format.
22
CA 02524728 2008-07-22
In operation, the embodiment of Figure 8 may be moved to a selected depth in
the
wellbore, and the one or more back up arms 63 may be extended to urge the
associated
unit's housing into contact with the casing. A seismic energy source 65
disposed at the
Earth's surface may be actuated at selected times and the signals detected by
the one or
more seismic receivers SR is recorded (indexed with respect to time of
actuation of the
source 65) for interpretation. The
22a
CA 02524728 2005-10-28
extension of the back up arm(s) 63, actuation of the source 65 and signal
recording may be
repeated at different selected depths in the wellbore.
Similarly, measurements of voltage drop and current amplitude may be made
using the one or
more auxiliary units 62 as the array is positioned at each one of a plurality
of selected depths in
the wellbore, while seismic data recordings are being made. Voltage
measurements may also be
made while the array is moving through the wellbore, depending on the type of
electrodes used
on each one of the units 60, 62.
While the embodiment shown in Figure 8 includes seismic receivers SR and a
back up arm 63 in
each one of the central control unit 60 and the auxiliary units 62, it should
be clearly understood
that any one or more of the units 60, 62 may include a seismic receiver and
back up arm.
A possible advantage of using a back up arm 63 in each of the units 60, 62 as
shown in Figure 8
is that each back up arm 63 may serve both the purpose of providing good
mechanical contact
between the unit housing and the casing to enhance acoustic coupling
therebetween, and to
provide back-up force such that the electrodes (see Figure 2) may be urged
into firm contact with
the interior of the casing to enhance electrical contact therebetween. An
combination through-
casing resistivity measuring and borehole seismic instrument configured as
shown in Figure 8
may provide the advantage of substantial time saving during operation, because
both seismic
surveys and resistivity measurements may be made in a single insertion of the
instrument into the
wellbore. Such time savings may be substantial in cases where conveyance other
than by gravity
are used, for example, well tractors or drill pipe.
The embodiment shown in Figure 8 may also include a gravity sensor, shown
generally at G, in
one or more of the central control unit 60 and the auxiliary units. The one or
more gravity
sensors G may be a total gravitational field sensor, or a differential gravity
sensor. Suitable types
of gravity and differential gravity sensors are disclosed, for example, in
U.S. Patent No.
6,671,057 issued to Orban. An instrument configured as shown in Figure 8
including resistivity,
gravity and seismic sensors may be used, for example, in fluid displacement
monitoring of
subsurface reservoirs.
One embodiment of an electrode system for making electrical contact with the
interior of a
casing is shown in Figure 9. The electrode system includes an electrical
insulating layer 90
23
CA 02524728 2005-10-28
coupled to the exterior surface of the instrument housing 18. A plurality of
resilient, electrically
conductive wires 90 are mechanically coupled to the insulating layer 92 so as
to protrude
laterally outward from the exterior of the housing 18 and insulating layer 92.
The wires 90 are
all in electrical contact with each other. The wires 90 are preferably made
from a corrosion
resistant, high strength and "spring like" alloy, and have a length such that
a free diameter
traversed by the wires 90 is slightly larger than the expected maximum
internal diameter of the
pipe or casing (14 in Figure 1) in the wellbore to be surveyed. The wires 90
will thus be urged
into a scratching or scraping contact with the interior of the pipe or casing.
While some of the
wires 90 may not penetrate scale, deposits or corrosion present on the
interior of the pipe, some
of the wires 90 are likely to make such penetration and thus provide good
electrical contact to the
pipe or casing.
One possible configuration for the wires 90 so as to be in electrical contact
with each other and
insulated from the exterior surface of the housing is shown in Figure 10. The
wires 90 are
bonded to an electrically conductive substrate 92B. The substrate 92B is
insulated from the
exterior surface of the housing (18 in Figure 9) by a lower insulation layer
92A. The substrate
92B may be covered on its exterior surface by an upper insulating layer 92C to
prevent electrical
contact between the housing (18 in Figure 9) and the substrate 92B. The wires
90 thus act as a
single electrode at the location of the substrate 92B and insulating layers
92A, 92C.
One embodiment of a system for estimating quality of contact between
extensible/retractable
type electrodes and the interior surface of the pipe is shown in Figure 11.
The electrode 106 in
Figure 11 in extended and retracted by a piston 102 disposed in an hydraulic
cylinder 100.
Alternatively, a solenoid or other similar electromagnetic device may be used
to extend and
retract the electrode 106. The piston 102 in the present embodiment is sealed
with respect to the
cylinder 100 by o-rings 100 or the like. Disposed proximate the contact tip of
the electrode 106
is an insulating mandrel 114 which includes an electromagnetic transmitter
antenna 110 and an
electromagnetic receiver antenna 112. The antennas 110, 112 can be wire coils.
The transmitter
coil 110 is coupled to a source of alternating current (AC) 108. The AC source
108 preferably
has a frequency selected to make a voltage induced in the receiver coil 112
related to a distance
between the coils 110, 112 and the pipe or casing 14. The voltage is
determined by a voltage
measuring circuit 116 coupled to the receiver coil 112. Additionally, a
resistance measuring
24
CA 02524728 2005-10-28
circuit, which can be a direct current (DC) or preferably and AC type, is
electrically coupled
between the electrode 106 and the pipe 14. The pipe connection may be at the
Earth's surface, or
through a different one of the electrodes on the instrument (10 in Figure 1).
Quality of electrical
contact is determined when the voltage detected by the voltage measuring
circuit 116 remains
steady, indicating no further movement of the electrode 106 toward the pipe
14, and the
resistance measured by the resistance circuit 118 reaches a minimum.
As will be readily appreciated by those skilled in the art, sometimes the
condition of the interior
of the pipe in the wellbore is such that it may prove difficult or even
impossible to provide
sufficient electrical contact between the electrodes on the instrument and the
conductive pipe.
Much operating time can be consumed in attempts to make such electrical
contact in sections of
the pipe which are sufficiently deteriorated or covered in mineral and/or
hydrocarbon deposits so
as to make electrical contact poor at best. One embodiment of an apparatus
according to the
invention may include one or more types of wellbore imaging device to assist
the system
operator in determining whether any particular portion of a wellbore pipe is
unlikely to provide a
sufficient basis for good electrical contact. One example of a wellbore
imaging subsystem is
shown in Figure 12. The imaging subsystem 7 may include any one or all of the
embodiments of
wellbore wall imaging devices shown therein. The imaging subsystem 7, and
additional ones of
the imaging subsystem, may in some embodiments be contained in one or more of
the auxiliary
units 62, the central unit 60, or in a single housing system such as shown in
Figure 1.
The imaging subsystem 7 may be contained in a housing having therein a
conventional strength
and load bearing portion 122, and an acoustically and/or optically transparent
window section
120. The transparent window section 120 may include therein an optical video
camera 134, and
an ultrasonic transducer 132 either or both of which may be coupled to a motor
130 for rotating
the camera 134 and transducer 132 to enable imaging over the entire interior
circumference of
the pipe. Output of the camera 134 and transducer 132 may be coupled to
conventional signal
processing circuits 128 disposed, preferably, in the housing 122.
A portion of the housing may include one or more electrically insulating
contact pads 124,
coupled to the housing 122 by extensible arms and linkages, shown generally at
124A. The
linkages 124A may be of any type known in the art for extending a pad or
contact device
laterally outward from the housing 122. Each of the one or more pads 124 may
include a
CA 02524728 2008-07-22
plurality of spaced apart electrodes 126 for making galvanic resistance
measurements
therebetween or with reference to a selected potential point, such as the
housing 122.
Imaging devices for making electrical and ultrasonic images of the interior
surface of a
wellbore, including a pipe, are disclosed in U.S. Pat. No. 5,502,686 issued to
Dory et al.,
incorporated herein by reference Video imaging devices for use in a wellbore
are
disclosed in U.S. Pat. No. 5,134,471 issued to Gendron et al. Various
embodiments of a
through-pipe resistivity measurement apparatus according to the invention may
include
any one or more, or all of the imaging systems shown in Figure 12.
During operation, the system operator may observe a visual representation,
such as by a
graphic print or video display, of measurements made by the one or more
imaging
systems shown in Figure 12. If the system operator determines that a
particular portion of
the wellbore is likely to be difficult to establish good electrical contact,
the operator may
instead more the instrument (10 in Figure 1) to a different portion of the
wellbore.
Alternatively, a record of the measurements made by the one or more imaging
systems
may be made with respect to depth in the wellbore, along with the measurements
of
potential as explained previously herein. Image representations may then be
used in
combination with the potential measurements to evaluate whether the potential
measurements are more likely representative of the true resistivity of the
Earth formations
outside the pipe, or whether such potential measurements are more likely to
have been
materially affected by the condition of the interior of the pipe. Such imaging
can
therefore improve the quality of interpreted results by providing a way to
resolve
ambiguous measurements where pipe condition is suspected to have affected the
potential
measurements.
While the invention has been described with respect to a limited number of
embodiments,
those skilled in the art, having benefit of this disclosure, will appreciate
that other
embodiments can be devised which do not depart from the scope of the invention
as
disclosed herein. Accordingly, the scope of the invention should be limited
only by the
attached claims.
26
