Note: Descriptions are shown in the official language in which they were submitted.
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OPTICAL FIBER EQUIPPED TUBING AND METHODS OF MAKING AND USING
FIELD OF THE INVENTION
[0001] The present invention relates generally to oilfield operations and more
particularly
methods and apparatus using fiber optics in coiled tubing operations in a
wellbore.
BACKGROUND OF THE INVENTION
[0002] Coiled tubing operations are used commonly in the oilfield industry,
for example to
pump fluids to a desired location in the wellbore or to manipulate oilfield
assemblies. One
advantage of coiled tubing is that it is provided on reels such that coiled
tubing .is unreeled as it
is inserted into a wellbore for a particular use and then reeled or spooled
back on the reel as it is
extracted from the wellbore. Coiled tubing reels may be conveniently stored or
moved, and
spooled coiled tubing may be transported on a trailer, flat, or truck. The use
of coiled tubing as a
different type of wellbore conveyance in wellbore applications is increasing,
resulting in an
increasing need for downhole apparatus and methods adapted for use with coiled
tubing.
Difficulties inherent with using conventional downhole electromechanical
apparatus with coiled
tubing include lack of power to the downhole apparatus and the lack of
telemetry from the
downhole apparatus to the surface.
[0003] It is known to use conventional wireline in coiled tubing to provide
communications
between downhole operations and the surface, including transmitting uphole
data measured by a
variety of wellbore tools and transmitting commands downhole to effect a
variety of operations.
Use of wireline cable in coiled tubing presents logistical challenges,
however, such as
installation of the wireline cable in the coiled tubing and the reduced fluid
capacity of the coiled
tubing owing to the space taken by the wireline cable.
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100041 The addition of wireline to a coiled tubing string significantly
increases the weight of a
coiled tubing sting. Installation of the wireline into the coiled tubing
string is difficult and the
wireline is prone to bunch into a "bird nest" within the coiled tubing. This,
and the relatively
large outer diameter of wireline compared to the internal diameter of coiled
tubing, can
undesirably obstruct the flow of fluids through the coiled tubing, such flow
through the coiled
tubing frequently being an integral part of the wellbore operation.
Furthermore, some fluids
routinely pumped through coiled tubing, such as acid, cement and proppant-
bearing fracturing
fluids, may have an adverse affect on the integrity or performance of wireline
cable. In addition,
pumping fluid down the coiled tubing can create a drag force on the wireline
cable owing to the
frictional force between the fluid and the surface of the cable. I =
[0005] Installation of wireline or other electrical cable into coiled tubing
is difficult and
cumbersome as its weight and bending stiffness can contribute to a high
friction force between =
the cable and the interior of the coiled tubing. Methods for installing
wireline in coiled tubing are
discussed in U.S. Patent 5,573,225 and U.S. Patent 5,699,996.
The methods described in each of these patents require a significant
installation apparatus at the surface to overcome the high frictional force
between the cable and
the coiled tubing and to convey the cable into the coiled tubing. The size of
such an apparatus
makes it unfeasible for use in some operations, particularly in offshore
operations.
[0006] Use of optical fiber in various applications and operations is
increasing. Optical fiber
provides many advantages over wireline when used as a transmission medium such
as small size,
lightweight, large bandwidth capacity, and high speed of transmission. A
significant challenge
to using optical fibers in subterranean oiffield operations is that the free
hydrogen ions will cause
darkening of the fiber at the elevated temperatures that are commonly found in
subterranean
wells. The use of optical fiber in wireline cable is known such as that
described in U.S. Patent
No 6,690,866 incorporated herein in its entirety by reference. This patent
teaches adding a
hydrogen absorbing material or scavenging gel to surround the optical fibers
inside a first metal
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tube. Thii patent also teaches that wireline cable disclosed therein requires
significant tensile
strength and teaches that this strength can be obtained by rigidly attaching
the first metal tube to
the interior of a second metal tube. Both teachings can significantly add to
the cost and weight
of the cable. In U.S. Patent No 6,557,630, a
method of deploying a remote measurement apparatus in a wellbore, the
apparatus comprising a
conduit in which a fiber optic sensor and a fiber optic cable is disposed, the
cable being
propelled along the conduit by fluid flow in a conduit. In GB Patent 236290g,
a method is proposed for placing sensors that relies upon first
installing first a hollow conduit into the coiled tubing and then subsequently
pumping a single =
fiber into that conduit. None of these patents teach or suggest propelling an
optically enabled
conduit or cable into a tubular using fluid flow.
[00071 Methods of installing optical fibers in tubulars often are directed
towards installing the
optical fiber by pumping or dragging the fiber into the tubular. In U.S.
Patent Application
Publication 2003/0172752, methods for
installing an optical fiber through a conduit in a wellbore application using
a fluid, wherein a
seal is provided bdtween the optical fiber and the conduit are described. To
install an optical
fiber in coiled tubing using these methods would require 1) unreeling the
coiled tubing, 2)
extending the coiled tubing (either in a wellbore or on the surface) and 3)
deploying the optical
fiber. Such a process is directed toward the installation of a single optical
fiber in a tubular; it is
time consuming and thus costly from an operational perspective. Furthermore,
these methods
are directed toward installing a single optical fiber in a tubular and are not
conducive to
installation of multiple fibers in a tubular. In addition, these methods do
not contemplate
recovery or reuse of the optical fiber.
[0008) Use of multiple optic fibers however may provide advantages in many
situations over the
use of a single optical fiber. Using multiple fibers provides operational
redundancy in the event
that any particular fiber becomes damaged or broken. Multiple fibers provide
increased
transmission capacity over a single fiber and permit flexibility to segregate
different types of
transmissions to different fibers. These advantages may be particularly
important in downhole
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applications where access is limited, environmental conditions may be extreme,
and dual-
direction (uphole and downhole) transmission is required. Using multiple
optical fibers also
allows an individual optical fiber to be used for a specific apparatus or
sensor. This
configuration is useful as some sensors, such as Fabry-Perot devices, require
a dedicated optical
fiber. The configuration also is useful for sensors with digital telemetry for
which a separate
fiber may be required. Sensors using Fiber-Bragg grating for example require a
separate fiber
from the fiber used for carrying digital optical telemetry.
[0010] For clarity, the term "duct" is used herein to identify a small tube or
hollow carrier that
encompasses an optical fiber or fibers. The term "optical fiber" refers to a
fiber or a waveguide
capable of transmitting optical energy. The term "fiber optic tube" or "fiber
optic tether" is used
to identify the combination of an optical fiber or multiple optical fibers
disposed in a duct. The
term "fiber optic cable" refers to a cable, wire, wireline or slickline that
comprises one or more
= optical fibers. "Tubular" and "tubing" refers to a conduit to any kind of
a round hollow.
apparatus
=
apparatus in general, and in the area of oilfield applications to casing,
drill pipe, metal tube, or =
= coiled tubing or other such apparatus.
= [00111 Various methods of manufacturing fiber optic tubes are known. Two
examples are laser
welding, such as described in U.S. Patent No. 4,852,790, and
tungsten inert gas welding (TIG) such as described in U.S. Patent No.
4,366,362.
Neither patent teaches or suggests the insertion of such tubes into a spooled
tubular by fluid flow.
[0012] Therefore it may be seen that there exists a need for an apparatus,
methods of making,
and methods of using fiber optic tubing disposed in a tubular, and in
particular, a need for such
an apparatus and methods of using in wellbore applications.
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SUMMARY OF THE INVENTION
100131 In one aspect, the present invention comprises optical fiber
equipped tubing
and methods of making and using the same. In a broad sense, the present
invention comprises
an optical fiber equipped tubing comprising a fiber optic tube deployed within
a tubular. In
many embodiments, the fiber optic tube comprises a metallic material, and in
some
embodiments, the fiber optic tube comprises more than one optical fiber. In
many
embodiments, the fiber optic tube will be constructed in an inert nitrogen
environment so that
the optical fiber or fibers therein are not exposed to hydrogen or water
during manufacturing.
The tubular may be, in particular, coiled tubing. In another embodiment, the
present
invention relates to a method of making an optical fiber equipped tubing
comprising pumping
a fluid into a tubular, deploying a fiber optic tube into the fluid as pumped
in the tubular, such
that the flow of the pumped fluid propels the tube along the tubular. When the
tubular is
coiled tubing, the fiber optic tube may be deployed in the coiled tubing while
the tubing is
spooled on a reel or while the tubing is deployed in a wellbore. In another
embodiment, the
present invention provides a method of communicating in a wellbore comprising
deploying an
optical fiber equipped tubing having at least one optical fiber disposed
therein, the fiber optic
tubing being disposed in the tubing by fluid flow; determining a property in
the wellbore; and
transmitting the determined property via at least one of the optical fibers
disposed in the fiber
optic tubing. In some embodiments, the least one optical fiber senses the
information for
transmitting. The method may also comprise disposing at least one sensor in
the wellbore,
with the sensor determining the property, and the sensed information
transmitted to the
surface via the optical fiber in the fiber optic tube. In other embodiments,
more than one
sensor may be disposed in the wellbore, each sensor transmitting its sensed
property over a
different optical fiber in the coiled tubing. In many embodiments the optical
fiber or fibers
will be attached to a wireless communication device via a pressure bulkhead so
that the
optical signal can readily transmitted to a surface computer while the coiled
tubing is being
spooled into and out of the wellbore.
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In some embodiments, the present invention provides an apparatus that is
deployed into the
wellbore and in communication with the surface for receiving signals or
transmitting sensed
information over the fiber optic tubing.
[0013a1 In another aspect, the present invention provides a method of
making
measurements in a wellbore, the method comprising the steps of: providing a
fiber optic tube
comprising at least one optical fiber disposed in a duct; providing a coiled
tubing disposed on
a reel; pumping fluid into the coiled tubing; deploying the fiber optic tube
directly into the
coiled tubing with fluid as the fluid is pumped into the coiled tubing while
the coiled tubing is
disposed on the reel, whereby the pumped fluid propels the fiber optic tube
along the coiled
tubing, wherein the fiber optic tube is disposed in an unrestrained manner in
the pumped fluid
and is permitted to self-locate in the coiled tubing without the use of
external apparatus while
the fiber optic tube is deployed; terminating the fiber optic tube at a
downhole termination of
.the coiled tubing, the downhole termination comprising a borehole apparatus
or sensor
attached to the coiled tubing; terminating the fiber optic tube at a surface
termination;
deploying the optical fiber equipped coiled tubing into the wellbore, wherein
the fiber optic
tube remains attached to the downhole termination and within the coiled tubing
while
deployed in the wellbore; performing a wellbore operation with the optical
fiber equipped
coiled tubing; determining a property of the wellbore related to the wellbore
operation; and
transmitting the determined property via the at least one optical fiber or via
one of the optical
fibers.
[0013b] In another aspect, the present invention provides a method of
communicating
in a wellbore, the method comprising the steps of: providing a fiber optic
tube comprising at
least one optical fiber disposed in a duct; providing a coiled tubing disposed
on a reel;
pumping fluid into the coiled tubing; deploying the fiber optic tube directly
into the coiled
tubing with fluid as the fluid is pumped into the coiled tubing while the
coiled tubing is
disposed on the reel, whereby the pumped fluid propels the fiber optic tube
along the coiled
tubing, wherein the fiber optic tube is disposed in an unrestrained manner in
the pumped fluid
and is permitted to self-locate during deployment in the coiled tubing without
the use of
external apparatus while the fiber optic tube is deployed; terminating the
fiber optic tube at a
downhole termination of a borehole apparatus or sensor; terminating the fiber
optic tube at a
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surface termination, wherein the downhole and surface terminations provide a
physical and
optical connection between a surface of the wellbore and the borehole
apparatus or sensor;
deploying the optical fiber equipped coiled tubing and the borehole apparatus
or sensor into
the wellbore after terminating the fiber optic tube at the uphole and downhole
terminations,
wherein the fiber optic tube remains attached to the downhole termination and
the borehole
apparatus or sensor and within the coiled tubing while deployed in the
wellbore; performing a
wellbore operation with the optical fiber equipped coiled tubing; and
transmitting a signal
from the surface of the wellbore to the apparatus via the at least one optical
fiber during the
wellbore operation.
5b
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[0014] While a particular embodiment and area of application is presented as
an exemplar,
namely that of fiber optic equipped coiled tubing useful for wellbore
applications, the present
invention is not limited to this embodiment and is useful for any application
wherein a fiber optic
equipped tubing is desirable.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] Fig 1 shows an embodiment of the apparatus of the present invention.
[0016] Fig 2A is a cross-sectional view of an embodiment of the present
invention.
[0017] Fig 2B is a cross-sectional view of another embodiment of the present
invention.
[0018] Fig 3 shows a typical configuration for coiled tubing operations.
DETAILED DESCRIPTION
[0019] The present invention provides an optical fiber equipped tubing and
methods of making
and using. The optical fiber equipped tubing of the present invention
comprises one or more
fiber optic tubes disposed in a tubular. An embodiment comprises a method for
installing one or
more fiber optic tubes in reeled or spooled tubing such as coiled tubing.
Another embodiment
provides a method for installing one or more fiber optic tubes in coiled
tubing deployed in a
wellbore.
[0020] Within the present invention is the unexpected recognition that a fiber
optic tube may be
deployed a tubular by pumping the fiber optic tube in a fluid without
additional structure or
protection. Methods of pumping cables into a tubular are generally considered
infeasible owning
to the inherent lack of compressional stiffness of cables. Furthermore, the
teachings of fiber
optic cables suggest that a fiber optic tube needs additional protection or
structure for use in a
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wellbore environment. Thus it is counter-intuitive to consider deploying a
fiber optic tube
directly in a tubular without encapsulating the tube in additional layers,
providing a protective
coating, or encompassing it in armor. Similarly it is counter-intuitive to
consider deploying a
fiber optic tube directly through fluid pumping.
[0021] An advantage of the optical fiber equipped tubing of the present
invention is that the fiber
optic tube possesses a certain level of stiffness in compression, leading it
to behave more similar
mechanically to coiled tubing than does cable or optical fiber alone. As such,
use of a fiber optic
tube inside coiled tubing avoids many of the slack management challenges
presented by other
transmission mechanism. Furthermore, the cross-section of a fiber optic tube
is relatively small
compared to the inner area within coiled tubing, thus limiting the possible
physical influence that
the fiber optic tube could have on the mechanical behavior of coiled tubing
during deployment
and retrieval. The small relative diameter of the fiber optic tube combined
with its light weight
make it more tolerant of pumping action, which is advantageous to avoid the
"bird-nesting" or
bundling within the coiled tubing that commonly occurs when installing
wireline in coiled
tubing. Moreover, as slack management problems are avoided in the present
invention, optical
fiber equipped coiled tubing may be deploying into and retrieved from a
wellbore at a quicker
rate than coiled tubing with wireline.
[0022] Referring now to FIG 1, optical fiber equipped tubing 200 is shown
having tubular 105
within which is disposed fiber optic tube 211. In FIG 1, fiber optic tube 211
is shown
comprising duct 203 in which a single optical fiber 201 is disposed. In other
embodiments, more
than one optical fiber 201 may be provided within fiber optic duct 203.
Surface termination 301
or downhole termination 207 may be provided for both physical and optical
connections between
optical fiber 201 and one or more borehole apparatus or sensor 209. The
optical fibers may be
multi-mode or single-mode. Types of borehole apparatus or sensor 209 may
include, for
example, gauges, valves, sampling devices, temperature sensors, pressure
sensors, distributed
temperature sensors, distributed pressure sensors, flow-control devices, flow
rate measurement
devices, oil/water/gas ratio measurement devices, scale detectors, actuators,
locks, release
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mechanisms, equipment sensors (e.g., vibration sensors), sand detection
sensors, water detection
sensors, data recorders, viscosity sensors, density sensors, bubble point
sensors, composition
sensors, resistivity array devices and sensors, acoustic devices and sensors,
other telemetry
devices, near infrared sensors, gamma ray detectors, H2S detectors, CO2
detectors, downhole
memory units, downhole controllers, perforating devices, shape charges, firing
heads, locators,
and other devices.
[0023] Referring to FIG 2A, a cross-sectional view of the fiber optic equipped
tubing 200 of
FIG 1 is shown. Within tubing 105 is shown a fiber optic tube 211 comprising
optical fiber 201
located inside duct 203. Referring to FIG 2B, another embodiment of the
present invention is
shown in cross-sectional view in which fiber optic equipped tubing 200 has
more than one fiber
optic tube 211 is disposed in tubular 105 and in which more than one optical
fiber 201 is
disposed within duct 203 in at least one of the fiber optic tube 211.
[0024] In fiber optic tube 211, an inert gas such as nitrogen may be used to
fill the space
between the optical fiber or fibers 201 and the interior of the duct 203. The
fluid may be
pressurized in some embodiments to decrease the susceptibility of the fiber
optic tube to
localized buckling. In a further embodiment, this laser-welding technique is
performed in an
enclosed environment filled with an inert gas such as nitrogen to avoid
exposure to water or
hydrogen during manufacturing, thereby minimizing any hydrogen-induced
darkening of the
optical fibers during oilfield operations. Using nitrogen to fill the space
offers advantages of
lower cost and greater convenience over other techniques that may require a
buffer material, gel,
or sealer in the space. In one embodiment, the duct 203 is constructed by
bending a metal strip
around the optical fiber or fibers 201 and then welding that strip to form an
encompassing duct
using laser-welding techniques such as described in US Patent No 4,852,790.
This gives a
significant reduction in the cost and weight of the resulting fiber optic tube
211 compared to
other optical cables previously known in the art. A small amount of gel
containing palladium or
tantalum can optionally be inserted into either end of the fiber optic tube to
keep hydrogen ions
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away from the optical fiber or fibers 201 during transportation of the
optically enabled tubing
200.
[0025] Materials suitable for use in duct 203 in fiber optic tube 211 of the
present invention
provide stiffness to the tube, are resistant to fluids encountered in oilfield
applications, and are
rated to withstand the high temperature and high pressure conditions found in
some wellbore
environments. Typically duct 203 in a fiber optic tube 211 is a metallic
material, and in some
embodiments, duct 203 comprises metal materials such as JnconelTM, stainless
steel, or
HasetloyTM. While fiber optic tubes manufactured by any method may be used in
the present
invention, laser welded fiber optic tubes are preferred as the heat affected
zone generated by
laser welding is normally less than that generated by other methods such as
TIG, thus reducing
the possibility of damage to the optical fiber during welding.
[0026] While the dimensions of such fiber optic tubes are small (for example
the diameter of
such products commercially available from K-Tube, Inc of California, U.S.A.
range from 0.5
mm to 3.5 mm), they have sufficient inner void space to accommodate multiple
optical fibers.
The small size of such fiber optic tubes is particularly useful in the present
invention as they do
not significantly deduct from the capacity of a tubular to accommodate fluids
or create obstacles
to other devices or equipment to be deployed in or through the tubular.
[0027] In some embodiments, fiber optic tube 211 comprises a duct 203 with an
outer diameter
of 0.071 inches to 0.125 inches (3.175 mm) formed around one or more optical
fibers 201. In a
preferred embodiment, standard optical fibers are used, and duct 203 is no
more than 0.020
inches (0.508 mm) thick. While the diameter of the optical fibers, the
protective tube, and the
thickness of the protective tube given here are exemplary, it is noteworthy
that the inner diameter
of the protective tube can be larger than needed for a close packing of the
optical fibers.
[0028] In some embodiments of the present invention, fiber optic tube 211 may
comprise
multiple optical fibers may be disposed in a duct. In some applications, a
particular downhole
apparatus may have its own designated optical fiber, or each of a group of
apparatuses may have
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their own designated optical fiber within the fiber optic tube. In other
applications, a series of
apparatus may use a single optical fiber.
[0029] Referring now to FIG 3, a typical configuration for wellbore operations
is shown in
which coiled tubing 15 is suitable for use as tubular 105 in the present
invention. Surface
handling equipment includes an injector system 20 on supports 29 and coiled
tubing reel
assembly 10 on reel stand 12, flat, trailer, truck or other such device. The
tubing is deployed into
or pulled out of the well using an injector head 19. The equipment further
includes a levelwind
mechanism 13 for guiding coiled tubing 15 on and off the reel 10. The coiled
tubing 15 passes
over tubing guide arch 18 which provides a bending radius for moving the
tubing into a vertical
orientation for injection through wellhead devices into the wellbore. The
tubing passes from
tubing guide arch 18 into the injector head 19 that grippingly engages the
tubing and pushes it
into the well. A stripper assembly 21 under the injector maintains a dynamic
and static seal
around the tubing to hold well pressure within the well as the tubing passes
into the wellhead
devices under well pressure. The coiled tubing then moves through a blowout
preventor (BOP)
stack 23, a flow tee 25 and wellhead master valve or tree valve 27. When
coiled tubing 15
disposed on coiled tubing reel 10 is deployed into or retrieved from a
borehole 8, the coiled
tubing reel 10 rotates.
[0030] Fiber optic tube 211 may be inserted into the coiled tubing 15 through
any variety of
means. One embodiment comprises attaching a hose to the reel 10 to the other
end of which
hose is attached a Y-joint. In this configuration, fiber optic tube 211 may be
introduced into one
leg of the Y and fluid pumped into the other leg. The drag force of the fluid
on fiber optic tube
211 then propels the tube down the hose and into the reel 10. It has been
found, that in
preferred embodiments wherein the outer diameter of the tether is less than
0.125 inches (3.175
mm), a pump rate as low as 1-5 barrels per minute (2.65 ¨ 13.25 liters per
second) is sufficient to
propel the tether the full length of the coiled tubing even while it is
spooled on the reel.
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[0031] In the method and apparatus of the present invention, a fluid, such as
gas or water, may
be used to propel a fiber optic tube 211 in a tubular 105. Typically, fiber
optic tube 211 is
disposed in an unrestrained manner in the pumped fluid. As the fluid is pumped
into the tubular,
the fiber optic tube is permitted to self-locate in the tubular without the
use of external apparatus
such as pigs for conveyance or placement or restricting anchors. In particular
embodiments, the
fluid is pumped and the fiber optic tube or tubes are deployed into coiled
tubing while it said
coiled tubing is configured in a spooled state on a reel. These embodiments
provide logistical
advantages as the fiber optic tube or tubes can be deployed into the coiled
tubing at a
manufacturing plant or other location remote from a wellsite. Thus the optical
fiber equipped
tubing of the present invention may be transported and field-deployed as a
single apparatus,
thereby reducing costs and simplifying operations.
[0032] The optical fiber equipped tubing 200 of the present invention may be
used in
conventional wellbore operations such as providing a stimulation fluid to a
subterranean
formation through coiled tubing. One advantage of the present invention is
that fiber optic tube
211 tolerates exposure to various well treatment fluids that may be pumped
into the coiled
tubing; in particular, the fiber optic tube or tubes of the present invention
can withstand abrasion
by proppant or sand and exposure to corrosive fluids such as acids. Preferably
the fiber optic
tube is configured as a round tube having a smooth outer diameter, this
configuration providing
less opportunity for degradation and thus a longer useful life for the fiber
optic tube.
[0033] The optical fiber equipped tubing of the present invention is useful to
perform a variety
of wellbore operation including determining a wellbore property and
transmitting information
from the wellbore. Determining includes, by way of example and not limitation,
sensing using
the optical fiber, sensing using a separate sensor, locating by a downhole
apparatus, and
confirming a configuration by a downhole apparatus. The optical fiber equipped
tubing of the
present invention may further comprise sensors such as fiber optic temperature
and pressure
sensors or electrical sensors coupled with electro-optical converters,
disposed in a wellbore and
linked to the surface via a fiber optic tube 211. Wellbore conditions that are
sensed may be
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transmitted via fiber optic tube 211. Data sensed by electrical sensors may be
converted to
analog or digital optical signals using pure digital or wavelength, intensity
or polarization
modulation and then provided to the optical fiber or fibers in fiber optic
tube 211. Alternatively,
optical fiber 201 may sense some properties directly, for example when optical
fiber 201 serves
as a distributed temperature sensor or when optical fiber 201 comprises Fiber-
Bragg grating and
directly senses strain, stress, stretch, or pressure. =
[00341 The information from the sensors or the property information sensed by
optical fiber 201
may be communicated to the surface via fiber optic tube 211. Similarly,
signals or commands
may be transmitted from the surface to a downhole sensor or apparatus via
fiber optic tube 201. =
In one embodiment of this invention, the surface communication includes a
wireless telemetry
link such as described in U.S. Patent Publication No. 2006/A10044156.
In a further embodiment, the wireless telemetry apparatus may be
mounted to the reel so that the optical signals can be 'transmitted while the
reel is rotating
without the need of a complicated optical collector apparatus. In yet a
further embodiment, the
wireless apparatus mounted to the reel may include additional optical
connectors so that surface
optical cables can be attached when the reel is not rotating.
[0035] It is to be appreciated that the embodiments of the invention described
herein are given
by way of example only, and that modifications and additional components can
be provided to
enhance the performance of the apparatus without deviating from the overall
nature of the
invention disclosed herein.
_
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