Note: Descriptions are shown in the official language in which they were submitted.
DOWNHOLE ELECTROMAGNETIC TELEMETRY APPARATUS
[0001]
Technical Field
[0002] This application relates to subsurface drilling, specifically to
apparatus for
telemetry of information from downhole locations. Embodiments are applicable
to
drilling wells for recovering hydrocarbons.
Background
[0003] Recovering hydrocarbons from subterranean zones relies on the process
of drilling
wellbores.
[0004] Wellbores are made using surface-located drilling equipment which
drives a drill
string that eventually extends from the surface equipment to the formation or
subterranean
zone of interest. The drill string can extend thousands of feet or meters
below the surface.
The terminal end of the drill string includes a drill bit for drilling (or
extending) the
wellbore. Drilling fluid usually in the form of a drilling "mud" is typically
pumped
through the drill string. The drilling fluid cools and lubricates the drill
bit and also carries
cuttings back to the surface. Drilling fluid may also be used to help control
bottom hole
pressure to inhibit hydrocarbon influx from the formation into the wellbore
and potential
blow out at surface.
[0005] Bottom hole assembly (BHA) is the name given to the equipment at the
terminal
end of a drill string. In addition to a drill bit a BHA may comprise elements
such as:
apparatus for steering the direction of the drilling (e.g. a steerable
downhole mud motor or
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rotary steerable system); sensors for measuring properties of the surrounding
geological
formations (e.g. sensors for use in well logging); sensors for measuring
downhole
conditions as drilling progresses; systems for telemetry of data to the
surface; stabilizers;
heavy weight drill collars, pulsers and the like. The BHA is typically
advanced into the
wellbore by a string of metallic tubulars (drill pipe).
[0006] Telemetry information can be invaluable for efficient drilling
operations. For
example, telemetry information may be used by a drill rig crew to make
decisions about
controlling and steering the drill bit to optimize the drilling speed and
trajectory based on
numerous factors, including legal boundaries, locations of existing wells,
formation
properties, hydrocarbon size and location, etc. A crew may make intentional
deviations
from the planned path as necessary based on information gathered from downhole
sensors
and transmitted to the surface by telemetry during the drilling process. The
ability to
obtain real time data allows for relatively more economical and more efficient
drilling
operations.
[0007] Various techniques have been used to transmit information from a
location in a
bore hole to the surface. These include transmitting information by generating
vibrations
in fluid in the bore hole (e.g. acoustic telemetry or mud pulse telemetry) and
transmitting
information by way of electromagnetic signals that propagate at least in part
through the
earth (EM telemetry). Other telemetry systems use hardwired drill pipe or
fibre optic
cable to carry data to the surface.
[0008] A typical arrangement for electromagnetic telemetry uses parts of the
drill string as
an antenna. The drill string may be divided into two conductive sections by
including an
insulating joint or connector (a "gap sub") in the drill string. The gap sub
is typically
placed within a bottom hole assembly such that metallic drill pipe in the
drill string above
the BHA serves as one antenna element and metallic sections in the BHA serve
as another
antenna element. Electromagnetic telemetry signals can then be transmitted by
applying
electrical signals between the two antenna elements. The signals typically
comprise very
low frequency AC signals applied in a manner that codes information for
transmission to
the surface. The electromagnetic signals may be detected at the surface, for
example by
measuring electrical potential differences between the drill string and one or
more ground
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rods. A challenge with EM telemetry is that the generated signals are
significantly
attenuated as they propagate to the surface. Further, the electrical power
available to
generate EM signals may be provided by batteries or another power source that
has limited
capacity. Therefore, it is desirable to provide a system in which EM signals
are generated
efficiently.
[0009] Design of the gap sub is an important factor in an EM telemetry system.
The gap
sub must provide electrical isolation between two parts of the drill string as
well as
withstand the extreme mechanical loading induced during drilling and the high
differential
pressures that occur between the center and exterior of the drill pipe. Drill
string
components are typically made from high strength, ductile metal alloys in
order to handle
the loading without failure. Most electrically-insulating materials suitable
for electrically
isolating different parts of a gap sub are weaker than metals (e.g. rubber,
plastic, epoxy) or
quite brittle (ceramics). This makes it difficult to design a gap sub that is
both configured
to provide efficient transmission of UM telemetry signals and has the
mechanical
properties required of a link in the drill string.
[0010] The following references describe various telemetry systems: US
3323327; US
4176894; US 4348672; US 4496174; US 4684946; US 4676773; US 4739325; US
5130706; US 5138313; US 5236048; US 5406983; US 5467832; US 5520246: US
5749605; US 5883516; US 6050353; IJS 6098727; US 6158532; US 6404350; US
6446736; US 6515592; US 6727827; US 6750783; US 6926098; US 7151466; US
7243028; US 7255183; US 7252160; US 7326015; US 7387167; US 7573397; US
7605716; US 7836973; US 7880640; US 7900968; US 8154420 US 2004/0104047; US
2005/0217898; US 2006/0202852; US 2006/003206; US 2007/0235224; IJS
2007/0247328; US 2009/0023502; US 2009/0065254; US 2009/0066334; US
.. 2010/0033344; US 2011/025469; US 2011/0309949; US 2012/0085583;
W02006/083764; W02008/116077; W02009/086637; W02011/049573;
W02010/121345; W02010/121346; W02011/133399; W02012/042499;
W02011/049573; W02012/045698; W02012/082748.
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[0011] Despite work that has been done to develop systems for subsurface
telemetry there
remains a need for practical subsurface telemetry systems and there remains a
need to
provide such systems that offer improved efficiency and/or greater range.
Summary
[0012] The invention has several aspects. One aspect provides EM telemetry
apparatus
for downhole applications. Another aspect provides methods for subsurface
drilling.
[0013] Apparatus according to one aspect provides a subsurface drilling
assembly
comprising a downhole probe and a gap sub. The gap sub comprises an
electrically-
conducting uphole part comprising an uphole coupling for coupling into a drill
string, an
electrically-conducting downhole part comprising a downhole coupling for
coupling into
the drill string, a bore extending through the gap sub from the uphole
coupling to the
downhole coupling and an electrically-insulating gap portion electrically
isolating the
uphole part of the gap sub from the downhole part of the gap sub. The probe
extends
within the bore. The probe comprises an elongated housing enclosing
electronics
including a signal generator. The probe comprises first and second electrical
contacts
spaced apart longitudinally on an outside of the housing. The apparatus
comprises a fluid-
carrying channel bypassing the probe. Walls of the fluid-carrying channel are
electrically-insulating at least in a section of the channel extending
longitudinally from a
location above the electrically-insulating gap portion to a location below the
electrically-
insulating gap portion.
[0014] Apparatus according to another aspect provides a probe for use in
subsurface
drilling. The probe comprises an elongated metallic housing. The housing
encloses
electronics, including a telemetry signal generator. The housing comprises
first and
second electrical contacts spaced apart longitudinally on the outside of the
housing and an
electrically-insulating gap comprising an electrically-insulating material
providing
electrical isolation between first and second parts of the metallic housing.
The gap is
located between the first and second electrical contacts. The probe also
comprises an
electrically-insulating layer on an outside surface of the metallic housing.
The electrically
insulating layer at least partially covers the electrically-insulating gap and
extends
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continuously to cover an outside surface of the metallic housing on at least
one side of the
gap. In some embodiments the covering extends for a distance of at least 1
meter. In
some embodiments the probe is combined with a gap sub. The gap sub (which may
comprise one component or a plurality of separable components comprises an
electrically-
conducting uphole part comprising an uphole coupling for coupling into a drill
string, an
electrically-conducting downhole part comprising a downhole coupling for
coupling into
the drill string, a bore extending through the gap sub from the uphole
coupling to the
downhole coupling and an electrically-insulating gap portion electrically
isolating the
uphole part of the gap sub from the downhole part of the gap sub. In the
combination, the
probe is located within the bore of the gap sub and the first electrical
contact is in
electrical contact with the uphole part of the gap sub and the second
electrical contact is in
electrical contact with the downhole part of the gap sub.
[0015] Apparatus according to another aspect provides a subsurface drilling
assembly
comprising a gap sub. The gap sub comprises an electrically-conducting uphole
part
comprising an uphole coupling for coupling into a drill string, an
electrically-conducting
downhole part comprising a downhole coupling for coupling into the drill
string, a bore
extending through the gap sub from the uphole coupling to the downhole
coupling and an
electrically-insulating gap portion electrically isolating the uphole part of
the gap sub from
the downhole part of the gap sub. An EM telemetry signal generator is housed
within a
wall of the gap sub. The EM telemetry signal generator has output leads
electrically
coupled to the uphole and downhole parts of the gap sub. An electrically-
insulating sleeve
lines at least a portion of the bore of the gap sub adjacent to the
electrically-insulating gap.
The electrically insulating sleeve covers at least one interface between the
electrically
insulating gap portion of the gap sub and one of the uphole and downhole parts
of the gap
sub and extends continuously along the one of the uphole and downhole parts of
the gap
sub.
[0016] A method according to a further aspect provides a subsurface drilling
method
performed using a drill string comprising a gap sub and an electronics package
located in a
bore of the gap sub. The electronics package comprises electrical contacts
that are in
electrical contact with electrically-conductive parts of the gap sub. The
method
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comprises passing a drilling fluid down a bore of the drill string and, at the
location of the
electronics package, channeling the drilling fluid into a channel that is
electrically
insulated from both the electrically conductive parts of the gap sub and
electrically
conductive parts of the housing of the electronics package.
[0017] Further aspects of the invention and features of example embodiments
are
illustrated in the accompanying drawings and/or described in the following
description.
Brief Description of the Drawings
[0018] The accompanying drawings illustrate non-limiting example embodiments
of the
invention.
[0019] Figure 1 is a schematic view of a drilling operation according to an
example
embodiment.
[0020] Figure 2 is a longitudinal cross sectional view of a gap sub according
to an
example embodiment.
[0021] Figures 3A-3D are cutaway views of a portion of a gap sub according to
an
example embodiment.
[0022] Figure 4 is a schematic view of an equivalent electrical circuit for a
telemetry
signal generator and gap sub according to an example embodiment.
[0023] Figure 5 is a cutaway view of a gap sub with radially-inwardly
extending parts
according to an example embodiment.
[0024] Figure 5A is an axial cross sectional view of a gap sub with radially-
inwardly
extending parts according to an example embodiment.
[0025] Figure 6 shows schematically an example embodiment in which an
electronics
package is located in a cavity in a wall of a gap sub.
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Description
[0026] Throughout the following description specific details are set forth in
order to
provide a more thorough understanding to persons skilled in the art. However,
well
known elements may not have been shown or described in detail to avoid
unnecessarily
obscuring the disclosure. The following description of examples of the
technology is not
intended to be exhaustive or to limit the system to the precise forms of any
example
embodiment. Accordingly, the description and drawings are to be regarded in an
illustrative, rather than a restrictive, sense.
[0027] While a number of exemplary aspects and embodiments have been discussed
.. above, those of skill in the art will recognize certain modifications,
permutations, additions
and sub-combinations thereof. It is therefore intended that the following
appended claims
and claims hereafter introduced are interpreted to include all such
modifications,
permutations, additions and sub-combinations as are within their true spirit
and scope.
[0028] Figure 1 shows schematically an example drilling operation. A drill rig
10 drives a
drill string 12 which includes sections of drill pipe that extend to a drill
bit 14. The
illustrated drill rig 10 includes a derrick 10A, a rig floor 10B and draw
works 10C for
supporting the drill string. Drill bit 14 is larger in diameter than the drill
string above the
drill bit. An annular region 15 surrounding the drill string is typically
filled with drilling
fluid. The drilling fluid is pumped through a bore in the drill string to the
drill bit and
returns to the surface through annular region 15 carrying cuttings from the
drilling
operation. As the well is drilled, a casing 16 may be made in the well bore. A
blow out
preventer 17 is supported at a top end of the casing. The drill rig
illustrated in Figure 1 is
an example only. The methods and apparatus described herein are not specific
to any
particular type of drill rig.
[0029] Drill string 12 includes a gap sub 20. An EM signal generator 18
located inside the
drill string (for example in an electronics probe contained within the bore of
the drill
string) is electrically connected across the electrically-insulating gap of
the gap sub 20.
The signals from the EM signal generator result in electrical currents 1 9A
and electric
fields 19B that are detectable at the surface. In the illustrated embodiment a
signal
receiver 13 is connected by signal cables 13A to measure potential differences
between
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electrical grounding stakes 13B and the top end of drill string 12. A display
11 may be
connected to display data received by the signal receiver 13.
[0030] Figure 2 shows an example arrangement of a gap sub 20. Gap sub 20 has
an
electrically-conducting uphole portion 20A and an electrically conducting
downhole
portion 20B separated by gap 20C filled with an electrically-insulating
material.
Couplings 21 for coupling to adjacent elements of the drill string are
provided at the
uphole and downhole ends of gap sub 20. An electronics package 22 comprising
an EM
telemetry signal generator (not shown in Figure 2) is supported in a bore 20D
of gap sub
20.
[0031] Electronics package 22 has a metal housing 23 comprising first and
second parts
23A and 23B that are electrically insulated from one another by an
electrically-insulating
gap 23C. First and second electrodes 24A and 24B are connected to the
telemetry signal
generator and are respectively in contact with the uphole portion 20A and the
downhole
portion 20B of gap sub 20. Electrode 24A may be, but is not necessarily, in
electrical
contact with first part 23A of the housing of electronics package 22.
Electrode 24B may
be, but is not necessarily in electrical contact with second part 23B of the
housing of
electronics package 22.
[0032] An electrically-insulating layer 25 at least partially covers
electrically-insulating
gap 23C of electronics package 22. Electrically insulating layer 25 extends
over the
outside surface of electronics package 22 and continuously covers the outside
surface of
conductive housing 23 of electronics package 22 for a distance beyond
electrically-
insulating gap 23C on one or both sides of electrically-insulating gap 23C. In
some
embodiments the length of continuous coverage of electrically-insulating layer
25 is at
least 1 meter and preferably at least 1 1/2 meters or 2 meters. In some
example
embodiments the length of continuous coverage of electrically-insulating layer
25 is 3 to 4
meters.
[0033] In some embodiments, electrically-insulating layer 25 continuously
covers at least
60% or 70% or 80% of that portion of the outside surface of electronics
package 22 that
lies between electrodes 24A and 24B. In some embodiments electrically
insulating layer
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25 continuously covers substantially all of that portion of the outside
surface of electronics
package 22 that lies between electrodes 24A and 24B. Here, 'substantially all'
means at
least 95%.
[0034] In some embodiments, electrically-insulating layer 25 comprises a
coating applied
.. to electronics package 22, a sleeve or tube extending around electronics
package 22, or the
like. The material of layer 25 may be any electrically insulating material
suitable for
exposure to downhole conditions. Some non-limiting examples are suitable
thermoplastics, epoxies, ceramics, elastomeric polymers, and rubber. Layer 25
may
comprise a coating that is applied to, or bonded to electronics package 22 or
a pre-formed
component (formed e.g. by extrusion, injection molding, or the like which is
subsequently
attached to, affixed around, or supported around electronics package 22. The
material of
layer 25 should be capable of withstanding downhole conditions without
degradation. The
ideal material can withstand temperature of up to at least 150C (preferably
175C or 200C
or more), is chemically resistant or inert to any drilling fluid to which it
will be exposed,
does not absorb fluid to any significant degree and resists erosion by
drilling fluid. An
example of a suitable material is PET (polyethylene terephthalate) or PEEK
(polyether
ether ketone).
[0035] A second electrically-insulating layer 26 is provided between
electronics package
22 and the inner surfaces of the electrically-conducting uphole and/or
downhole parts 20A
and 20B of gap sub 20. Electrically insulating layer 26 extends to at least
partially cover
the inner side of electrically-insulating gap 20C and extends continuously to
cover
electrically-conductive parts of the bore wall on at least one side of
electrically-insulating
gap 20C. In some embodiments electrically insulating layer 26 continuously
covers a part
of the bore wall that includes the inner side of electrically-insulating gap
20C and extends
continuously to cover parts of both uphole and downhole parts 20A and 20B of
gap sub
20. In some embodiments electrically insulating layer 26 comprises a coating
applied to
the inside of gap sub 20, a sleeve or tube extending around the inside of gap
sub 20, or the
like.
[0036] As with layer 25, the material of layer 26 may be any electrically
insulating
material suitable for exposure to downhole conditions. Some non-limiting
examples are
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suitable thermoplastics, epoxies, ceramics, elastomeric polymers, and rubber.
Layer 26
may comprise a coating that is applied to, formed on or bonded to the inner
wall of gap
sub 20 or a pre-formed component (formed e.g. by extrusion, injection molding,
or the
like) which is subsequently attached to, affixed around, supported around the
inside of the
bore of gap sub 20. An example of a suitable material is PET (polyethylene
terephthalate)
or PEEK (polyether ether ketone).
[0037] The inventors have determined that low impedance paths within the bore
of a gap
sub can provide a significant source of inefficiency in the transmission of EM
telemetry
signals. The provision of electrically insulating layer 25, especially in
combination with
the provision of electrically insulating layer 26 has been found to
dramatically reduce
losses arising from conduction currents within the bore of the gap sub. With
electrically-
insulating layers 25 and 26 lining electrically-conductive surfaces within
bore 27, the
shortest path through the fluid in bore 27 electrically connecting parts 20A
and 20B of gap
sub 20 is at least the length of the shorter one of electrically-insulating
layers 25 and 26.
[0038] Figures 3A to 3D illustrate possible electrical conduction paths
through which
current originating from electrodes 24A and 24B could pass. It can be seen
that all of
these possible electrical conduction paths are blocked by at least one of
electrically-
insulating layer 25, electrically-insulating layer 26, electrically-insulating
gap 23C, and
electrically-insulating gap 20C.
[0039] By providing electrically insulating barriers on conductive surfaces of
electronics
package 22 and/or gap sub 20 that would otherwise be exposed to the drilling
fluid in the
bore of gap sub 20, considerable improvements in the efficiency of EM
transmission may
be achieved. The lengths of insulating layers 25 and 26 should be sufficient
to raise the
impedance of the conductive paths through the bore fluid to a desired degree.
Providing
electrically insulating layers 25 and 26 that are at least approximately 2
meters (6 feet)
long has been shown to reduce power lost as a result of current flowing inside
the borehole
by 90% or more in some cases.
[0040] In example embodiments, insulating layers 25 and 26 are at least 1
meter in length
(although they could be shorter in some embodiments). In some embodiments
insulating
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layer 26 extends for a length that is at least 75% of the length of
electrically insulating
layer 25. In preferred embodiments, electrically insulating layer 26 is at
least as long as
electrically insulating layer 25. In some embodiments, electrically insulating
layer 26
covers substantially the entire inside of that portion of the bore of gap sub
20 lying
between electrodes 24A and 24B.
[0041] Figure 4 illustrates schematically an equivalent electrical circuit for
the telemetry
signal generator and gap sub 20 (neglecting capacitive and inductive effects).
Resistor RIN
represents the available current paths within the bore 20D of the gap sub 20
and resistor
RouT represents the available current paths external to the gap sub 20. Dual
non-
conductive layers 25 and 26 provide an effectively large internal isolation
path (a large
value for RIN) thus increasing the electrical efficiency of the gap sub 20 EM
telemetry by
providing an internal resistance (RIN) between antenna elements of the gap sub
20 that is
large compared to the resistance of the external gap (RouT).
[0042] Another advantage of providing non-conductive layers on both the inner
surface of
gap sub 20 and the outer surface of electronics package 22 is that layers 25
and 26 prevent
conductive outer surfaces of electronics package 22 from making electrical
contact with
inner surfaces of gap sub 20 as might possibly occur in cases where the
electronics
package and gap sub are subjected to high shocks and/or vibration. Such
contact could
damage a telemetry signal generator (e.g. by shorting its output) and/or
interfere with
telemetry of downhole information.
[0043] A centralizer may optionally be provided to maintain electronics
package 22
central in bore 20D of gap sub 20. Various centralizer designs are used. Any
suitable
centralizer may be used. In some embodiments one or both of layers 25 and 26
is
integrated with a centralizer. For example, centralizing members such as
longitudinally-
extending ridges or bumps or other protrusions may be provided on one or both
of layers
25 and 26 to maintain electronics package 22 centered in the bore of gap sub
20. The
centralizing members may comprise a resilient elastomeric or vibration
dampening
material such as rubber or a suitable plastic, for example.
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[0044] Providing electrically-insulating layers 25 and/or 26 also allows the
minimum
spacing between the inner surfaces of electrically conducting parts 20A and
20B of gap
sub 20 and the outer surface of the housing 23 of electronics package 22 to be
reduced
significantly without causing losses due to conduction through the fluid
within the bore of
.. gap sub 20 to increase significantly. This is particularly significant
where the drilling
fluids being used are of a type that provides relatively low electrical
impedance. Water-
based drilling fluids tend to have lower electrical impedance.
[0045] Providing electrically-insulating layers 25 and/or 26 also allows the
width of gap
20C inside the bore of gap sub 20 and the width of gap 23C to be reduced.
Reducing the
widths of gaps 20C and/or 23C can result in more robust apparatus since most
available
electrically-insulating materials suitable for gaps 23C and 20C are less
robust than the
materials (most typically metals) used for other parts of gap sub 20 and
housing 23.
[0046] Electrically-insulating layers 25 and 26A also alleviate any need to
align gap 20C
of gap sub 20 with gap 23C of electronics package 22. In some embodiments gap
20C is
longitudinally spaced apart from Gap 23C. Thus the provision of electrically-
insulating
layers 25 and 26 allows the longitudinal position of electronics package 22 to
be adjusted
without causing problems that might otherwise arise from the misalignment of
gaps 20C
and 23C. Furthermore, the location of gap 23C on electronics package 22 may be
selected
for optimum mechanical properties and/or for optimum placement of electronics
systems
and components within electronics package 22 when it is unnecessary for gap
23C to be
aligned longitudinally with gap 20C..
[0047] In some embodiments, electrically conducting parts 20A and 20B of gap
sub 20 are
formed to provide parts that extend radially inwardly to provide support to
electronics
package 22. The radially-inwardly extending parts may be integrally formed
with parts
20A and 20B of the same metal.
[0048] Figure 5 illustrates an example apparatus 50 comprising a gap sub 20
that is
formed to provide radially-inwardly extending parts in the form of rounded
lobes 52 that
extend longitudinally within bore 20D of gap sub 20. Lobes 52 may extend for
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substantially the full length of electronics package 22. Lobes 52 may be
formed, for
example, by hobbing.
[0049] Figure 5A shows an example embodiment wherein an electrically
insulating layer
25 is provided on the outside of electronics package 22. Another electrically
insulating
layer 26A is preferably but optionally provided on the inside of the bore of
gap sub 20
covering lobes 52.
[0050] As shown in Figure 5A, lobes 52 are dimensioned such that electronics
package 22
is firmly held within their inwardly-facing tips. Electrically-insulating
layers 25 and/or
26A may be of materials that provide mechanical damping as well as electrical
insulation.
Mechanically coupling electronics package 22 to gap sub 20 continuously along
its length
can substantially reduce flexing and vibration of electronics package 22
caused by lateral
accelerations of the drill string, flow of drilling fluid, or the like.
[0051] Apparatus as described herein may be applied in a wide range of
subsurface
drilling applications. For example, the apparatus may be applied to provide
telemetry in
logging while drilling ('LWD') and/or measuring while drilling ('1V1WD')
applications.
Providing apparatus as described herein in which electrical current flow
between different
antenna elements within the bore of a drill string is significantly diminished
reduces the
load on a telemetry signal generator. This in turn may permit the same
telemetry signal
generator to operate with a reduced power output and/or to provide a higher-
voltage signal
to the antenna elements, thereby facilitating one or more of extended battery
life, reduced
power consumption, improved telemetry signal strength at the surface and
reduced
telemetry error rate. Extended battery life in downhole applications is very
significant
since battery replacement or recharging may require withdrawal of the
electronics package
from the hole. This can be time consuming and labor intensive. Thus, increased
battery
life can result in a longer run length during drilling operations with fewer
service intervals
needed.
[0052] Another aspect of the invention provides a subsurface drilling method.
The
method is performed using a drill string comprising a gap sub and an
electronics package
located in a bore of the gap sub. The electronics package has electrical
contacts that are in
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electrical contact with electrically-conductive parts of the gap sub. The
method involves
passing a drilling fluid down a bore of the drill string and, at the location
of the electronics
package, channeling the drilling fluid into a channel that is electrically
insulated from both
the electrically conductive parts of the gap sub and electrically conductive
parts of the
housing of the electronics package. In some embodiments, examples of which are
described above, the channel is an annular channel that surrounds that portion
of the
electronics package between the electrodes. This is not mandatory, however.
[0053] A wide range of alternatives are possible. For example, it is not
mandatory that the
gap sub be a single component. In some embodiments a gap sub comprises a
plurality of
.. components that can be assembled together into the drill string to provide
electrical
insulation between two parts of the drill string. A probe may extend fully or
partially
through one, two, three, or more coupled-together sections of the drill
string.
[0054] In some embodiments, electronic systems which may include a telemetry
signal
generator are provided in a package located in a cavity formed in a wall of a
drill collar or
gap sub. Such embodiments may not have a separate probe mounted in a bore of
the drill
collar or gap sub. Electrical connections between an EM telemetry signal
generator
housed in a wall of a drill string section and uphole and downhole portions
20A and 20B
of the gap sub may be made by way of conductors embedded in the wall of the
gap sub.
Figure 6 shows schematically an example embodiment in which an electronics
package 60
is located in a cavity 61 in a wall of a gap sub 20. In such embodiments
efficiency of EM
telemetry may be improved by providing an electrically-insulating layer 26
that at least
partially covers the inside of electrically-insulating gap 20C and extends to
continuously
cover parts of one or both of the inner surfaces of the electrically-
conducting uphole and
downhole parts 20A and 20B of gap sub 20 that are adjacent to electrically-
insulating gap
20C. The electrically-insulating layer 26 covers at least one of the
interfaces 62 between
electrically-insulating gap 20C and uphole and downhole parts 20A and 20B.
With
electrically-insulating layer 26 lining bore 27, the shortest path through the
fluid in bore 27
electrically connecting parts 20A and 20B of gap sub 20 is at least the length
of
electrically-insulating layer 26.
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Interpretation of Terms
[0055] Unless the context clearly requires otherwise, throughout the
description and the
claims:
= "comprise", "comprising", and the like are to be construed in an
inclusive
sense, as opposed to an exclusive or exhaustive sense; that is to say, in the
sense of "including, but not limited to".
= "connected", "coupled", or any variant thereof, means any connection or
coupling, either direct or indirect, between two or more elements; the
coupling
or connection between the elements can be physical, logical, or a combination
thereof.
= "herein", -above", "below", and words of similar import, when used to
describe this specification shall refer to this specification as a whole and
not to
any particular portions of this specification.
= "or", in reference to a list of two or more items, covers all of the
following
interpretations of the word: any of the items in the list, all of the items in
the
list, and any combination of the items in the list.
= the singular forms "a7, "an- and "the- also include the meaning of any
appropriate plural forms.
[0056] Words that indicate directions such as "vertical", "transverse",
"horizontal",
.. "upward", "downward", "forward", "backward", "inward", "outward",
"vertical",
"transverse", "left", "right", "front", "back", "top", "bottom", "below",
"above", "under",
and the like, used in this description and any accompanying claims (where
present) depend
on the specific orientation of the apparatus described and illustrated. The
subject matter
described herein may assume various alternative orientations. Accordingly,
these
directional terms are not strictly defined and should not be interpreted
narrowly.
[0057] Where a component (e.g. a circuit, module, assembly, device, drill
string
component, drill rig system etc.) is referred to above, unless otherwise
indicated, reference
to that component (including a reference to a "means") should be interpreted
as including
as equivalents of that component any component which performs the function of
the
described component (i.e., that is functionally equivalent), including
components which
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are not structurally equivalent to the disclosed structure which performs the
function in the
illustrated exemplary embodiments of the invention.
[0058] Specific examples of systems, methods and apparatus have been described
herein
for purposes of illustration. These are only examples. The technology provided
herein
can be applied to systems other than the example systems described above. Many
alterations, modifications, additions, omissions and permutations are possible
within the
practice of this invention. This invention includes variations on described
embodiments
that would be apparent to the skilled addressee, including variations obtained
by: replacing
features, elements and/or acts with equivalent features, elements and/or acts;
mixing and
matching of features, elements and/or acts from different embodiments;
combining
features, elements and/or acts from embodiments as described herein with
features,
elements and/or acts of other technology; and/or omitting combining features,
elements
and/or acts from described embodiments.
[0059] It is therefore intended that the following appended claims and claims
hereafter
introduced are interpreted to include all such modifications, permutations,
additions,
omissions and sub-combinations as may reasonably be inferred. The scope of the
claims
should not be limited by the preferred embodiments set forth in the examples,
but should
be given the broadest interpretation consistent with the description as a
whole.
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