Improved systems, methods and apparatus for downhole monitoring of production conditions using em telemetry

Abstract

A gap sub for use in oil, gas or water production wells comprises an electrical isolation structure to electrically isolate two metallic ends of the gap sub with a housing mechanically and electrically coupled at one end close to or against the inside wall of one metallic end of the gap sub and offset from a centre of the gap sub. An "internal gap" is incorporated in the other end of the housing with a toolstring accommodated inside the housing to acquire in-situ measurements of wellbore and near-wellbore conditions and to transmit the measurements to surface using EM telemetry. An antenna rod connected to the "internal gap" is electrically isolated from the housing and electrically coupled to the other metallic end of the gap sub.

Description

TITLEIMPROVED SYSTEMS, METHODS AND APPARATUS FOR DOWNHOLE MONITORING OF PRODUCTION CONDITIONS USING EM TELEMETRYFIELD OF THE INVENTION

[0001] The present invention relates to technology useful in wireless monitoring of in-situ conditions in conventional and unconventional oil and gas production and monitoring wells. In particular, but not exclusively, the present invention relates to an electrically insulating, gap sub assembly, used for electromagnetic telemetry between surface and subsurface locations, wherein the sensors, electronics and power source are self-contained in a pressure- tight housing, filled with air or inert gas.BACKGROUND TO THE INVENTION

[0002] FIG. 1A and FIG. 1B illustrates a system 100 for wireless monitoring of in-situ conditions in conventional and unconventional oil and gas production and monitoring wells, known from the Applicant’s earlier patent application no. PCT / AU2022 / 051169. The apparatus comprises a hollow composite isolation joint 101 between metal end members 105 and 106, collectively referred to as the gap sub, with a toolstring 102, comprising electronic sensors, EM transceiver and battery pack, positioned inside a separate pressure housing 103 mounted on the outside of end member 106. The pressure housing 103 is connected to antenna rod 108, through an antenna isolator assembly 107, the purpose of which is to electrically isolate antenna rod 108 from end member 106. The antenna rod 108 straddles composite isolation joint 101, with the other end of the antenna rod 108 mechanically and electrically coupled to end member 105. The bore of the pressure housing 103 is isolated from wellbore fluids surrounding it by means of the antenna isolator assembly 107 and pressure and temperature transducer 104. Fully assembled, apparatus 100 is disposed to be inserted into a tubing string (not shown) that is installed into the well to facilitate production of reservoir fluids to surface, or injection offluids from surface into one or more reservoirs, or to monitor wellbore equipment or reservoir characteristics and properties.

[0003] FIGS. 2A and 2B illustrate, in assembled and disassembled form respectively, cross-sectional views of a composite isolation joint gap sub 200, also known from the Applicant’s earlier patent application no.PCT / AU2022 / 051169. In FIG. 2A, gap sub 200 comprises pin end members 201 and 202 separated by an electrical isolation structure 203. A pressure housing 204, containing toolstring 102, is welded or affixed using other standard electrically conductive techniques to pin end member 202. Antenna isolator assembly 205 and pressure transducer 206 create a pressure seal, isolating the void 207 shown in FIG. 2B inside pressure housing 204 from wellbore fluids inside and outside the gap sub 200. The pin end members 201 and 202 are made from an electrically conductive material, which can be magnetic or non-magnetic. The electrical isolation structure 203 is a composite collar made from a non-conductive composite material, such as glass-fibre, although other non-conductive materials can be used.

[0004] As noted, FIG. 2B illustrates gap sub 200 from FIG. 2A in disassembled form. All part numbers in FIG. 2A are illustrated in FIG. 2B by the same part number. FIG. 2B shows composite collar 203 including threaded box connections 208 at each end, configured to mate with threaded pin connections 209 at opposite end of threaded pin connections 210 on pin end members 201 and 202 to create a pressure-tight seal.

[0005] One or more ports 211 are included in pressure housing 204 that enables any build-up of gas pressure in the void space 207 inside the housing to be vented to the outside in a safe and controlled manner when the gap sub is retrieved to surface. This is achieved by backing out the threaded antenna isolator assembly 205 until seals 212 clear ports 211. The position and length of the internal threads 213 in pressure housing 204 is set to ensure that the pressure housing 204 is still engaged to these threads when seals 212 clear ports 211.

[0006] Adjustable antenna isolator assembly 205 is attached to electrically conductive antenna rod 214, using either a threaded or pinned connection. The adjustable antenna isolator assembly 205 electrically isolates antenna rod214 from pin end member 202. The rod can be hollow or solid. The other end of antenna rod 214 is mechanically and electrically coupled to pin member 201 by sandwiching the rod between a metallic clamp block base 215, which is welded to pin end member 201, and a clamp block cap 216, which is bolted to the clamp block base 215. The clamp block cap 216 is made from an electrically non-conductive composite material, such as glass-filled PA66, although other non-conductive materials can be used.

[0007] Pressure transducer 206 senses wellbore pressure surrounding pressure housing 204 via the hollow pressure port adapter 217. Multiple seals218 on the pressure transducer 206 prevent wellbore fluid from entering the internal void 207 between the pressure transducer and adjustable antenna isolator assembly 205. Pressure transducer 206 also incorporates a shoulder219 that presses against internal lip 220 on the inside of the pressure housing 204 due to action of wellbore fluid pressure on the end face of pressure transducer 206. Pressure transducer 206 is secured in place by means of a circlip 221 (not shown). The circlip could be replaced with a threaded lock ring instead to press pressure transducer 206 against internal lip 220.

[0008] The systems 100, 200 of FIGS. 1A, 1B, 2A, 2B provide the benefits of full-bore access for fluids produced through the gap sub into the tubing string extending to surface above the gap sub from geological formations intersected by the well. The full-bore access also enables fluids to be pumped down the same tubing string from surface to stimulate the geological formations or to circulate produced solids that have accumulated in the well back to surface. The systems 100, 200 also prevent the toolstring from being exposed to corrosion, abrasion risk and / or erosion risk from the fluids inside the gap sub and provide improved electrical isolation of the metal end members, thus reducing signal losses and improving the quality of the EM telemetry.

[0009] However, mounting the toolstring 102 on the outside of the gap sub 100, 200 increases the total width of system 100. This means the through- bore diameter of gap sub 200 may need to be smaller than the through-bore diameter of the tubing string or other equipment, such as a submersible pump, positioned above it, in order to fit inside the well. This smaller bore couldrestrict production of fluids from prolific formations. Less prolific wells with a small diameter production casing, and wells which have metal or composite casing patches installed inside a larger diameter casing to isolate defects, can impose a more severe restriction on the through-bore diameter of gap sub.

[0010] In such circumstances, consideration could be given to repositioning the toolstring 102 inside a larger diameter gap sub that will still fit inside the well and has an internal flow area past the toolstring 102 that is still larger than the flow area through any submersible pump positioned above it. Apparatus and methods adopting this approach are known. For example, U.S. patent application US2016 / 0032660 A1 to Logan et al. discloses a probe which, while not described explicitly, is a toolstring comprising one or more sensors, an EM telemetry device and a power source. The probe shown in FIG.2 to Logan et al. has ported metallic rings at each end, with one ring suspending the probe inside the gap sub, and both rings keeping the probe centralised in the gap sub bore. The two rings also electrically couple both ends of the probe with the two metal end members of the gap sub. This same arrangement is also shown in FIG. 17 of U.S patent 10,400,520 B2, also to Logan et al. Rather than using a suspension device, U.S. patent 10,450,858 to Petrovic et al. discloses a toolstring in FIG.4 which is set in an anchoring device instead.

[0011] However, such centralised toolstrings known from Logan et al. and Petrovic et al. create restricted flow areas through / around the centralised toolstring, as illustrated in FIGS 2C and 2D. Ported rings, an example of which is shown in FIG. 2C, and “spider” hangers, an example of which is shown in FIG. 2D, have the restricted flow areas for the fluid to pass through as represented by the dark circles / areas. Ported rings, “spider” hangers, or any other type of device straddling the bore to suspend or anchor a toolstring in the centre of a gap sub will also be exposed to erosion risk from fluids passing through / around them, and / or abrasion if the fluids have entrained solids. The “spider” type of suspension device show in FIG 2D provides a greater flow area past it than the ported ring shown in FIG 2C. However, the radial struts are exposed to erosion and abrasion, and if the struts erode completely, the probe will fall down the well. Furthermore, an annulargeometry for fluid flow around the centralised probe increases the risk of solids bridging in this space in situations where appreciable solids are suspended in the fluid further restricting and possibly blocking fluid flow.OBJECT OF THE INVENTION

[0012] It is a preferred object of the present invention to provide a system, a method and / or an apparatus that addresses or at least ameliorates one or more of the aforementioned problems of the prior art and / or provides a useful commercial alternative.

[0013] Another preferred object of the present invention is to provide an improved gap sub design that accommodates the toolstring in its entirety inside a single gas-filled housing on the inside of the gap sub.

[0014] Another preferred object of the present invention is to provide a gap sub that eliminates use of a device straddling the bore to suspend or anchor the toolstring in the gap sub bore, thereby alleviating erosion and abrasion risk.

[0015] Another preferred object of the present invention is to provide a gap sub and internal toolstring such that the net flow-through area inside the gap sub can be, but not necessarily, at least equal to the flow area through any submersible pump positioned above it.

[0016] Another preferred object of the present invention is to provide a gap sub incorporating an electrical isolation structure (“the gap”) to electrically isolate two metallic ends of the gap sub, with one metallic end accommodating a toolstring, comprising one or more sensors, an EM transceiver and an electrical power source, and clamped to the inside wall of the metallic end.SUMMARY OF THE INVENTION

[0017] Generally, the present invention relates to systems, methods and / or apparatus for acquiring in-situ measurements at the bottom of an oil, gas or water production or monitoring well, with measurements transmitted to surface using EM telemetry, by a toolstring situated entirely within a pressure-tight,gas-filled housing mounted on an inside wall of one metallic end of the gap sub and offset from a centre of the gap sub. The gap sub is incorporated between tubing joints in a tubing string at a desired depth in a well, and below any submersible pumps, with measurements acquired from the sensors inside the gap sub transmitted to surface using ULF EM telemetry.

[0018] In one form, although not necessarily the broadest form, the invention resides in a gap sub for wellbore fluids, the gap sub comprising: an electrical isolation structure to electrically isolate two metallic ends of the gap sub; a housing mechanically and electrically coupled to the inside wall of one metallic end of the gap sub and offset from a centre of the gap sub, the housing accommodating an integral “internal gap” and a toolstring for acquiring in-situ measurements of wellbore and near-wellbore conditions and transmitting the measurements to surface using EM telemetry; and an antenna rod connected to the “internal gap” in the housing, the antenna rod straddling the electrical isolation structure in the gap sub to be electrically isolated from the housing and mechanically and electrically coupled to the other metallic end of the gap sub.

[0019] In particular, the housing is coupled to the inside wall of the one metallic end of the gap sub such that the housing abuts the inside wall, or is adjacent and in very close proximity to the inside wall such that the housing is off-centre and non-coaxial with the gap sub.

[0020] Suitably, the toolstring comprises: one or more electronic sensors able to measure the wellbore and near-wellbore conditions; an EM transceiver for encoding measurements from the sensors, modulating an EM waveform and transmitting the EM waveform to surface; and a power source, such as a battery pack, to provide electrical power to the sensors and the EM transceiver.

[0021] Suitably, the one or more electronic sensors are integrated with the EM transceiver in a single module or provided in separate modules that can be coupled together.

[0022] Suitably, the housing is in the form of a pressure housing comprising a pressure transducer at the lower end thereof.

[0023] Suitably, the lower end of the pressure housing is mechanically and electrically coupled to the inside wall of one metallic end of the gap sub via a first clamp element.

[0024] Suitably, the “internal gap” is in the form of an adjustable antenna isolator assembly adjustably accommodated at least partly within an upper end of the pressure housing.

[0025] Suitably, the adjustable antenna isolator assembly is attached to a first end of the electrically conductive antenna rod to electrically isolate the antenna rod from the pressure housing and one end of the gap sub.

[0026] Suitably, a second end of the electrically conductive antenna rod is mechanically and electrically coupled to the inside wall of the other metallic end of the gap sub via a second clamp element.

[0027] Suitably, the first and second electrically conductive clamp elements used to secure the pressure housing and antenna rod to the inside wall of the two metallic ends of the gap sub have hydrodynamic shapes with no flow ports.

[0028] Suitably, the first and second electrically conductive clamp elements are made from a material imbued with wear-resistance.

[0029] Suitably, the first and second electrically conductive clamp elements used to secure the pressure housing and antenna rod close to or against the inside wall of the two metallic ends of the gap sub result in a crescent-shaped bypass flow geometry within the gap sub that substantially reduces risk of solids entrained in fluid flow through this region from bridging or plugging this space.

[0030] Suitably, an internal diameter of the gap sub and an outside diameter of the pressure housing are chosen to result in a flow area through the gap sub that can be, but is not necessarily, equal to or greater than a flow area through any submersible pump positioned in the tubing string above the gap sub.

[0031] Suitably, the gap sub comprises upper and lower metal tubes and the electrical isolation structure is in the form of a hollow composite electrical isolation collar in between the upper and lower metal tubes.

[0032] In some embodiments, the lower metal tube comprises two separate parts joined together with a second electrical isolation structure.

[0033] Suitably, the second electrical isolation structure is in the form of a threaded collar comprising threaded box connections at each end configured to mate with threaded pin connections on pin end members of the two separate parts of the lower metal tube.

[0034] Suitably, the second electrical isolation structure is: a) metallic; or b) a composite when the inside of the gap sub is filled with an electrically conductive fluid.

[0035] Preferably, the gap sub comprises an assembly for creating an electrical connection between a transmit / receive line on the EM transceiver with one end of the gap sub.

[0036] Preferably, the gap sub comprises an assembly for establishing an electrical connection between a ground plane on the EM transceiver with the other end of the gap sub.

[0037] Suitably, the gap sub comprises threaded pin and / or box connections at either end, configured to be inserted at a desired position within a tubing string, below any submersible pump.

[0038] Preferably, the adjustable antenna isolator assembly is accommodated at least partly within the pressure sensor housing to enable fine adjustment of an axial position of the antenna isolator assembly in the pressure housing. Suitably, this is achieved with external threads on an outer collar of the antenna isolator assembly engaging with internal threads in the pressure housing.

[0039] Preferably, the pressure housing comprises one or more ports to bleed-off any trapped pressure inside the pressure housing when the gap sub is retrieved to surface. Suitably, the adjustable antenna isolator assembly comprises one or more seals, wherein adjustment of the axial position of the adjustable antenna isolator assembly in the pressure housing until the one ormore seals clear the respective one or more ports provides a vent mechanism to bleed-off any trapped pressure inside the pressure housing.

[0040] Suitably, the module comprising the one or more sensors comprises a shoulder that presses against an internal lip of the pressure housing under wellbore fluid pressure to rigidly secure the toolstring inside the pressure housing.

[0041] Suitably, the gap sub comprises a thread locking compound applied to threads of the threaded pin and / or box connections to ensure torque rating of the gap sub equals or exceeds the torque rating of the tubing string above and below the gap sub.

[0042] In another form, the invention resides in a method of acquiring in-situ measurements of wellbore and near-wellbore conditions and transmitting the measurements to surface using EM telemetry, the method comprising: providing a gap sub between tubing joints in a tubing string and below any submersible pump for wellbore fluids, the gap sub comprising an electrical isolation structure to electrically isolate two metallic ends of the gap sub; mechanically and electrically connecting a housing to an inside wall of one metallic end of the gap sub such that the housing is offset from a centre of the gap sub, the housing accommodating a toolstring with the EM telemetry and an “internal gap”; and connecting an antenna rod to the “internal gap” to electrically isolate the antenna rod from the housing, the antenna rod straddling the electrical isolation structure in the gap sub, with the other end of the antenna rod mechanically and electrically connected to the other metallic end member.

[0043] In a further form, the invention resides in a system to acquire in-situ measurements of wellbore and near-wellbore conditions and transmitting the measurements to surface using EM telemetry, the system comprising: a gap sub between tubing joints in a tubing string for wellbore fluids, the gap sub comprising an electrical isolation structure to electrically isolate two metallic ends of the gap sub;a housing mounted on an inside wall of one metallic end of the gap sub and offset from a centre of the gap sub, the housing accommodating an “internal gap” and a toolstring providing the EM telemetry; and an antenna rod connected to the “internal gap”, the antenna rod straddling the electrical isolation structure in the gap sub to be electrically isolated from one metallic end of the gap sub and electrically coupled to the other metallic end of the gap sub.

[0044] In yet another form, the invention resides in a gap sub for wellbore fluids, the gap sub comprising: an electrical isolation structure to electrically isolate two metallic ends of the gap sub; a housing mounted against the inside wall of the gap sub and offset from a centre of the gap sub, the housing accommodating an “internal gap”, and a toolstring for acquiring in-situ measurements of wellbore and nearwellbore conditions and transmitting the measurements to surface using EM telemetry; wherein a flow area through the gap sub is equal to or greater than a flow area through any submersible pump positioned in a tubing string above the gap sub.

[0045] Further forms and / or features of the present invention will become apparent from the following detailed description.BRIEF DESCRIPTION OF THE DRAWINGS

[0046] In order that the invention may be readily understood and put into practical effect, reference will now be made to preferred embodiments of the present invention with reference to the accompanying drawings, wherein like reference numbers refer to identical elements. The drawings are provided by way of example only, wherein:

[0047] FIG. 1A is a diagram illustrating an apparatus according to one embodiment of the prior art;

[0048] FIG. 1B is a cross sectional view of the apparatus shown in FIG. 1A;

[0049] FIGS. 2A and 2B illustrate, in assembled and disassembled form respectively, cross-sectional views of a hollow composite isolation joint gap sub, including a non-conductive composite insert, known from the prior art;

[0050] FIGS 2C and 2D are cross sectional end views of a ported ring hanger and a “spider” hanger, respectively used to suspend or anchor a toolstring in the centre of a gap sub known from the prior art illustrating the reduced flow areas;

[0051] FIG. 3A is a diagram illustrating an apparatus according to one embodiment of the present invention;

[0052] FIG. 3B is a cross sectional view of the apparatus shown in FIG. 3A;

[0053] FIGS. 4A and 4B illustrate, in assembled and disassembled form respectively, cross-sectional views of a hollow composite isolation joint gap sub, including a non-conductive composite insert, shown in FIG. 3B;

[0054] FIG. 5 is a diagram of the toolstring shown in FIG. 3B;

[0055] FIG. 6 is an alternative embodiment of the toolstring shown in FIG. 3B;

[0056] FIGS. 7A and 7B are cross-sectional views of the gap sub according to an embodiment of the present invention showing the clamping element and housing; and

[0057] FIGS. 8A and 8B illustrate, in assembled and disassembled form respectively, cross-sectional views of a hollow composite isolation joint gap sub, including at least one non-conductive composite insert, according to another embodiment.

[0058] Skilled addressees will appreciate that the drawings may be schematic and that elements in the drawings are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the relative dimensions of some of the elements in the drawings may be distorted to help improve understanding of embodiments of the present invention.DETAILED DESCRIPTION OF THE INVENTION

[0059] The present invention relates to systems, methods and apparatus for acquiring in-situ measurements at the bottom of an oil, gas or water producing or monitoring well, with measurements transmitted to surface using EM telemetry, using a toolstring situated entirely within a gap sub.

[0060] FIGS. 3A and 3B illustrate an apparatus 300 according to one embodiment of the invention. The apparatus comprises a hollow composite isolation joint 301 between metal end members 305 and 306, collectively referred to as the gap sub, with a toolstring 302, comprising electronic sensors, EM transceiver and battery pack, positioned inside a separate housing 303. In particular, the housing is in the form of a pressure housing 303, which is electrically and mechanically coupled to an inside wall 305A of end member 305 via clamp element 309. The pressure housing 303 is coupled to the inside wall 305A of end member 305 such that it is offset from a centre of the gap sub. In particular, the pressure housing 303 is coupled to the inside wall 305A of the one metallic end of the gap sub such that the housing 303 abuts the inside wall 305A, or is adjacent and in very close proximity to the inside wall such that the housing 303 is off-centre and noncoaxial with the gap sub.

[0061] The pressure housing 303 is connected to antenna rod 308, through an antenna isolator assembly 307, the purpose of which is to electrically isolate antenna rod 308 from pressure housing 303 and end member 305. The antenna rod 308 straddles composite isolation joint 301, with the other end of the antenna rod 308 mechanically and electrically coupled to an inside wall 306A of end member 306 via clamp element 310. The bore of the pressure housing 303 is isolated from wellbore fluids surrounding it by means of an antenna isolator assembly 307 and pressure and temperature transducer 304.

[0062] Fully assembled, apparatus 300 is disposed to be inserted into a tubing string and below any submersible pump that is installed into the well to facilitate production of reservoir fluids to surface, or injection of fluids from surface into one or more reservoirs, or to monitor wellbore equipment or reservoir characteristics and properties. It will be appreciated that theorientation of apparatus 300 will in no way impact the ability of the EM transceiver to transmit modulated EM waveforms from downhole to surface or receive EM signals from surface. As such, apparatus 300 can be inserted into the tubing string with end member 305 facing upwards or downwards, with end member 306 facing the opposite way.

[0063] FIGS. 4A and 4B illustrate, in assembled and disassembled form respectively, cross-sectional views of one embodiment of a composite isolation joint gap sub 400. In FIG. 4A, gap sub 400 comprises pin end members 401 and 402 separated by an electrical isolation structure 403. A pressure housing 404, containing toolstring 425, is mechanically and electrically affixed to clamp element 415, which is bolted to pin end member 401 using bolts 417, or can be affixed using other standard mechanically and electrically conductive techniques. Antenna isolator assembly 405 and pressure transducer 406 create a pressure seal, isolating the void 407 shown in FIG. 4B inside pressure housing 404 from wellbore fluids inside the gap sub 400. The pin end members 401 and 402 are made from an electrically conductive material, which can be magnetic or non-magnetic. The electrical isolation structure 403 is a composite collar made from a non-conductive composite material, such as glass-fibre, although other non-conductive materials can be used, and the present invention is not limited in any way to use of this specific composite. Antenna isolator assembly 405 is made from an electrically non-conductive composite material, such as glass-filled PA66.

[0064] As noted, FIG. 4B illustrates gap sub 400 from FIG. 4A in disassembled form. All part numbers in FIG. 4A are illustrated in FIG. 4B by the same part number. FIG. 4B shows composite collar 403 including threaded box connections 408 at each end, configured to mate with threaded pin connections 409 at the opposite end to threaded pin connection 410 on pin end member 401 and at the opposite end of threaded box connection 411 on pin end member 402 to create a pressure-tight seal. A thread locking compound (not shown) is applied to threads 408 and 409 to increase torque rating. Suitable thread locking compounds include ForumLok from Forum Energy Technologies, although the present invention is not limited in any way to use of this specific compound.

[0065] Nothing in this disclosure should be interpreted to place any limitation on the size, shape, geometry or pin / box configurations of the threaded connections on 408, 409, 410 or 411. It will be appreciated that the dimensional specifics of the threads may be varied to suit individual applications, responsive to parameters such as, for example, the dimensions and thicknesses of surrounding conductive portions, the electrical conductivity (or non-conductivity) and other electrical characteristics of the materials being used in the assembly, size and type of tubing used above and below the gap sub, the size and type of other equipment integrated into the tubing string above and below the gap sub, and / or the expected dynamic loads exerted on the gap sub in-situ. However, for exemplary guidance only, it has been observed that serviceable results may be obtained when the threaded connections 408, 409, 410 and 411 are in a range of between 2 inches (5.08cm) and 6 inches (15.24cm) in length when the corresponding outside diameter of the assembly is in a range of between 2.5 inches (6.35cm) and 6.5 inches (16.51cm). It should also be obvious to the skilled addressee that threaded connections 408 and 409 can instead have pin and box configurations respectively, and that either of threaded connections 410 and 411 may have pin or box configurations, or both connections having the same box configuration also.

[0066] The flow area through gap sub 400 is determined not only by the outside diameter of gap sub 400, but also the size of the pressure housing 404. It has been observed that serviceable results may be obtained when the outside diameter of this housing is in a range of between 1.25 inches (3.175 cm) and 2.5 inches (5.08 cm). However, serviceable results may be obtained outside these ranges.

[0067] One or more ports 412 are included in pressure housing 404 that enables any build-up of gas pressure in the void space 407 inside the housing to be vented to the outside in a safe and controlled manner when the gap sub is retrieved to surface. This is achieved by backing out the threaded antenna isolator assembly 405 until seals 413 clear ports 412. The position and length of the internal threads 414 in pressure housing 404 is set to ensure that thepressure housing 404 is still engaged to these threads when seals 413 clear ports 412.

[0068] Adjustable antenna isolator assembly 405 is attached to electrically conductive antenna rod 423, using either a threaded or pinned connection. The adjustable antenna isolator assembly 405 electrically isolates antenna rod 423 from pin end member 410. The rod can be hollow or solid. The other end of antenna rod 423 is mechanically and electrically coupled to pin member 402 by clamp element 416, which is bolted to pin end member 402 using bolts 417, or can be affixed using other standard mechanically and electrically conductive techniques.

[0069] Pressure transducer 406 senses wellbore pressure surrounding pressure housing 404 via the hollow pressure port adapter 418. Multiple seals419 on the pressure transducer 406 prevent wellbore fluid from entering the internal void 407 between the pressure transducer and adjustable antenna isolator assembly 405. Pressure transducer 406 also incorporates a shoulder420 that presses against internal lip 421 on the inside of the pressure housing 404 due to action of wellbore fluid pressure on the end face of pressure transducer 406. Pressure transducer 406 is secured in place by bulkhead adapter 422.

[0070] In some embodiments, clamp elements 415 and 416 are made from an electrically conductive material that has been subjected to a surface hardening treatment process, such as Boronizing (also known as Bonding), to greatly improve wear resistance of these parts. Furthermore, clamp elements 415 and 416 have hydrodynamic geometries or shapes with no flow ports to minimise wear and abrasion potential from fluids flowing in either direction through the bore of gap sub 400. For example, clamp elements 415 and 416 can comprise sloping, inclined and / or rounded surfaces to promote fluid flow and minimise wear, abrasion and erosion.

[0071] With additional reference to FIGS. 7A and 7B, the electrically conductive clamp elements 309 and 310 and 415 and 416 used to secure the pressure housing 303 and antenna rod 423 close to or against the inside wall 305A of the two metallic ends 401 and 402 of the gap sub result in a crescentshaped bypass flow geometry within the gap sub that substantially reducesthe risk of solids entrained in fluid flow through this region from bridging or plugging this space. The cross section of the clamp elements ostensibly overlaps the cross section of the pressure housing 303, instead of extending away from the pressure housing, as with the ported ring and “spider” hangers of the prior art, thereby significantly increasing flow area through the gap sub.

[0072] In some embodiments, an internal diameter of the gap sub and an outside diameter of the pressure housing 303 are chosen to result in a flow area through the gap sub that can be, but is not necessarily, equal to or greater than a flow area through any submersible pump positioned in the tubing string above the gap sub.

[0073] FIG. 5 shows an example toolstring 500. It comprises a pressure and temperature transducer 501 with a multi-pin electrical connector 502 that attaches to a mating multi-pin electrical connector 503 at one end of battery carrier 504. For compactness, pressure and temperature transducer 501 incorporates the EM transmitter and receiver circuitry 507 (not shown) to send and receive encoded EM signals. The battery carrier 504 houses individual cells, connected in series and parallel as required, and is manufactured from an electrically insulating material, such as plastic. Alternatively, the interconnected cells can be shrouded in a sleeve also made from an electrically insulating material, such as rubber. Both insulating methods fulfill the function of an “internal gap”, but not possessing any mechanical strength or containing any seal arrangements required of traditional “internal gaps” described previously. In particular embodiments, to assist in reducing the size of the toolstring 500, smaller sized cells can be used, and other miniaturization techniques can be employed, such as elongating PCBs used in the toolstring.

[0074] An electrical antenna wire 505 (not shown) extends from the EM transceiver circuitry inside pressure and temperature transducer 501 through the mating multi-pole connectors 502 and 503 and along the length of the battery carrier 504 and terminates into electrical socket 506 at the other end of battery carrier 504. The electrical socket 506 engages with electrical pin 424 at end of the antenna rod 423. EM signals generated by the EM transceivercircuity 507 are transmitted along the antenna wire 505 and injected into pin end member 402 of gap sub 400, via antenna rod 423 and clamp 416.

[0075] FIG. 6 shows an alternative toolstring embodiment 600 in which the EM transceiver circuitry is incorporated in a separate module 607 equipped with the same 2-part multi-pole connectors 602, 603 at either end. The EM transceiver module 607 is sandwiched between the pressure and temperature transducer 601 and battery carrier 604. All other parts remain the same as described for toolstring 500 shown in FIG. 5, including electrical socket 606.

[0076] FIGS. 8A and 8B show in assembled and disassembled form respectively, cross-sectional views of a hollow composite isolation joint gap sub 800, including at least one non-conductive composite insert, according to another embodiment.

[0077] Gap sub 800 comprises many of the same features as the embodiments shown in FIGS. 4A and 4B including metallic pin end members 801, 802 separated by an electrical isolation structure 803. A pressure housing 804, containing toolstring 825, is mechanically and electrically affixed to clamp element 815, which is bolted to pin end member 801 using bolts 817, or other standard mechanically and electrically conductive techniques. Antenna isolator assembly 805 and pressure transducer 806 create a pressure seal, isolating the void 807 shown in FIG. 8B inside pressure housing 404 from wellbore fluids inside the gap sub 400. The pin end members 401 and 402 are made from an electrically conductive material, which can be magnetic or non-magnetic.

[0078] FIG. 8B shows composite collar 803 including threaded box connections 808 at each end, configured to mate with threaded pin connections 809 at the opposite end to threaded pin connection 810 on pin end member 801 and at opposite end of threaded box connection 811 on pin end member 802 to create a pressure-tight seal.

[0079] One or more ports 812 are included in pressure housing 804 that enables any build-up of gas pressure in the void space 807 inside the housing to be vented to the outside in a safe and controlled manner when the gap sub is retrieved to surface. This is achieved by backing out the threaded antennaisolator assembly 805 until seals 813 clear ports 812. The position and length of the internal threads 814 in pressure housing 804 is set to ensure that the pressure housing 804 is still engaged to these threads when seals 813 clear ports 812.

[0080] Hollow or solid adjustable antenna isolator assembly 805 is attached to electrically conductive antenna rod 823, using either a threaded or pinned connection. Adjustable antenna isolator assembly 805 electrically isolates antenna rod 823 from pin end member 801. The other end of antenna rod 823 is mechanically and electrically coupled to pin member 802 by clamp element 816, which is bolted to pin end member 802 using bolts 817 or can be affixed using other standard mechanically and electrically conductive techniques.

[0081] Pressure transducer 806 senses wellbore pressure surrounding pressure housing 804 via the hollow pressure port adapter 818. Multiple seals819 on the pressure transducer 806 prevent wellbore fluid from entering the internal void 807 between the pressure transducer and adjustable antenna isolator assembly 805. Pressure transducer 806 also incorporates a shoulder820 that presses against internal lip 821 on the inside of the pressure housing 804 due to action of wellbore fluid pressure on the end face of pressure transducer 806. Pressure transducer 806 is secured in place by bulkhead adapter 822.

[0082] Comparing with the embodiments shown in FIGS. 4A and 4B, the only difference is metallic end member 401 of the earlier embodiment is now split into three sections: metallic end members 801 and 801A, attached in the middle by a threaded collar 827. In one embodiment, threaded collar 827 is also metallic. The motivation for splitting metallic end member 410 into three metallic members is to reduce manufacturing cost, because member 801A can be a commercially available piece of rolled pipe. Threaded collar 827 includes threaded box connections 828 at each end, configured to mate with threaded pin connection 826 on pin end member 801 at the opposite end to threaded pin connection 810 and at opposite end of connection 809 on pin end member 801A to create a pressure-tight seal.

[0083] However, if when installed in a well, the void inside this assembly is filled with an electrically conductive fluid, such as brine, then in anotherembodiment it is advantageous to replace the metallic threaded collar 827 with a composite version, which can be identical to the electrical isolation structure 803 in the form of a threaded collar connecting metallic end member 801 A to metallic end member 802.

[0084] This has the effect of increasing the dipole antenna impedance created by the composite isolation gap sub 800, when it is filled with electrically conductive fluid, because dipole antenna impedance is greatly influenced by the separation distance D between the metallic end members. In the aforementioned embodiment of the composite isolation gap sub 400 shown in FIGS 4A and 4B, the separation distance D is merely the gap between the end faces of connections 409 at end of metallic end members 410 and 402. However, in composite isolation gap sub 800, the separation distance D is significantly increased to the gap between the end face of connection 826 on metallic end member 810 and the end face of connection 809 on metallic end member 802. Increasing dipole antenna impedance increases the strength of EM signals transmitted by the toolstring 825 to surface, which is greatly advantageous because this improves reliability of surface detection and also enables the invention to be deployed to greater depths below ground level. For sake of clarity, when the gap sub is filled with an electrically non-conductive fluid, such as oil or gas, the distance D has little / no effect on antenna impedance.

[0085] It should be appreciated that in other embodiments, additional sensors can be integrated into pressure and temperature gauge 501, or incorporated as separate modules between pressure and temperature transducer gauge 601 and a separate EM transceiver module 607, and connected together using suitable multi-pole electrical connectors.

[0086] Hence, embodiments of the present invention address or at least ameliorate one or more of the aforementioned problems of the prior art by positioning the toolstring with an “internal gap” inside the gap sub coupled to an inside wall of the gap sub and offset from a centre of the gap sub to maximise the flow area through the gap sub. Since the toolstring is not positioned centrally in the gap sub fluid production rates are not hindered by bridging and the size of the toolstring is reduced to contribute to the flow area.Although the toolstring is positioned inside the gap sub corrosion, abrasion risk and / or erosion risk from the fluids in the gap sub are reduced by the hydrodynamically shaped clamp elements, which may also be treated to minimise wear resistance. Further, by relocating the toolstring from the outside wall of the gap sub to the inside wall of the gap sub, it is possible to increase the outside diameter and the inside diameter of the gap sub, such that it still fits inside the well and has an internal flow area that is at least as large as the flow area through any submersible pump positioned above the gap sub. Yet further, electrical isolation of the drill string is improved compared with at least some of the prior art solutions, thus reducing losses and improving the quality of the EM telemetry. In some embodiments in which the gap sub is filled with an electrically conductive fluid, the quality of the EM telemetry is also improved by splitting one of the pin end members into two which are attached by a threaded collar. This has the effect of increasing the dipole antenna impedance created by the composite isolation gap sub.

[0087] The reference to any prior art in this specification is not, and should not be taken as, an acknowledgement or any form of suggestion that the prior art forms part of the common general knowledge.

[0088] In this specification, adjectives such as first and second, and the like may be used solely to distinguish one element or action from another element or action without necessarily requiring or implying any actual such relationship or order. Where the context permits, reference to an integer or a component or step (or the like) is not to be interpreted as being limited to only one of that integer, component, or step, but rather could be one or more of that integer, component, or step etc.

[0089] In this specification, the terms “comprises”, “comprising” or similar terms are intended to mean a non-exclusive inclusion, such that an apparatus that comprises a list of elements does not include those elements solely, but may well include other elements not listed.

[0090] Throughout the specification the aim has been to describe the invention without limiting the invention to any one embodiment or specific collection of features. Persons skilled in the relevant art may realize variationsfrom the specific embodiments that will nonetheless fall within the scope of the invention as defined by the accompanying claims.

Claims

CLAIMS1 . A gap sub for wellbore fluids, the gap sub comprising: an electrical isolation structure to electrically isolate two metallic ends of the gap sub; a housing coupled to the inside wall of the gap sub and offset from a centre of the gap sub, the housing accommodating an “internal gap”, and a toolstring for acquiring in-situ measurements of wellbore and near-wellbore conditions and transmitting the measurements to surface using EM telemetry; and an antenna rod connected to the “internal gap” in the housing, the antenna rod straddling the electrical isolation structure in the gap sub to be electrically isolated from the housing and mechanically and electrically coupled to the other metallic end of the gap sub.

2. The gap sub of claim 1 , wherein the housing is coupled to the inside wall of the one metallic end of the gap sub such that the housing abuts the inside wall, or is adjacent and in very close proximity to the inside wall such that the housing is off-centre and non-coaxial with the gap sub.

3. The gap sub of claim 1 or 2, wherein the toolstring comprises: one or more electronic sensors able to measure the wellbore and near-wellbore conditions; an EM transceiver for encoding measurements from the sensors, modulating an EM waveform and transmitting the EM waveform to surface; and a power source, such as a battery pack, to provide electrical power to the sensors and the EM transceiver.

4. The gap sub of claim 3, wherein the one or more electronic sensors are integrated with the EM transceiver in a single module or provided in separate modules that can be coupled together.

5. The gap sub of any preceding claim, wherein the housing is in the form of a pressure housing comprising a pressure transducer at a lower end thereof.

6. The gap sub of claim 5, wherein the lower end of the pressure housing is mechanically and electrically coupled to the inside wall of one metallic end of the gap sub via a first clamp element.

7. The gap sub of any preceding claim, wherein the “internal gap” is in the form of an adjustable antenna isolator assembly adjustably accommodated at least partly within a lower end of the pressure housing.

8. The gap sub of claim 7, wherein the adjustable antenna isolator assembly is attached to a first end of the antenna rod to electrically isolate the antenna rod from the pressure housing and one end of the gap sub.

9. The gap sub of claim 8, wherein a second end of the electrically conductive antenna rod is mechanically and electrically coupled to the inside wall of the other end of the gap sub via a second clamp element.

10. The gap sub of claim 9, wherein the first and second electrically conductive clamp elements used to couple the pressure housing and antenna rod to the inside wall of the two metallic ends of the gap sub have hydrodynamic shapes with no flow ports.11 . The gap sub of claim 9 or 10, wherein the first and second electrically conductive clamp elements are made from a material imbued with wear-resistance.

12. The gap sub of any of claims 9 to 11 , wherein the first and second electrically conductive clamp elements used to couple the pressure housing and antenna rod close to or against the inside wall of the two metallic ends of the gap sub result in a crescent-shaped bypass flow geometry within the gap sub that substantially reduces risk of solids entrained in fluid flow through this region from bridging or plugging this space.

13. The gap sub of any preceding claim, wherein an internal diameter of the gap sub and an outside diameter of the housing are chosen to result in a flow area through the gap sub that is equal to or greater than a flow area through any submersible pump positioned in the tubing string above the gap sub.

14. The gap sub of any of claims 7 to 13, wherein the adjustable antenna isolator assembly is accommodated at least partly within the pressure sensor housing to enable fine adjustment of an axial position of the antenna isolator assembly in the pressure housing.

15. The gap sub of claim 14, wherein the fine adjustment is achieved with external threads on an outer collar of the antenna isolator assembly engaging with internal threads in the pressure housing.

16. The gap sub of any preceding claim, wherein the housing comprises one or more ports to bleed-off any trapped pressure inside the housing when the gap sub is retrieved to surface.

17. The gap sub of claim 16, wherein the adjustable antenna isolator assembly comprises one or more seals, wherein adjustment of the axial position of the adjustable antenna isolator assembly in the pressure housing until the one or more seals clear the respective one or more ports provides a vent mechanism to bleed-off any trapped pressure inside the pressure housing.

18. The gap sub of any of claims 4 to 17, wherein the module comprising the one or more electronic sensors comprises a shoulder that presses against an internal lip of the housing under wellbore fluid pressure to rigidly secure the toolstring inside the housing.

19. The gap sub of any preceding claim, wherein the gap sub comprises upper and lower metal tubes and the electrical isolation structure is in the form of a hollow composite electrical isolation collar in between the upper and lower metal tubes.

20. The gap sub of claim 19, wherein the lower metal tube comprises two separate parts joined together with a second electrical isolation structure.21 .The gap sub of claim 20, wherein the second electrical isolation structure is in the form of a threaded collar comprising threaded box connections at each end configured to mate with threaded pin connections on pin end members of the two separate parts of the lower metal tube.

22. The gap sub of claim 21 , wherein the second electrical isolation structure is: a) metallic; or b) a composite when the inside of the gap sub is filled with an electrically conductive fluid.

23. The gap sub of any preceding claim, wherein the gap sub comprises an assembly for creating an electrical connection between a transmit / receive line on the EM transceiver with one end of the gap sub.

24. The gap sub of any preceding claim, wherein the gap sub comprises an assembly for establishing an electrical connection between a ground plane on the EM transceiver with the other end of the gap sub.

25. The gap sub of any preceding claim, wherein the gap sub comprises threaded pin and / or box connections at either end, configured to be inserted at a desired position within a tubing string below any submersible pump.

26. The gap sub of claim 25, wherein the gap sub comprises a thread locking compound applied to threads of the threaded pin and / or box connections to ensure torque rating of the gap sub equals or exceeds the torque rating of the tubing string above and below the gap sub.

27. A method of acquiring in-situ measurements of wellbore and nearwellbore conditions and transmitting the measurements to surface using EM telemetry, the method comprising: providing a gap sub between tubing joints in a tubing string for wellbore fluids, the gap sub comprising an electrical isolation structure to electrically isolate two metallic ends of the gap sub; mounting a housing to an inside wall of one metallic end of the gap sub and offset from a centre of the gap sub, the housing accommodating an “internal gap” and a toolstring for the EM telemetry; and connecting an antenna rod to the “internal gap” to electrically isolate the antenna rod from the housing, the antenna rod straddlingthe electrical isolation structure in the gap sub, with the other end of the antenna rod mechanically and electrically connected to the other metallic end.

28. A system to acquire in-situ measurements of wellbore and nearwellbore conditions and transmitting the measurements to surface using EM telemetry, the system comprising: a gap sub between tubing joints in a tubing string for wellbore fluids, the gap sub comprising an electrical isolation structure to electrically isolate two metallic ends of the gap sub; a housing mounted on an inside wall of one metallic end of the gap sub and offset from a centre of the gap sub, the housing accommodating an “internal gap” and a toolstring providing the EM telemetry; and an antenna rod connected to the “internal gap”, the antenna rod straddling the electrical isolation structure in the gap sub to be electrically isolated from one metallic end of the gap sub and electrically coupled to the other metallic end of the gap sub.

29. The system of claim 28, wherein the gap sub comprises any one or more of the features of claims 2 to 26.

30. A gap sub for wellbore fluids, the gap sub comprising: an electrical isolation structure to electrically isolate two metallic ends of the gap sub; a housing mounted against the inside wall of the gap sub and offset from a centre of the gap sub, the housing accommodating an “internal gap”, and a toolstring for acquiring in-situ measurements of wellbore and near-wellbore conditions and transmitting the measurements to surface using EM telemetry;wherein a flow area through the gap sub is equal to or greater than a flow area through any submersible pump positioned in a tubing string above the gap sub.31 . The gap sub of claim 30, wherein the gap sub comprises any one or more of the features of claims 2 to 12 and 14 to 26.

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