Cooling for geothermal well drilling

Abstract

To provide a method and system for drilling geothermal wells.SOLUTION: A method for drilling a geothermal well in a subsurface zone includes drilling a geothermal well bore in the subsurface zone with a drill string. Intrinsic temperature of rock near a rock face located at a downhole end of the well bore is at least 250 degrees Celsius, and during the drilling step, a drilling fluid is flowed at the rock face at a temperature such that the difference between the intrinsic temperature of the rock near the rock face and the temperature of the drilling fluid at the rock face is at least 100 degrees Celsius.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present disclosure relates to geothermal well drilling. [Background technology]

[0002] Wells drilled for geothermal systems can experience high formation temperatures that can cause problems with rate of penetration, operation of downhole electronics, and other factors. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] US Patent Application Publication No. 2019 / 0154010A1 [Patent Document 2] US Patent Application Publication No. 2019 / 0346181A1 [Patent Document 3] US Patent Application Publication No. 2020 / 0011151A1 [Patent Document 4] U.S. Patent No. 4,741,405 [Patent Document 5] U.S. Patent No. 9,027,669 [Patent Document 6] U.S. Patent No. 9,279,322 [Patent Document 7] U.S. Patent No. 10,060,195 [Patent Document 8] US Patent Application Publication No. 2020 / 0299562A1 [Patent Document 9] WO2008 / 003092 [Patent Document 10] WO2010 / 027866 [Patent Document 11] WO2014 / 008483 [Patent Document 12] WO2018 / 136033 [Patent Document 13] WO2020 / 236189 Summary of the Invention [Problem to be solved by the invention]

[0004] A method and system for drilling a geothermal well is provided. [Means for solving the problem]

[0005] Some aspects of the subject matter herein may be implemented as a method for drilling a geothermal well in a subterranean zone. The method includes drilling a geothermal well bore in a subterranean zone with a drill string. The intrinsic temperature of rock near a rock face located at a downhole end of the well is at least 250 degrees Celsius. During the drilling step, a drilling fluid is flowed at the rock face at a temperature such that the difference between the intrinsic temperature of the rock near the rock face and the temperature of the drilling fluid at the rock face is at least 100 degrees Celsius.

[0006] An aspect, which may be combined with any other aspect, may include the following features: A difference between the inherent temperature of the rock near the rock face and the temperature of the drilling fluid at the rock face induces a thermally induced stress in the rock at the rock face that is higher than the tensile strength of the rock at the rock face.

[0007] An embodiment, which may be combined with any other embodiment, may include the following features: The downhole end of the well is located at a measured depth of at least 4000 meters.

[0008] An embodiment, which may be combined with any other embodiment, may include the following features: The downhole end of the well is located at a vertical depth of at least 6000 meters.

[0009] An embodiment, which may be combined with any other embodiment, may include the following features: The difference between the inherent temperature of the rock near the rock face and the temperature of the drilling fluid at the rock face is at least 175 degrees Celsius.

[0010] An embodiment, which may be combined with any other embodiment, may include the following features: The inherent temperature of the rock near the rock face is at least 350 degrees Celsius, and the difference between the inherent temperature of the rock near the rock face and the temperature of the drilling fluid at the rock face is at least 200 degrees Celsius.

[0011] An embodiment, which may be combined with any other embodiment, may include the following features: The inherent temperature of the rock near the rock face is at least 500 degrees Celsius, and the difference between the inherent temperature of the rock near the rock face and the temperature of the drilling fluid at the rock face is at least 350 degrees Celsius.

[0012] An aspect, which may be combined with any other aspect, may include the following features: The wellbore is a lateral wellbore.

[0013] An aspect, which may be combined with any other aspect, may include the following features: The downhole end of the drill string includes a rotary drill bit.

[0014] One aspect, which can be combined with any other aspect, can include the following features: The downhole end of the drill includes a non-contact drill bit configured to break formation components at the rock face without requiring contact between the bit and the rock face.

[0015] An aspect, which may be combined with any other aspect, may include the following features: A closed-loop geothermal well system is formed that includes a well.

[0016] An aspect that can be combined with any other aspect may include the following features: The well bore is a lateral well bore. Forming the closed loop system includes drilling a lateral well bore from a first surface well bore and connecting the first surface well bore to a second surface well bore by the lateral well bore.

[0017] An aspect, which may be combined with any other aspect, may include the following features: a difference between an inherent temperature of the rock near the rock face and a temperature of the drilling fluid at the rock face induces a radial tensile fracture in at least a portion of a wall of the wellbore, and the method further includes sealing the radial tensile fracture with a sealing material.

[0018] An aspect that may be combined with any other aspect may include the following features: A drill string comprising a plurality of tube segments, at least one of the tube segments comprising a coating layer at least partially covering a circumferential surface of the tube segment, and a length-normalized thermal resistance of a coated wall portion of the tube string of at least 0.002 meter-Kelvin / Watt.

[0019] One embodiment, which may be combined with any other embodiment, may include the following features: The coated wall portion has a thermal resistance normalized to the length of at least 0.01 meter Kelvin / Watt.

[0020] One aspect, which may be combined with any other aspect, may include the following features: the plurality of tube segments are interconnected at an interface joint, the coating layer at least partially covering one or more circumferential surfaces within the interface joint.

[0021] An aspect that may be combined with any other aspect may include the following features: The well bore is a first well bore. And, the method further includes forming a second well bore intersecting the first well bore. A second drilling fluid stream is flowed down the second well bore, the second fluid stream constituting at least a portion of the drilling fluid flowing at the rock face. In addition to or as an alternative to the second fluid stream, a return drilling fluid stream is diverted upward from the downhole end of the first well bore to the second well bore.

[0022] One aspect, which may be combined with any other aspect, may include the following features: The method further includes positioning an intermediate tubing string in the wellbore and positioning a drill string within the intermediate tubing string, such that an inner annular gap is formed between an exterior of the drill string and an interior of the intermediate tubing string, the inner annular gap extending at least partially downhole along a length of the drill string, and the method further includes at least partially filling the inner annular gap with an insulating material.

[0023] One aspect, which can be combined with any other aspect, can include the following features: The insulating material is or includes a gas.

[0024] An aspect, which may be combined with any other aspect, may include the following features: Also, the method further includes adding a phase change material to the drilling fluid near the downhole end of the drill string, the phase change material being characterized to undergo a phase change.

[0025] One aspect, which may be combined with any other aspect, may include the following features: The drill string includes an uphole portion of a first plurality of tube segments and a downhole portion of a second plurality of tube segments. A majority of the first plurality of tube segments has a tensile strength that is at least 25% greater than a tensile strength of a majority of the second plurality of tube segments. A majority of the second plurality of tube segments is at least 35% lighter than a majority of the first plurality of tube segments.

[0026] Some aspects of the subject matter herein may be implemented as a method for drilling a geothermal well in a subterranean zone. The method includes drilling a first surface well and a second surface well. A lateral well is drilled from the first surface well to connect the first surface well to the second surface well in the subterranean zone. Drilling the lateral well includes positioning a drill string in the lateral well. The drill string defines a conduit for displacing fractured formation members from the rock face by flowing drilling fluid to the rock face at a downhole end of the lateral well. The method also includes drilling the lateral well farther into the subterranean zone with the drill string. The characteristic temperature of the rock near the rock face located at the downhole end of the lateral well is at least 250 degrees Celsius. A drilling fluid is flowed into the lateral wellbore at a temperature at the rock face that is at least 100 degrees Celsius below the inherent temperature of the rock near the rock face. The drill string is removed from the lateral wellbore and working fluid is circulated in a closed loop through the first surface wellbore, the second surface wellbore, and the lateral wellbore.

[0027] One embodiment, which can be combined with any other embodiment, can include the following features: Thermal energy is extracted from a working fluid.

[0028] Some aspects of the subject matter herein may be implemented as a system for drilling a geothermal well in a subsurface zone, the intrinsic temperature of the rock near a rock face located at a downhole end of the well being at least 250 degrees Celsius, the system including a drill string having a drill bit for fracturing a formation at the rock face, and a drilling fluid circulated to the rock face at a temperature such that the difference between the intrinsic temperature of the rock near the rock face and the temperature of the drilling fluid at the rock face is at least 100 degrees Celsius.

[0029] An aspect, which can be combined with any other aspect, may include the following features: A difference between the inherent temperature of the rock near the rock face and the temperature of the drilling fluid at the rock face induces a thermally induced stress in the rock at the rock face that is higher than the tensile strength of the rock at the rock face.

[0030] An embodiment, which may be combined with any other embodiment, may include the following features: The downhole end of the well is located at a measured depth of at least 4000 meters.

[0031] An embodiment, which may be combined with any other embodiment, may include the following features: The difference between the inherent temperature of the rock near the rock face and the temperature of the drilling fluid at the rock face is at least 175 degrees Celsius.

[0032] An embodiment, which may be combined with any other embodiment, may include the following features: The inherent temperature of the rock at the rock face is at least 350 degrees Celsius, and the difference between the inherent temperature of the rock near the rock face and the temperature of the drilling fluid at the rock face is at least 200 degrees Celsius.

[0033] An aspect, which may be combined with any other aspect, may include the following features: The wellbore is a lateral wellbore. [Brief description of the drawings]

[0034] [Figure 1A] FIG. 1 is a schematic diagram of a closed loop geothermal system according to the concepts herein. [Figure 1B] FIG. 1B is a plan view of the closed-loop geothermal system shown in FIG. 1A. [Diagram 2] FIG. 1 is a schematic diagram of a drilling system according to the concepts herein. [Figure 3A] FIG. 1 is a schematic diagram of a drill bit in accordance with the concepts herein. [Figure 3B] 1 is a schematic cross-sectional view of a drill bit cone in accordance with the concepts herein; [Figure 4A] 1 is a graph showing the temperature-pressure relationship for the brittle-quasi-brittle transition in plastic rock according to the concepts herein. [Figure 4B] 1 is a graph showing the brittle-quasi-brittle transition in brittle igneous rocks according to concepts herein. [Diagram 5]1 is a graph showing the effects of strain and stress on brittle and plastic rocks in accordance with the concepts herein; [Figure 6A] 1 is a graph showing the relationship between rock brittleness and penetration rate. [Figure 6B] FIG. 13 is a diagram showing the relationship between rock damage due to an excavation operation and cooling temperature difference. [Figure 7] 1 is a graph showing laboratory test results of rate of penetration versus temperature difference between the drilling fluid and the rock being drilled. [Figure 8A] FIG. 1 illustrates a coated tube segment of a tubing string for drilling according to the concepts herein. [Figure 8B] FIG. 1 illustrates a coated tube segment of a tubing string for drilling according to the concepts herein. [Figure 9A] FIG. 13 illustrates the relationship between vertical depth and temperatures of the drill pipe, the annular gap, and the rock when using tube segments with certain coating configurations according to the concepts herein. [Figure 9B] FIG. 13 illustrates the relationship between vertical depth and temperatures of the drill pipe, the annular gap, and the rock when using tube segments with certain coating configurations according to the concepts herein. [Figure 9C] FIG. 13 illustrates the relationship between vertical depth and temperatures of the drill pipe, the annular gap, and the rock when using tube segments with certain coating configurations according to the concepts herein. [Figure 9D] FIG. 13 illustrates the relationship between vertical depth and temperatures of the drill pipe, the annular gap, and the rock when using tube segments with certain coating configurations according to the concepts herein. [Figure 10] FIG. 13 illustrates the relationship between maximum drillable rock temperature and thermal gradient for different tube segment coating configurations according to the concepts herein. [Figure 11] 1A-1C are schematic diagrams of annular gaps and thermal conduction resistances for different tube configurations in accordance with the concepts herein. [Figure 12] FIG. 1 is a schematic diagram of a wellbore system using a second insulating annular gap in accordance with the concepts herein. [Figure 13A] FIG. 13 illustrates the thermal effect of the second insulating annular gap of FIG. [Figure 13B] FIG. 13 illustrates the thermal effect of the second insulating annular gap of FIG. [Figure 14] FIG. 1 is a schematic diagram of a well system for drilling with a second well serving as an inlet and / or outlet for drilling fluid according to concepts herein. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0035] 1A is a diagram illustrating a closed-loop geothermal system according to concepts herein. The closed-loop geothermal well system can be, for example, a system such as that by Eavor Technologies Inc, Calgary, Alberta, which consists of a network of sealed horizontal wells that act as radiators or heat exchangers between the downhole formation. Descriptions of methods and apparatus used in some examples of such closed-loop geothermal systems can be found, for example, in U.S. Patent Application Publication No. 2019 / 0154010A1, U.S. Patent Application Publication No. 2019 / 0346181A1, and U.S. Patent Application Publication No. 2020 / 0011151A1, the contents of which are incorporated herein by reference.

[0036] Referring to FIG. 1A, a closed-loop geothermal system 100 includes an inlet surface well 104 and an outlet surface well 106 that are connected in a subterranean zone 108 by a network of lateral wells 110. The subterranean zone 108 is a formation, a portion of a formation, or multiple formations. In the illustrated example, the surface wells 104 and 106 are substantially vertical, while in other examples of the disclosure, one or both of the surface wells may be other than substantially vertical. In the illustrated example, the lateral wells 110 that connect the surface wells 104 and 106 are substantially horizontal, while in some examples of the disclosure, some or all of the lateral wells may be other than substantially horizontal, may be substantially straight or curved, or may have a helical or other configuration. The lateral wells 110 may be sealed, and a working fluid may be added to the closed loop as a circulating fluid. A power plant 112 is disposed on the earth's surface 114 between the inlet surface well 104 and the outlet surface well 106 to complete the closed loop system. Heat from the subterranean zone 108 is recovered from the working fluid circulating in the loop circuit 116, which is then used to generate electricity at the power plant 112 using a generator (not shown). In some examples of the present disclosure, the lateral well 110 can be 2000 meters to 8000 meters long and 1000 meters to 20000 meters deep below the earth's surface 114.

[0037] FIG. 1B is a plan view of lateral wells 110 forming part of the closed-loop geothermal system 100 of FIG. 1A. Referring to FIG. 1B, the lateral wells 110 are disposed radially spaced apart within the subterranean zone 108. Each lateral well 110 is commonly connected to an inlet well 104 and an outlet well 106 in the closed loop. In some examples of the present disclosure, some or all of the inlet wells 104 and outlet wells 106 are cased. In some examples of the present disclosure, the lateral wells 110 are not cased, but instead are sealed without the use of casing by forming an interface between the lateral well and a substantially fluid-impermeable layer.

[0038] Although Figures 1A and 1B show the inlet wells 104 spaced apart from the outlet wells 106, in other examples of the disclosure, the wells 104 and 106 can be located close together and a network of lateral wells 110 can be stacked or interleaved and intersect at their leading ends.

[0039] Drilling of geothermal systems such as those shown in Figures 1A and 1B can involve drilling through very hard polycrystalline rocks, such as granite, at very high temperatures (greater than 250°C, and in some environments greater than 400°C or 800°C). Such hard, hot rocks may be encountered, such as when drilling deep horizontal wellbore segments such as those shown in Figures 1A and 1B.

[0040] FIG. 2 is a schematic diagram of a well drilling system 200 according to one example of the present disclosure. The well drilling system 200 may be suitable for drilling the inlet surface well 104, the outlet surface well 106, and / or the lateral well 110 of FIGS. 1A and 1B. With reference to FIG. 2, a well 202 is formed in a subterranean zone 204 by drilling with a drill string 206 positioned within the well 202. The drill string 206 includes a bottom hole assembly (BHA) 210 at a downhole end. The BHA 210 includes a drill bit 208 and may further include a drill collar, a directional drilling tool, and various electrical and electronic components for operating and / or controlling the drill bit 208. The interior of the drill string 206 defines a conduit for displacing fractured formation elements from the rock face 214 by flowing drilling fluid 212 to the downhole end of the wellbore, where the formation elements are then carried by the drilling fluid 212 upwardly along an annular gap 216 defined between the exterior of the drill string 206 and the interior surface of the wellbore 202.

[0041] The drill string 206 includes a number of tube segments 220 connected to one another at connection joints 222. In some examples of the present disclosure, the connection joints 222 comprise threaded box-pin joints or other suitable connections.

[0042] Heat transfer, illustrated by arrows 224, may proceed from the subsurface zone 204 into the annular gap 216 and from the annular gap 216 into the interior of the drillstring 206 and into the drilling fluid 212 flowing down the drillstring 206. Thus, heat transfer from the subsurface zone 204 to the annular gap 216 and from the annular gap 216 to the interior of the drillstring 206 contributes to increasing the temperature of the drilling fluid 212 by a countercurrent exchange mechanism before it is delivered to the drill bit 208.

[0043] In some examples of the present disclosure, the drill bit 208 is a contact type drill bit, such as a polycrystalline diamond compact (PDC) drill bit, a rotary drill bit, and / or other types of drill bits that rely on contact with rock to achieve drilling. One example of a suitable contact type drill bit is a tricone bit 300 shown in FIGS. 3A and 3B. As shown in FIG. 3A, the tricone bit 300 includes three cones 302, each of which has a plurality of cutting elements 304. FIG. 3B is a detailed view of a cross section taken along line 305A-305B shown in FIG. 3A. In this example, each cone 302 includes a plurality of cutting elements 304 spaced apart in a particular spaced array across the face of each cone. A series of additional cutting elements 306 may be provided to facilitate extended service life and long uninterrupted drilling. In this example, the additional cutting elements 306 are positioned below the cutting elements 304 such that the tips of the additional cutting elements 306 are adjacent the contact points at the bottoms of the cutting elements 304 that they overlay. In this arrangement, as elements 304 wear, the tips of the additional cutting elements 306 emerge. This can be further accelerated by incorporating materials of different hardness at least in the gaps 310 between adjacent elements 304 and 306. This configuration allows the cutting surface of each cone to self-renew. Further benefits of this feature include more uniform wear of the bit 300, which reduces the chance of eccentric drilling progression and jamming or stuck / unrecoverable conditions in the borehole being formed.

[0044] In other examples of the disclosure, the drill bit 208 of FIG. 2 can be a non-contact drill bit configured to spall formation components at the rock face 214 of the subterranean zone 204 located at the downhole end of the wellbore 202 without requiring contact between the bit 208 and the rock face 214, and in some examples of the disclosure can consist of an electronic spallation bit for electronic pulse drilling. Examples of non-contact drilling systems include plasma drilling (e.g., a plasma drilling system developed by GA Drilling, AS), laser drilling (e.g., a laser drilling system developed by Foro Energy), microwave drilling (e.g., a microwave drilling system developed by Quaise), thermal spallation techniques such as supercritical water jet or flame jet, and electronic pulse drilling (e.g., an electronic pulse drilling system developed by Tetra Corporation). (It will be understood that portions of the non-contact drill bit can periodically impact, scrape, or otherwise come into contact with the formation during the drilling process.)

[0045] For example, an electronic pulse drilling system developed by Tetra Corporation uses an electronic spalling bit having multiple electrodes that generate high energy sparks to spall formation components and thereby remove the spalled formation components from the path of the drill assembly. The bit is capable of generating multiple sparks per second by using a specific excitation current profile that causes a transient spark to form an arc that passes through the most conductive portion of the rock face located at the downhole end of the wellbore. The arc breaks or disintegrates the portion of the rock face it passes through and causes it to be removed by the drilling fluid flow. A high resistivity drilling fluid is used for such electronic pulse drilling. Descriptions of some electronic pulse drill bits, drilling fluids, and related systems and methods can be found, for example, in U.S. Pat. No. 4,741,405, U.S. Pat. No. 9,027,669, U.S. Pat. No. 9,279,322, U.S. Pat. No. 10,060,195, U.S. Patent Application Publication No. 2020 / 0299562A1, PCT Patent Applications WO2008 / 003092, WO2010 / 027866, WO2014 / 008483, WO2018 / 136033, and WO2020 / 236189, the contents of which are incorporated herein by reference. Electronic pulse drilling and other forms of non-contact drilling fracture rock in tension (as opposed to compression or shear), which may provide additional synergistic benefits with the cooling effect discussed in more detail below.

[0046] The rate of penetration (ROP) may be reduced when the rock is at very high confining pressures and / or has plastic characteristics due to the high temperatures that may be encountered in drilling deep geothermal environments, such as when drilling a lateral well 110 in a closed loop system (for example) as shown in FIGS. 1A and 1B. Such high temperatures may also interfere with the operation of downhole electronics and / or downhole sensors. Furthermore, drilling multiple lateral wells, such as those shown in FIG. 1B, may require extensive use of directional drilling techniques. Magnetometers or other downhole equipment used for such directional drilling may be adversely affected by high downhole temperatures. Some downhole components of a directional drilling system have temperature limits of 150-250° C. Other downhole components may have different (higher or lower) temperature limits or temperature ranges.

[0047] In some examples of the present disclosure, as described below, a combination of coatings, wellbore geometries, downhole equipment, and / or additives are used to deliver a drilling fluid flow to the downhole end of the wellbore at a drilling temperature such that the difference between the intrinsic temperature of the rock near the rock face (i.e., the temperature of the rock in front of the drill bit that is being drilled inherently, excluding the cooling effect of the drilling fluid) and the temperature of the drilling fluid at the rock face is at least 100° C. The fluid temperature at the rock face is the bulk fluid temperature at which convective cooling of the rock face occurs, such as within about 1 cm of the rock face being drilled. In some examples of the present disclosure, such a temperature difference may be in a geothermal environment where the intrinsic temperature of the rock near the rock face is at least 250° C. at measurement depths of 4000 meters or more, i.e., at measurement depths through surface and lateral wellbores. (As used herein, this measurement depth is the length along the path of the wellbore, not the total vertical depth of a wellbore other than a complete vertical wellbore.) In some examples of the present disclosure, the temperature difference can be even greater. For example, in some examples of the present disclosure where the inherent temperature of the rock near the rock face is at least about 500° C., the difference between the inherent temperature of the rock near the rock face and the temperature of the drilling fluid at the rock face can be at least about 350° C. In other examples of the present disclosure, this temperature difference can be a larger or smaller amount. Such a large temperature difference can increase the ROP due to the shock cooling effect, thereby thermally contracting the rock face. This stresses the rock in tension, reducing the effective confining pressure at the rock face. This can also cause tensile microfractures in the rock matrix.

[0048] For example, FIG. 4A is a graph showing the temperature-pressure relationship for the brittle-quasi-brittle transition in plastic rock. Plastic rock transitions to a more brittle state when the temperature is reduced or the pressure is reduced. When a rapid thermal cooling treatment is applied to a hot brittle rock, the internal temperature of the rock is reduced and the rock transitions to a more brittle state compared to the untreated rock. FIG. 4B shows a generalized representation of this result. Thus, this zonal change from brittle, semi-brittle, and any combination within the zone due to temperature manipulation makes the treated rock embrittled compared to the initial untreated state.

[0049] Rock strength (the stress required to cause irreversible deformation) does not necessarily change with increasing brittleness, as shown in Figure 5. However, the deformation mode of brittle rocks is rapid failure and fragmentation, and for more brittle rocks, this failure mode is one that undergoes higher plastic deformation before failure.

[0050] As shown in Figure 6A, in general, the penetration rate increases with the brittleness of the rock, regardless of the drilling method. In particular, pulsed electron drilling systems or other non-contact drilling systems may be particularly well suited for brittle rocks.

[0051] FIG. 6B shows the relationship of internal damage in rock to cooling temperature (e.g., during a drilling operation). Note that internal damage is a separate and additional effect in addition to the embrittlement effect described above. In contrast to simple embrittlement, a larger cooling temperature difference is required to cause irreversible damage in rock. Irreversible damage is manifested as microcracks, fractures, and displacements between rock grains due to differential thermal contraction. With sufficient thermal cooling, both embrittlement and subsequent irreversible damage can be induced in rock during drilling.

[0052] FIG. 7 is a graph showing laboratory test results of rate of penetration versus temperature difference between the drilling fluid and the rock being drilled. These laboratory tests were performed on granite blocks with a diameter of 10 inches. The blocks were heated in an oven to a target temperature. The blocks were then placed in a pressurized chamber, and the chamber was pressurized to simulate a depth of about 1000 m in terms of both confining pressure (applied against a sleeve surrounding the rock sample) and hydrostatic pressure of the drilling fluid. The rock sample was then drilled at a constant bit load, rpm, and flow rate with drilling fluid at ambient temperature. As can be seen from FIG. 7, the rate of penetration (ROP) increases substantially when the difference between the rock temperature and the drilling fluid temperature during drilling exceeds about 175 degrees Celsius. Such a temperature difference can cause the thermally induced stress in the rock at the rock face to be higher than the tensile strength of the rock at the rock face, which can embrittle the rock and cause cracks in the rock, thereby increasing the rate of penetration. Further increase in temperature difference further improves the ROP.

[0053] In addition, shock cooling can reduce the effective lithostatic confining pressure at the rock face due to thermal contraction. In bench-scale tests without shock cooling, the ROP of the excavation typically decreases with increasing confining pressure. Thus, the shock cooling effect alone can improve performance in deep rocks under high confining pressure.

[0054] In some examples of the present disclosure, the difference between the inherent temperature of the rock near the rock face and the drilling fluid temperature at the rock face is sufficient to induce embrittlement of the formation at the rock face. When the embrittled rock breaks, it may fracture abruptly and without plastic deformation of the components.

[0055] In some examples of the present disclosure, the difference between the inherent temperature of the rock near the rock face and the cooling fluid temperature at the rock face is sufficient to reduce the tensile strength of the rock and / or damage the rock microstructure (which can reduce rock strength due to small micro-fractures and brittleness in the rock matrix) and / or induce spalling of the rock face due to thermal contraction of the rock. In some examples of the present disclosure, the temperature difference is sufficient to reduce the confining pressure at the rock face (by thermally contracting the rock and inducing failure). When thermal contraction occurs to a level where failure occurs in the rock face, the rock face loses confining pressure and becomes more easily fractured.

[0056] In some examples of the present disclosure, the difference between the inherent temperature of the rock near the rock face and the drilling fluid temperature at the rock face is sufficient to maintain the bottom hole assembly (BHA) at a low and relatively constant temperature, even when drilling rock at temperatures of 250° C.-500° C. or more and at depths of 2-14 km or more. Such cooling is particularly advantageous in examples of electronic pulse drilling, because such technology inherently requires the generation and transmission of electrical power within the BHA, and increasing temperatures increase electrical resistance. Despite this rock fracturing method, some downhole electronics, circuit boards, batteries, and other components may have temperature limits of 150-200° C. (some downhole components may have different (higher or lower) temperature limits). By utilizing the cooling system of the present disclosure, these components are maintained below their temperature limits, even when drilling very hot rock.

[0057] Similarly, some examples described in this disclosure may enable directional drilling applications in hotter rock environments than previously possible by cooling magnetometers and other downhole components of the directional drilling system.

[0058] Thus, the enhanced cooling systems and methods disclosed herein may enable the utilization of drilling system (e.g., electronic pulse drilling) and directional drilling components to drill multiple horizontal wells (such as those shown in FIG. 1B ) of a closed-loop geothermal system in high formation temperature environments with better downhole electronics performance and higher ROP by increasing the temperature differential between the rock near the rock face and the drilling fluid at the rock face.

[0059] In some examples of the present disclosure, enhanced cooling can be used to drill all of the wells of the system shown in Figures 1A and 1B. Because hot rocks may be encountered in the formation drilling the lateral wells 110, in some examples of the present disclosure, unimproved conventional cooling can be used to drill the vertical well components shown in Figures 1A and 1B, and enhanced cooling can be used to drill some or all of the lateral wells 110. Impact cooling can have a greater effect on ROP when the rock is hot, e.g., above 250°C. Thus, enhanced cooling can be particularly suitable for wells where the majority of the drilling is in very hot rocks. This ROP advantage may be reduced in a single vertical well or off-reach well, but can be significant when a network of wells is drilled deep into hot rocks, such as the examples shown in Figures 1A and 1B.

[0060] FIG. 8A illustrates a cooling coating applied to the tube segments 220 of the drill string 206 of FIG. 2 according to an example of the disclosure. The tube segments 220 are interconnected at connection joints 222 and include a main body 802. In some examples of the disclosure, the main body 802 is comprised of a carbon steel body. In other examples of the disclosure, the main body 802 may be comprised of an aluminum alloy, a titanium alloy, and / or a fiber composite (e.g., a polymeric binder composite including carbon fiber, aramid fiber, fiberglass, E-glass, and / or other structural fibers), as described in more detail below. An inner coating layer 804 at least partially covers the inner circumferential surface of the tube segments 220. In the example shown, the inner coating layer covers the entire length of the tube segments 220 and even the inner surface of the connection joints 222. In some examples of the disclosure, the connection joints 222 may be a significant heat transfer area. By covering the inner surface of the connection joint 222 with an inner coating layer 804, heat transfer at the connection joint 222 is reduced.

[0061] The outer coating layer 806 at least partially covers the outer circumferential surfaces of the tube segments 220. In the illustrated example, the connection joints 222 have a larger diameter than the main portion of the body 802 and may therefore be subject to higher contact and therefore higher frictional forces against the wellbore or other components of the well system. In the illustrated example, the outer coating layer 806 covers the portions of the tube segments 220 between the connection joints 222, but not the larger diameter areas around the connection joints 222. In this manner, the outer coating layer 806 is less exposed to the friction that occurs at the connection joints 222.

[0062] In some examples of the present disclosure, the inner coating layer 804 is made of one or more of epoxy novolac resins TK340XT and CP-2060, and epoxy phenolic resins TK34XT and CP-2050. The TK products are commercially available from NOV, Inc., and the CP products are commercially available from Aremco products Inc. The thickness of the epoxy phenolic resin inner coating layer 804 can be in the range of 150-250 μm, and the thickness of the epoxy novolac resin inner coating layer 804 can be in the range of 400-1270 μm. The epoxy phenolic resin can have an average thermal conductivity of about 0.8 W / mk, and the epoxy novolac resin can have an average thermal conductivity of about 0.4 W / mk. Insulating particles can be added to these resins or others to further reduce the heat transfer rate.

[0063] In some examples of the present disclosure, the outer coating layer 806 is comprised of a fiber composite overwrap (e.g., carbon fiber, E-glass composite, and / or other fiber composites) approximately 2540 μm thick. These coatings are commercially available from ACPT Inc. and / or Seal for Life Industries. E-glass may have a thermal conductivity of approximately 0.288 W / mk and carbon fiber may have a thermal conductivity of approximately 0.8 W / km.

[0064] In some examples of the present disclosure, the length-normalized thermal resistance of the wall of the tubing string is at least about 0.002 meter Kelvin / Watt. In some examples of the present disclosure, the length-normalized thermal resistance of the wall of the tubing string is at least about 0.01 meter Kelvin / Watt. With reference to FIG. 8A, this wall thickness 810 is defined by the inner surface of the inner coating layer 804 and the outer surface of the outer coating layer 806. In this disclosure, "length-normalized thermal resistance" is the effective conductive thermal resistance of the string for radial heat transfer, taking into account material changes along the length, and is the temperature difference required to transfer 1 watt of energy over 1 meter of axial material length.

[0065] Below is the thermal resistance normalized by wall length of a tubing string in several examples of the present disclosure having a steel inner body 802 and inner coating layer 804 (and no outer coating layer 806) of the suggested materials and thicknesses:

[0066] [Table 1]

[0067] Below is the thermal resistance normalized by wall length of a tubing string for several examples of the present disclosure having an inner body 802 and inner coating layer 804 made of steel of the suggested materials and thicknesses, and an outer coating layer 806 ("jacket") made of E-glass of the suggested thickness:

[0068] [Table 2]

[0069] In one example of the present disclosure, the drill string 206 of Figure 2 includes tube segments 220 as shown in Figure 8A, which include an inner coating layer 804 of epoxy novolac resin TK340XT having a thickness of about 400 microns and an outer coating layer 806 of E-glass having a thickness of about 5 millimeters. In such an example, given a subsurface zone with a thermal gradient of about 60°C / km, a drill string with a length of about 8000 m, a water-based drilling fluid with a circulation rate of about 3 m3 / min, and a rock face temperature of about 490°C, such a drill string 206 of such tube segments 220 may result in a temperature difference of about 346°C between the rock near the rock face and the drilling fluid at the rock face.

[0070] In another example of the present disclosure, the drill string 206 of Figure 2 includes tube segments 220 as shown in Figure 8A, which include an inner coating layer 804 of epoxy novolac resin TK34XT having a thickness of about 250 microns and an outer coating layer 806 of E-glass having a thickness of about 2.5 millimeters. In such an example, given a subsurface zone with a thermal gradient of 40°C / km, a tube string having a length of about 9000 m, a water-based drilling fluid with a circulation rate of about 3.5 m3 / min, and a rock face temperature of about 370°C, such a drill string 206 of such tube segments 220 may result in a temperature difference of about 196°C between the rock near the rock face and the drilling fluid at the rock face.

[0071] In other examples of the present disclosure, the inner coating layer 804 and / or the outer coating layer 806 can have a greater or lesser thickness and / or can comprise other types of coatings, such as, for example, a ceramic inorganic coating, such as a silicate-bonded ceramic.

[0072] Figure 8B illustrates a cooling coating applied to tube segments 220 of the drill string 206 of Figure 2 according to another example of the disclosure. In the example shown in Figure 8B, the tube segments 220 are composite pipe segments that include a composite main body 850 (which may be made of steel, titanium, aluminum, fiber composite, or other suitable material) that is connected to a connection joint 222 that may also be made of steel, titanium, aluminum, fiber composite, or other suitable material.

[0073] 8B, the inner coating layer 854 at least partially covers the inner circumferential surface of the tube segment 220. In this illustrated example, the inner coating layer 854 covers the inner circumferential surface of the tube segment 220 only at and near the connection joint 222. By covering an area of ​​the inner circumferential surface of the segment 220 at and near the connection joint 222 with the inner coating layer 804, heat transfer at the connection joint 222 is reduced. In other examples of the present disclosure, the inner coating layer 854 covers the entire inner circumferential surface of the tube segment 220.

[0074] In some examples of the present disclosure, the inner coating layer 854 of Figure 8B can be of the same material and thickness as described with reference to the inner coating layer 804 of Figure 8A. In some examples of the present disclosure, the inner coating layer 854 can be of other suitable materials or thicknesses. In some examples of the present disclosure, the tube segment 220 can comprise a vacuum insulated tube (VIT), where the insulation is provided by a vacuum layer within the tube segment 220 instead of or in addition to the inner coating layer 804 (or 854) and the outer coating layer 806.

[0075] In some examples of the present disclosure, main body 802 and / or main body 850 may comprise a high strength-to-weight ratio steel drill pipe, such as, for example, UD165 steel drill pipe available from NOV, Inc. In some examples of the present disclosure, such steel drill pipe may be UD-165 steel drill pipe, which may have a yield strength of about 165,000 psi (1,138 MPa), a tube tensile strength of about 1,000,000 lbf (4.45 MN), a length-normalized joint air load of 24.76 lbf / ft (361.3 N / m), and a joint strength-to-weight ratio of about 900 lbf / lbf (900N / N) for a 5.875 inch (14.92 cm) OD drill pipe.

[0076] In some examples of the present disclosure, main body 802 and / or main body 850 may comprise drill pipe made from titanium alloy. In some examples of the present disclosure, such titanium alloy drill pipe may be made from Ti-6Al-4V titanium alloy and have a yield strength of about 120,000 psi (827 MPa), a tube tensile strength of about 750,000 lbf (3.34 MN), a length-normalized joint air load of 16 lbf / ft (233.8 N / m), and a joint strength-to-weight ratio of about 1,000 lbf / lbf (1000 N / N) for a 5.875 inch (14.92 cm) OD drill pipe.

[0077] In some examples of the present disclosure, the main body 802 and / or the main body 850 may comprise drill pipe made from an aluminum alloy. In some examples of the present disclosure, such aluminum alloy drill pipe may be made of Al-Zn-Mg II aluminum alloy and have a yield strength of about 70,000 psi (483 MPa), a tube tensile strength of about 600,000 lbf (2.67 MN), a length-normalized joint air load of 15.5 lbf / ft (226 N), and a joint strength-to-weight ratio of about 825 lbf / lbf (825 N / N) for a 5.787 inch (14.699 cm) OD drill pipe. In some examples, such aluminum alloy pipe may be made of FarReach® drill pipe available from Alcoa Energy Systems. In some examples of the present disclosure, such aluminum alloy drill pipe may be made of aluminum drill pipe available from Aluminum Drill Pipe, Inc.

[0078] In some examples of the present disclosure, main body 802 and / or main body 850 may comprise carbon fiber composite drill pipe. In some examples of the present disclosure, such carbon fiber composite drill pipe may comprise Advance Composite Drill Pipe available from Advance Composite Products & Technology, Inc.

[0079] In some examples of the disclosure, the drill string 206 of FIG. 2 includes tube segments 220 that may each include a main body 802 and / or 850 that is made of steel, titanium, aluminum, and / or a fiber composite material, as described above. For example, in some examples of the disclosure, each segment of the drill string 206 includes a main body 802 and / or 850 that is made of a single material, such as an aluminum alloy. In other examples of the disclosure, the drill string 206 may include various sections, each including multiple segments 220 that are made of different materials. For example, in some examples of the disclosure, some of the tube segments 220 of the drill string 206 may be made of one main body material (e.g., an aluminum alloy) and the remaining tube segments 220 of the drill string 206 may be made of another main body material (e.g., steel). In some examples of the disclosure, the drill string 206 may include two, three, or more sections, each of which includes a tube segment 220 having a main body that is made of a different material than the other sections. In another example, the majority of the segments 220 closer to the drill bit 208 have a main body made of a lighter material on a length-normalized air-loaded basis than the segments further uphole from the drill bit 208. For the above example, titanium drill pipe is about 35% lighter than steel and aluminum drill pipe is about 37% lighter. Those segments 220 located further uphole from the drill bit 208 may have a main body having a higher strength material. For the above example, UD-165 drill pipe has about 67% higher tensile strength than aluminum and titanium drill pipe has about 25% higher tensile strength than aluminum. Furthermore, the difference in tensile strength and the difference in length-normalized load can be achieved with a single material but by using telescopically varying thickness / diameter drill pipe in the uphole portion compared to the downhole portion.In one example of the present disclosure, the majority of the segments 220 in the portion located near the drill bit are approximately 35% lighter than the majority of the segments 220 in the uphole portion, and the majority of the segments 220 in the uphole portion have approximately 25% higher tensile strength than the majority of the segments 220 in the portion located near the drill bit. This use of different drill string materials allows drilling into much hotter rocks at deeper depths, which may require an insulated drill string with sufficient tensile strength to extend to such depths. The above materials, when properly combined, allow drilling at depths greater than 9 km, including up to 14 km or more. Due to the geothermal gradient of the earth, rocks at deeper depths have higher temperatures. This impingement cooling technique provides a way to improve the rate of penetration and drilling performance in hot rocks. Thus, the deeper depth drilling possible by combining drill pipe segments of different weight / strength, combined with the cooling techniques described herein, provides a synergistic effect. Closed-loop multilateral wells can be drilled at sufficient depths (and therefore rock temperatures) to allow for shock cooling effects, thus significantly reducing the drilling time and costs of multilateral wells.

[0080] FIG. 9A shows the results of a thermodynamic simulation of the heat transfer rate in fluids flowing in a downhole tubular drill string comprising standard carbon steel pipe located in a cased well in a subterranean zone. The simulation assumes water drilling fluid pumped at 3.5 m3 / min down the tubular string and a temperature gradient of 50° C. / km from the surface (i.e., from a surface location located at the uphole end of the well) to the rock face (located at the downhole end of the well). With reference to FIG. 9A, the curve labeled "Drill Pipe" is the temperature of the fluid flowing in the tubular drill string at a given depth, the curve labeled "Annular Space" is the temperature of the fluid flowing in the annular space between the tubing and the casing at a given depth, and the curve labeled "Rock" is the characteristic temperature of the rock at a given depth. As shown in Figure 9A, some insulation is provided to prevent heat transfer such that the temperature of the fluid at the rock face at the bottom hole is about 206°C and the rock temperature is 260°C, resulting in a temperature difference of about 54°C. However, such a temperature difference may be insufficient to provide a sufficient cool drilling fluid flow to cool downhole electronics or directional drilling equipment, or to provide an impingement cooling effect at the rock face, or to realize other benefits of cool drilling fluid flow for drilling at the downhole end of the well, as discussed above. In Figure 9A, the fluid temperature at the rock face is equivalent to the annular void temperature at the bottom hole after the fluid exits the drill bit.

[0081] The coatings and coating geometries described with reference to Figures 8A and 8B can reduce heat exchange between cooler fluids traveling down the tube string and hotter fluids returning in the annulus during drilling, resulting in at least a 100°C difference between the intrinsic temperature of the rock near the rock face and the temperature of the drilling fluid located at the rock face, even in geothermal environments where the intrinsic temperature of the rock adjacent the rock face is at least 250°C. For example, Figures 9B-9D show the results of a thermodynamic simulation of heat transfer to a downhole tube, given a tube segment as shown in Figure 8A, in various examples of the present disclosure as described below. In the simulations shown in Figures 9B-9D, the inner coating layer 804 covers the entire length of the tube segment, including the inner surface of the connection joint. In Figures 9B-9D, the curve labeled "Tube" is the temperature of the fluid flowing in the tubing drill string at a given depth, the curve labeled "Casing" is the temperature of the fluid flowing in the annulus between the drill pipe tubing and the casing at a given depth, and the curve labeled "Rock" is the characteristic temperature of the rock at a given depth. Drilling fluid flows into the tubing at the surface, through the BHA, through the drill bit, past the rock face and into the annulus. The temperature of the drilling fluid at the rock face is approximately the same as the fluid temperature in the annulus at the bottom of the well.

[0082] Figure 9B shows a thermodynamic heat transfer simulation for an example consisting of a standard carbon steel pipe with an internal coating 804 (without an external coating 806) of 400 μm thick epoxy novolac (TK340XT). The simulation assumes a water drilling fluid pumped at about 3.5 m3 / min and a temperature gradient of about 50°C / km from the surface to the rock face. As shown in Figure 9B, this example achieves a higher temperature difference compared to Figure 7, namely about 91°C at 5000 meters.

[0083] Figure 9C shows a thermodynamic heat transfer simulation for an example consisting of a standard carbon steel pipe with an inner coating 804 of 400 μm thick epoxy novolac TK340XT and an outer coating 806 of 5 mm thick E-glass jacket. The simulation assumes a water drilling fluid pumped at about 3 m3 / min and a temperature gradient of about 60° C. / km from the surface to the rock face. As shown in Figure 9C, the example achieves a temperature difference of about 346° C. at 8000 meters.

[0084] Figure 9D shows a thermodynamic heat transfer simulation for an example consisting of a standard carbon steel pipe with an inner coating 804 of 250 μm thick epoxy novolac TK34 and an outer coating 806 of 2.5 mm thick E-glass jacket. The simulation assumes a water drilling fluid pumped at about 3.5 m3 / min and a temperature gradient of about 40° C. / km from the surface to the rock face. As shown in Figure 9D, the example achieves a temperature difference of about 196° C. at 9000 meters.

[0085] FIG. 10 shows the maximum rock temperature at which a well can be drilled for the example described with reference to FIGS. 9C and 9D versus the thermal gradient from the surface to the rock face (located at the downhole end of the well), assuming that the temperature of the drilling fluid leaving the drill bit at the rock face should not exceed about 150° C. The upper curve 1002 corresponds to the example described with reference to FIG. 9C. For example, using the example described with reference to FIG. 9C, at point 1004, the temperature gradient is about 60° C. / km and the maximum drillable rock temperature is about 483° C. The lower curve 1006 corresponds to the example described with reference to FIG. 9D. For example, using the example described with reference to FIG. 9D, at point 1008, the temperature gradient is about 40° C. / km and the maximum drillable rock temperature is about 335° C.

[0086] In some examples of the present disclosure, instead of or in addition to coating layers 804 and 806 on tube segment 220, a phase change material, such as water ice or dry ice, may be added to the drilling fluid of the drilling system (such as drilling fluid 212 of FIG. 2). These phase change materials may absorb thermal energy by undergoing a phase change (e.g., melting). In some examples of the present disclosure, the drilling fluid may be pumped at a sufficient flow rate such that the phase change material undergoes a phase change in the vicinity of the drill bit.

[0087] In some examples of the present disclosure, a heat exchanger may be added to the system of Figure 2 to cool the drilling fluid 212 as it is recirculated back downhole from the well 202. In some examples of the present disclosure, such a heat exchanger may be positioned at a surface location.

[0088] Not all of the drilling fluid necessarily must flow through the drill bit to achieve the results described herein, and a portion of the drilling fluid may pass from the tubing into the annular gap via a port or other device located near the bit or BHA. Such a configuration may allow for higher flow rates if components in the bottom hole assembly have flow restrictions.

[0089] FIG. 11 shows the heat transfer resistance or thermal resistance through the annulus and four different configurations of tubes. As the cool drilling fluid circulates back downhole through the tube and up through the annulus, the drilling fluid in the annulus is heated by the surrounding rocks of the underground zone to TANNULUS. The drilling fluid in the annulus then heats the drilling fluid inside the tube to TTUBING. In each example, the primary heat transfer mechanism through the drilling fluid filled annulus is convection. The flowing drilling fluid, an imperfect convection medium, exhibits a thermal resistance RCONVECTION,ANNULUS. In the case of a section of uninsulated carbon steel tube (carbon steel), the steel is an imperfectly conductive medium and exhibits an additional (series) of thermal resistances RCONDUCTION,STEEL. Finally, the primary heat transfer mechanism in the tube filled with drilling fluid, which heats the flowing drilling fluid itself, is convection. The drilling fluid exhibits an additional (series) of thermal resistances RCONVECTION,TUBING. When the tube is completely covered with a thermal insulation coating (coated carbon steel), this coating presents an additive (series) thermal resistance RCONDUCTION,COATING. When composite tubes with steel collars (joints) are used (composite and composite+coated collar), the composite tube and carbon steel present parallel thermal resistances (RCONDUCTION,COMPOSITE and RCONDUCTION,STEEL COLLAR, respectively). In other words, materials with lower thermal resistance (i.e. less insulating materials) have a greater effect on the total thermal resistance of the tube segment. Typically, composite materials have a higher thermal resistance than steel, so coating only the steel collar can result in a significant increase in the total thermal resistance of the tube segment.

[0090] FIG. 12 illustrates the formation of a second insulating annular gap according to one example of the disclosure. This example of the disclosure, which is described with reference to FIG. 12, is described with reference to the components of the drilling system 200 of FIG. 2. With reference to FIG. 12, a well 202 is being drilled using a drill bit 208 at the downhole end of a drill string 206. Drilling fluid 212 flows down the drill string 206 and out of the drill bit 208. An annular gap 216 is defined in a lower portion of the well 202 such that it is located between the exterior of the drill string 206 and the well 202. An intermediate tube string 1202 is positioned within the well such that the drill string 206 is positioned within the intermediate tube string 1202, thereby forming an inner annular gap 1204 between the exterior of the drill string 206 and the interior of the intermediate tube string 1202, and an outer annular gap 1206 between the exterior of the intermediate tube string 1202 and the well 202. The inner annular gap 1204 and the outer annular gap 1206 each extend at least partially downhole along the length of the drill string 206. In some examples of the present disclosure, the inner annular gap 1204 may be filled with an insulating material. In some examples of the present disclosure, the insulating material is a gas, thus forming a "gas blanket." In other examples of the present disclosure, the inner annular gap 1204 may be filled with foam, insulating oil, or any fluid or material having low conductivity instead of or in addition to gas.

[0091] The inner annular gap 1204 insulates the drilling fluid 212 flowing downward from the heated fluid flowing upward in the outer annular gap 1206. Figures 13A and 13B are a comparison of wellbore fluid temperatures in one wellbore system. Figure 13A shows the expected drilling fluid temperature versus depth in a drilling system without an insulating intermediate tube string, and Figure 13B shows the drilling fluid temperature versus depth in a drilling system with an insulating intermediate tube string forming an inner annular gap filled with insulating gas as described with reference to Figure 12. Although the bottom portion of the wellbore continues to heat up due to the lack of insulation at the bottom of the gas blanket, the rock face temperature still cools dramatically. In this example, the inner annular gap 1204 only extends to the bottom of the last casing string, but significant cooling is still achieved at the rock face (of the main wellbore and / or of lateral wells drilled from the main wellbore). The insulating fluid blanket stays in place due to its much lower density, and therefore essentially floats on top of the drilling fluid.

[0092] Due to the lower density of the "blanket fluid", this blanket fluid is pressurized in the wellbore (not shown) on the surface. Controlled Pressure Drilling (MPD) technology is a system that maintains pressure in the annular gap around the rotating drill pipe. The main challenge is to seal the fluid from leaking past the rotating pipe. MPD systems have improved enough in recent years to hold the pressurized fluid blanket in place. Thus, where the fluid blanket is to fill the inner annular gap coaxial with the rotating drill pipe, the latest MPD systems can preferably be used.

[0093] One variation of this is to install another casing string to form two inner annular gaps (not shown). The inner annular gap is positioned coaxially and adjacent to the rotating drill pipe, a secondary inner annular gap that can be filled with blanket fluid, and an outer annular gap through which heated drilling fluid returns. This setup requires the cost and hassle of a larger well to create room for the additional annular gaps, but avoids the use of a high pressure MPD system because the inner annular gap can be filled with drilling fluid.

[0094] Another example according to the present disclosure for reducing countercurrent heat transfer from the annular gap to the tubes is to utilize a second wellbore that acts as an inlet and / or outlet for the drilling fluid. Figure 14 shows a schematic of this "slipstream wellbore."

[0095] 14 and as described further with reference to FIG. 2, a well bore 202 is being drilled using a drill bit 208 at the downhole end of a drill string 206. Drilling fluid 212 flows down the drill string 206 to the drill bit 208. An annular gap 216 is defined between the exterior of the tubing string and the well bore 202. The well bore 202 may comprise a main well bore and / or a lateral well bore.

[0096] 14, the well 202 is a first well and a second well 1402 is drilled to intersect the first well 202. A second drilling fluid stream 1404 is flowed down the second well 1402. The second drilling fluid stream 1404 constitutes at least a portion of the drilling fluid flowing at the downhole end of the well 202, i.e., at the rock face near the drill bit 208. The second well 1402 is located far enough from the first well 202 to reduce or eliminate heat transfer, such that the second drilling fluid stream 1404 provides additional cooling to the downhole end of the well 202.

[0097] In some examples of the present disclosure, drilling fluid and cuttings may be sent up the second wellbore 1402 and back to the surface. In this variation, heated fluid does not flow upward into the annular gap 216 and therefore no countercurrent heat exchange occurs above the crossover point 1406. This directional flow is indicated in dashed lines by numeral 1408.

[0098] It will be appreciated that the second well 1402 may be drilled and used to cool any number of additional wells / conduits from a surface location. For example, a closed loop geothermal well system may be constructed by drilling four corner wells. After completion of drilling one of the four corner wells, the "slipstream" intersecting section is plugged and abandoned, and another intersecting segment is drilled to intersect another one of the corner wells. In this manner, a single well may be used multiple times to cool other wells; only the interconnecting segment needs to be drilled each time.

[0099] Shock cooling of hot rock using the techniques described herein can create several challenges to the drilling process behind the bit. Cooling increases the compressive strength of the wellbore but reduces the tensile strength. Significant temperature differences between the circulating drilling fluid and the wellbore wall can cause cooling-induced tensile fractures that are radially away from the wellbore. These tensile fractures may need to be sealed or controlled with wellbore strengthening materials such as graphite or calcium carbonate or other lost circulation materials. Additionally, these fractures may need to be sealed with sealant chemistries such as sodium silicate or potassium silicate. Another method is to perform the drilling process underbalanced, which may be utilized separately or in combination with other techniques disclosed to mitigate the tensile fracture effects behind the bit. A system design particularly suited to electronic pulse drilling would utilize a pressure-controlled drilling system and an oil-based drilling fluid with high electrical resistivity and an equivalent circulating specific gravity less than the hydrostatic pressure. This allows the freedom to control the downhole pressure while providing a drilling fluid suitable for electronic fracturing.

[0100] Another difficulty associated with shock cooling is the possibility that the induced tensile fractures may propagate and cause shear failure or further problems, resulting in the removal of large amounts of cuttings or rock fragments of various sizes from the wellbore behind the bit. Highly viscous drilling fluids and high flow rates (>2.5m3 / min) in combination with other methods can remove the excess debris generated by the shock cooling process. In other examples of the present disclosure, the drilling fluid may have a Marsh Funnel viscosity of at least 80-100 seconds. Also, various slugs or sweeps of high viscosity fluid volume passing through the system aid in the removal of excess debris. Successful circulation of larger debris to the surface may be correlated with two key parameters: annular pore fluid velocity (determined by flow rate and annular pore volume) and fluid rheology (plastic viscosity / yield point (PV / YP), which adds transport capacity / reduces sliding velocity, and gel strength, which provides interlocking and suspension). Periodic circulation of low volume / high viscosity sweeps can transport and suspend large debris to the surface, where, in some examples of the present disclosure, these debris can be captured (i.e., filtered and removed) to prevent or reduce contamination of the base drilling fluid.

[0101] By reducing heat exchange between the cooler fluid moving down the tube string and the hotter fluid returning within the annular gap during drilling, the coatings and coating geometries described with reference to Figures 8A and 8B, and the methods and systems described with reference to Figures 12 and 14, can further reduce the negative effects of higher formation temperatures on the tensile strength and other properties of tube segments of the tube string (such as tube segment 220 of drill string 206 in Figure 2).

[0102] The above-described methods, systems, and apparatus for enhanced cooling of drilling fluids may be used alone or in combination with each other.

[0103] In this disclosure, the terms "a" or "the" are used to include one or more, unless the context clearly indicates otherwise. The term "or" is used to refer to a non-exclusive "or", unless otherwise indicated. The phrase "at least one of A and B" has the same meaning as "A, B, or A and B". Furthermore, it should be understood that the phrases or terms used in this disclosure, unless otherwise defined, are for descriptive purposes only and not for limiting purposes. Any section headings used are intended to aid in the reading of the literature and should not be construed as limiting. Information associated with a section heading may relate to that section or elsewhere.

[0104] Although the present disclosure includes many specific implementation details, these should not be construed as limitations on the subject matter or on the scope that may be claimed, but rather as descriptions of features that may be specific to a particular implementation. Also, some features described in the present disclosure in the context of separate implementations may be implemented in combination or in a single implementation. In contrast, various features described in the context of a single implementation may also be implemented in multiple implementations, each separately or in any suitable subcombination. Furthermore, although the features described above may be described as functioning in some combinations and even initially claimed as such, in some instances one or more features in a claimed combination may be deleted from the combination, and the claimed combination may be directed to a subcombination or a variation of the subcombination.

[0105] Specific implementations of the subject matter have been described. However, it should be understood that various modifications, substitutions, and alterations may be made. Although operations may be shown in a particular order in the drawings or claims, this should not be understood as mandating that the operations be performed in the particular order or sequence shown to achieve desirable results, nor should it be understood as requiring that all operations shown be performed (some operations may be considered optional). Thus, the exemplary implementations described above do not define or limit the scope of the present disclosure. [Explanation of symbols]

[0106] 100 Closed Loop Geothermal System 104 Inlet surface well 106 Outlet Surface Well 108 Underground Zone 110 Horizontal wells 112 Power Plant 114 Ground surface 116 Loop Circuit 200 Well Drilling System Well 202, Well No. 1 204 Underground Zone 206 Drill String 208 Drill Bit 210 Bottom Hole Assembly 212 Drilling Fluids 214 Rock surface 216 Annular Gap 220 Tube Segment 222 Connecting joint 224 Arrow 300 Tri-cone Bit 302 Corn 304 Cutting elements 306 Additional cutting elements 310 Gap 802 Main body, steel inner body 804 Inner coating layer, inner coating 806 Outer coating layer, outer coating 810 Wall thickness 850 Composite material main body 854 Inner coating layer 1004 points 1006 curve 1008 points 1202 Intermediate Tube String 1204 Inner annular gap 1206 Outer annular gap 1402 Second Well 1404 Second drilling fluid flow 1406 Crossing Point 1408 Directional Flow

Claims

1. 1. A method of forming a geothermal well system, comprising: forming a wellbore in a formation within a subsurface zone, the wellbore having an interface between the wellbore and the substantially fluid impermeable formation, the forming of the wellbore comprising: drilling into the formation by fracturing rock at a rock face forward of the drill bit with a drill bit at a downhole end of a drill string, wherein the inherent temperature of the rock near the rock face forward of the drill bit is at least 250 degrees Celsius; while fracturing the rock at the rock face with the drill bit, flowing a water-based or oil-based drilling fluid having a temperature at the rock face that is lower than the intrinsic temperature of the rock near the rock face in front of the drill bit, wherein the difference between the intrinsic temperature of the rock near the rock face in front of the drill bit and the temperature of the drilling fluid at the rock face in front of the drill bit is at least 100 degrees Celsius; having The method further comprises: sealing the wellbore without the use of casing to form the interface; circulating a geothermal working fluid through the well in a closed loop; A method comprising:

2. the drill string comprising composite tubing; The method of claim 1.

3. the composite tube having a normalized thermal resistance of at least 0.002 meter-Kelvin / Watt; The method of claim 2.

4. the composite tube having a normalized thermal resistance of at least 0.01 meter-Kelvin / Watt; The method of claim 2.

5. The drill bit is a non-contact drill bit. The method of claim 1.

6. The downhole end of the well is located at a measured depth of at least 4000 meters. The method of claim 1.

7. a difference between the inherent temperature of the rock near the rock face in front of the drill bit and the temperature of the drilling fluid at the rock face in front of the drill bit is at least 175 degrees Celsius; The method of claim 1.

8. the inherent temperature of the rock near the rock face in front of the drill bit is at least 350 degrees Celsius, and the difference between the inherent temperature of the rock near the rock face in front of the drill bit and the temperature of the drilling fluid at the rock face in front of the drill bit is at least 200 degrees Celsius; The method of claim 1.

9. the inherent temperature of the rock near the rock face in front of the drill bit is at least 500 degrees Celsius, and the difference between the inherent temperature of the rock near the rock face in front of the drill bit and the temperature of the drilling fluid at the rock face in front of the drill bit is at least 350 degrees Celsius; The method of claim 1.

10. The well is a lateral well. The method of claim 1.

11. the temperature of the drilling fluid is greater than 25 degrees Celsius as it flows through the drill string at a surface location located at the uphole end of the well; The method of claim 1.

12. 1. A system for forming a geothermal system comprising a well bore in a geothermal well in a subsurface zone, the system comprising: a rock adjacent a rock face located at a downhole end of the well bore, the rock having an intrinsic temperature of at least 250 degrees Celsius; a drill string including a drill bit for fracturing the rock at the rock face; a water-based or oil-based drilling fluid, the system being configured to flow while the drill bit fractures the rock at the rock face, the drilling fluid having a temperature that is lower than an intrinsic temperature of the rock near the rock face in front of the drill bit, and the difference between the intrinsic temperature of the rock near the rock face in front of the drill bit and the temperature of the drilling fluid at the rock face in front of the drill bit is at least 100 degrees Celsius; a sealing material configured to seal the wellbore after drilling without casing such that an interface between the wellbore and the formation is substantially fluid impermeable and such that a geothermal working fluid can flow through the wellbore in a closed loop; A system equipped with

13. the drill string comprising composite tubing; The system of claim 12.

14. the composite tube having a normalized thermal resistance of at least 0.002 meter-Kelvin / Watt; The system of claim 13.

15. the composite tube having a normalized thermal resistance of at least 0.01 meter-Kelvin / Watt; The system of claim 13.

16. The drill bit is a non-contact drill bit. The system of claim 12.

17. the downhole end of the well is located at a measured depth of at least 4000 meters; The system of claim 12.

18. a difference between the inherent temperature of the rock near the rock face in front of the drill bit and the temperature of the drilling fluid at the rock face in front of the drill bit is at least 175 degrees Celsius; The system of claim 12.

19. the inherent temperature of the rock near the rock face in front of the drill bit is at least 350 degrees Celsius, and the difference between the inherent temperature of the rock near the rock face in front of the drill bit and the temperature of the drilling fluid at the rock face in front of the drill bit is at least 200 degrees Celsius; The system of claim 12.

20. The well is a lateral well. The system of claim 12.

21. the inherent temperature of the rock near the rock face in front of the drill bit is at least 500 degrees Celsius, and the difference between the inherent temperature of the rock near the rock face in front of the drill bit and the temperature of the drilling fluid at the rock face in front of the drill bit is at least 350 degrees Celsius; The system of claim 12.

22. the temperature of the drilling fluid is greater than 25 degrees Celsius as it flows through the drill string at a surface location located at the uphole end of the well; The system of claim 12.

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