Geothermal Mining System

Abstract

A geothermal mining system (100) may operate within a single primary borehole (110) within a geothermal reservoir (120). A primary fluid loop (130) may include a low-temperature working fluid line (132) leading to the primary borehole (110) and a high-temperature working fluid line (134) exiting the primary borehole (110). A secondary fluid loop (140) may be positioned below the primary borehole (110), the secondary fluid loop (140) being in thermal contact with the geothermal reservoir (120). A downhole heat mining device (150) may control the rate of heat transfer from the secondary fluid loop (140) to the primary fluid loop (110) by selectively controlling fluid flow through the primary fluid loop (110), the secondary fluid loop (140), or both.

Description

[Technical field]

[0001] Related Applications This application claims priority to U.S. Provisional Patent Application No. 62 / 878,578, filed July 25, 2019, which is incorporated herein by reference. [Background technology]

[0002] Heat within the Earth's crust provides an effectively inexhaustible, carbon-free energy source that can be used to generate electricity, heat and cool buildings, desalinate water, produce hydrogen fuel, and meet other human energy needs. However, the use of geothermal energy is currently limited due to the difficulty of reliably harvesting large reservoirs, or volumes of the Earth's crust that contain the targeted thermal energy density. Extracting enough geothermal energy to economically and reliably generate electricity is often difficult due to the intersection of geological realities and current technological limitations. Many techniques for extracting geothermal energy are only suitable for use in areas of the continental crust that have high heat sources, such as magmatic or volcanic sources. Additionally, many drilling methods currently in use have limited depth ranges.

[0003] Most economical geothermal power systems are located in unique geographic environments with very high crustal heat sources, such as volcanic intrusions, surrounded by fractured, permeable, fluid-saturated or steam-saturated rock bodies that can be replenished by fluid injection. Some developments are also focused on "recharged geothermal systems", which involve the formation of new fracture networks in hot, dry rock.

[0004] Improving the efficiency and economic predictability of geothermal energy extraction can be useful from several perspectives. Geothermal energy is a renewable source of dispatchable electricity with sufficient baseload quality to provide grid stability. Utilization of geothermal energy can also help mitigate climate change concerns because the extraction process does not produce carbon dioxide. In many processes for extracting geothermal energy, i.e. "thermal mining", fluids are heated while percolating through a permeable fracture network in a hot rock mass. The heated fluids reemerge at the surface through drilled holes for fluid / vapor recovery. If a permeable fracture network is not available, a fracture network can be created in the "hot dry rock mass" (HDR) by hydraulic and thermal fracturing methods. The drilling and created fracture network in the HDR is referred to as an "enhanced geothermal system" (EGS). Some limitations of current EGS methods are (1) the potential for reactivating existing faults inducing significant seismicity (this is true for any geothermal reservoir system), (2) the difficulty in controlling the geometry of the fracture network generated to enhance recovery, and (3) the potential for generating relatively large fractures that act as high-velocity fluid paths where available heat is rapidly diminished, resulting from the hydraulic fracturing process. Another limitation of EGS is that it is increasingly difficult to drill at greater depths because the increasing ductility of rock masses as ambient temperatures increase, along with other technical difficulties, inhibits fracture formation necessary for the mechanical drilling process. Almost all approaches involve drilling multiple holes for separate injection and production, expanding drilling costs and uncertainties in the generation of fracture networks to connect injection and production boreholes. Despite advances in EGS and other geothermal technologies, challenges remain for cost-effective and reliable recovery of heat from geothermal reservoirs to generate electricity. Furthermore, the efficiency of a wide range of turbine-generators is strongly dependent on the temperature of the fluid / steam recovered from the midstream. An important goal is the extraction of supercritical fluids, as this would significantly increase generator efficiency. Summary of the Invention

[0005] This disclosure describes systems, devices, and methods for extracting geothermal energy, also known as geothermal mining. The geothermal mining system may include a single primary borehole extending from the earth's surface into a geothermal reservoir. A primary fluid loop may include a low temperature working fluid line leading to the primary borehole and a high temperature working fluid line exiting the primary borehole. A secondary fluid loop may be disposed below the primary borehole. The secondary fluid loop may be in thermal contact with the geothermal reservoir. The system may also include an in-hole heat mining device for controlling a rate of heat transfer from the secondary fluid loop to the primary fluid loop by selectively controlling fluid flow through the primary fluid loop, the secondary fluid loop, or both.

[0006] The downhole geothermal mining device may include a device body shaped to descend a primary borehole, a low temperature working fluid inlet, a high temperature working fluid outlet, and a controller. The low temperature working fluid inlet may be connectable to a low temperature working fluid line leading downwardly to the primary borehole. The high temperature working fluid outlet may be connectable to a high temperature working fluid line leading upwardly to the primary borehole. The low temperature working fluid line and the high temperature working fluid line may form a primary fluid loop. The controller may be adapted to control a rate of heat transfer from the secondary fluid loop to the primary fluid loop. The secondary fluid loop may be disposed below the primary borehole, and the secondary fluid loop may be in thermal contact with the geothermal reservoir. The controller may control the rate of heat transfer by selectively controlling fluid flow through the primary fluid loop, the secondary fluid loop, or both.

[0007] The geothermal mining method may include forming a single primary borehole; disposing a geothermal mining device in the primary borehole; forming a primary fluid loop including a low temperature working fluid line leading to the primary borehole and a high temperature working fluid line exiting the primary borehole; forming a secondary fluid loop located below the primary borehole, the secondary fluid loop being in thermal contact with the geothermal reservoir; transferring heat from the secondary fluid loop through the primary fluid loop to an energy storage or production unit outside the primary borehole; and controlling a rate of heat transfer from the secondary fluid loop to the primary fluid loop by selectively controlling fluid flow through the primary fluid loop, the secondary fluid loop, or both.

[0008] Although multiple vertical (primary) boreholes can be used, this technique allows geothermal extraction from each vertical borehole in such a multi-borehole system. Such an approach can reduce or eliminate difficulties with fluid permeability through rock masses between adjacent primary vertical boreholes. Furthermore, any number of additional secondary boreholes can be formed from vertical boreholes at corresponding target reservoir depths. Thus, a system can be comprised of multiple primary boreholes feeding an energy conversion system, but the primary boreholes are not connected to each other in the subsurface reservoir, eliminating difficulties with controlling fluid flow through rock masses connecting adjacent boreholes.

[0009] There has thus been outlined, rather broadly, the more important features of the present invention in order that the detailed description thereof that follows may be better understood, and in order that the present contribution to the art may be better appreciated. Other features of the present invention will become more apparent from the following detailed description of the invention, taken in conjunction with the accompanying drawings and claims, or may be learned by the practice of the invention. [Brief description of the drawings]

[0010] [Figure 1] 1 is a schematic diagram of an exemplary geothermal mining system according to one embodiment of the present invention. [Diagram 2]2 is another schematic diagram of an exemplary geothermal mining system according to one embodiment of the present invention. [Figure 3A] FIG. 2 illustrates a side cross-sectional view of an exemplary downhole thermal mining device according to one embodiment of the present invention. [Figure 3B] FIG. 2 illustrates a top cross-sectional view of an exemplary downhole thermal mining device according to one embodiment of the present invention. [Figure 4] 1 is a side cross-sectional view of another exemplary downhole heat mining device including an inclined lateral secondary borehole, according to one embodiment of the present invention. [Diagram 5] 1 is a side cross-sectional view of another exemplary downhole thermal mining device according to one embodiment of the present invention. [Figure 6] FIG. 2 is a schematic diagram of another exemplary geothermal mining system including multiple lateral secondary fluid loops, according to an embodiment of the present invention. [Figure 7] FIG. 2 is a schematic diagram of another exemplary geothermal mining system having one completed secondary fluid loop and another secondary fluid loop in the process of being formed, according to one embodiment of the present invention. [Figure 8] 1 is a cross-sectional side view of another exemplary downhole thermal mining device in which a fractured layer forms part of a secondary fluid loop, according to one embodiment of the present invention. [Figure 9] 1 is a side cross-sectional view of another exemplary downhole thermal mining device including tertiary drilling, according to one embodiment of the present invention. [Figure 10] 1 is a side cross-sectional view of another exemplary downhole thermal mining device according to one embodiment of the present invention. [Figure 11] FIG. 2 is a schematic diagram of another exemplary geothermal mining system including a vertically spaced secondary loop directly fluidly coupled to the primary fluid loop, in accordance with one embodiment of the present invention. [Figure 12] FIG. 2 is a schematic diagram of another exemplary geothermal mining system that enables simultaneous heat recovery and main borehole drilling in accordance with an embodiment of the present invention. [Figure 13] 2 is a schematic diagram of another exemplary geothermal mining system according to an embodiment of the present invention. [Figure 14] 1 is a flow chart illustrating an exemplary geothermal mining method according to one embodiment of the present invention. [Figure 15] 2 is a schematic diagram of another exemplary geothermal mining system including multiple primary boreholes, according to one embodiment of the present invention. [Figure 16] 1 is a close-up view of a cross section of a primary borehole and a secondary borehole including a thermal mining device, according to one embodiment of the present invention. [Figure 17A] FIG. 17 is a cross-sectional view taken along dashed line "B" in FIG. [Figure 17B] FIG. 17B is a perspective cross-sectional view of the narrow borehole segment shown in FIG. 17A. [Figure 18] FIG. 17 is an enlarged view of box "C" in FIG. [Figure 19] FIG. 13 is a schematic diagram of another exemplary method of drilling a primary wellbore and adding secondary wellbore and fluid loops without enlarging the initial wellbore, according to an embodiment of the present invention. [Figure 20] FIG. 2 is a schematic diagram of a method for incremental enlargement of an initial wellbore according to one embodiment of the present invention. [Figure 21A] 2 is a side view of a second example of an exemplary downhole thermal mining device in a borehole according to one embodiment of the present invention. [Figure 21B] FIG. 21B is a cross-sectional view of the example shown in FIG. 21A. [Figure 22A] 1 is a cross-sectional view of an exemplary heat exchanger according to one embodiment of the present invention. [Figure 22B] FIG. 22B is a perspective cross-sectional view of the exemplary heat exchanger shown in FIG. 22A. [Figure 23A] FIG. 2 is a cross-sectional view of an exemplary valve mounting unit according to one embodiment of the present invention. [Figure 23B] FIG. 23B is a perspective cross-sectional view of the valve installation unit shown in FIG. 23A. [Figure 24A] FIG. 1 is a schematic diagram of a computational model of heat transfer in a secondary loop with a rock conduit without a heat exchanger. [Figure 24B] FIG. 1 is a schematic diagram of a computational model of heat transfer in a secondary loop with a rock conduit and a heat exchanger. [Figure 24C] FIG. 1 is a schematic diagram of parameters used in the computational model. [Figure 24D]FIG. 1 is a schematic diagram of parameters used in the computational model. [Figure 25A] FIG. 13 is a schematic and temperature plot of a computational model without a heat exchanger. [Figure 25B] FIG. 1 is a schematic diagram and temperature plot of a computational model with a heat exchanger. [Figure 25C] FIG. 4 is a graph of output temperature as a function of time. [Figure 25D] FIG. 2 is a graph of generator efficiency as a function of output temperature. [Figure 25E] FIG. 1 is a graphical illustration of power as a function of number of lateral loops. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] These drawings are provided to illustrate various aspects of the present invention and are not intended to be limiting in scope with respect to dimensions, materials, configurations, arrangements or proportions unless specifically limited by the claims.

[0012] Although these exemplary embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, it should be understood that other embodiments may be realized and various modifications may be made to the invention without departing from the spirit and scope of the invention. Accordingly, the following more detailed description of embodiments of the invention is not intended to limit the scope of the invention as claimed, but is presented for purposes of illustration only, and not limitation, to describe the features and characteristics of the invention, to illustrate the best mode of operation of the invention, and to fully enable those skilled in the art to practice the invention. Accordingly, the scope of the invention should be defined only by the appended claims.

[0013] definition In describing and claiming the present invention, the following terminology is used.

[0014] The term "geothermal" is used to refer to any heat present beneath the Earth's surface. Geothermal energy can include heat of a wide range of temperatures that can exist over a wide range of depths beneath the Earth's surface. As used herein, "geothermal reservoir" refers to an area beneath the Earth's surface that has a temperature (or thermal energy density) high enough to allow useful thermal energy to be generated from that area. Typically, a geothermal reservoir can be naturally heated, or in other words, contains heat supplied by the Earth (i.e., thermal energy flowing from deep within the Earth toward the Earth's surface) rather than introduced from a surface heat source. A geothermal reservoir can include rock bodies with fluids present in fractures and / or other forms of porosity or without such porosity ("wet" or "dry", respectively). For example, a wet geothermal reservoir can include an underground aquifer or a hot wet rock body (HWR) that is at a temperature high enough to generate thermal energy therefrom. A dry geothermal reservoir can include a hot dry rock body (HDR), which can be a rock formation that contains little or no fluid. The system described herein can be effectively used in either HWR or HDR. The lateral wellbore embodiment can be used to adaptively access heat in either HWR or HDR (or where mixing / permeability and fluid content vary temporally and spatially). In either case, the system can be implemented over a wide range of target temperatures, but is intended for use in geothermal reservoirs, particularly at very high temperature targets (>350°C). The system can be particularly useful for power generation, but other applications can include, but are not limited to, energy storage through the production of hydrogen fuel (electrolysis), direct production of steam, direct heating, etc.

[0015] As used herein, "thermal contact" may refer to a functional connection between two bodies or fluids that allows heat to be transferred from one body or fluid to the other. In some instances, thermal contact may be established by direct physical contact. For example, water injected directly into a crack in a hot, dry rock formation may be in thermal contact with the hot, dry rock body because heat is transferred from the hot, dry rock body to the water during the direct physical contact. In other instances, thermal contact may be established without direct physical contact, but in which case there is an intermediate medium through which heat can be conducted. For example, water may be enclosed within a pipe that is in physical contact with the hot, dry rock body. The water may be in thermal contact with the hot, dry rock body because heat can be conducted from the hot, dry rock body through the pipe wall to the water without direct physical contact between the water and the hot, dry rock body.

[0016] As used herein, "fluidically connected" can refer to fluid bodies, fluid lines, or fluid containers that are physically connected to allow mass transfer of fluids from one to the other. Similarly, "fluidically isolated" can refer to fluid bodies, fluid lines, or fluid containers that are separated such that fluids cannot be transferred from one to the other.

[0017] As used herein, whenever any property is mentioned that can have a distribution between different values, such as temperature distribution, pore size distribution, etc., the property being mentioned represents the average of the distribution, unless otherwise stated.

[0018] It should be noted that as used in this specification and the appended claims, the singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to a "layer" includes one or more of such features, reference to a "particle" includes reference to one or more of such elements, and reference to "producing" includes reference to one or more of such steps.

[0019] As used herein, the terms "about" and "approximately" are used to provide flexibility, such as to indicate that a given value at a numerical range endpoint may be "slightly above" or "slightly below" the endpoint. The degree of flexibility for a particular variable can be easily determined by those skilled in the art based on the context. However, unless otherwise specified, the term "about" generally means flexibility of less than 5%, and in some cases less than 2%.

[0020] As used herein, the term "substantially" refers to a complete or nearly complete degree or degree of an action, characteristic, property, state, structure, item, or result. The exact acceptable degree of deviation from absolute perfection may depend on the particular situation, as the case may be. However, approximation to perfection generally results in having the same overall result as if absolute and total perfection had been obtained. "Substantially" refers to a degree of deviation that is small enough so as not to measurably impair the identified property or condition. The exact acceptable degree of deviation may depend on the particular situation, as the case may be. The use of "substantially" is equally applicable when used in a negative sense, referring to a complete or nearly complete lack of an action, characteristic, property, state, structure, item, or result.

[0021] As used herein, "adjacent" refers to the proximity of two structures or elements. In particular, elements identified as "adjacent" may abut or be connected. Such elements may also be proximate or close to one another without necessarily touching one another. The exact degree of proximity may, in some cases, depend on the particular situation. Additionally, adjacent structures or elements may, in some cases, be separated by additional structures or elements between the adjacent structures or elements.

[0022] As used herein, a plurality of items, structural elements, compositional elements, and / or materials may be presented in a common list for convenience. However, these lists should be construed as though each member of the list is individually identified as a separate and unique member. Thus, the individual members of such lists should not be construed as de facto equivalents of any other members of the same list solely based on their presentation in a common group, unless indicated to the contrary.

[0023] Concentrations, amounts, and other numerical data may be presented in a range format herein. It should be understood that such range formats are used merely for convenience and brevity, and should be interpreted flexibly to include not only the numerical values ​​expressly recited as the limits of the range, but also all individual numerical values ​​or subranges subsumed within the range, as if each numerical value and subrange were expressly recited. For example, a numerical range of about 1 to about 4.5 should be interpreted to include not only the explicitly recited limits of 1 to about 4.5, but also the individual numbers such as 2, 3, 4, and subranges such as 1 to 3, 2 to 4, etc. The same principle applies to ranges reciting only one numerical value, such as "less than about 4.5," which should be interpreted to include all of the values ​​and ranges recited above. Moreover, such interpretation should be applied regardless of the breadth of the range or characteristic being described.

[0024] Any steps recited in any method or process claim may be performed in any order and are not limited to the order presented in the claims. Means-plus-function or step-plus-function limitations are used for a particular claim limitation only if all of the following conditions are present within that limitation: a) the "means for" or "step for" is explicitly recited, and b) the corresponding function is explicitly recited. The structure, material, or acts supporting the means-plus-function are explicitly described in the description herein. Thus, the scope of the present invention should be determined solely by the appended claims and their legal equivalents, and not by the descriptions and examples given herein.

[0025] Reference will now be made to the exemplary embodiments illustrated, and specific language will be used herein to describe the same. It will nevertheless be understood that no limitation of the scope of the technology is intended thereby. Further features and advantages of the technology will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, which together illustrate, by way of example, features of the technology.

[0026] By way of the general examples set forth in the Summary above, it should be noted that in this disclosure, when describing systems, or related devices or methods, individual or separate descriptions are considered applicable to one another, whether or not explicitly described in the context of a particular example or embodiment. For example, when describing a device itself, other device, system, and / or method embodiments are also included in such description, and vice versa.

[0027] Furthermore, various modifications and combinations can be derived from the present disclosure and examples, and therefore the following figures should not be considered limiting.

[0028] Geothermal Mining System The present disclosure describes systems and methods for extracting geothermal energy. In various examples, the systems and methods can allow for improved efficiency and control of geothermal energy production. In some examples, the systems and methods can utilize a single borehole, thereby reducing drilling compared to systems that utilize separate boreholes for injection and production. The systems and methods can allow for energy production from hot dry rock without hydraulic fracturing or with minimal hydraulic fracturing. Thus, the systems and methods can reduce the induction of seismicity compared to processes that include more hydraulic fracturing. In certain examples, the systems and methods described herein can allow for drilling a borehole and beginning to generate geothermal heat from the borehole while simultaneously drilling the borehole deeper. This capability can allow for energy production to begin more quickly while also drilling deeper to access more geothermal reservoirs at increasingly higher temperatures. The systems and methods can also be successfully used to access geothermal reservoirs at greater depths and higher temperatures than many other geothermal generation processes.

[0029] In some examples, the geothermal mining system may include a single primary wellbore in the geothermal reservoir. The primary fluid loop may be comprised of a low temperature working fluid line leading to the primary wellbore and a high temperature working fluid line exiting the primary wellbore. The system may also include a secondary fluid loop disposed below the primary wellbore. The secondary fluid loop may be in thermal contact with the geothermal reservoir. The system may also include an in-hole heat mining device that may be used to control a rate of heat transfer from the secondary fluid loop to the primary fluid loop by selectively controlling fluid flow through the primary fluid loop, the secondary fluid loop, or both. More specifically, controlling the rate of fluid flow through the secondary fluid loop may include circulating the fluid in thermal contact with the geothermal reservoir such that the fluid may be heated by the geothermal reservoir. The rate at which heat is transferred from the geothermal reservoir to the fluid in the secondary fluid loop may depend on various factors, such as the flow rate of the fluid through the secondary fluid loop, the relative temperatures of the fluid in the secondary fluid loop and the geothermal reservoir, the presence of any material that physically separates the fluid from the geothermal reservoir, the local structure of the rock body adjacent to the interface between the fluid and the rock body (e.g., porosity and fracture density and geometry that affect the effective permeability and effective heat transfer process), etc. Additionally, in various examples, the secondary fluid loop may be in thermal contact with the primary fluid loop, or the secondary fluid loop may be selectively in thermal contact with the primary fluid loop. In some examples, the downhole heat mining device may adjust the flow rate of the fluid in the primary and secondary flow loops to control the rate of heat transfer to the primary fluid loop. The heat transferred to the primary fluid loop may be used on the surface to generate electricity, or for direct heating, hydrolysis to generate hydrogen fuel, or any purpose for which geothermal energy can be used.

[0030] FIG. 1 illustrates an exemplary geothermal mining system 100. The system includes a single primary borehole 110 in a geothermal reservoir 120. A primary fluid loop 130 can include a low temperature working fluid line 132 leading to the primary borehole and a high temperature working fluid line 134 exiting the primary borehole. In this example, the high temperature working fluid line is used to power a generator 102 on the surface. A secondary fluid loop 140 is disposed below the primary borehole. The secondary fluid loop is in thermal contact with the geothermal reservoir. This thermal contact is represented by a dotted box 142. The secondary fluid loop can also be in thermal contact with the primary fluid loop. This thermal contact is represented by a dotted box 144. The system can also include a downhole heat mining device 150 that can control the rate of heat transfer from the secondary fluid loop to the primary fluid loop by selectively controlling fluid flow through the primary fluid loop, the secondary fluid loop, or both. In various examples, the system can include a variety of different arrangements and designs of boreholes, fluid loops, downhole heat mining devices, and the like. Some examples are described in more detail below.

[0031] Hot rock bodies in a geothermal reservoir may, in some cases, exhaust heat in a localized area where heat is being extracted. For example, a secondary fluid loop may be used to transfer heat from the geothermal rock body to a fluid in the secondary fluid loop. If the rate of heat transfer from the rock body to the fluid exceeds the rate at which heat can be conducted from the surrounding rock bodies to the location of the secondary fluid loop, the rock body may become cooler in the localized area of ​​the secondary fluid loop. In some examples, the flow rate of the fluid in the secondary fluid loop may be controlled so that heat transfer is not too fast and the localized surrounding rock body does not cool to a temperature where the heat can no longer be used. In other examples, the system may include multiple secondary fluid loops at different locations in the geothermal reservoir. The rock body may cool as heat is extracted by one of the secondary fluid loops. Once the rock body has cooled to a specified threshold temperature (i.e., the depletion temperature), the fluid flow in that particular secondary loop may be reduced or stopped to allow the temperature of the geothermal rock body at the location of that secondary fluid loop to be increased by thermal diffusion from adjacent rock volumes of the geothermal reservoir. Although conditions may vary depending on the surface equipment, heating fluid, and desired target temperature, typical depletion temperatures may range from about 150°C to about 400°C, and often below about 300°C. As a general guideline, the outlet temperature (i.e., at the surface) may be above about 150°C to generate electricity using standard current turbine systems, and in some cases may be above about 400°C. To compensate for lost production of heat from a secondary fluid loop that is temporarily shut down, another secondary fluid loop may be operated simultaneously. In some examples, the system may include a sufficient number of secondary fluid loops such that geothermal energy can be continuously generated within the target ranges of mass flow rate and outlet temperature. Additionally, the amount of geothermal energy generated may be controlled by controlling the fluid flow rate through the multiple secondary fluid loops and the primary loop.

[0032] In some examples, the primary and secondary fluid loops can be fluidly isolated from each other, meaning that fluid flowing through the secondary loop does not physically cross over into the primary fluid loop, or vice versa. In these cases, heat can be transferred from the secondary fluid loop to the primary fluid loop, for example, via a heat exchanger. In other words, the heat exchanger is physically oriented downhole within the housing of the downhole heat mining device. In certain examples, the downhole heat mining device can include a heat exchanger connected to the primary and secondary fluid loops. The heat exchanger can have a hot side and a cold side. The hot side can be connected to the secondary fluid loop and the cold side can be connected to the primary fluid loop. Heat can be transferred from the secondary fluid loop on the hot side through the heat exchanger to the primary fluid loop on the cold side. In certain examples, the downhole heat mining device can also include a pump to control the fluid flow rate through the secondary fluid loop. In one example, insulation can be selectively oriented within the mining device to minimize heat loss across inlet and outlet pipes to and from the surface.

[0033] 2 shows an exemplary geothermal mining system 100 that includes an downhole thermal mining device 150 that includes a secondary fluid loop contained within the thermal mining device (also shown in FIG. 1). A low temperature working fluid line 132 leads down the primary wellbore 110 to the thermal mining device, and a high temperature working fluid line 134 leads up from the thermal mining device to the surface.

[0034] FIG. 3A shows an enlarged cross-sectional side view of an in-hole heat mining device 150. The low temperature working fluid line 132 leads to a low temperature working fluid inlet 156 at the top of the heat mining device, and the high temperature working fluid line 134 exits at a high temperature working fluid outlet 158 ​​at the top of the heat mining device. In this example, the heat mining device includes a first pump 152 for circulating fluid in the primary fluid loop by pumping fluid from the low temperature working fluid line into the device towards the high temperature working fluid line. The device also includes a heat exchanger 160. The heat exchanger has a high temperature side 162 and a low temperature side 164. In this example, the low temperature working fluid line leads to the low temperature side of the heat exchanger. The high temperature side of the heat exchanger is connected to an outer shell fluid flow chamber 166, which may optionally be in thermal contact with the surrounding rock mass, to operate as a secondary heat exchanger. The high temperature side of the heat exchanger and the outer shell fluid flow chamber together constitute the secondary fluid loop in this example. The secondary fluid loop is thus fluidly isolated from the primary fluid loop and contained within the heat mining device. The device also includes a second pump 154 ​​for circulating fluid in the secondary fluid loop. The fluid in the secondary fluid loop can draw heat from the geothermal reservoir surrounding the heat mining device. In certain examples, the shell fluid flow chamber can include mechanisms for increasing heat transfer, such as coils, fins, tubes, baffles, and the like. In one example, the shell fluid flow chamber can include a coil or corkscrew shaped flow passage to increase the surface area and increase the time the fluid is in contact with the hot shell of the heat mining device. For example, the flow tubing is wrapped around the inner surface of the shell. This configuration can extract heat from the surrounding rock mass (thermal perturbations induce localized convection of fluid in the connected porosity of the surrounding rock mass or free fluid located between the device and the rock mass, both of which promote heat extraction). This example can extract usable thermal energy to the surface and can also reduce the ambient temperature inside the device to improve the operability of electronics and machinery inside the device.

[0035] 3B is a top cross-sectional view of the exemplary heat mining device 150 shown in FIG. 3A. As shown in this figure, a heat exchanger 160 is disposed inside the device, with the low temperature working fluid line 132 and the high temperature working fluid line 134 connected to the low temperature side 164 of the heat exchanger. The high temperature side 162 of the heat exchanger is connected to an outer shell fluid flow chamber 166 to form a secondary fluid loop. The heat mining device can control the rate at which heat is transferred to the primary fluid loop by controlling the flow rate of fluid through the primary and secondary fluid loops.

[0036] The heat exchanger shown in the above figures is shown as a simple design with two chambers separated by a wall. However, any type of suitable heat exchanger can be used to effectively transfer heat from the secondary fluid loop to the primary loop. In various examples, the heat exchanger may include a parallel flow, counter flow, cross flow, or hybrid flow heat exchanger. In further examples, the heat exchanger may include a shell-and-tube heat exchanger, a double tube heat exchanger, a plate heat exchanger, or another suitable heat exchanger.

[0037] In other examples, the geothermal mining system can include secondary fluid loops that run through the geothermal reservoir itself, rather than being entirely contained within the downhole heat mining device. Flow paths can be formed that branch outward from the primary borehole, and the secondary fluid loop can include these flow paths. In certain examples, the system can include lateral secondary boreholes formed in the geothermal reservoir that extend from the primary borehole. In some cases, the lateral secondary boreholes can be at a 90° angle to the primary borehole. However, in other examples, the lateral secondary boreholes can be angled upward or downward. Thus, the term "lateral" is not limited to completely horizontal boreholes, and the secondary boreholes can be formed at various angles to the primary borehole.

[0038] In a particular example, two lateral boreholes can be formed extending from the primary borehole, and the secondary boreholes can be oriented such that they merge with each other after passing some distance through the geothermal reservoir. Both of these secondary boreholes can be connected as part of a secondary fluid loop, such that fluid in the loop flows out of the heat mining device, through the geothermal reservoir and back to the heat mining device.

[0039] FIG. 4 shows a side cross-sectional view of another exemplary downhole heat mining device 150 with inclined lateral secondary boreholes. This example includes a heat exchanger 160 as in the previous example. The heat exchanger includes a hot side 162 and a cold side 164. The cold side is connected to the cold working fluid line 132 and the hot working fluid line 134. A first pump 152 can circulate working fluid from the cold working fluid line to the hot working fluid line. The system also includes a lateral secondary borehole 112 extending from the primary borehole into the geothermal reservoir. The lateral secondary boreholes in this example are inclined relative to the primary perforation lines such that the secondary boreholes merge to form a fluid path. The secondary boreholes are connected to the hot side of the heat exchanger to form a secondary fluid loop. A second pump 154 ​​can be used to circulate fluid through the secondary fluid loop.

[0040] FIG. 5 illustrates an exemplary downhole heat mining device 150 that can be used to form a lateral secondary wellbore as shown in FIG. 4. The heat mining device includes a lateral drilling head 170 directed outward from the heat mining device. The lateral drilling head can be controlled and / or launched from a secondary drilling unit 172 and can be tethered to a portion of the secondary drilling unit by lines that carry power and / or fluids to the drilling unit. The lateral drilling head can extend outward to drill a secondary wellbore in a cross direction relative to the primary wellbore. Again, the secondary wellbore can be drilled at various angles from the primary wellbore. Thus, the term "cross direction" is not limited to a secondary wellbore that is at a 90° angle from the primary wellbore. In some examples, the lateral drilling head can be steerable so that the direction of drilling can be changed to form a curved secondary wellbore. This exemplary heat mining device can also connect the lateral secondary wellbore after it has been drilled to the hot side 162 of a heat exchanger 160 to form a secondary fluid loop. The secondary fluid loop can transfer heat to the primary fluid loop, which includes the low temperature working fluid line 132, the low temperature side of the heat exchanger 164, and the high temperature working fluid line 134. Methods and devices for fluidly connecting the secondary boreholes to the fluid lines and piping within the device are described in more detail in the text associated with FIG. 14. Any operating device has the ability to install the necessary connections and piping by a variety of methods, including transporting pre-fabricated components from the surface via mechanical connections ("component delivery lines") and / or 3D printing the components within the device, including mechanisms for installing, mating, and sealing the components.

[0041] While examples including secondary lateral wellbore are shown with a single secondary fluid loop, other examples can include systems with multiple secondary fluid loops. For example, a downhole heat mining device can drill several lateral secondary wellbore holes to form multiple secondary fluid loops at a common depth, multiple different depths, or both. In some examples, a downhole heat mining device can include valves or pumps to simultaneously control fluid flow through multiple secondary fluid loops, or the device can selectively flow fluid through the secondary fluid loops one at a time or in any order for the purpose of optimizing heat extraction and viable duration of the reservoir. Although a brief description of the drawings is provided above, it is emphasized that the scale of these drawings is for illustrative clarity only and does not represent the dimensions of typical wells and devices. For example, a primary wellbore may be 20 cm in diameter, while a lateral loop may be hundreds of meters long.

[0042] FIG. 6 illustrates an exemplary geothermal mining system 100 having multiple secondary fluid loops 140 formed by drilling lateral secondary boreholes outward from the primary borehole. In this example, the secondary fluid loops are configured in a six-loop star pattern radiating outward from the downhole thermal mining device 150. In some examples, the downhole thermal mining device can include multiple lateral drilling heads so that all secondary boreholes can be formed simultaneously. In other examples, one or two lateral drilling heads can be used to drill the secondary boreholes, and then the downhole thermal mining device can rotate to drill additional boreholes in different directions. In further examples, additional arrays of secondary boreholes can be formed upward (i.e., further up the primary borehole) and / or downward (i.e., further down the primary borehole) at various depths. The spacing and distribution of these secondary fluid loops can vary significantly depending on thermodynamic and economic considerations, including the desired heat recovery rate, the desired duration, which depends on the rock mass type, rock mass porosity, depth, reservoir temperature (which may vary with depth and radial direction), borehole diameter, and other factors.

[0043] FIG. 7 illustrates a different exemplary geothermal mining system 100, also having multiple secondary loops 140. In this example, the secondary loops include curved lateral secondary boreholes. The curved secondary boreholes can be formed using a steerable lateral drilling head 170. The steerable lateral drilling head can drill secondary boreholes having a curved loop shape, and then an in-hole thermal mining device can connect both ends of the curved secondary boreholes to a secondary fluid loop. Each of the curved secondary boreholes can form a single separate secondary fluid loop. The lateral boreholes can be drilled by existing mechanical drilling heads (diamond or other drill bit material), thermal / flame fracturing drilling, hydrothermal fracturing (hot or cold fluid jets) drilling, metal shot abrasive drilling, chemically enhanced drilling, or other techniques, if operable in the particular conditions. Hydrothermal fracturing can be particularly useful in very high temperature conditions, and is described below.

[0044] In a further example, the secondary fluid loop can incorporate fractures in the geothermal formation near the primary borehole. In some examples, the primary borehole can be drilled into an area of ​​the geothermal formation that contains microfractures distributed throughout the formation. In such examples, the downhole heat mining device can be used to drill lateral secondary boreholes outward from the primary borehole. These lateral secondary boreholes can be part of a secondary fluid loop in which fluid is pumped into one lateral secondary borehole to percolate through the microfractures, and then the fluid can flow through a second lateral secondary borehole back to the heat mining device.

[0045] In examples where the geothermal formation contains multiple microcracks in all directions from the primary borehole, the lateral secondary boreholes can be drilled in any direction to access the microcracks. In other examples, localized pockets of microcracks may exist in the geothermal formation. The lateral secondary boreholes can be drilled in the appropriate direction to reach these localized pockets of microcracks. A first lateral secondary borehole can be formed to pump fluids into the pockets of microcracks, and a second lateral secondary borehole can be formed to produce (or collect) fluids exiting the pockets of microcracks. In further examples, the microcracks can be generated in the geothermal formation using an appropriate fracturing method. In certain examples, the downhole thermal mining device and / or the lateral drilling head can include a micro-damage head capable of creating microcracks in the surrounding rock body. The micro-damage head can form the microcracks by mechanical damage (e.g., impacting a rotating mass against the rock body surface), cryofracturing, hydraulic fracturing, sonic fracturing, or other existing methods.

[0046] In some examples, the lateral drilling head can include passages for propelling fluid from the lateral drilling head into the surrounding rock body. In certain examples, the fluid can be propelled perpendicularly outward relative to the direction in which the lateral drilling head is drilling. Propelling fluid at steady or pulsed high pressure into the surrounding rock body can form a network of small cracks that propagate into the rock body. In some examples, small holes can be drilled first to initiate the propagation of the cracks. These cracks can form a "converging crack network" that can locally surround a secondary borehole or intersect with other secondary boreholes or crack networks around other secondary boreholes.

[0047] FIG. 8 illustrates one hypothetical example downhole heat mining device 150 forming lateral secondary boreholes into pockets of microcracks 122. In this example, lateral drilling heads 170 begin drilling at secondary drilling units 172 and drill lateral secondary boreholes into the geothermal formation. One drilling head drills into the top of the formation containing microcracks. The other drills into the bottom of the area of ​​the formation containing microcracks. After drilling the lateral secondary boreholes, the drilling heads can be retracted into the secondary drilling units. Pump 154 ​​can then be used to pump fluid into one of the lateral secondary boreholes. The fluid can then proceed through the microcracks to the other lateral secondary borehole. Thus, the lateral secondary boreholes and the microcrack network are part of a secondary fluid loop. As with the other examples shown above, heat from the secondary fluid loop can be transferred to the primary fluid loop through heat exchanger 160.

[0048] Another method that can be used to form a secondary fluid loop in a geothermal reservoir can include forming a tertiary borehole that extends from a secondary borehole. In some examples, the downhole heat mining device can include a secondary drilling unit having a lateral drilling head that can extend outward from the heat mining device. In certain examples, the lateral drill head can also include a tertiary drilling unit having a tertiary drilling head. The tertiary drilling head can be smaller than the secondary lateral drilling head. In some examples, two secondary lateral boreholes can be formed, and then a tertiary borehole can be formed that extends from the secondary borehole. These tertiary boreholes can be connected to each other or to other secondary boreholes, or the tertiary boreholes can increase the porosity of the geothermal rock body or increase the core network of thermal-hydraulic fractures so that fluids can pass through other secondary boreholes. In some examples, multiple tertiary boreholes can be formed that connect two secondary boreholes. The additional tertiary boreholes can increase the surface area of ​​the geothermal rock body that is contacted by the fluids flowing through the secondary fluid loop. This level of multi-scale drilling can increase the rate of heat transfer from the geothermal rock mass to the fluid.

[0049] FIG. 9 illustrates an exemplary downhole heat mining device 150 that can form tertiary boreholes extending from secondary lateral boreholes. The device includes a lateral drilling head 170 that can extend from a secondary drilling unit 172 to form lateral secondary boreholes in the geothermal formation. After drilling some distance into the geothermal formation, the lateral drilling head can deploy a tertiary drilling head 174 that can drill another borehole outward from the secondary borehole. In this example, two tertiary drilling heads are used to drill tertiary boreholes connecting the secondary boreholes. In another example, the tertiary drilling heads can be retracted after forming the tertiary boreholes, and then the tertiary drilling heads can be used again multiple times to form multiple tertiary boreholes connecting the secondary boreholes. The secondary and tertiary boreholes can form a secondary fluid loop, which can transfer heat to the primary fluid loop through a heat exchanger 160 as in the example above.

[0050] In further examples, the geothermal mining system can include a processor to help control fluid flow through the primary fluid loop and one or more secondary fluid loops. As mentioned above, in some examples, the downhole thermal mining device can include a pump for pumping fluid through the secondary fluid loop. In other examples, the downhole thermal mining device can also include a pump for pumping fluid through the primary fluid loop, but this can also be performed by a pump at the surface. The operation of these pumps can be controlled by control signals from the processor in some examples. In one example, the processor can be integrated within the downhole thermal mining device. In another example, the processor can be outside the primary borehole, for example, on the surface. The processor can send control signals that can be received by the downhole thermal mining device to control the pump. In further examples, the processor can generate control signals to control various other operations of the downhole thermal mining device, such as drilling, steering the steering drilling head, operating the micro-damage head, opening and closing valves, moving up and down in the primary borehole, and rotating the thermal mining device.

[0051] The geothermal mining system may also include sensors. In some examples, the processor described above may be connected to sensors to receive information about the geothermal formation, the borehole, the local rock mass and the fluid in the primary and secondary fluid loops including fluid temperature, stress in the rock mass, pressure in the fluid, elastic vibrations due to microseismic activity, electrical conductivity, and any other relevant information. For example, the sensor may include a temperature sensor to measure the temperature of the fluid in the primary or secondary fluid loop. Temperature sensors may also be installed to measure the temperature of the geothermal rock mass at various locations. Other types of sensors that may be used may include pressure sensors, position sensors, and combinations of these types of sensors. In certain examples, the processor may be programmed to generate control signals to instruct the heat mining device to take appropriate action based on the information provided by the sensor. For example, the processor may accelerate the fluid flow in the secondary fluid loop if additional heat transfer to the primary fluid loop is desired. In another example, the processor may slow down the fluid flow in the secondary fluid loop if the temperature of the geothermal formation falls below a predetermined threshold (e.g., depletion temperature) to allow the geothermal formation time to reheat to a higher temperature.

[0052] FIG. 10 illustrates an exemplary downhole thermal mining device 150 including an integrated processor 176. The processor is connected to a temperature sensor 178 to receive temperature measurement signals from the temperature sensor. The processor is also connected to a pump 154 ​​to allow the processor to control the pump. In this example, the temperature sensor can measure the temperature of the fluid in the secondary fluid loop 140. The processor can control the pump based on the temperature measurements. The temperature sensor can be made from a high temperature bimetallic thermocouple with wiring that is locally insulated and / or cooled to minimize signal degradation at high temperatures, or other methods can be used. The electronics for transmitting and processing all signals can be implemented in either a computer in a chamber in the device that is cooled by multiple methods to a safe operating temperature for the electronics (currently less than 200° C.). For example, Peltier elements can be used to actively cool the electronics to prevent damage to such electronics and maintain proper operation.

[0053] The above examples include a secondary fluid loop that is separate from the primary fluid loop. Heat can be transferred from the secondary fluid loop to the primary fluid loop, but the fluid in the secondary loop does not physically flow into or mix with the fluid in the primary loop. Similarly, the fluid in the primary loop does not physically flow into the secondary loop. In some examples, there may be exceptions to this approach where fluid can be transferred between the primary and second loops. For example, after drilling a lateral secondary borehole to form the secondary fluid loop, the secondary fluid loop may initially be filled with fluid provided by the primary fluid loop. However, upon the secondary loop being filled with fluid, heat transfer may be achieved primarily through a heat exchanger while the primary and secondary fluid loops are isolated from each other during normal operation of the system. In further examples, fluid may leak from the secondary fluid loop over time through natural fractures in the geothermal rock mass or through incomplete seals in the borehole or heat mining device. Thus, in some examples, fluid from the primary fluid loop may be used to replenish the fluid in the secondary fluid loop.

[0054] In other examples, the geothermal mining system may utilize a secondary fluid loop that is fluidly connected to the primary fluid loop such that fluid can flow from the primary fluid loop to the secondary fluid loop and back to the primary fluid loop. In such examples, heat transfer from the secondary fluid loop to the primary fluid loop may be achieved primarily by mass transfer of fluid that physically flows from the secondary fluid loop to the primary fluid loop. In some examples, the secondary fluid loop may include secondary boreholes, tertiary boreholes, microcrack networks, or combinations thereof, as previously described with respect to the integral heat exchanger. Thus, the secondary fluid loop may be similar to any of the examples described above. However, in further examples, the secondary fluid loop may be directly connected to the primary fluid loop to allow fluid to flow from the secondary fluid loop to the primary fluid loop. In certain examples, the secondary fluid loop may be connected to the primary fluid loop using a valve that can selectively start, stop, and adjust the flow rate through the secondary fluid loop.

[0055] The downhole thermal mining device may include a tool used to form a secondary fluid loop. As in the examples above, the downhole thermal mining device may include a lateral drilling head, a tertiary drilling head, a micro-damage head, and the like. In a further example, the downhole thermal mining device may also include a valve installation unit. After the thermal mining device drills the secondary wellbore, the valve installation unit may install a valve connecting the secondary wellbore to the primary fluid loop. In particular, the valve may connect the secondary wellbore to either the low temperature working fluid line or the high temperature working fluid line of the primary fluid loop. In a particular example, the thermal mining device may form a secondary fluid loop made up of one or more secondary wellbore. The valve installation unit may then install a first valve connecting one end of the secondary fluid loop to the low temperature working fluid line. The valve installation unit may also install a second valve connecting the other end of the secondary fluid loop to the high temperature working fluid line. In various examples, the valve may be an on / off valve, a controllable analog valve, or a simple junction such as a T-junction (i.e., a valve that is always on).

[0056] In certain examples, the valve installation unit can include an additive manufacturing device such as a 3D printer. The 3D printer can manufacture valves that connect the low temperature and high temperature working fluid lines to the secondary fluid loop. In some examples, the 3D printer can continuously print the low temperature and high temperature working fluid lines as the thermal mining device moves down the borehole. Such a 3D printer can also build other parts directly into place as the drilling process continues. For example, a borehole lining is not required, but in some cases, a borehole lining can be printed along the exposed surface of the borehole as the device moves down. The 3D printer can be adapted to form parts from build materials that can withstand the high temperatures in the primary borehole. In some examples, the build materials can include high temperature polymers, ceramics, metals, carbon fiber, etc. The unit can also function to repair parts. Alternatively, a remote control unit may transport parts, repair damaged components, patch leaks, etc.

[0057] In one example, the interior of the valve installation unit, so that the electronic elements of the tools described herein can operate, can be cooled by a coiled loop in contact with the inside surface of the device's outer shell, as described in Figure 3. Secondary cooling methods can be used, such as by circulating a cryogenic fluid through a device built into the housing of the electronics, or by passing an electric current through a thermoelectric (Peltier) device built into a similar housing around the operating electronics.

[0058] In other examples, the valve installation unit can be adapted to receive components that can be lowered down a borehole from the surface. For example, a valve can be lowered down a borehole, and the valve installation unit can receive the valve and then connect the valve to a low temperature working fluid line or a high temperature working fluid line. In this case, fluid circulation can be stopped below the last, deepest secondary loop to allow for the connection of a subsequent lower section. In some options, flow can be stopped completely, but optionally, multiple sets of valves or three-way valves can be used to allow for temporary diversion of fluid while connections are being added.

[0059] FIG. 11 illustrates another exemplary geothermal mining system 100 in which a secondary fluid loop is fluidly connected to the primary fluid loop. The system includes a primary borehole 110 within which is a downhole thermal mining device 150 having two lateral drilling heads 170. The lateral drilling heads are deployed from a lateral drilling unit 172 to drill the lateral secondary boreholes. The lateral drilling heads in this example may be steerable to be able to drill the looped secondary boreholes 112. The downhole thermal mining device may also include a valve installation unit 180. The valve installation unit is adapted to install valves 182 connecting the looped secondary boreholes to the low temperature working fluid line 132 and the high temperature working fluid line 134. The valves may be controllable to be able to regulate the fluid flow through each looped secondary borehole. The valve installation unit may also install pipe joints 184 on the side of the primary borehole that connect the valves to the secondary boreholes. In this example, the working fluid may flow down a low temperature working fluid line, and then a portion of the low temperature working fluid may be sent through a secondary borehole. The working fluid may be heated due to contact with the geothermal formation in the secondary borehole. The heated working fluid may then flow back into the high temperature working fluid line and be carried by the high temperature working fluid line back to the surface.

[0060] As shown in the previous examples, any number of secondary boreholes can be created and connected to the primary fluid loop. In some cases, using more secondary boreholes can allow for more heat generation since a larger surface area of ​​the geothermal formation is used to heat the working fluid. In a further example, the valves leading to the secondary boreholes can be adjusted depending on the demand for thermal energy. Thus, the system can be flexible to allow the energy generation to match the demand. The valves can also be adjusted to minimize subcooling around any of the secondary boreholes. If the geothermal formation reaches too low a temperature near a secondary borehole, the valve leading to that borehole can be temporarily shut off to allow the adjacent geothermal formation time to reheat. In particular, the use of transport through insulated pipes means that the fluid does not come into direct contact with the primary borehole for the majority of the length of its flow, thus avoiding thermal shock that can cause chemical / mechanical degradation of the borehole walls when changing the flow rate and temperature of the fluid. This design can reduce or eliminate the need for a borehole lining system.

[0061] Additionally, the level of control of the fluid mass flux through all parts of the system allows for a high degree of control over the fluid pressure. This level of control can allow the system to maintain the fluid in a supercritical state throughout if the fluid pressure and temperature conditions allow this state to be stable. By allowing the supercritical fluid to reach the surface, for example by using a "triple expansion" system, much higher power generation efficiencies can be enabled.

[0062] The above examples show multiple secondary boreholes formed as looped boreholes by using a steerable lateral drilling head. However, other forms of secondary fluid loops can be formed and connected to the primary fluid loop using valves. Any of the exemplary arrangements of secondary fluid loops shown above can be used, including a loop formed by drilling two secondary boreholes that merge into one another, or a loop formed by drilling a secondary borehole into an existing microcrack network, or a loop formed by forming a new microcrack network.

[0063] The downhole heat mining device can also be adapted to increase the depth of the primary wellbore. In some examples, the downhole heat mining device can increase the depth of the primary wellbore while simultaneously generating thermal energy from the geothermal reservoir. FIG. 12 shows an example geothermal mining system 100 including a downhole heat mining device 150 having a drill head 190 for drilling the primary wellbore. The drill head is positioned at the downhole end of the heat mining device. In this example, the drill head can be extended downward from the heat mining device such that the depth of the primary wellbore can be increased while the heat mining device can simultaneously perform other operations such as installing a valve between the primary and secondary fluid loops 140. The heat mining device can be locked in place while drilling continues. For example, spikes, cables, and / or wheels can be used to keep the body of the heat mining device in place (whether or not it includes a heat exchanger) while the primary drill head 190 continues to operate. The primary fluid loop includes a low temperature working fluid line 132 and a high temperature working fluid line 134. Also included in this example is a slurry line 136 that is fluidly isolated from the primary loop fluid. The drill head in this example is a cryogenic fracturing drill head that can use cold water to micro-fracture hot rock masses of a geothermal reservoir. The cryogenic working fluid line and the slurry line extend to the drill head. Once the drill head uses the cryogenic working fluid to remove small pieces of rock mass, a slurry comprised of rock masses and working fluid can be extracted through the slurry line and pumped up to the surface. Additionally, both the primary and secondary drill heads can be fluidly connected to the slurry line.

[0064] Any suitable type of drill head may be used to form the primary borehole. In some examples, the drill head may include a rotary mechanical drill head, a percussion drill head, a hot fracturing drill head such as a hydrothermal fracturing drill head, a cold fracturing drill head, or a combination thereof. A cold fracturing drill head may be particularly useful when the thermal mining device is in a high temperature geothermal reservoir, such as 500° C. or higher, in the so-called brittle-ductile transition zone, where the temperature depends on the rock composition. The cold fracturing drill head may include jets for injecting cold fluid, such as water, into the geothermal rock mass. Injecting cold water into the hot rock mass may cause a thin layer of the rock mass to cool and contract very quickly, resulting in small cracks in the rock mass. In some cases, the cracks may break up particles in the rock mass. In certain examples, the cold fluid may be pressurized to high pressures, which may cause the fluid to expand in the fractures and aid in the removal of the fracturing particles. In some examples, these particles may be removed from the borehole via a slurry line. In further examples, a combination of cold fluid jets and mechanical forces may be used. For example, a cryogenic fluid can be injected into the rock body to form a fracture, and then the drill head can use a rotational motion or a striking force to break up the fractured rock body. The cryogenic jet can also be pulsed with sufficient time between pulses or groups of pulses to allow the geothermal rock body to reheat between pulses or groups of pulses. Allowing the rock body to reheat can promote fracture formation by ensuring a large temperature difference between the rock body and the cryogenic fluid jet. The fluid can also be pulsed at increasingly higher pressures. In other examples, high temperature hydrothermal fracturing can be used. For example, a pressurized fluid that is hotter than the rock body can be used. For example, an initial shallow region of the rock body may be at a relatively low temperature, in which case high temperature hydrothermal fracturing drilling can be useful until the temperature increases enough for cryogenic fracturing drilling to become more efficient. The pressurized fluid can enlarge the fracture, stripping away small rock fragments, and removing the fractured material as a slurry to expose a new rock surface.

[0065] The cryofracturing drill head can also be configured to be steerable. In some examples, the cryofracturing drill head can include multiple fluid jets pointed in different directions at angles to the borehole line axis. To steer the drill head, the fluid jets pointed in a desired direction can be used to preferentially remove rock mass in that particular direction. This can change the direction of the borehole and steer the drill head. Such steerable drill heads can be used for the main drill head that drills the primary borehole, as well as the secondary and tertiary drill heads that are used to drill the secondary and tertiary boreholes. In further examples, the secondary and tertiary drill heads can be any other suitable type of drill head, such as a rotary drill head, a percussion drill head, a cryofracturing drill head, or the like.

[0066] To more precisely steer the primary, secondary, and / or tertiary drilling directions, the position sensors can provide feedback to the processor to adjust the drilling direction to obtain the desired main borehole line shape and direction, secondary line shape, and / or tertiary line shape. For example, inductive sensors, magnetic sensors, echo location, distributed acoustic sensing (DAS), beamforming of seismic noise sources (e.g., acoustic emissions from the drill head, electromagnetic energy from electric motors, etc.), and the like, can be used in a method to identify the drill head position relative to the main borehole line and / or relative to a corresponding drill head base unit. In some cases, a set of three spaced apart position sensors can accurately determine the relative position of the drill head.

[0067] In a further example, the downhole thermal mining device may be self-assembled or semi-self-assembled. This capability may allow for a relatively small diameter initial borehole to be drilled first. After reaching a desired depth, such as a depth where the temperature of the rock mass is about 500° C., the bottom of the initial borehole may be enlarged by various methods. In some examples, the downhole thermal mining device that has been lowered down the initial, narrower borehole may include a layered, expandable cylindrical shell with a drill head that self-expands using material lowered on the component transport elevator and / or a drill head that can be progressively enlarged. The drill head may then be used to increase the borehole depth at the wider borehole width. Any lines or tubes for the low temperature working fluid line, high temperature working fluid line, slurry, lines, and power or control signals, etc., as well as the component transport elevator may be thinned sufficiently to pass through the narrower initial borehole section. As the borehole drilling unit progresses drilling a larger diameter cavity, the downhole thermal mining device may self-assemble an operable unit at the enlarged borehole diameter. This self-assembly is accomplished by robotic addition of components down a component elevator.

[0068] In a further example, the geothermal mining system can include a combination of features of examples including a secondary fluid loop fluidly connected to the primary fluid loop and examples including a heat exchanger that transfers heat between the secondary fluid loops fluidly isolated from the primary fluid loop. In one example, the downhole heat mining device can include an internal heat exchanger with a low temperature working fluid line connected to the low temperature side of the heat exchanger, as in some of the examples above. However, the downhole heat mining device can also include a valve installation unit. The system can include multiple secondary loops that directly connect to the low temperature working fluid and the high temperature working fluid via valves installed by the valve installation unit. At the same time, the downhole heat mining device can transfer additional heat to the working fluid using the internal heat exchanger. Heat can be transferred from a secondary fluid loop contained within the downhole heat mining device, such as a fluid loop that collects heat from an outer shell of the downhole heat mining device. Alternatively or additionally, heat can be transferred from another secondary loop formed by drilling a secondary borehole into the geothermal reservoir. Thus, a single system can include a combination of the features described above.

[0069] In another example, a heat exchanger may be included in the primary borehole but separate from the downhole heat mining device. FIG. 13 shows one such exemplary geothermal mining system 100. The system includes a narrow initial borehole 114 that is enlarged to transition to a wider primary borehole 110. After the borehole is enlarged, the downhole heat mining device 150 can self-assemble in the wider section. The downhole heat mining device includes a drill head 190 that can be used to increase the depth of the primary borehole. The downhole heat mining device can also assemble an independent heat exchanger 168 in the primary borehole that is separate from the drill head 190 and the valve installation unit 180. The low temperature working fluid line 132 and the high temperature working fluid line 134 can be connected to the low temperature side of this heat exchanger. The high temperature side of the heat exchanger can be connected to additional fluid lines 133, 135 that extend below the heat exchanger. These additional fluid lines are further connected to the lateral looped secondary borehole 112 using valves. These secondary boreholes can be used to circulate fluids to extract heat from the geothermal reservoir, as described above. However, in this example, the entire group of secondary boreholes and additional fluid lines below the heat exchanger can be considered to be the secondary fluid loop, and the low and high working fluid lines leading to the heat exchanger can be considered to be the primary fluid loop. The fluid lines that make up the secondary fluid loop are shaded with diagonal crosshatch lines in this illustration. In this example, the primary and secondary working fluid loops are fluidically isolated from each other. Again, this can mean that the loops are fluidically isolated during normal operation to transfer heat through the heat exchanger, although there can be exceptions, such as when working fluid needs to initially fill new secondary boreholes or replenish fluid that has leaked from the secondary fluid loop.

[0070] Geothermal mining method In addition to the geothermal mining systems and devices described above, the present disclosure also describes a consequential geothermal mining method. The method can include forming a geothermal mining system as described above and using the system to generate geothermal energy using the system. More specifically, the method can include controlling fluid flow through a primary fluid loop, a secondary fluid loop, or both, thereby controlling a rate of heat production from the system.

[0071] 14 is a flow chart illustrating an exemplary geothermal mining method 200. The method may include forming 210 a single primary borehole, downhole positioning 220 a geothermal mining device within the primary borehole, forming 230 a primary fluid loop including a low temperature working fluid line leading to the primary borehole and a high temperature working fluid line exiting the primary borehole, forming 240 a secondary fluid loop located below the primary borehole, the secondary fluid loop being in thermal contact with the geothermal reservoir, transferring 250 heat from the secondary fluid loop through the primary fluid loop to an energy storage or production unit outside the primary borehole, and controlling 260 a rate of heat transfer from the secondary fluid loop to the primary fluid loop by selectively controlling fluid flow through the primary fluid loop, the secondary fluid loop, or both.

[0072] In some examples, a working fluid can be pumped from the surface into the primary borehole. The working fluid can be, for example, water, liquid CO 2or other high heat capacity coolants. The working fluid may optionally include additives used to adjust the properties of the fluid. Non-limiting examples of additives may include surfactants, anti-flocculants, pH adjusters, colorants, viscosity modifiers, and the like. In certain examples, the working fluid may be pumped down the wellbore through the low temperature working fluid line. The working fluid may be heated by transferring heat from the secondary fluid loop downhole, and then the heated working fluid may be produced down the wellbore through the high temperature working fluid line. The working fluid may be at a relatively low temperature in the low temperature working fluid line compared to the high temperature working fluid line. In various examples, the working fluid in the low temperature working fluid line may have a temperature of about 0° C. to about 200° C. when the working fluid enters the primary wellbore. The operation in the high temperature working fluid line may have a higher temperature, which may be about 150° C. to about 500° C. in some examples. In one example, the working fluid may be heated to a supercritical aqueous fluid at above about 400° C. and 25 MPa. In some examples, the low temperature working fluid line and the high temperature working fluid line may form a closed loop, and if so, may be of a different composition than the working fluid in the secondary loop. An energy generation or storage unit may be located at the surface, and the thermal energy in the high temperature working fluid line may be transferred to or used in the energy generation or storage unit. For example, the working fluid in the high temperature working fluid line may be steam or a supercritical fluid, and the energy production unit may be a steam turbine generator. The steam may power the steam turbine generator and then condense into liquid water, which may flow down the low temperature working fluid line.

[0073] In further examples, the working fluid from the cryogenic working fluid line can also be used by the cryogenic fracturing drill head. For example, a bleed line can be connected to the cryogenic working line via a valve to divert a portion of the cryogenic working fluid to the drill head. The drill head can inject the cryogenic working fluid into the rock body to form fractures, as described above. In some examples, the cryogenic fracturing drill head can also include a cooling device that can further reduce the temperature of the cryogenic working fluid. For example, a thermoelectric element, also known as a Peltier element, can be used to cool the fluid just before it enters the pump of the drill head. By reducing the temperature of the fluid injected from the cryogenic fracturing drill head, the thermal shock stress and therefore the effectiveness of the fluid in fracturing the hot rock body can be increased. In certain examples, the fluid injected by the cryogenic fracturing drill head can include water and can be at a temperature of about -10°C to about 50°C, or about 0°C to about 20°C.

[0074] The primary borehole can be drilled to any desired depth underground where the temperature is in a suitable range for extracting geothermal energy. In some examples, the primary borehole can be drilled to a depth where the target temperature is about 300°C to about 600°C, often 450°C to 500°C. The depth of the primary borehole can be very shallow, less than 1 to 4 km below the surface, in areas with shallow magma intrusions (or where the user seeks low temperature reservoirs). The present invention can reliably make high temperature geothermal resources accessible, so one desirable target is about 500°C. For example, in cooler areas with a geothermal gradient of about 25°C / km, 500°C is reached at about 20 km. In areas with higher gradients (about 75°C / km), this target is achieved at 6 to 7 km.

[0075] The primary borehole may have a diameter sufficient to accommodate the downhole thermal mining device, especially in the simpler examples. In some examples, the primary borehole may have a diameter of about 20 to about 40 cm and can accommodate a smaller diameter thermal mining device. In certain examples, a narrower initial borehole may be drilled and then enlarged. The initial borehole may be drilled to a depth where the target temperature is reached at a minimum diameter of about 20 cm, as described above, and then enlarged to a width sufficient to accommodate the downhole thermal mining device, which may use a diameter of about 50 to 100 cm or even more depending on the consistency of the surrounding rock mass and the corresponding mechanical support. The wider borehole may then extend further down, about 100 to 1000 m or more, depending on the number of lateral loops the system can accommodate. The lateral secondary borehole may have a diameter of about 2 to 10 cm and can have a much longer length of about 10 to 1000 m or more.

[0076] The downhole thermal mining device can have a variety of configurations and shapes. In some examples, the downhole thermal mining device can have a housing that is cylindrical or has a generally cylindrical shape. The housing can be made from a material that can withstand high temperatures and pressures, such as 600° C. and 500 mPa (although the material likely will not need to withstand rock pressures). Non-limiting examples of such housing materials can include nickel-chromium alloys (e.g., INCONEL, HASTELLOY types), martensitic metal alloys, high carbon steels, various ceramics, and the like.

[0077] The systems and methods described herein can be particularly useful because they can enable geothermal energy to be accessed by a single primary borehole. Furthermore, geothermal energy production can begin while the downhole heat mining device is still in the process of deepening the primary borehole and / or forming additional secondary boreholes and assembling fluid lines to transfer more thermal energy to the surface. In comparison, many other geothermal generation systems utilize two boreholes to recover heat, one injection borehole and one production borehole. No geothermal energy is produced by such systems until both the injection borehole and the production borehole are drilled and fluidly connected. There is a large uncertainty in the accessible heat when connecting the two boreholes by industrial production. Thus, the downhole systems described herein can enable each of the one or more boreholes to generate geothermal energy. Each of these one or more boreholes can be fluidly isolated from one another. Furthermore, many other geothermal generation methods use large-scale hydraulic fracturing to increase the permeability of the geothermal rock body and allow working fluid to flow between the injection borehole and the production borehole. This hydraulic fracturing can cause undesirable seismic activity as a side effect. The systems and methods described herein can reduce this undesirable seismic activity by using fluid loops that do not contain shaped fractures or that contain shaped fractures in small localized areas. Thus, these systems and methods can provide great flexibility and predictability for extracting geothermal energy over long periods of time (decades).

[0078] While the systems and methods described herein can be used with a single primary borehole, in some instances it may be useful to drill multiple primary boreholes as part of a larger energy generation system. When multiple primary boreholes are used, each of the boreholes can include a low temperature working fluid line leading to the borehole and a high temperature working fluid line exiting the borehole. Thus, boreholes are not used in pairs as injection and production boreholes as in other processes. In the systems described herein, each of the multiple primary boreholes can include an in-hole heat mining device and other equipment described herein so that the primary boreholes can operate in parallel. This can increase production capacity and flexibility. The energy generated by each individual primary borehole can be controlled and adjusted to meet energy demands, for example. Additionally, the location of each borehole can be adjusted based on the available geothermal reservoir configuration, which is often not uniformly distributed within the formation.

[0079] FIG. 15 illustrates one exemplary geothermal mining system 100 including multiple primary boreholes 110. The boreholes are drilled into the geothermal reservoir 120 to a depth where the temperature is between about 500° C. and about 600° C. An in-hole heat mining device is used in each borehole to form multiple secondary fluid loops 140. Fluid can flow through the secondary fluid loops to collect heat from the geothermal reservoir and then transfer the heat to the high temperature working fluid lines in any of the above manners. The high temperature working fluid lines from all the primary boreholes can lead to a power generation station 102. The power generation station can generate electricity using thermal energy from the high temperature working fluid. The electricity can be transmitted using power lines 104 used in the power grid. Although these examples include about 30 secondary loops, any number of secondary loops may be used. As a general guideline, between 2 and 100 secondary loops can be used, and in some cases, between about 10 and about 50 secondary loops can be included in a single primary borehole.

[0080] FIG. 16 shows an enlarged view of a cross section of one of the primary boreholes 110. In this example, the initial narrow borehole 114 is enlarged to form the full diameter primary borehole 110. A heat exchanger 168 is installed within the borehole, with a low temperature working fluid line 132 and a high temperature working fluid line 134 connected to the cold side of the heat exchanger. Additional fluid lines are formed below the heat exchanger, including the looped secondary borehole 112. In this example, the geothermal formation is also fractured around the secondary borehole to increase the permeability of the rock body. An in-hole heat mining device 150 is also installed within the borehole. The device includes a valve installation unit 180 that can install valves 182 and pipe joints 184 to connect the secondary boreholes. This view also shows microcracks 186 in the rock formation directly adjacent to the secondary borehole. The device also includes a drilling unit 172 that deploys a secondary drilling head 170 to drill the looped secondary borehole. At the in-hole end of the device is a primary drilling head 190 that includes a low temperature fracturing fluid jet.

[0081] FIG. 17A shows a cross section of the narrow initial borehole looking towards the plane shown as dashed line "B" in FIG. 16. Several pipes extend down the initial borehole. These include a low temperature working fluid line 132 and a high temperature working fluid line 134. These pipes may be wrapped with a thermal insulation layer to prevent undesired heat transfer. A slurry line 136 is used to remove the slurry of crushed rock formed by the low temperature fracturing drilling. Data and power cables 179 are routed adjacent to the low temperature working fluid lines. A component transport elevator 188 is also used to provide a way to lower parts and / or materials down the borehole for self-assembly of the downhole thermal mining device. A perspective cross section of a segment of the narrow borehole 114 is shown in FIG. 17B. This view shows how the fluid lines, data cables, and component transport elevators are arranged in the narrow cross section of the borehole extending through the geothermal formation 120.

[0082] Figure 18 shows an expanded view of the portion indicated in box "C" in Figure 16. Cold fluid line 133 contains the fluid flowing down and hot fluid line 135 contains the fluid flowing up. A portion of the cold fluid is diverted by valve 182 to a pipe junction 184 which connects to looped secondary borehole 112. The secondary borehole passes through the geothermal formation 120 and connects back to the hot fluid line through another pipe junction.

[0083] FIG. 19 illustrates an exemplary method for drilling a primary borehole and generating geothermal energy (without enlarging the initial borehole). In phase 1, a primary borehole 110 is first drilled to a desired depth using any suitable drilling method. As explained above, the initial depth may be sufficient to reach a target temperature to begin extracting geothermal energy. In some examples, the initial borehole may be drilled to a depth where the temperature of the surrounding formation is about 500° C. or greater. In phase 2, the downhole heat mining device 150 is lowered to the primary borehole. If the initial drilling method is unable to achieve the desired depth, the device may continue drilling. In phase 3, the downhole heat mining device begins drilling a lateral secondary borehole to form a secondary fluid loop 140. In phase 4, the heat exchanger section 168 is removed from the downhole heat mining device and secured in place to serve as a connection between the primary loop and all heat mining activities taking place at greater depths. The remainder of the device is moved to greater depths by drilling the primary borehole using a primary drill head 190. The downhole heat mining device also continues to drill secondary wellbore to form additional secondary fluid loops and connects the secondary fluid loops to the low temperature and high temperature working fluid lines using valves. In stages 5 and 6, the downhole heat mining device continues to drill deeper while adding additional secondary fluid loops. The system can simultaneously generate geothermal energy by transferring heat from the secondary fluid loops to the primary fluid loop through a heat exchanger and using the heat to generate energy at the surface.

[0084] FIG. 20 illustrates an exemplary method of enlarging an initial borehole and incorporating a downhole heat mining device into the enlarged borehole. In phase 1, the initial primary start borehole may be drilled to a desired target temperature or to a maximum depth that the drilling method is capable of (if not the target depth). In phase 2, the downhole heat mining device with the primary borehole drilling unit is lowered into the start borehole. The possibility that this unit may continue drilling until the target depth is reached is not included in the figure. In phase 3, the drilling unit begins to enlarge the borehole diameter. In phase 4, the drilling unit has reached the desired diameter and is self-assembling using components lowered by the component transport elevator and / or mechanical expansion of units, sections, and components to continue drilling at the new diameter. In phase 5, the downhole heat mining unit begins self-assembly of additional units, which may proceed according to the method illustrated in the following figures.

[0085] FIG. 21A is a side view of another example showing a cross section of an initial narrow borehole 114 expanded to a wider primary borehole 110 with a downhole heat mining device 150 in the primary borehole. This example is similar to the example shown in FIG. 16. FIG. 21B is a cross section of the same example. As shown in these figures, a low temperature working fluid line 132 and a high temperature working fluid line 134 are connected to a heat exchanger 168 in the primary borehole. Additional low temperature fluid line 133 and a high temperature fluid line 135 are connected below the heat exchanger so that heat can be transferred from these lines through the heat exchanger to the working fluid line above the heat exchanger. The lateral secondary borehole 112 is connected to the low temperature and high temperature fluid lines by valves 182 and pipe joints 184. The valves and pipe joints are installed in the downhole heat mining device by a valve installation unit 180. The secondary borehole is drilled by a lateral drilling head 170 from a secondary drilling unit 172. The primary borehole is drilled by a primary drill head 190 .

[0086] FIG. 22A is a cross-sectional view of an exemplary heat exchanger 168 that may be placed in the primary wellbore. The heat exchanger may be connected to a low temperature working fluid line 132, a high temperature working fluid line 134, an additional low temperature fluid line 133, and an additional high temperature fluid line 135. The heat exchanger may include piping that allows for multiple uses of the low temperature downflow fluid, which may be cold water in some examples. In certain examples, cold water may be used to cool the shell heat exchanger, and / or the cold water may flow through the low temperature side of this heat exchanger to be heated and then flow back up the high temperature working fluid line, and / or the cold water may bypass the heat exchanger and flow down the additional low temperature fluid line. The zipper-shaped interface between the low temperature and high temperature sides of the heat exchanger represents a generalized complex shape that the heat exchanger may take to maximize heat transfer over short distances, but no particular shape is preferred. The shell of the heat exchanger may consist of coiled tubing that serves to cool the interior of this unit to ensure that the electronics operate and add to the heat extraction capabilities. When the unit is stationary, due to the lack of heat from the surrounding rock mass, this cooling does not significantly increase power generation capacity, but it may provide cooler temperature conditions that extend the life of the device.

[0087] 22B is a perspective cross-sectional view of the primary borehole 110 segment in which the heat exchanger 168 is located. The component transport elevator 188, data and power cables 179, and slurry lines 136 pass through the heat exchanger as shown. The low temperature working fluid line 132 and the high temperature working fluid line 134 are connected using piping that allows the working fluid to flow through the low temperature side of the heat exchanger and be isolated from the fluid on the high temperature side of the heat exchanger, or that allows the working fluid to partially or completely bypass the heat exchanger, as determined by a processor that optimizes the balance between heat extraction and resource duration.

[0088] FIG. 23A is a cross-sectional view of an exemplary valve installation unit 180 that may be part of a downhole thermal mining device. The entire unit may be cooled by fluid flowing through tubes or baffles, as described for the heat exchanger unit. In this example, a cold fluid line 133 and a hot fluid line 135 pass through the valve installation unit. The valve installation unit may connect these lines to the lateral secondary wellbore 112. The robots performing these tasks may operate in cooled vessels 137 and 139 that surround the fittings and valve locations. Cooling of these vessels may be achieved by circulating a cold fluid around the chambers and / or thermoelectric (Peltier) elements lining the chambers. A schematic of the robotic elements performing the assembly is shown. The pipes and valve components are manufactured at the surface and transported through a component elevator or 3D printed within the device.

[0089] FIG. 23B is a perspective cross-sectional view of the valve installation unit 180 positioned in the primary wellbore 110. In this view, the parts transport elevator 188, the slurry line 136, and the data / power line 179 can be seen passing through the valve installation unit. The cold fluid line 133 and the hot fluid line 135 also pass through, and the valve installation unit connects these lines to the lateral secondary wellbore. Two cooled vessels 137 and 139 where the robotic assembly takes place are shown surrounding the valve location. Upon completion and when the next lateral wellbore loop drilling is completed, the unit can be lowered to that location. The unit's shell can also have baffles or tubes to cool the inside of the unit, which can be configured to allow for the slots required to pass the horizontal pipes as the unit is lowered. Other embodiments not shown can also be used. EXAMPLES

[0090] As an illustration of these principles, a simple thermodynamic model of the two elements of the present invention is presented here, highlighting the steady fluid flow and time-dependent behavior of heat transfer from the rock conduit, through the device, and then to the surface. As shown in Figures 24A-24D, the constant thermal conductivity k in the surrounding rock mass is assumed. s( (which depends strongly on the crack density) and the solid-to-fluid heat transfer coefficient h at the interface of the conduit sf The constant mass velocity, together with the approximation of

[0091] Heat equation solution: Using the governing heat equation in cylindrical coordinates, a cylinder of rock mass of radius R is driven by a constant velocity v in the x direction. f and radius r c This paper describes the heat transfer of a solid to a fluid (water) flowing through a cylindrical borehole (conduit).

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[0092] With this conductive boundary condition at the fluid-solid interface,

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[0093] In a first exemplary calculation (FIG. 24A), in which no heat exchanger is present, the fluid entering the secondary loop conduit in the rock mass is at approximately the temperature T at which the fluid enters the primary fluid loop at the surface. f 0 This calculation assumes that the fluid leaves the secondary loop, enters the primary loop, and is transported isothermally to the surface at constant volume.

[0094] Heat exchanger solution: In an exemplary calculation with a heat exchanger (FIG. 24B), the fluid exiting the secondary loop flows directly into the heat exchanger "hot side" and the fluid exiting the hot side of the heat exchanger then flows back into the secondary loop in a closed flow loop. The fluid enters the conduit at a higher temperature than without the heat exchanger so that the rock body cools slower. The length and shape of the heat exchanger is optimized based on the mass flux of the hot and cold sides. It can also be of adaptable / variable length. In the "cold side", the fluid enters from the surface at a constant temperature, draws heat from the hot side, and exits back to the surface. The heat exchange is calculated by the following set of partial differential equations in one dimension for the hot and cold sides, respectively:

number

[0095] These equations are solved using the method of lines incorporating an ordinary differential equation solver. The conduit and heat exchanger equations are coupled together through the input and output temperature assignments as described above and shown in FIG. 24B.

[0096] These equations were used to calculate the performance of the thermal mining system using the parameters outlined in Table 1. Subscripts: s = solid, f = fluid, c refers to the properties of the conduit through the rock mass containing the heat extraction cells, he refers to the properties of the heat exchanger, and hs, cs refer to the hot and cold sides of the heat exchanger. [Table 1]

[0097] A comparison of the results in Figures 25A, 25B, and 25C shows that without a heat exchanger, the fluid entering the conduit always flows at T f 0 but the mass flux h sf , and for a given set of conduit dimensions, the fluid exits at a temperature close to the rock temperature. In the heat exchanger of FIG. 25B, the fluid exits at a temperature T equal to the temperature of the fluid exiting the hot side of the heat exchanger. f 0The heat energy enters the conduit at a much higher temperature than in the case without the heat exchanger, meaning that the thermal energy is extracted at a slower rate than in the case without the heat exchanger. In Figure 25C, it can be seen that the output temperature as a function of time for both cases rises rapidly and then drops rapidly to a level where it declines very slowly, a few degrees per week, by day 30. Thus, the downhole heat exchanger results in a lower output temperature, but a longer reservoir duration.

[0098] Energy extraction and conversion can be determined as the thermal energy extracted by the device per unit time,

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[0099] As shown in FIG. 25D, efficiency is a strongly nonlinear function of recovered fluid temperature that is estimated to reflect various turbine systems optimized for different conditions. The large step upwards at about 350° C. (and 25 MPa) reflects the production of supercritical fluid rather than steam or liquid water. The two points in FIG. 25D show that the example without the heat exchanger reaches high efficiency conditions. A jump in efficiency is one goal of this system, which can be optimized by controlling the fluid pressure in the primary and secondary loops to keep the fluid supercritical as it leaves the unit at the surface.

[0100] As mentioned above, variations of the device are designed to increase energy extraction over time by extracting heat from existing secondary loops while adding secondary loops as they drill to greater depths and pausing generation from those that are subcooled to allow for heat recharge. In the primary, electrical energy is simply generated by the model output P of one conduit. e , the number of effective conduits N as shown in FIG. 25E for the conditions shown in the two examples. c It can be estimated by multiplying

[0101] These two examples, and N c Based on simple scaling estimates, the achievement of supercritical conditions would result in much higher energy output at a scale sufficient for production. This calculation is not intended to be interpreted as an economic model, which may include other factors.

[0102] A comparison of the two examples can reinforce the value of the example shown in Figures 21A, 21B, which combines the two capabilities, and the heat exchanger that can be bypassed allows for optimization of the balance between heat extraction rate and life. This configuration can be advantageous where the output fluid can be maintained in a supercritical state, but utilizes the heat exchanger path to maximize the duration of production of the existing secondary loop. Many aspects of the device and system can be controlled, including the fracture density in the rock conduit, which determines the physics of heat transfer. This can therefore be achieved by reducing the amount of CO in the atmosphere. 2 It is a system that can mine heat in a reliable and sustainable manner, with low economic and seismic risks, that could potentially help meet human energy needs without increasing thermal levels.

[0103] The described features, structures, or characteristics may be combined in any suitable manner in one or more examples. In the foregoing description, numerous specific details, such as examples of various configurations, have been provided to provide a thorough understanding of examples of the described technology. However, one of ordinary skill in the art will recognize that the technology can be practiced without one or more of the specific details, or with other methods, components, devices, etc. In other examples, well-known structures or operations have not been shown or described in detail to avoid obscuring aspects of the technology.

[0104] The above detailed description describes the invention with reference to certain exemplary embodiments. However, it will be understood that various modifications and changes can be made without departing from the scope of the invention as set forth in the appended claims. The detailed description and accompanying drawings should be regarded as merely illustrative and not restrictive, and all such modifications or changes, if any, are intended to be included within the scope of the invention as described and illustrated herein.

Claims

1. 1. A geothermal mining system comprising: a single primary borehole in a geothermal reservoir; a primary fluid loop including a low temperature working fluid line leading to the primary wellbore and a high temperature working fluid line exiting the primary wellbore; a secondary fluid loop located below the primary borehole, the secondary fluid loop being in thermal contact with the geothermal reservoir and adapted to circulate fluid separated from the primary fluid loop; a downhole heat mining device for controlling a rate of heat transfer from the secondary fluid loop to the primary fluid loop by selectively and independently controlling fluid flow through the primary fluid loop and fluid flow through the secondary fluid loop, the downhole heat mining device comprising a heat exchanger having the primary fluid loop connected to a low temperature side and the secondary fluid loop connected to a high temperature side; A geothermal extraction system comprising:

2. 10. The system of claim 1, wherein the heat mining device further comprises a pump in the secondary fluid loop for controlling fluid flow through the secondary fluid loop.

3. The system of claim 2 , wherein the secondary fluid loop flows within a lateral secondary borehole formed in the geothermal reservoir that extends from the primary borehole.

4. 4. The system of claim 3, wherein the system comprises a plurality of secondary fluid loops flowing within a plurality of lateral secondary wellbore holes, and the downhole heat mining device selectively controls fluid flow through the plurality of secondary fluid loops.

5. The system of claim 3 , wherein the secondary fluid loop further comprises a micro-fracture network connecting at least a portion of the lateral secondary wellbore holes, or tertiary wellbore holes extending from the secondary wellbore holes, or a combination of the above.

6. 4. The system of claim 3, wherein the thermal mining device further comprises a lateral drilling head directed outwardly from the thermal mining device for drilling the secondary borehole in a cross direction relative to the primary borehole.

7. 2. The system of claim 1, wherein the secondary fluid loop includes a lateral secondary borehole formed in the geothermal reservoir, and the heat mining device includes a valve fluidly connecting the secondary fluid loop to the primary fluid loop.

8. 8. The system of claim 7, wherein the thermal mining device further comprises a lateral drilling head directed outwardly from the thermal mining device for drilling the secondary borehole in a cross direction relative to the primary borehole.

9. the heat mining device comprises a processor integrated with the heat mining device for generating control signals for selectively controlling the fluid flow, and includes a thermoelectric cooling device for maintaining the processor at a safe operating temperature; or the thermal mining device controls the fluid flow based on a control signal received from a processor external to the primary borehole; or A combination of the above, The system of claim 1 .

10. The system of claim 9 , wherein the processor is in communication with a sensor.

11. The system of claim 10 , wherein the sensor is a pressure sensor, a temperature sensor, a position sensor, or a combination of the above.

12. 2. The system of claim 1, wherein the heat mining device comprises a drill head at an in-hole end of the heat mining device, the heat mining device adapted to extend the depth of the primary borehole using the drill head while simultaneously transferring heat from the secondary fluid loop to the primary fluid loop.

13. The system of claim 12 , wherein the drill head comprises a rotary drill head, a percussion drill head, a hot fracturing drill head, a cold fracturing drill head, or a combination thereof.

14. The system of claim 1 , wherein the cold working fluid line and the hot working fluid line are not concentric, and the cold working fluid line and the hot working fluid line are insulated.

15. 1. A downhole geothermal mining device, comprising: a device body shaped to descend into a primary borehole; a cryogenic working fluid inlet connectable to a cryogenic working fluid line leading down to the primary borehole; a high temperature working fluid outlet connectable to a high temperature working fluid line leading up to the primary borehole, the low temperature working fluid line and the high temperature working fluid line forming a primary fluid loop; a controller adapted to control a rate of heat transfer from a secondary fluid loop to the primary fluid loop, the secondary fluid loop being disposed down the primary borehole and adapted to circulate a fluid separated from the primary fluid loop, the secondary fluid loop being in thermal contact with a geothermal reservoir, the controller controlling the rate of heat transfer by selectively and independently controlling fluid flow through the primary fluid loop and fluid flow through the secondary fluid loop; a heat exchanger adapted to connect to the primary fluid loop on a low temperature side and adapted to connect to the secondary fluid loop on a high temperature side, the device further comprising a pump adapted to control the fluid flow through the secondary fluid loop.

16. 16. The device of claim 15, further comprising a lateral drilling head directed outwardly from the device for drilling a secondary borehole in a direction transverse to the primary borehole, the secondary fluid loop including the secondary borehole.

17. 17. The device of claim 16, further comprising a micro-damage head adapted to form micro-cracks fluidly connected along the secondary borehole, the secondary fluid loop including the micro-cracks.

18. The device, a lateral drilling head directed outwardly from the device for drilling a secondary borehole transversely to the primary borehole; a valve installation unit adapted to install a valve connecting the secondary wellbore to the low temperature working fluid line or the high temperature working fluid line, the secondary fluid loop including the secondary wellbore; and The device of claim 15 further comprising:

19. 20. The device of claim 18, wherein the valve installation unit is adapted to receive a valve down the primary borehole and connect the valve to the secondary fluid loop and the low temperature working fluid line or the high temperature working fluid line.

20. 20. The device of claim 18, wherein the valve installation unit comprises an additive manufacturing device for fabricating a valve downhole and connecting the valve to the secondary fluid loop and the low temperature working fluid line or the high temperature working fluid line.

21. 16. The device of claim 15, further comprising a drill head at an in-hole end of the device, the drill head comprising a rotary drill head, a percussion drill head, a hot-fracture drill, a cold-fracture drill head, or a combination thereof.

22. 22. The device of claim 21, wherein the drill head comprises a cryo-fracture drill head connected to the cryo-fracture working fluid inlet to expel cryo-fracture working fluid from the cryo-fracture drill head.

23. 1. A method for geothermal mining, comprising: forming a single primary borehole; placing a geothermal mining device in the primary borehole; forming a primary fluid loop including a low temperature working fluid line leading to the primary wellbore and a high temperature working fluid line exiting the primary wellbore; forming a secondary fluid loop located below the primary borehole, the secondary fluid loop being in thermal contact with a geothermal reservoir and adapted to circulate fluid separated from the primary fluid loop; transferring heat from the secondary fluid loop through the primary fluid loop to an energy storage or production unit outside the primary wellbore; controlling a rate of heat transfer from the secondary fluid loop to the primary fluid loop by selectively and independently controlling fluid flow through the primary fluid loop and fluid flow through the secondary fluid loop; wherein the heat mining device comprises a heat exchanger having the primary fluid loop connected to a low temperature side and the secondary fluid loop connected to a high temperature side.

24. 24. The method of claim 23, wherein the thermal mining device comprises a lateral drilling head directed outwardly from the thermal mining device, and forming the secondary loop comprises using the lateral drilling head to drill a lateral secondary borehole extending from the primary borehole.

25. 25. The method of claim 24, wherein the thermal mining device further comprises a cryo-fracturing drill head at a distal end of the thermal mining device, the method further comprising using the cryo-fracturing drill head to extend the depth of the primary wellbore or extend the length of the lateral secondary wellbore while simultaneously performing the heat transferring step.

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