Subterranean energy storage system

Abstract

A plurality of operational cycles are performed on a well. Each operational cycle includes an injection phase followed by a production phase. Each injection phase includes injecting a fluid into a fracture in a subterranean formation via the first well. Each production phase includes producing the fluid from the fracture, and generating electricity using the fluid. An amount of the fluid produced from the fracture during each production phase is less than an amount of the fluid remaining in the fracture at an end of each production phase.

Description

SUBTERRANEAN ENERGY STORAGE SYSTEMCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is an International Patent Application under the Patent Cooperation Treaty and claims priority to and the benefit of U.S. Provisional Patent Application Serial No. 63 / 620,604 filed January 12, 2024, titled Improved Subterranean Energy Storage System, the disclosure of which is incorporated herein in its entirety by this reference.BACKGROUNDField

[0002] Embodiments of the present disclosure generally relate to geothermal power systems and processes, and particularly relates to the recovery of geothermal energy to perform useful work, such as generating electricity.Description of the Related Art

[0003] Geothermal energy is a type of renewable energy generated within the earth. A geothermal fluid (such as water, steam, brine, or hydrocarbons) is heated in a subterranean geological formation by the earth’s natural internal temperature. The heated geothermal fluid is produced to the earth’s surface. The enthalpy of the geothermal fluid includes a heat energy component and a pressure-volume energy component. Typically, the heat energy component is greater than the pressure-volume energy component. At the earth’s surface, the heat energy component is used to perform useful work, such as heating buildings or generating electricity in a geothermal power system. However, the pressure-volume energy component is usually wasted, such as by venting. After performing useful work, the geothermal fluid is reinjected into the subterranean formation, reheated by the subterranean formation, then produced again to the earth’s surface to perform useful work.

[0004] Some geothermal power systems generate electricity by using the geothermal fluid to drive a steam turbine. However, where the producedgeothermal fluid is at or below about 180 degrees C, geothermal power systems typically incorporate a binary cycle power plant to generate electricity. A heat exchanger of the binary cycle power plant transfers heat, but not pressure, from the geothermal fluid to an operating fluid of the binary cycle power plant.

[0005] In an example, operation of the binary cycle power plant is based on the Brayton Cycle, in which the heated operating fluid passes through a turbine, which drives a generator. In another example, operation of the binary cycle power plant is based on the Organic Rankine Cycle, in which the heated operating fluid passes through an expander, which drives a generator. Typically, binary cycle power plants utilize only the heat energy component of the enthalpy of the geothermal fluid. The pressure-volume energy component is wasted, such as by venting.

[0006] The efficiency of a geothermal power system depends on the amount of energy (in the form of heat energy and pressure-volume energy) that can be transferred from the subterranean geological formation to the geothermal fluid, and depends on the proportion of that energy that is converted into useful work. The efficiency of a geothermal power system can be adversely affected by the time taken to heat a relatively cold geothermal fluid that is returned to subterranean geological formation. Typically, the efficiency of converting geothermal energy (in the form of heat energy plus pressure-volume energy) into electricity is less than 15 percent.

[0007] Thus, there is a need for improved systems and processes that facilitate the conversion of geothermal energy into electricity.SUMMARY

[0008] The present disclosure generally relates to geothermal power systems and processes, and particularly relates to the recovery of geothermal energy to perform useful work, such as generating electricity. In one implementation, a method includes performing a plurality of operational cycles on a first well. Each operational cycle including an injection phase followed bya production phase. Each injection phase includes injecting a fluid into a first fracture in a subterranean formation via the first well. Each production phase includes producing the fluid from the first fracture, and generating electricity using the fluid. An amount of the fluid produced from the first fracture during each production phase is less than an amount of the fluid remaining in the first fracture at an end of each production phase.

[0009] In another implementation, a method includes producing a fluid from a first fracture in a subterranean formation via a first well. An initial total amount of the fluid in the first fracture equals an amount of a first portion of the fluid plus an amount of a second portion of the fluid. The method includes generating electricity a first time using the fluid. The method includes leaving the second portion of the fluid remaining in the first fracture after producing the first portion of the fluid from the first fracture. The amount of the second portion of the fluid is greater than the amount of the first portion of the fluid. The method includes injecting the first portion of the fluid into a second fracture in the subterranean formation via a second well. At a start of the injecting, the second fracture contains a total amount of the fluid that is greater than the amount of the first portion of the fluid. The method includes producing a third portion of the fluid from the second fracture via the second well, and generating electricity a second time using the third portion of the fluid. The method includes injecting the third portion of the fluid into the first fracture via the first well.

[0010] In another implementation, a method includes producing a fluid a first time from a fracture in a subterranean formation via a well. An initial total amount of the fluid in the fracture equals an amount of a first portion of the fluid plus an amount of a second portion of the fluid. The method includes generating electricity a first time using the fluid. The method includes leaving the second portion of the fluid remaining in the fracture after producing the first portion of the fluid from the fracture. The amount of the second portion of the fluid is greater than the amount of the first portion of the fluid. The method includes injecting the first portion of the fluid back into the fracture via the well. The method includes producing the fluid a second time from the fracture via thewell. The method includes generating electricity a second time using the fluid. The method includes leaving a fourth portion of the fluid remaining in the fracture after producing a third portion of the fluid from the fracture. An amount of the fourth portion of the fluid is greater than an amount of the third portion of the fluid.BRIEF DESCRIPTION OF THE DRAWINGS

[0011] So that the manner in which the above recited features of the present disclosure can be understood in detail, a more particular description of the disclosure, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only exemplary embodiments and are therefore not to be considered limiting of the scope of the disclosure, as the disclosure may admit to other equally effective embodiments.

[0012] Figure 1A schematically illustrates a geothermal power system.

[0013] Figure 1 B schematically illustrates a manifold assembly that may be incorporated into the geothermal power system of Figure 1A.

[0014] Figure 1 C schematically illustrates another manifold assembly that may be incorporated into the geothermal power system of Figure 1A.

[0015] Figure 2 depicts a bar graph that illustrates two operating scenarios for wells, such as the wells depicted in Figure 1A.

[0016] Figure 3 depicts a graph that schematically illustrates aspects of production phases under the two operating scenarios of Figure 2.

[0017] Figure 4 schematically illustrates fluid dynamics during an injection phase under one scenario of the two operating scenarios of Figure 2.

[0018] Figure 5 schematically illustrates fluid dynamics during an injection phase under another scenario of the two operating scenarios of Figure 2.

[0019] Figure 6 depicts a composite of two graphs that schematically illustrates injection and production phases under one scenario of the two operating scenarios of Figure 2.

[0020] Figure 7 depicts a composite of two graphs that schematically illustrates injection and production phases under another scenario of the two operating scenarios of Figure 2.

[0021] Figure 8 depicts a graph that schematically illustrates the power generated by operating wells according to some embodiments.

[0022] Figure 9 depicts a graph that schematically illustrates the power generated by operating wells according to some embodiments.

[0023] Figure 10 is a flow chart of a method of operating a power generating system.

[0024] Figure 11 is a flow chart of a method of operating a power generating system.

[0025] Figure 12 is a flow chart of a method of operating a power generating system.

[0026] To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements and features of one embodiment may be beneficially incorporated in other embodiments without further recitation.DETAILED DESCRIPTION

[0027] The present disclosure concerns geothermal power systems and processes, and particularly relates to the recovery of geothermal energy to perform useful work, such as generating electricity.

[0028] Figure 1A schematically illustrates a geothermal power system 100. The geothermal power system 100 includes a power generation unit 20 locatedat the Earth’s surface 40. The power generation unit 20 may be of any suitable type that converts heat energy and / or pressure energy to electricity, and may include any one or more of an expansion unit 22, a turbine / expander 24, a generator 26, or a cooling unit 28. In some examples, the expansion unit 22 is omitted. In some examples, the cooling unit 28 is omitted.

[0029] Some examples of representative power generation units 20 include a direct dry steam plant, a flash plant, a binary plant, a combined-cycle or hybrid plant, etc., that receives heated fluid from surface or subsurface sources of heat. In some examples, the turbine / expander 24 drives the generator 26 to produce electricity. In some embodiments, the generator 26 produces DC electrical power. In some embodiments, the generator 26 produces AC electrical power. The generated electricity is routed to an electricity distribution system 30, such as a power grid.

[0030] The geothermal power system 100 utilizes a geothermal fluid (represented by arrows 12), such as water, steam, brine, a refrigerant, a supercritical fluid, carbon dioxide, ammonia, an organic compound (e.g., a hydrocarbon, a fluorocarbon, etc.), or any combination thereof. In some embodiments, the geothermal fluid 12 is heated, or is maintained at an elevated temperature, in a subterranean formation 42. In some examples, the temperature of the geothermal fluid 12 is at or about 150 degrees C or higher, such as 175 degrees C or higher, 200 degrees C or higher, 250 degrees C or higher, or 300 degrees C or higher.

[0031] In some embodiments, the geothermal fluid 12 is maintained at an elevated pressure in the subterranean formation 42. In some examples, the pressure of the geothermal fluid 12 in the subterranean formation 42 is at or about 3 MPa or higher, such as 5 MPa or higher, 10 MPa or higher, 20 MPa or higher, 30 MPa or higher, 40 MPa or higher, or 50 MPa or higher. In some embodiments, the geothermal fluid 12 is geopressured. In some examples, the geothermal fluid 12 may be a geopressured-geothermal fluid.

[0032] The geothermal fluid 12 flows from the subterranean formation 42 into a first well 44. As illustrated, the geothermal fluid 12 flows from the subterranean formation 42 into the first well 44 via one or more fractures 62 in the subterranean formation 42 at the first well 44. The first well 44 may be a geothermal well. The geothermal fluid 12 flows from the first well 44 to the power generation unit 20. In some embodiments, the temperature of the geothermal fluid 12 at a wellhead 45 of the first well 44 is at or near the temperature of the geothermal fluid 12 in the subterranean formation 42.

[0033] The geothermal fluid 12 is utilized by the geothermal power system 100 to produce electricity. In some embodiments, the geothermal fluid 12 powers the turbine / expander 24 which drives the generator 26. In some embodiments, the power generation unit 20 includes a binary plant that operates according to the Organic Rankine Cycle or the Brayton Cycle. In such embodiments, the power generation unit 20 transfers heat energy and / or pressure energy of the geothermal fluid 12 to a second fluid, and the second fluid powers the turbine / expander 24 which drives the generator 26 to generate electricity.

[0034] As illustrated, in some embodiments, the geothermal fluid 12 is conveyed from the power generation unit 20 to a second well 46. In some examples, one or more pumps of the geothermal power system 100 increase a pressure of the geothermal fluid 12, and inject the geothermal fluid 12 into the second well 46. The geothermal fluid 12 flows through the second well 46, and enters one or more fractures 64 in the subterranean formation 42. In some embodiments, the geothermal fluid 12 is returned to the subterranean formation 42 via the first well 44.

[0035] In some embodiments, the pressure of the geothermal fluid 12 at the wellhead 45 of the first well 44 is maintained at a magnitude such that the one or more fractures 62 remain open while flowing the geothermal fluid 12 to the power generation unit 20. In an example, the power generation unit 20 is operated such that a back-pressure is exerted on the geothermal fluid 12 at the wellhead 45 of the first well 44. In some embodiments, the pressure of thegeothermal fluid 12 at a wellhead 47 of the second well 46 is maintained at a magnitude such that the one or more fractures 64 remain open.

[0036] As illustrated, in some embodiments, the geothermal fluid 12 does not flow within the subterranean formation 42 from the second well 46 to the first well 44. In some examples, the one or more fractures 62 do not intersect with the one or more fractures 64. In some examples, the one or more fractures 62 are not contiguous with the one or more fractures 64.

[0037] In some embodiments, after flowing the geothermal fluid 12 from the first well 44, the geothermal power system 100 is reconfigured to flow the geothermal fluid 12 from the second well 46 to the power generation unit 20, and to flow the returning geothermal fluid 12 from the power generation unit 20 to the first well 44. In some examples, the geothermal power system 100 includes a manifold assembly, such as described below.

[0038] Figure 1 B schematically illustrates an example manifold assembly 150 that facilitates reconfiguring the geothermal power system 100 to flow the geothermal fluid 12 from the second well 46 to the power generation unit 20, and to flow the returning geothermal fluid 12 from the power generation unit 20 to the first well 44. In some embodiments, geothermal power system 100 incorporates the manifold assembly 150.

[0039] The manifold assembly 150 includes a production manifold 160 and an injection manifold 170. Line 162 conveys the geothermal fluid 12 from the production manifold 160 to the power generation unit 20. Line 172 conveys the geothermal fluid 12 from the power generation unit 20 to the injection manifold 170. Line 164 conveys the geothermal fluid 12 from the first well 44 to the production manifold 160. Line 174 conveys the geothermal fluid 12 from the injection manifold 170 to the first well 44. When geothermal fluid 12 is flowing from the first well 44 to the production manifold 160, valve 182 in line 164 is open, and valve 184 in line 174 is closed. When geothermal fluid 12 is flowing from the injection manifold 170 to the first well 44, valve 184 in line 174 is open, and valve 182 in line 164 is closed.

[0040] Line 166 conveys the geothermal fluid 12 from the second well 46 to the production manifold 160. Line 176 conveys the geothermal fluid 12 from the injection manifold 170 to the second well 46. When geothermal fluid 12 is flowing from the second well 46 to the production manifold 160, valve 186 in line 166 is open, and valve 188 in line 176 is closed. When geothermal fluid 12 is flowing from the injection manifold 170 to the second well 46, valve 188 in line 176 is open, and valve 186 in line 166 is closed.

[0041] In some embodiments, reconfiguring the geothermal power system 100 is prompted by a trigger condition. In an example, the trigger condition includes the flow rate of geothermal fluid 12 from the first well 44 to the power generation unit 20 diminishing to or beyond a threshold level. In another example, the trigger condition includes the pressure of geothermal fluid 12 at the wellhead 45 of the first well 44 diminishing to or beyond a threshold level. In another example, the trigger condition includes the flow rate of geothermal fluid 12 into the second well 46 diminishing to or beyond a threshold level. In another example, the trigger condition includes the pressure of geothermal fluid 12 at the wellhead 47 of the second well 46 rising to or beyond a threshold level.

[0042] When reconfiguring the geothermal power system 100 to flow the geothermal fluid 12 from the second well 46 to the geothermal power system 100, the first well 44 is closed-in, such as by closing the valve 52. In some embodiments, the second well 46 is closed-in, such as by closing valve 54. The manifold assembly 150 is operated such that valve 182 is closed, valve 184 is opened, valve 186 is opened, and valve 188 is closed. Valve 52 is opened to allow geothermal fluid 12 to be injected into the first well 44. If the second well 46 is closed-in, valve 54 is opened to allow geothermal fluid 12 to be produced from the second well 46. The geothermal fluid 12 flows from the subterranean formation 42 via the second well 46 to the power generation unit 20, as described above. The returning geothermal fluid 12 flows into the subterranean formation 42 via the first well 44, as described above.

[0043] In some embodiments, more than two wells are coupled to the production manifold 160 and to the injection manifold 170. In an example, production of geothermal fluid 12 is started from a third well while the second well 46 remains closed-in. The geothermal fluid 12 flows to the geothermal power system 100 as described above. The geothermal fluid 12 may then be injected into the first well 44 or into a fourth well.

[0044] In some embodiments, the first well 44 and the geothermal power system 100 are operated in a repeating alternating sequence of production of the geothermal fluid 12 from a specific location in the subterranean formation 42, then reinjection of the geothermal fluid 12 into the subterranean formation 42 at the same specific location. Such a sequence may be referred to as “huff and puff.” In an example, the first well 44 may be coupled to the power generation unit 20 via another manifold assembly 190, as schematically illustrated in Figure 1 C.

[0045] In the manifold assembly 190 of Figure 1 C, line 192 conveys the geothermal fluid 12 from the first well 44 to the power generation unit 20. Line 194 conveys the geothermal fluid 12 from the power generation unit 20 back to the first well 44. When geothermal fluid 12 is flowing from the first well 44 to the power generation unit 20, valve 196 in line 192 is open, and valve 198 in line 194 is closed. When geothermal fluid 12 is flowing from the power generation unit 20 to the first well 44, valve 198 in line 194 is open, and valve 196 in line 192 is closed.

[0046] The geothermal fluid 12 is produced from the subterranean formation 42 via the first well 44, and flows to the power generation unit 20 through line 192. The geothermal fluid 12 is utilized (as described above) in the geothermal power system 100, and then is stored in a reservoir, such as a tank, a pond, or a subterranean formation different from the subterranean formation 42. Then the flow of the geothermal fluid 12 from the first well 44 is ceased. Valve 196 of the manifold assembly 190 is closed, and valve 198 of the manifold assembly 190 is opened. Then one or more pumps convey the geothermal fluid 12 from the reservoir back into the first well 44, and inject the geothermal fluid 12 intothe first well 44, and into the subterranean formation 42 at the one or more fractures 62. Valve 198 of the manifold assembly 190 is closed, and valve 196 of the manifold assembly 190 is opened. Then the sequence is repeated.

[0047] In some embodiments, the one or more fractures 62, 64 (Figure 1A) provide self-contained storage of the geothermal fluid 12 such that the geothermal fluid 12 is hindered from leaking off into pores of the subterranean formation 42. In some examples, the subterranean formation 42 is substantially impermeable, such as a so-called “Hot Dry Rock” such as granite. In some examples, the one or more fractures 62, 64 are created by fluids containing particulates that plug pores of the subterranean formation 42 that are exposed to the created fractures.

[0048] During operation of the geothermal power system 100, the geothermal fluid 12 is flowed in a repeating cycle of an injection phase followed by a production phase. In the injection phase, the geothermal fluid 12 is injected into the one or more fractures 62, 64 to be heated by the geothermal heat of the subterranean formation 42. In the production phase, the heated geothermal fluid 12 is produced from the one or more fractures 62, 64. It has been found that the effectiveness of heating the geothermal fluid 12 in the subterranean formation 42 is at least in part dependent upon a residence time of the geothermal fluid 12 in the one or more fractures 62, 64.

[0049] Figure 2 depicts a bar graph 200 that illustrates operating scenarios for wells, such as the first well 44 or the second well 46. A volume of geothermal fluid 12 present in a fracture (such as the one or more fractures 62, 64) is measured by the Y axis. The Figure illustrates comparative fluid volumes under two scenarios, Scenario 1 and Scenario 2.

[0050] Scenario 1 represents an operation in which a majority of the geothermal fluid 12 contained in the fracture is produced out of the fracture during the production phase. The bar 212 at 1 A represents a residual volume of the geothermal fluid 12 contained in the fracture at the end of the production phase, and prior to the injection phase. The amount 232 of the geothermal fluid12 remaining in the fracture is equal to the residual volume 212 of the geothermal fluid 12. The bar 214 at 1 B represents a working volume of the geothermal fluid 12 contained in the fracture at the end of the injection phase that is to be produced during the subsequent production phase. The amount 234 of the geothermal fluid 12 in the fracture at the end of the injection phase and prior to commencing the production phase is equal to the residual volume 212 plus the working volume 214 of the geothermal fluid 12.

[0051] In Scenario 1 , the size of the working volume 214 is larger than the size of the residual volume 212. In some examples, the size of the working volume 214 is five, six, seven, eight, nine, ten, or more times the size of the residual volume 212. In some examples, the size of the working volume 214 is from 300 to 500 m3, and the size of the residual volume 212 is from 30 to 100 m3.

[0052] Scenario 2 represents an operation in which a minority of the geothermal fluid 12 contained in the fracture is produced out of the fracture during the production phase. The break lines across the bar graph 200 allow the bars at 2A and 2B to be plotted according to the same scale as the bars at 1 A and 1 B. The bar 222 at 2A represents a residual volume of the geothermal fluid 12 contained in the fracture at the end of the production phase, and prior to the injection phase. The amount 236 of the geothermal fluid 12 remaining in the fracture is equal to the residual volume 222 of the geothermal fluid 12.

[0053] The bar 224 at 2B represents a working volume of the geothermal fluid 12 contained in the fracture at the end of the injection phase that is to be produced during the subsequent production phase. The amount 238 of the geothermal fluid 12 in the fracture at the end of the injection phase and prior to commencing the production phase is equal to the residual volume 222 plus the working volume 224 of the geothermal fluid 12.

[0054] In the illustrated examples, the size of the working volume 224 in Scenario 2 is the same as the size of the working volume 214 in Scenario 1 . In Scenario 2, the size of the residual volume 222 is larger than the size of theworking volume 224. In some examples, the size of the residual volume 222 is two, three, four, five, six, seven, eight, nine, ten, or more times the size of the working volume 224. In some examples, the size of the working volume 224 is from 300 to 600 m3, and the size of the residual volume 222 is from 600 to 6000 m3

[0055] Figure 3 depicts a graph 300 that schematically illustrates aspects of the production phases under Scenarios 1 and 2. The Y axis represents a temperature of the geothermal fluid 12 at the wellhead (such as wellhead 45, 47) during the production phase. The X axis represents a duration in which a well (such as first well 44 or second well 46) is shut-in between the end of the injection phase and the start of the production phase. In other words, the X axis represents the length of a delay between the end of the injection phase and the start of the production phase.

[0056] The temperature of the geothermal fluid 12 produced by the well under Scenario 1 is represented by line 310. Point 312 represents the temperature of the geothermal fluid 12 in the case that there is no delay between finishing the injection phase and starting the production phase. The temperature at point 312 is lower than a maximum temperature 302 because the cool fluid of the working volume (214, Figure 2) in the fracture (such as fracture 62, 64) has not yet been heated by the subterranean formation 42.

[0057] The longer the well is kept shut-in before commencing the production phase, the higher the temperature (up to the maximum temperature) of the geothermal fluid 12 when the production phase begins. In some embodiments, the power generation unit 20 is operated when the temperature of the geothermal fluid 12 is at least at a first threshold 304. The well is kept shut-in for a required shut-in period 314 while the subterranean formation 42 heats the geothermal fluid 12 in the fracture until the temperature of the geothermal fluid 12 reaches at least the first threshold 304. In some examples determined through testing and modeling, the shut-in period 314 is several hours, such as 12 hours, 18 hours, 24 hours, or more.

[0058] In some embodiments, the well is shut-in until the temperature of the geothermal fluid 12 reaches a second threshold 306 that is lower than the first threshold 304. The graph 300 shows that the well would remain shut-in for a required shut-in period 316 (that is less than the shut-in period 314) before the temperature of the geothermal fluid 12 reaches at least the second threshold 306. In some examples determined through testing and modeling, the shut-in period 316 is 6 hours, 12 hours, 18 hours, or more.

[0059] Although the shut-in period 316 is shorter than the shut-in period 314, operating the power generation unit 20 with the geothermal fluid 12 at the second threshold 306 is less effective than operating the power generation unit 20 with the geothermal fluid 12 at the first threshold 304. In some examples, the power generation unit 20 produces less electrical power. In some examples, the power generation unit 20 produces electrical power for a shorter duration of time.

[0060] The temperature of the geothermal fluid 12 produced by the well under Scenario 2 is represented by line 320. Point 322 represents the temperature of the geothermal fluid 12 in the case that there is no delay between finishing the injection phase and starting the production phase. The temperature at point 320 is greater than the temperature at point 312 for the comparative example of Scenario 1 .

[0061] In embodiments in which the well is shut-in until the temperature of the geothermal fluid 12 reaches the first threshold 304, the graph 300 shows that the required shut-in period 324 is significantly shorter than the required shut-in period 314 of the comparative example of Scenario 1. In some examples determined through testing and modeling, the required shut-in period 324 is less than 2 hours, such as 1 .5 hours or less, 1 hour or less, or 0.5 hours or less. In embodiments in which the well put into production when the temperature of the geothermal fluid 12 reaches the second threshold 306, the graph 300 shows that the well could be opened to commence the production phase immediately after the injection phase has ended.

[0062] It has been determined through testing and modeling, that the greater the ratio of the size of the residual volume (222, Figure 2) to the size of the working volume (224, Figure 2), the greater the temperature at point 322 (i.e. cases in which the production phase is commenced immediately after the injection phase has ended). Indeed, in some embodiments (such as in cases in which the size of the residual volume 222 is at least two times the size of the working volume 224), the temperature at point 322 may be substantially equal to the first threshold 304, such as within 20 degrees C, within 15 degrees C, within 10 degrees C, or within 5 degrees C of the first threshold 304. Furthermore, in some embodiments (such as in cases in which the size of the residual volume 222 is at least five times the size of the working volume 222), the temperature at point 322 may be substantially equal to the maximum temperature 302, such as within 20 degrees C, within 15 degrees C, within 10 degrees C, or within 5 degrees C of the maximum temperature 302.

[0063] Consequently, the greater the ratio of the size of the residual volume 222 to the size of the working volume 224, the shorter the required shut-in period 324 before commencing the production phase. In some embodiments (such as in cases in which the size of the residual volume 222 is at least five times the size of the working volume 224), the required shut-in period 324 may be reduced to zero.

[0064] The results described above and illustrated for Scenario 2 are surprising given that the injection phase involves pumping relatively cold geothermal fluid 12 into the fracture, and therefore it would take time for the relatively cold geothermal fluid 12 to be heated by the subterranean formation 42. The expectation would be that the temperature of the geothermal fluid 12 at point 324 would be similar to the temperature of the geothermal fluid 12 at point 314. Furthermore, the expectation would be that the line 320 would overlay, or at least be close to, the line 310.

[0065] Without being bound by theory, it is contemplated that the fluid dynamics in the fracture of Scenario 1 during the injection phase are significantly different from the fluid dynam ics in the fracture of Scenario 2 duringthe injection phase. Comparative fluid dynamics in the injection phase are schematically illustrated in Figures 4 and 5.

[0066] Figure 4 schematically illustrates fluid dynamics during the injection phase under Scenario 1 . Well 70 represents the first well 44 and / or the second well 46. Well 70 penetrates into the subterranean formation 42, and is flu idical ly coupled to the subterranean formation 42 by fracture 72. Geological stresses, such as the overburden, act to close the fracture 72. Removal of the working volume (214, Figure 2) of the geothermal fluid 12 during the production phase results in the fracture 72 closing and reducing in size. At the end of the production phase, the volume of the fracture 72 that remains open is equal to the size of the residual volume (212, Figure 2) of the geothermal fluid 12.

[0067] The flow of the working volume 214 of the geothermal fluid 12 during the injection phase is represented by arrows 402. As described above, the working volume 214 is larger than the residual volume 212, and larger than the volume of the fracture 72 that remained open at the end of the production phase. The injection of the working volume 214 into the fracture 72 causes the fracture 72 to grow in size. As described with respect to Figure 2, the total volume of the geothermal fluid 12 in the fracture 72 at the end of the injection phase is predominantly made up of the working volume 214. In some embodiments, the working volume 214 mixes with the residual volume 212. Additionally, or alternatively, as the working volume 214 enters the fracture 72, the flow 402 of the working volume 214 displaces the residual volume 212 deeper into the opening fracture 72.

[0068] Without being bound by theory, it is contemplated that the relatively larger size of the working volume 214 compared to the size of the residual volume 212 results in significant cooling of the residual volume 212. Consequently, convection of the geothermal fluid 12 within the fracture 72 can be minimal during the injection phase. Furthermore, at the end of the injection phase, the majority of the fluid in the fracture 72 is the relatively cold working volume 214, and therefore it takes time to heat the fluid in the fracture 72.

[0069] Figure 5 schematically illustrates fluid dynamics during the injection phase under Scenario 2. Well 70 penetrates into the subterranean formation 42, and is fluidically coupled to the subterranean formation 42 by fracture 74 instead of fracture 72. Geological stresses, such as the overburden, act to close the fracture 74. Removal of the working volume (224, Figure 2) of the geothermal fluid 12 during the production phase results in the fracture 74 closing and reducing in size. At the end of the production phase, the volume of the fracture 74 that remains open is equal to the size of the residual volume (222, Figure 2) of the geothermal fluid 12.

[0070] The fracture 74 is larger compared to fracture 72 of Scenario 1 . For example, the size of the residual volume 222 of the geothermal fluid 12 in Scenario 2 can be 20 to 1000 times the size of the residual volume 212 of the geothermal fluid 12 in Scenario 1 . Consequently, the volume of the fracture 74 that remains open at the end of the production phase under Scenario 2 can be 20 to 1000 times the volume of the fracture 72 that remains open at the end of the production phase under Scenario 1 .

[0071] The flow of the working volume 224 of the geothermal fluid 12 during the injection phase is represented by arrows 502. As described above, the working volume 224 is smaller than the residual volume 222, and smaller than the size of the fracture 74 that remained open at the end of the production phase. The total volume of the geothermal fluid 12 in the fracture 74 at the end of the injection phase is predominantly made up of the residual volume 222.

[0072] In some embodiments, the working volume 224 mixes with the residual volume 222. Additionally, or alternatively, as the working volume 224 enters the fracture 74, the flow 502 of the working volume 224 moves downwards as shown through a portion of the residual volume 222. At least a portion of the residual volume 222 is displaced upwards and around the working volume 224 (shown by arrows 504).

[0073] Without being bound by theory, it is contemplated that the relative sizes and temperatures of the working volume 224 and the residual volume 222affect the flow dynamics within the fracture 74 differently than for the fracture 72 of Scenario 1. The residual volume 222 has been heated by the subterranean formation 42, whereas the working volume 224 being injected into the fracture 74 is relatively cooler. The cooler fluid of the working volume 224 is more dense than the warmer fluid of the residual volume 222. The relatively smaller, cooler, and more dense fluid of the working volume 224 entering the relatively warmer, larger, and less dense fluid of the residual volume 222 can create a convection current (shown by arrows 502 and 504) within the fracture 74.

[0074] In comparison with Scenario 1 , the relatively smaller size of the working volume 224 compared to the size of the residual volume 222 does not result in significant cooling of the residual volume 222. The working volume 224 entering the fracture 74 moves downwards and away from the well 70, and a portion of the residual volume 222 moves upwards and towards the well 70.

[0075] As illustrated, in some embodiments, the fracture 74 extends downwards below a lowermost entry point 80 in the well 70 at which the working fluid 214 enters the fracture 74. Without being bound by theory, it is contemplated that the convection effect described above is enhanced by the greater the extent 76 of the fracture below the lowermost entry point 80 in the well 70.

[0076] Without being bound by theory, it is contemplated that the convection in the fracture 74 and the comparative lack of convection in the fracture 72 results in the temperature profile 320 under Scenario 2 being greater than the temperature profile 310 under Scenario 1 , as shown in Figure 3.

[0077] The portion of the residual volume 222, 504 that moves upwards in the fracture 74 and towards the well 70 becomes the working volume that is produced out of the fracture 74 during the subsequent production phase. This portion of the residual volume 222 has undergone heating in the subterranean formation 42 for at least the duration of the previous production phase in addition to the duration of the injection phase. In embodiments in which thesize of the residual volume 222 is significantly greater than the size of the working volume 224 (such as seven, eight, nine, ten, or more times greater), this portion of the residual volume 222 may have undergone heating in the subterranean formation 42 continuously for several consecutive cycles of production and injection.

[0078] By comparison, in Scenario 1 , a majority of the geothermal fluid 12 that is produced from the well 70 would have been heated by the subterranean formation 42 only for a portion of the duration of the injection phase. Thus, under Scenario 1 , the production phase must be delayed for longer than the shut-in period 324 of Scenario 2 to allow the geothermal fluid 12 in the fracture 72 to be heated to at least the appropriate second threshold 306 or the first threshold 304.

[0079] Figure 6 depicts a composite of two graphs that schematically illustrates injection and production phases of well 70 under Scenario 1. The X axis of each graph represents time. The Y axis of graph 610 represents the volume of geothermal fluid 12 in the fracture 72. The line 612 represents how the volume of geothermal fluid 12 in the fracture 72 varies over repeated cycles of the injection phase followed by the production phase. The Y axis of graph 620 represents the power generated at the power generation unit 20 during each production phase. The line 622 represents the power generated at the power generation unit 20 over repeated cycles of the production phase.

[0080] A first shut-in period 634 follows a first injection phase 632. The total volume 614 of geothermal fluid 12 in the fracture 72 equals the amount 616 of the working volume (214, Figure 2) plus the amount 618 of the residual volume (212, Figure 2). In the illustrated example, the amount 616 of the working volume 214 is 300 m3and the amount 618 of the residual volume is about 50 m3The first shut-in period 634 represents the shut-in duration necessary for the geothermal fluid 12 in the fracture 72 to be heated to at least the second threshold (306, Figure 3). The first shut-in period 634 is followed by a first production phase 636 in which the working volume 214 is produced from the fracture 72 via the well 70 to the power generation unit 20, which generateselectrical power. In the illustrated example, 100kW of electrical power is generated over 5 hours. At the end of the first production phase 636, only the residual volume 212 remains in the fracture 72. There follows a second injection phase 642, during which the working volume 214 is returned to the fracture 72.

[0081] A second shut-in period 644 follows the second injection phase 642. In the depicted example, the second shut-in period 644 is longer than the first shut-in period 634. The second shut-in period 644 represents the shut-in duration necessary for the geothermal fluid 12 in the fracture 72 to be heated to at least the first threshold (304, Figure 3). The second shut-in period 644 is followed by a second production phase 646, in which the working volume 214 is produced from the fracture 72 via the well 70 to the power generation unit 20, which generates electrical power. In the illustrated example, 200kW of electrical power is generated over 5 hours. As shown, the electrical power generated during the second production phase 646 is greater than during the first production phase 636, resulting from the working volume 214 produced from the fracture 72 being hotter during the second production phase 636.

[0082] Figure 7 depicts a composite of two graphs that schematically illustrates injection and production phases of well 70 under Scenario 2. The X axis of each graph represents time. The Y axis of graph 710 represents the volume of geothermal fluid 12 in the fracture 74. The line 712 represents how the volume of geothermal fluid 12 in the fracture 74 varies over repeated cycles of the injection phase followed by the production phase. Graph 710 is plotted according to the same scale as graph 610. The break lines across graph 710 allows line 712 to be plotted according to the same scale as line 612 in graph 610. The Y axis of graph 720 represents the power generated at the power generation unit 20 during each production phase. The line 722 represents the power generated at the power generation unit 20 over repeated cycles of the production phase. Graph 720 is plotted according to the same scale as graph 620.

[0083] Following a first injection phase 732, the total volume of geothermal fluid 12 in the fracture 72 (represented by line 713) equals the amount 716 of a first working volume (e.g., 224, Figure 2) plus the amount 718 of the residual volume (222, Figure 2). The amount 718 of the residual volume 222 in Scenario 2 is significantly greater than the amount 618 of the residual volume 212 in Scenario 1 , as described above. In the illustrated example, the amount 716 of the first working volume is 300 m3and the amount 718 of the residual volume is about 3500 m3

[0084] A first shut-in period 734 is shown to be of very short duration (such as a few minutes). The first shut-in period 734 represents the case in which immediately commencing a first production phase 736 after the first injection phase 732 produces geothermal fluid 12 from the facture 94 that is already at a temperature greater than the second threshold (306, Figure 3). The first working volume 224 is produced from the fracture 74 via the well 70 to the power generation unit 20, which generates electrical power. In the illustrated example, 200kW of electrical power is generated over 5 hours. The electrical power generated in Scenario 2 is greater than the electrical power generated in Scenario 1 due to the geothermal fluid 12 being hotter in Scenario 2 than in Scenario 1 despite the shorter duration of the first shut-in period 734 compared to the first shut-in period 634 of Scenario 1 .

[0085] At the end of the first production phase 736, only the residual volume remains 222 in the fracture 74. There follows a second injection phase 742, during which the first working volume 224 is returned to the fracture 74. In the illustrated example, the first working volume 224 is augmented with additional geothermal fluid 12 to create a second working volume. The amount 717 of the second working volume is 600 m3. A second shut-in period 744 follows the second injection phase 742. The second shut-in period 744 represents the shut-in duration necessary for the geothermal fluid 12 in the fracture 74 to be heated to at least the first threshold (304, Figure 3).

[0086] The second shut-in period 744 is followed by a second production phase 746, in which the second working volume is produced from the fracture74 via the well 70 to the power generation unit 20, which generates electrical power. In the illustrated example, 200kW of electrical power is generated over 10 hours.

[0087] There follows a third injection phase 752, during which the second working volume is returned to the fracture 74. A third shut-in period 754 follows the third injection phase 752. The third shut-in period 754 represents the shut- in duration necessary for the geothermal fluid 12 in the fracture 74 to be heated to at least the first threshold (304, Figure 3).

[0088] The third shut-in period 754 is followed by a third production phase 756, in which the second working volume is produced from the fracture 74 via the well 70 to the power generation unit 20, which generates electrical power. In the illustrated example, 400kW of electrical power is generated over 5 hours.

[0089] In some embodiments, the first well 44 and the second well 46 (Figures 1A, 1 B) are operated with alternating cycles of an injection phase followed by a production phase according to at least part of the sequence of operations depicted in Figure 7. In some examples, the first production phase of the first well 44 corresponds with the first injection phase of the second well 46. The first working volume (e.g., 224, Figure 2) of the geothermal fluid 12 produced by the first well 44 is injected into the fracture 64 via the second well 46. Additionally, the first production phase of the second well 46 may correspond to a second injection phase of the first well, such as the second injection phase. Each working volume (e.g., 224, Figure 2) of each of the first well 44 and the second well 46 is injected into the other of the first well 44 and the second well 46 after being used at the power generation unit 20 to produce electricity. It is contemplated that the first well 44 and the second well 46 may be operated according to any of the methods described herein.

[0090] Figure 8 depicts a graph 800 that schematically illustrates the power generated at the power generation unit 20 according to the operation of the first well 44 and the second well 46 with alternating cycles of an injection phase followed by a production phase, described above. The X axis represents time.The Y axis of graph 800 represents the power generated at the power generation unit 20 during each production phase of each of the first well 44 and the second well 46. The line 812 represents the power generated at the power generation unit 20 during each production phase of the first well 44. The line 814 represents the power generated at the power generation unit 20 during each production phase of the second well 46.

[0091] Using the systems and methods described herein, a power output 802 can be maintained continually by operating the first well 44 and the second well 46 with alternating cycles of an injection phase followed by a production phase. In some embodiments, the power output 802 is maintained 24 hours per day. In some embodiments, the power output 802 is maintained 7 days per week.

[0092] In some embodiments, the first well 44, the second well 46, and a third well are operated with alternating cycles of an injection phase followed by a production phase to facilitate the continual generation of electricity at the power generation unit 20. In some examples, when the first well 44 is undergoing a production phase, the third well is undergoing an injection phase, and the second well 46 is shut-in. Then the second well 46 undergoes a production phase, the first well 44 undergoes an injection phase, and the third well is shut-in. Then the third well undergoes a production phase, the second well 46 undergoes an injection phase, and the first well 44 is shut-in. Then the sequence is repeated. It is contemplated that the first well 44, the second well 46, and the third well may be operated according to any of the methods described herein.

[0093] Figure 9 depicts a graph 900 that schematically illustrates the power generated at the power generation unit 20 according to the operation of the first well 44, the second well 46, and the third well with alternating cycles of an injection phase followed by a production phase, described above. The X axis represents time. The Y axis of graph 900 represents the power generated at the power generation unit 20 during each production phase of each of the first well 44, the second well 46, and the third well. The line 912 represents thepower generated at the power generation unit 20 during each production phase of the first well 44. The line 914 represents the power generated at the power generation unit 20 during each production phase of the second well 46. The line 916 represents the power generated at the power generation unit 20 during each production phase of the third well.

[0094] Using the systems and methods described herein, a power output 902 can be maintained continually by operating the first well 44, the second well 46, and the third well with alternating cycles of an injection phase followed by a production phase. In some embodiments, the power output 902 is maintained 24 hours per day. In some embodiments, the power output 902 is maintained 7 days per week.

[0095] Figure 10 is a flow chart of a method 1000. Operation 1002 includes performing a plurality of operational cycles on a first well (such as first well 44), each operational cycle including an injection phase followed by a production phase. In some embodiments, each injection phase includes injecting a fluid into a first fracture in a subterranean formation via the first well. In some examples, the fluid is geothermal fluid 12. In some examples, the first fracture is fracture 62 or fracture 74. In some examples, the subterranean formation is subterranean formation 42.

[0096] In some embodiments, each production phase includes producing the fluid from the first fracture, and generating electricity using the fluid. In some embodiments, generating the electricity is performed using the power generation unit 20. In some embodiments, an amount of the fluid produced from the first fracture during each production phase is less than an amount of the fluid remaining in the first fracture at an end of each production phase.

[0097] In some embodiments, the fluid produced from the first fracture in each production phase is injected into the first fracture during each subsequent injection phase. In some embodiments, the fluid produced from the first fracture in each production phase is injected into the first fracture during each subsequent injection phase. In some embodiments, the amount of the fluidremaining in the first fracture at the end of each production phase is at least ten times greater than the amount of the fluid produced from the first fracture during each production phase.

[0098] In some embodiments, each production phase performed on the first well corresponds to an injection phase performed on a second well, such as second well 46. In some embodiments, each injection phase performed on the second well includes injecting the fluid produced from the first fracture into a second fracture in the subterranean formation via the second well.

[0099] In some embodiments, each injection phase performed on the first well corresponds to a production phase performed on the second well. In some embodiments, each production phase performed on the second well includes producing the fluid from the second fracture, and generating electricity using the fluid. In some embodiments, an amount of the fluid produced from the second fracture during each production phase performed on the second well is less than an amount of the fluid remaining in the second fracture at an end of each production phase performed on the second well.

[0100] In some embodiments, the amount of the fluid remaining in the second fracture at the end of each production phase performed on the second well is at least five times greater than the amount of the fluid produced from the second fracture during each production phase performed on the second well. In some embodiments, the amount of the fluid remaining in the second fracture at the end of each production phase performed on the second well is at least seven times greater than the amount of the fluid produced from the second fracture during each production phase performed on the second well. In some embodiments, the amount of the fluid remaining in the second fracture at the end of each production phase performed on the second well is at least ten times greater than the amount of the fluid produced from the second fracture during each production phase performed on the second well.

[0101] It is contemplated that method 1000 may include any one or more of the operations or activities described herein. It is contemplated that method1000 may be performed using at least some of the apparatus or systems described herein.

[0102] Figure 11 is a flow chart of a method 1100. Operation 1102 includes producing a fluid from a first fracture in a subterranean formation via a first well (such as first well 44). In some examples, the fluid is geothermal fluid 12. In some examples, the first fracture is fracture 62 or fracture 74. In some examples, the subterranean formation is subterranean formation 42. An initial total amount of the fluid in the first fracture equals an amount of a first portion of the fluid plus an amount of a second portion of the fluid.

[0103] Operation 1104 includes generating electricity a first time using the fluid. In some embodiments, generating the electricity is performed using the power generation unit 20.

[0104] Operation 1106 includes leaving the second portion of the fluid remaining in the first fracture after producing the first portion of the fluid from the first fracture. The amount of the second portion of the fluid is greater than the amount of the first portion of the fluid. In some embodiments, the amount of the second portion of the fluid is at least five times greater than the amount of the first portion of the fluid. In some embodiments, the amount of the second portion of the fluid is at least seven times greater than the amount of the first portion of the fluid. In some embodiments, the amount of the second portion of the fluid is at least ten times greater than the amount of the first portion of the fluid.

[0105] Operation 1108 includes injecting the first portion of the fluid into a second fracture in the subterranean formation via a second well. In some examples, the second well is second well 46. In some examples, the second fracture is fracture 64 or fracture 74. At a start of the injecting, the second fracture contains a total amount of the fluid that is greater than the amount of the first portion of the fluid.

[0106] Operation 1110 includes producing a third portion of the fluid from the second fracture via the second well. Operation 1112 includes generating electricity a second time using the third portion of the fluid. In some embodiments, generating the electricity is performed using the power generation unit 20. Operation 1114 includes injecting the third portion of the fluid into the first fracture via the first well.

[0107] In some embodiments, a time delay between injecting the first portion of the fluid into the second fracture and producing the third portion of the fluid from the second fracture is less than two hours. In some embodiments, a time delay between injecting the first portion of the fluid into the second fracture and producing the third portion of the fluid from the second fracture is less than one hour. In some embodiments, a time delay between injecting the first portion of the fluid into the second fracture and producing the third portion of the fluid from the second fracture is less than half an hour.

[0108] In some embodiments, method 1100 includes leaving a fourth portion of the fluid remaining in the second fracture after producing the third portion of the fluid from the second fracture. An amount of the fourth portion of the fluid is greater than an amount of the third portion of the fluid. In some embodiments, the amount of the fourth portion of the fluid is at least five times greater than the amount of the third portion of the fluid. In some embodiments, the amount of the fourth portion of the fluid is at least seven times greater than the amount of the third portion of the fluid. In some embodiments, the amount of the fourth portion of the fluid is at least ten times greater than the amount of the third portion of the fluid. It is contemplated that between operation 1108 and operation 1110, a total amount of the fluid in the second fracture equals an amount of the third portion of the fluid plus an amount of the fourth portion of the fluid.

[0109] In some embodiments, method 1100 includes producing a fifth portion of the fluid from the first fracture via the first well. In some embodiments, method 1100 includes generating electricity a third time using the fifth portionof the fluid. In some embodiments, generating the electricity is performed using the power generation unit 20.

[0110] In some embodiments, method 1100 includes injecting the fifth portion of the fluid into the second fracture via the second well. In some embodiments, a time delay between injecting the third portion of the fluid into the first fracture and producing the fifth portion of the fluid from the first fracture is less than two hours. In some embodiments, a time delay between injecting the third portion of the fluid into the first fracture and producing the fifth portion of the fluid from the first fracture is less than one hour. In some embodiments, a time delay between injecting the third portion of the fluid into the first fracture and producing the fifth portion of the fluid from the first fracture is less than half an hour.

[0111] It is contemplated that method 1100 may include any one or more of the operations or activities described herein. It is contemplated that method 1100 may be performed using at least some of the apparatus or systems described herein.

[0112] Figure 12 is a flow chart of a method 1200. Operation 1202 includes producing a fluid a first time from a fracture in a subterranean formation via a well (such as first well 44). In some examples, the fluid is geothermal fluid 12. In some examples, the fracture is fracture 62 or fracture 74. In some examples, the subterranean formation is subterranean formation 42. An initial total amount of the fluid in the fracture equals an amount of a first portion of the fluid plus an amount of a second portion of the fluid.

[0113] Operation 1204 includes generating electricity a first time using the fluid. In some embodiments, generating the electricity is performed using the power generation unit 20.

[0114] Operation 1206 includes leaving the second portion of the fluid remaining in the fracture after producing the first portion of the fluid from the fracture. The amount of the second portion of the fluid is greater than theamount of the first portion of the fluid. In some embodiments, the amount of the second portion of the fluid is at least five times greater than the amount of the first portion of the fluid. In some embodiments, the amount of the second portion of the fluid is at least seven times greater than the amount of the first portion of the fluid. In some embodiments, the amount of the second portion of the fluid is at least ten times greater than the amount of the first portion of the fluid.

[0115] Operation 1208 includes injecting the first portion of the fluid back into the fracture via the well. Operation 1210 includes producing the fluid a second time from the fracture via the well. Operation 1212 includes generating electricity a second time using the fluid. In some embodiments, generating the electricity is performed using the power generation unit 20.

[0116] Operation 1214 includes leaving a fourth portion of the fluid remaining in the fracture after producing a third portion of the fluid from the fracture. An amount of the fourth portion of the fluid is greater than an amount of the third portion of the fluid. In some embodiments, the amount of the fourth portion of the fluid is at least five times greater than the amount of the third portion of the fluid. In some embodiments, the amount of the fourth portion of the fluid is at least seven times greater than the amount of the third portion of the fluid. In some embodiments, the amount of the fourth portion of the fluid is at least ten times greater than the amount of the third portion of the fluid. It is contemplated that between operation 1208 and operation 1210, a total amount of the fluid in the fracture equals an amount of the third portion of the fluid plus an amount of the fourth portion of the fluid.

[0117] In some embodiments, a time delay between injecting the first portion of the fluid back into the fracture and producing the third portion of the fluid from the fracture is less than two hours. In some embodiments, a time delay between injecting the first portion of the fluid back into the fracture and producing the third portion of the fluid from the fracture is less than one hour. In some embodiments, a time delay between injecting the first portion of thefluid back into the fracture and producing the third portion of the fluid from the fracture is less than half an hour.

[0118] In some embodiments, method 1200 includes injecting the third portion of the fluid back into the fracture via the well. In some embodiments, method 1200 includes producing the fluid a third time from the fracture via the well. In some embodiments, method 1200 includes generating electricity a third time using the fluid. In some embodiments, generating the electricity is performed using the power generation unit 20.

[0119] It is contemplated that method 1200 may include any one or more of the operations or activities described herein. It is contemplated that method 1200 may be performed using at least some of the apparatus or systems described herein.

[0120] It is contemplated that any one or more elements or features of any one disclosed embodiment may be beneficially incorporated in any one or more other non-mutually exclusive embodiments. While the foregoing is directed to embodiments of the present disclosure, other and further embodiments of the disclosure may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.

Claims

What is claimed is:1 . A method, comprising: performing a plurality of operational cycles on a first well, each operational cycle including an injection phase followed by a production phase, wherein: each injection phase includes injecting a fluid into a first fracture in a subterranean formation via the first well; each production phase includes producing the fluid from the first fracture, and generating electricity using the fluid; and an amount of the fluid produced from the first fracture during each production phase is less than an amount of the fluid remaining in the first fracture at an end of each production phase.

2. The method of claim 1 , wherein the fluid produced from the first fracture in each production phase is injected into the first fracture during each subsequent injection phase.

3. The method of claim 1 , wherein the amount of the fluid remaining in the first fracture at the end of each production phase is at least five times greater than the amount of the fluid produced from the first fracture during each production phase.

4. The method of claim 1 , wherein the amount of the fluid remaining in the first fracture at the end of each production phase is at least ten times greater than the amount of the fluid produced from the first fracture during each production phase.

5. The method of claim 1 , wherein: each production phase performed on the first well corresponds to an injection phase performed on a second well; and each injection phase performed on the second well includes injecting the fluid produced from the first fracture into a second fracture in the subterranean formation via the second well.

6. The method of claim 5, wherein: each injection phase performed on the first well corresponds to a production phase performed on the second well; and each production phase performed on the second well includes producing the fluid from the second fracture, and generating electricity using the fluid.

7. The method of claim 6, wherein an amount of the fluid produced from the second fracture during each production phase performed on the second well is less than an amount of the fluid remaining in the second fracture at an end of each production phase performed on the second well.

8. The method of claim 7, wherein the amount of the fluid remaining in the second fracture at the end of each production phase performed on the second well is at least five times greater than the amount of the fluid produced from the second fracture during each production phase performed on the second well.

9. A method, comprising: producing a fluid from a first fracture in a subterranean formation via a first well, wherein an initial total amount of the fluid in the first fracture equals an amount of a first portion of the fluid plus an amount of a second portion of the fluid; generating electricity a first time using the fluid; leaving the second portion of the fluid remaining in the first fracture after producing the first portion of the fluid from the first fracture, wherein the amount of the second portion of the fluid is greater than the amount of the first portion of the fluid; injecting the first portion of the fluid into a second fracture in the subterranean formation via a second well, wherein at a start of the injecting, the second fracture contains a total amount of the fluid that is greater than the amount of the first portion of the fluid; producing a third portion of the fluid from the second fracture via the second well; generating electricity a second time using the third portion of the fluid; and injecting the third portion of the fluid into the first fracture via the first well.

10. The method of claim 9, wherein a time delay between injecting the first portion of the fluid into the second fracture and producing the third portion of the fluid from the second fracture is less than one hour.11 . The method of claim 9, wherein the amount of the second portion of the fluid is at least five times greater than the amount of the first portion of the fluid.

12. The method of claim 9, further comprising leaving a fourth portion of the fluid remaining in the second fracture after producing the third portion of the fluid from the second fracture, wherein an amount of the fourth portion of the fluid is greater than an amount of the third portion of the fluid.

13. The method of claim 12, wherein the amount of the fourth portion of the fluid is at least five times greater than the amount of the first portion of the fluid.

14. The method of claim 12, further comprising: producing a fifth portion of the fluid from the first fracture via the first well; generating electricity a third time using the fifth portion of the fluid; and injecting the fifth portion of the fluid into the second fracture via the second well.

15. The method of claim 14, wherein a time delay between injecting the third portion of the fluid into the first fracture and producing the fifth portion of the fluid from the first fracture is less than one hour.

16. The method of claim 15, wherein: a total amount of the fluid in the first fracture at a start of producing the fifth portion of the fluid equals an amount of the fifth portion of the fluid plus an amount of a sixth portion of the fluid; the amount of the sixth portion of the fluid is greater than the amount of the fifth portion of the fluid; andthe method further comprises leaving the sixth portion of the fluid remaining in the first fracture after producing the fifth portion of the fluid from the first fracture.

17. A method, comprising: producing a fluid a first time from a fracture in a subterranean formation via a well, wherein an initial total amount of the fluid in the fracture equals an amount of a first portion of the fluid plus an amount of a second portion of the fluid; generating electricity a first time using the fluid; leaving the second portion of the fluid remaining in the fracture after producing the first portion of the fluid from the fracture, wherein the amount of the second portion of the fluid is greater than the amount of the first portion of the fluid; injecting the first portion of the fluid back into the fracture via the well; producing the fluid a second time from the fracture via the well; generating electricity a second time using the fluid; and leaving a fourth portion of the fluid remaining in the fracture after producing a third portion of the fluid from the fracture, wherein an amount of the fourth portion of the fluid is greater than an amount of the third portion of the fluid.

18. The method of claim 17, wherein a time delay between injecting the first portion of the fluid back into the fracture and producing the third portion of the fluid from the fracture is less than one hour.

19. The method of claim 17, further comprising injecting the third portion of the fluid back into the fracture via the well.

20. The method of claim 19, further comprising: producing the fluid a third time from the fracture via the well; and generating electricity a third time using the fluid.

Images