Geothermal isothermal power generation

Abstract

In a in a geothermal well having a surface wellbore extending between a terranean surface and geothermal subterranean zone and a lateral wellbore extending from the surface wellbore into the subterranean zone, circulating a heat transfer working fluid through the well from the surface, to the subterranean zone and back to the surface. While circulating the heat transfer fluid, extracting energy from the heat transfer fluid with an isothermal type power generation cycle and producing electricity.

Description

GEOTHERMAL ISOTHERMAL POWER GENERATIONTECHNICAL FIELD

[0001] This disclosure relates to configuring and control of geothermal well systems for isothermal power generation.BACKGROUND

[0002] Geothermal wells produce hot fluid from a hot, subterranean zone, which in turn, can be used to produce electricity or for heating. Electricity is typically produced using an Organic Rankine Cycle (ORC). The hot fluid from the well heats a working fluid of the ORC, which in turn expands the heated fluid through a turbine that drives a generator. The generator produces electricity. ORCs are suitable for most types of geothermal wells, because they maximize net power production from heat sources that exhibit a large temperature glide. Closed loop geothermal wells, however, have different flow and heat production characteristics than other types of wells. These different characteristics can support other types of thermodynamic cycles for power generation.SUMMARY

[0003] This disclosure relates to configuring and control of geothermal well systems for isothermal power generation.

[0004] Certain aspects encompass a method performed with a geothermal well having a surface wellbore extending between a terranean surface and geothermal subterranean zone and a lateral wellbore extending from the surface wellbore into the geothermal subterranean zone. In the method, a heat transfer fluid is circulated through the well from the surface, to the subterranean zone and back to the surface. Heat is extracted from the heat transfer fluid with an isothermal type power generation cycle and electricity is produced from the isothermal type power generation cycle. The heat transfer fluid is circulated at a rate based on a specified temperature associated with the isothermal type power generation cycle.

[0005] Certain aspects encompass a system having a geothermal well and an isothermal type power generation cycle. The geothermal well has a surface wellbore extending between a terranean surface and a geothermal subterranean zone, and a lateral wellbore extending from the surface wellbore into the geothermal subterranean zone. The geothermal well is configured to circulate a heat transfer fluid through the well from the surface to the subterranean zone and back to the surface. An isothermal type power generation cycle is coupled to the geothermal well to receive the heat transfer fluid and generate electricityfrom heat from the heat transfer fluid. The geothermal well is configured to circulate the heat transfer fluid at a rate based on a specified temperature associated with isothermal type power generation cycle.

[0006] Certain aspects encompass geothermal system having a closed loop geothermal well and an isothermal type power generation cycle coupled to the geothermal well to generate electricity from heated fluid produced from the well. The geothermal well is configured to circulate the fluid at a rate based on a specified temperature associated with the isothermal type power generation cycle.

[0007] The aspects above can include some, none or all of the following features. In certain instances, the wellbores of the geothermal well collectively have a hydraulic flow resistance normalized for vertical depth of less than 15 Pa / kg / s per meter of total vertical depth of the geothermal well, and in certain instances, less than 5 Pa / kg / s per meter of total vertical depth of the geothermal well. In certain instances, a ratio of the average surface wellbore diameter to lateral wellbore diameter is 1.3 or greater. In certain instances, the heat transfer fluid is circulated entirely by the thermosyphon effect (i.e., without needing pumping). In certain instances, the heat transfer fluid is circulated to maintain a temperature differential of the fluid at the surface of 70° C or less. In certain instances, the geothermal well comprises a second surface wellbore and the lateral wellbore connects the first mentioned surface wellbore to the second surface wellbore. In such a well, the heat transfer fluid is circulated from the surface, down the first mentioned surface wellbore, through the lateral wellbore and back to the surface through the second surface wellbore. In certain instances, the isothermal type power generation cycle includes a flooded turbine in a first fluid loop comprising a recuperator, condenser and pump and the flooded turbine is also in a second fluid loop having a heat exchanger and pump. In such an instance, extracting energy from the heat transfer fluid entails circulating the heat transfer fluid from the geothermal well through the heat exchanger to transfer heat to the fluid in the second loop. In certain instances, the isothermal type power generation cycle includes a Stirling engine. In certain instances, the heat transfer fluid is configured to undergo a phase change in the lateral wellbore or configured to undergo a reversible endothermic chemical reaction in the lateral wellbore.

[0008] Other features and aspects are described below.BRIEF DESCRIPTION OF DRAWINGS

[0009] FIG. 1A is a schematic side cross-sectional view of an example closed loop geothermal system in accordance with the concepts herein.

[0010] FIG. IB is a schematic side cross-sectional view of another example closed loop geothermal system in accordance with the concepts herein.

[0011] FIG. 2 is a schematic of a geothermal system having a closed loop geothermal well and an isothermal type power generation cycle.

[0012] Like reference numbers in the drawings represent like elements.DETAILED DESCRIPTION

[0013] A closed loop geothermal system can use an isothermal type power generation cycle in converting heat extracted from the geothermal subterranean zone into electricity. Isothermal type power generation cycles can be more efficient than isentropic expansion processes, such as steam Rankine cycles and Organic Rankine Cycles. The closed loop geothermal well enables system configuration (e.g., inlet / outlet temperature, flowrate, vertical wellbore diameter, and / or other parameters) and control of operating parameters to take advantage of the operating efficiencies of isothermal type power generation cycles. For example, the closed loop geothermal well can be designed and operated to output heated heat transfer fluid in a manner that matches the temperature profile needed for an isothermal power generation cycle. This matching enables use of an isothermal power generation cycle to generate electricity from the heated geothermal heat transfer fluid and enables resultant improved electrical production by the overall system. The matching enables a high level of recuperation within the isothermal cycle, which further improves the electric production efficiency. Additionally, the closed loop geothermal well can be operated with a higher inlet temperature to improve cycle efficiency, where the higher inlet temperatures lead to higher outlet temperatures and higher well cycle efficiency.

[0014] Turning to FIG. 1A, an example closed loop geothermal system 100 is shown in the schematic, side cross-sectional view in accordance with the concepts herein. In certain instances, the closed loop geothermal wellbore system can be, for example, a system such as that developed by Eavor Technologies Inc. of Calgary, Alberta, which includes a network of sealed lateral (e.g., horizontal, sloped or otherwise deviated) wellbores that exchange heat with the subterranean zone.

[0015] System 100 includes a closed geothermal well 102 drilled into the Earth through a geothermal subterranean zone of interest 104. In certain instances, the zone 104 has a temperature of 200° C or higher. In certain instances, the subterranean zone is a dry, impermeable (matrix permeability of 0. 1 millidarcy or less) formation, portion of formation or multiple formations having little to no naturally occurring recoverable fluids. In certain instances, the subterranean zone is in a crystalline basement formation. In certain instances, the rock of the subterranean zone is in a granitic formation (e.g., granite). In the illustrated instance, well 102 includes an inlet surface wellbore 120 and an outlet surface wellbore 130 in close proximity, each extending between the terranean surface and the subterranean zone 104. The inlet surface wellbore 120 and outlet surface wellbore 130 are connected within the subterranean zone 104 by one or more connecting wellbores 140. In the illustrated instance, connecting wellbores 140 define a multilateral pattern of wellbores, including a plurality of pairs of lateral wellbores 150, a subset of which are kicked offfrom the inlet wellbore 120 and a subset of which are kicked off from the outlet wellbore 130. The pairs of lateral wellbores 150 each intersect at a respective junction 154 at or near their respective toes. Thus, the inlet wellbore 120, outlet wellbore 130 and connecting wellbores 140 define a closed loop.

[0016] The inlet wellbore 120 and the outlet wellbore 130 can be drilled from the same drilling pad and / or reside on the same well site. In certain instances, the wellbores 120, 130 are drilled within 10, 25, 50 or 100 meters of one another. In other instances, the inlet surface wellbore 120 and the outlet surface wellbore 130 can be separated by a longer distance. For example, FIG. IB, discussed in more detail below, shows a configuration where the surface wellbores 120, 130 and the connecting wellbores 140 define a U- shape configuration. In certain instances, the inlet surface wellbore 120 and the outlet surface wellbore 130, when the geothermal well 102 is configured as a U-shape, are drilled 3,000 meters or more apart.

[0017] In the illustrated instance, inlet surface wellbore 120 and outlet surface wellbore 130 are vertical wellbores, drilled substantially straight (i.e., without the use of directional drilling methods or equipment). In other instances, one or both of the surface wellbores are other than vertical (e.g., slanted) and / or may be drilled with the use of directional drilling techniques. The connecting wellbores 140 are drilled using directional drilling techniques through the surface wellbores 120, 130, and include a curve in their trajectory beginning at a kickoff 148 at surface wellbores 120, 130. Although shown as slanted downward, in some instances, some or all of the connecting wellbores are horizontal. In some instances, the connecting wellbores 140 follow the geological dip of the formation in the subterranean zone. In some instances, lateral wellbores 150 are anywhere from 2,000 meters to 10,000 meters or more in length and from 1,000 meters to 8,000 meters or more in depth from the surface.

[0018] FIG. 1A shows each pair of lateral wellbores 150 parallel to one another extending in the same direction (azimuth) from their respective surface wellbore 120, 130. The lateral wellbores 150 extending from the inlet surface wellbore 120 are shown above the lateral wellbores 150 extending from the outlet surface wellbore 130. In some instances, the upper lateral wellbores 150 are directly above their (and are, in some instances, directly above a respective one of the lower lateral wellbores 150. In FIG. 1A the upper lateral wellbores 150 each turn to intersect its adjacent lower lateral wellbore 150 pair at the junction 154 to connect the surface wellbores 120, 130. In other instances, one or more of the lower lateral wellbores 150 could intersect the upper lateral wellbores 150. Regardless, the configuration of connecting wellbores 140, one set atop the other defines a stacked wellbore pattern, with one sub-pattern of wellbores above and one sub-pattern of wellbores below. In certain instances, one or more additional sets of stacked patterns can be drilled from the surface wellbores 120, 130 at different depths (i.e., with different kickoffs 148). In FIG. 1A, the lower lateral wellbores 150 extend past and below the junction 154 to define a sump 152. The sump 152 provides a location for debris to accumulate outside of the flow path through the wellbores. In otherinstances, one or more of the upper lateral wellbores 150 could extend past the junction to define the sump 152.

[0019] FIG. IB another embodiment of a geothermal well system 100’ having lateral wellbores 150 extending, respectively, from the inlet and outlet surface wellbores 120, 130 toward one another. The pairs of lateral wellbores 150, once intersected, together with the inlet and outlet wellbores 120, 130, define a generally U-shape. The configuration of connecting wellbores 140 defines a pattern of wellbores, in certain instances, in the same plane. In certain instances, one or more additional patterns of connecting wellbores can be drilled between the surface wellbores 120, 130 at different depths (i.e., with different kickoffs 148).

[0020] Referring to FIGS. 1A and IB, collectively, in some instances, the surface wellbores 120, 130 are cased (at least partially or entirely), and the connecting wellbores 140, including the junctures at the kickoffs 148 are open hole (i.e., without casing or liner or a junction liner). In some instances, the connecting wellbores 140 can be at least partially lined (e.g., include a liner or casing in those portions where the subterranean zone 104 is fractured, susceptible to collapse, unconsolidated or otherwise needing a liner). The connecting wellbores 140, including the junctures to the inlet and outlet surface wellbores 120, 130 are sealed (entirely or substantially) with a sealant against exchange of fluids with the surrounding subterranean zone 104. The sealant is designed such that all or substantially all of the heat transfer fluid circulated through the well 102 during operation is recovered to the surface, and no or little naturally occurring fluids from the subterranean zone 104 are recovered. In other words, the resulting well 102 is closed loop from the surface inlet to the surface outlet. In certain instances, the sealant can be applied to the wellbores during drilling the connecting wellbores 140, e.g., included in the drilling fluid and / or supplied in fluid slugs distinct from the drilling fluid. Alternatively, or additionally, the sealant is applied after drilling and / or during operation of the well. In certain instances, the sealant can be included in the heat transfer working fluid and / or supplied in fluid slugs, distinct from the heat transfer fluid.

[0021] In the illustrated instance, system 100 further includes a facility 110 disposed between inlet surface wellbore 120 and outlet surface wellbore 130. Well 102 can be sealed and a working fluid added to the closed loop and circulated in the system such that it absorbs heat from subterranean zone 104. In certain instances, facility 110 includes valves and pumps for controlling the flow of the working fluid through the well 102, as well as a heat exchanger for extracting the heat from the working fluid and conveying it into a related process, such as a Rankine cycle (e.g., Organic Rankine Cycle), an isothermal heat cycle or other heat cycle that generates electricity, a steam generation process for industrial, agricultural or residential use, or another process. In certain instances, instead of, or in addition to a heat exchanger, facility 110 directly uses the heated working fluid, such as by passing it through an expander (e.g., a turbine) that drives an electric generator or directly using the heat of the working fluid in an industrial, agricultural or residentialprocess. In some instances, facility 110 is disposed at or near the Earth’s surface; in other instances, facility 110 may be disposed partially or fully within a subsurface location. The facility 110 need not be housed in one location, and, for example as shown in FIG. IB, it can be split between one or more discrete locations (shown as facility 110a, 110b) connected by piping.

[0022] In either instance, FIG. 1A or IB, the geothermal well is constructed by drilling and, if cased, casing the inlet wellbore 120 and outlet wellbore 130. The connecting wellbores 140 are drilled as intersecting lateral wellbores 150 from the inlet wellbore 120 and the outlet wellbore 130. In particular, a connecting wellbore 140 is constructed as a lateral wellbore 150 beginning at the sidewall of the inlet wellbore 120, i.e., kicking off from the inlet wellbore 120, and a lateral wellbore 150 beginning at the sidewall of the outlet wellbore 130, i.e., kicking off from the outlet wellbore. The lateral wellbores 150 are drilled to intersect at a junction 154 to define a connecting wellbore 140. Additional connecting wellbores 140 can be drilled as lateral wellbores 150 kicking off from the inlet wellbore 120, outlet wellbore 130 and / or other of the connecting wellbores 140 (or lateral wellbore 150 that may become a connecting wellbore 140). The lateral wellbores 150 are drilled using directional drilling techniques with a drilling string extending from the terranean surface through the inlet wellbore 120 and outlet wellbore 130, respectively. In certain instances, a whipstock is used to kick off the lateral wellbores 150 from their respective surface wellbore 120, 130.

[0023] The drilling the inlet and outlet wellbores 120, 130 and connecting wellbores 140 can be conducted sequentially with one drill rig and drilling string, or concurrently and, in some instances simultaneously, with the two drilling strings each operating to drill their respective lateral wellbore 150 at the same time. In some instances, the inlet wellbore 120, the outlet wellbore 130 and the connecting wellbores 140 are drilled with two drilling rigs, one atop the inlet wellbore 120 and one atop the outlet wellbore 130. In other instances, the wellbores are drilled with a single drilling rig configured to drill two wellbores at the same time (e.g., having two masts, two top drives and / or two rotary tables, etc.). In certain instances, such as when using coiled tubing, the drilling rig can be a coiled tubing rig. In certain instances, such as when constructing the drilling string from both jointed tubing and coiled tubing, the drilling rig can be a hybrid jointed tubing and coiled tubing rig. For example, such a rig can have jointed pipe handling capability and a drive (e.g., top drive, rotary table drive and / or another tubing drive), as well as a tubing spool handling capability and a continuous injector system, and be able to switch between the two as different types of tubing, jointed or coiled, are assembled into the drill string.

[0024] FIG. 2 shows an example flow diagram of a closed loop geothermal system 200 having a closed loop geothermal well 202 that extracts heat from a geothermal subterranean zone and feeds that heat to an isothermal type power generation cycle 206 to generate electricity. The geothermal well 202 is configuredto circulate a heat transfer fluid between a terranean surface and the subterranean zone, collecting heat from the zone in the heat transfer fluid primarily by conductive heat transfer of the fluid with the rock of the subterranean zone. The heated fluid is then transported to the surface and circulated through the isothermal type power generation cycle 206, where the heat is used by the power generation cycle 206 to generate electricity. The geothermal well 202 is a closed loop type well, configured to limit the loss of heat transfer fluid injected and withdrawn from the well, for example, by being placed in an impermeable formation and / or by being cased, lined, sealed with a sealant and / or otherwise sealed against loss of the fluid. In certain instances, the geothermal system 200 employs a geothermal well 202 such as the wells 102 described above and the isothermal type power generation system 206 is incorporated into a surface facility 110.

[0025] T he isothermal type power generation cycle 206 would be one that operates on an isothermal thermodynamic cycle, having a relatively constant temperature throughout the expansion phase of the cycle. For example, a Carnot cycle is an example of an isothermal thermodynamic cycle, but others exist and are within the concepts herein. By contrast, a Rankine cycle, more typically used for electric generation from geothermal heat, depends on the temperature differential between its heat sink and heat source to convert heat into kinetic energy. An isothermal thermodynamic cycle cannot reach its efficiency potential if driven by a heat source that, as heat is withdrawn from the source, the temperature output by the heat source drops. The temperature drop leads to a temperature mismatch between the heat source and temperature of the isothermal thermodynamic cycle, in turn leading to losses and diminished efficiency. Therefore, conventional, open loop geothermal wells (e.g., hydrothermal wells) would not benefit from using an isothermal thermodynamic cycle because they cannot maintain a constant temperature as fluids are withdrawn from the subterranean zone.

[0026] While ideal isothermal power generation cycles are not possible, there are multiple different isothermal type power generation cycles that exist that can be used in accordance with the concepts herein in the isothermal type power generation system 206. In certain instances, the isothermal type power generation cycle is one that is configured to approximate a Carnot cycle or an isothermal Stirling cycle. In certain instances, the isothermal type power generation cycles can have efficiencies that are approximately 75% of an ideal Carnot cycle. Some examples include: W02014020595A2, “Energy conversion system,” WO2022049573A2, “Heat engine,” WO2022234554A1, “Heat-transfer-liquid-operated turbine and compressor,” WO2023228173, “Two-phase heat engine,” WO2024028878A1, “Heat engine using a liquid- vapor-phase-changing material,” WO20222256302, “Stirling engine with near isothermal working spaces.”

[0027] FIG. 2 shows an example cycle 206 that utilizes a flooded turbine 208, similar to those described in the publications above. The turbine 208 works by mixing a first light, working fluid (low boiling point) with a second, heavier working fluid, i.e., the flooding fluid, with a high thermal capacity (high flowrate,high heat capacity, high boiling point). The second, heavier working fluid is heated by an external process, and supplies heat to the first, light working fluid. In certain instances, the flowrate of the second, heavier working fluid is 5 or more or 10 or more times the flow rate of the first, light working fluid. As heat from the second working fluid is absorbed by the first, light working fluid, the first, light fluid (nearly) isothermally expands, and the fluid mixture jets out of a nozzle. The kinetic energy from expansion and jetting of the fluid mixture is extracted as work. For example, the nozzle can be mounted on a rotor of a generator, or directly or indirectly (e.g., through a gearbox) to a rotor of a generator, so that as the fluid jetting from the nozzle propels the nozzle, it turns the rotor and generates electricity. In certain instances, multiple nozzles performing the same process can be coupled to the rotor, so that the multiple nozzles extract the isothermal expansion as work. After exiting the nozzle, the fluid mixture is separated at the discharge of the turbine 208 back into the first, light working fluid and second, heavier working fluid.

[0028] The example cycle 206 includes two loops, one light fluid loop 212 that carries the first light working fluid to / from the turbine 208 and one heavy fluid loop 214 that carries the second, heavier working fluid to / from the turbine 208. In certain instances, the first light working fluid is n-pentane, and vapor when heated in the nozzle, while the second, heavier working fluid is liquid ethylene glycol. In yet other examples, the first light working fluid is isopentane, cyclopentane, R1234zEe, R1233zdE, R227ea, dimethyl ether, R1234yf, R1234zeZ, isobutane, butane, propane, hexane, R152a, R1243zf, propylene , R1243zf and / or another light working fluid. In other examples, the second, heavier working fluid is thermal oil, water and / or another working fluid. In certain instances, the second, heavier working fluid is one that remains liquid at the designated temperature of the cycle and has a higher thermal capacity than the first light working fluid.

[0029] The heavy fluid loop 214 includes a heat exchanger 216 and a pump 218. The pump 218 circulates the second, heavier working fluid flowing from the heavy fluid discharge of the turbine 208, through the loop 214, through the heat exchanger 216, and into a heavy fluid inlet of turbine 208. The heat exchanger 216 receives a flow of heated geothermal heat transfer fluid from the geothermal well 202 (e.g., from an outlet surface wellbore 130), and heats the second, heavy working fluid by heat transfer with the geothermal heat transfer fluid. The geothermal heat transfer fluid is then returned to the geothermal well 202 (e.g, to an inlet surface wellbore 120) to be reheated by the geothermal subterranean zone (e.g., zone 104).

[0030] The light fluid loop 212 includes a recuperator heat exchanger 220, a condenser heat exchanger 222 and a pump 224. The first, light working fluid exits the light fluid discharge of the turbine 208 in vapor form, supplies heat to the recuperator 220, and is condensed to a liquid in the condenser 222. The pump 224 then pumps the first, light working fluid as a liquid from the condenser 222, through the recuperator 220(where it recovers heat from the fluid exiting the turbine 208), and back to the light fluid inlet of turbine 208. In certain instances, the example cycle 206 can yield 70% or better efficiency relative to an ideal Carnot, where the efficiency is defined as a ratio of the temperature in the nozzle of the turbine 208 relative to the temperature of the first, light working fluid exiting the condenser 222.

[0031] In certain instances, the system can use multiple isothermal type power generation cycles 206 in series (2, 3 or more), where the geothermal heat transfer fluid flows through the heat exchanger 216 of one cycle 206 and into the heat exchanger 216 of an adjacent cycle, and so on. The resulting arrangement can, in certain instances, result in a reduced temperature difference across each heat exchanger. A lower temperature difference across each isothermal cycle facilitates a closer match between the geothermal well heat transfer fluid temperature and the temperature of the second, heavier working fluid, thereby improving efficiency by reducing heat exchanger losses.

[0032] In certain instances, the second heavier working fluid can be the geothermal heat transfer fluid. In such a case, the heat exchanger 216 can be omitted, and the fluid can be circulated directly from the geothermal well 202 into the turbine 208.

[0033] In operation, the second, heavier working fluid mixes with the cooler first isothermal working fluid in the flooded turbine 208. The combined fluid reaches an intermediate “blended” temperature. The “blended” temperature in the turbine 208 is controlled by the flowrate and temperature of the second, heavier working fluid and the selected parameters of the first light working fluid (e.g., the pentane pressure, flowrate, size of recuperator, ambient air / water temperature and / or other parameters). All these factors can be controlled in conjunction with the geothermal working fluid circulating fluid rate and geothermal well 202 design (including the system hydraulic resistance, discussed below). The objective is to reduce (e.g., minimize) heat exchanger losses between the geothermal working fluid and the first light working fluid, and increase (e.g., maximize) the “blended” temperature in the turbine 208 to result in a high (e.g., highest) net power production.

[0034] The configuration and operation of the geothermal well 202 can be adapted for use with an isothermal type power generation cycle 206 by configuring and controlling the well 202 to produce specified temperatures based on the needs of the isothermal type power generation cycle 206. The well 202, being a closed loop well, responds to higher temperature geothermal heat transfer fluid supplied to its inlet by producing higher temperature heat transfer fluid at its outlet. Increasing the rate of circulation of the heat transfer fluid through the well 202 offsets this effect, and allows the well 202 to approach an isothermal output temperature profile. Also, there is a reduced thermal duty trade-off with higher inlet temperatures in closed loop as compared to other types of wells, such as enhanced geothermal (EGS) and conventional, hydrothermal wells. EGS / hydrothermal resources do not exhibit a strong feedback effect between injectionand production temperatures, because they have a higher hydraulic resistance relative to their heat transfer. Therefore increasing injection temperatures in these types of wells corresponds to a large reduction in thermal output (or wasted energy) as production temperature is largely dictated by the reservoir temperature.

[0035] In certain instances, the flow rate of the geothermal heat transfer fluid through the geothermal well 202 can be controlled to maintain a near isothermal temperature in the connecting wellbores in the geothermal subterranean zone (e.g., in connecting wellbores 140). Additionally, the flow rate through the well 202 can be controlled to match the temperature of the geothermal heat transfer fluid output at the outlet wellbore to a specified temperature of fluid for the heat exchanger 216 (or heat exchangers). Typically, higher cycle efficiency is achieved through configuring the geothermal well 202 to have a high inlet temperature (more work can be extracted from higher temperature sources), and a closer temperature match in the heat exchanged 16 via a high geothermal well 202 flowrate and well design with a low hydraulic resistance (to facilitate the high flowrate).

[0036] The temperature differential between the geothermal heat transfer fluid at the outlet wellbore and inlet wellbore and / or between the inlet and outlet of the heat exchanger(s) 216 can also be matched to a specified temperature differential. In certain instances, the specified temperature differential between the inlet and outlet wellbore is 70° C or less. In certain instances, the specified temperature differential between the inlet and outlet of the heat exchangers(s) 216 is 30° C or less. The temperature differential between the inlet and outlet wellbores and / or across the heat exchanger(s) 316 can be decreased by increasing the flow rate through the geothermal well 202.

[0037] In certain instances, the well 202 flows entirely by thermosyphon effect, or primarily by thermosyphon effect with some assistance by pump, as needed. The thermosyphon effect is the tendency of the well 202 to circulate heat transfer fluid between the inlet and outlet surface locations (i.e. , surface openings of the inlet and outlet wellbores) due to denser, cooler heat transfer fluid supplied down the inlet wellbore, overcoming the flow resistance through the well 202, and pushing the less dense, heated fluid up the outlet wellbore. The thermosyphon effect, and the resulting circulation rate, depends on the thermal characteristics of the geothermal subterranean zone, the vertical depth of the well 202, the density and thermal characteristics of the heat transfer fluid, and the flow resistance through the well 202. Although the well 202 may, initially, need pumping to more quickly begin the thermosyphon effect, once flow has been established, a well 202 can then continue circulating entirely on thermosyphon effect without pumping. Once the thermosyphon effect circulation is established, the flow rate of the circulating fluid can be controlled with one or more valves (e.g., valve 226) metering the flow out of the well 202.

[0038] The flow rates to attain the specified temperature and / or temperature differentials discussed above can be achieved, for example, by sizing the diameters of inlet wellbore and outlet wellbores toaccommodate the needed flow via thermosyphon. The largest hydraulic pressure drop in the well 202 occurs in the inlet and outlet wellbores (e.g., wellbores 120, 130 of FIG. 1A), and there is significantly less hydraulic pressure drop through the connecting wellbores. Thus, in certain instances, the well 202 can be constructed with inlet and / or outlet wellbores having an average flow diameter that is larger than the connecting wellbores, and in some instances, 1.3 times the connecting wellbore diameters. Also, the casing design in well 202 can be configured without a tieback tubing string or other concentric tubing in the inlet / outlet wellbores that may reduce flow area. The number of connecting wellbores also matters. In certain instances, the inlet, outlet, and connecting wellbore flow diameters and the number of connecting wellbores can be configured to allow at least 125 kg / s, and in some instances 150-200 kg / s, 250-300 kg / s or greater, of fluid flow through the well 202 entirely by thermosyphon under the thermal characteristics of the geothermal subterranean zone. In certain instances, the inlet, outlet, and connecting wellbore flow diameters and number of connecting wellbores can be configured to achieve a hydraulic resistance normalized for vertical depth less than 15 Pa / kg / s per meter total vertical depth of the geothermal well 202. In certain instances, the well configurations described herein facilitate design to achieve hydraulic resistance normalized for vertical depth of less than 10, less than 5, less than 3 or even less than 2 Pa / kg / s per meter total vertical depth of the geothermal well 202. The hydraulic resistance normalized for vertical depth can be calculated by determining the hydraulic pressure drop through the well at a target flow rate, dividing by the target flow rate, and dividing by the deepest total vertical depth reached by the deepest location of the deepest connecting wellbore. In certain instances, the well 202 is operated at or within 10% or 5%of the thermosyphon limit to help achieve the high flow rates. For example, in certain instances, the well 202 is operated with 500kPa or less pressure differential between the fluid in the inlet and outlet wellbores.

[0039] In certain instances, the geothermal well 202 and isothermal type power generation cycle 206 can be configured and operated to maintain higher inlet temperatures. For example, as noted above, the target temperature differential between the inlet and outlet wellbore can be 70° C or less. Thus, in certain instances, for a target specified temperature at the outlet wellbore of 120° C, the well 202 and cycle 206 can be configured and operated to achieve an inlet wellbore temperature of at least 50° C, and in some instances at least 90° C. Such a higher inlet temperature can improve cycle efficiency and provide higher outlet temperatures. The higher outlet temperatures, in turn, enable a high level of recuperation within the cycle 206, which further improves efficiency.

[0040] In certain instances, the geothermal heat transfer fluid is aqueous, such as water or brine, and can include treatment additives to reduce friction, to suppress corrosion and / or for other purposes. In certain instances, the geothermal heat transfer fluid is a fluid selected for its phase change characteristics so that it changes from a liquid to a vapor or solid in the connecting wellbores, given the thermal characteristics of thesubterranean zone and the flow rates through the well. In certain instances, the geothermal heat transfer fluid is a fluid selected to undergo a reversible endothermal chemical reaction in the connecting wellbores. In either instance, the fluid is configured to absorb the heat from the subterranean zone with little or no temperature change. Use of such fluids enables most closely a perfect temperature match to the isothermal power generation cycle 206, and thus the maximum possible power production with this system.

[0041] While this disclosure contains many specific implementation details, these should not be construed as limitations on the subject matter or on what may be claimed, but rather as descriptions of features that may be specific to particular implementations. Certain features that are described in this disclosure in the context of separate implementations can also be implemented, in combination, or in a single implementation. Conversely, various features that are described in the context of a single implementation can also be implemented in multiple implementations, separately, or in any suitable subcombination. Moreover, although previously described features may be described as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can, in some cases, be excised from the combination, and the claimed combination may be directed to a subcombination or variation of a sub-combination.

[0042] Particular implementations of the subject matter have been described. Nevertheless, it will be understood that various modifications, substitutions, and alterations may be made. While operations are depicted in the drawings or claims in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed (some operations may be considered optional), to achieve desirable results. Accordingly, the previously described example implementations do not define or constrain this disclosure.

Claims

What is claimed is:

1. A method, comprising: in a geothermal well comprising a surface wellbore extending between a terranean surface and geothermal subterranean zone and a lateral wellbore extending from the surface wellbore into the geothermal subterranean zone, circulating a heat transfer fluid through the well from the surface, to the subterranean zone and back to the surface; extracting heat from the heat transfer fluid with an isothermal type power generation cycle and producing electricity; and circulating the heat transfer fluid at a rate based on a specified temperature associated with the isothermal type power generation cycle.2 The method of claim 1, where the wellbores of the geothermal well collectively have a hydraulic flow resistance normalized for vertical depth of less than 15 Pa / kg / s per meter of total vertical depth of the geothermal well.3 The method of claims 1 or 2, where a ratio of the average surface wellbore diameter to lateral wellbore diameter is 1.3 or greater.4 The method of any of claims 1 to 3, where circulating the heat transfer fluid comprises circulating the heat transfer fluid entirely by the thermosyphon effect.5 The method of any of claims 1 to 4, where circulating the heat transfer fluid comprises circulating the heat transfer fluid to maintain a temperature differential of the fluid at the surface of 70° C or less.6 The method of any of claims 1 to 5, where the geothermal well comprises a second surface wellbore and the lateral wellbore connects the first mentioned surface wellbore to the second surface wellbore; and where circulating the heat transfer fluid comprises circulating the heat transfer fluid from the surface, down the first mentioned surface wellbore, through the lateral wellbore and back to the surface through the second surface wellbore.

7. The method of any of claims 1 to 6. where the isothermal type power generation cycle comprises a flooded turbine in a first fluid loop comprising a recuperator, condenser and pump and where in the flooded turbine is also in a second fluid loop comprising a heat exchanger and pump; and where extracting energy from the heat transfer fluid comprises circulating the heat transfer fluid from the geothermal well through the heat exchanger to transfer heat to the fluid in the second loop.8 The method of any of claims 1 to 7, where the isothermal type power generation cycle comprises a Stirling engine.9 The method of any of claims 1 to 8, where the heat transfer fluid comprises a fluid configured to undergo a phase change in the lateral wellbore or that is configured to undergo a reversible endothermic chemical reaction in the lateral wellbore.10 The method of any of claims 1 to 9, where the wellbores of the geothermal well collectively have a hydraulic flow resistance normalized for vertical depth of less than 5 Pa / kg / s per meter of total vertical depth of the geothermal well.

11. A system, comprising: a geothermal well comprising a surface wellbore extending between a terranean surface and a geothermal subterranean zone and a lateral wellbore extending from the surface wellbore into the geothermal subterranean zone, the geothermal well configured to circulate a heat transfer fluid through the well from the surface to the subterranean zone and back to the surface; an isothermal type power generation cycle coupled to the geothermal well to receive the heat transfer fluid and generate electricity from heat from the heat transfer fluid; and where the geothermal well is configured to circulate the heat transfer fluid at a rate based on a specified temperature associated with isothermal type power generation cycle.

12. The system of claim 11, where the flow area of the wellbores of the geothermal well collectively have a hydraulic flow resistance normalized for vertical depth of less than 15 Pa / kg / s per meter of total vertical depth of the geothermal well.

13. The system of claims 11 or 12, where a ratio of the average surface wellbore diameter to lateral wellbore diameter is 1.3 or greater.

14. The system of any of claims 11 to 13, where the geothermal well is configured to circulate the heat transfer fluid entirely by the thermosyphon effect.

15. The system of any of claims 11 to 14, where the geothermal well is configured to maintain a temperature differential of the fluid at the surface of 70° C or less.

16. The system of any of claims 11 to 15, where the geothermal well comprises a second surface wellbore and the lateral wellbore connects the first mentioned surface wellbore to the second surface wellbore.

17. The system of any of claims 11 to 16, where the isothermal type power generation cycle comprises a flooded turbine in a first fluid loop comprising a recuperator, condenser and pump and where the flooded turbine is also in a second fluid loop comprising a heat exchanger and pump; and wherein the geothermal well is coupled to the heat exchanger to circulate heat transfer fluid to the heat exchanger and heat to the fluid in the second fluid loop.

18. The system of any of claims 11 to 16, where the isothermal type power generation cycle comprises a Stirling engine.

19. The system of any of claims 11 to 18, comprising a heat transfer fluid configured to undergo a phase change in the lateral wellbore or that is configured to undergo a reversible endothermic chemical reaction in the lateral wellbore.

20. A geothermal system, comprising: a closed loop geothermal well; an isothermal type power generation cycle coupled to the geothermal well to generate electricity from heated fluid produced from the well; and where the geothermal well is configured to circulate the fluid at a rate based on a specified temperature associated with the isothermal type power generation cycle.

21. The geothermal system of claim 20, where wellbores of the geothermal well collectively have a hydraulic flow resistance normalized for vertical depth of less than 15 Pa / kg / s per meter of total vertical depth of the geothermal well.

22. The geothermal system of claim 20, where wellbores of the geothermal well collectively have a hydraulic flow resistance normalized for vertical depth of less than 5 Pa / kg / s per meter of total vertical depth of the geothermal well.

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