METHOD FOR A RADIATOR EGS TO HARVEST GEOTHERMAL ENERGY

Abstract

An embodiment in accordance with the present invention includes an EGS configured to allow the commercial production of electrical energy. One criteria of an EGS according to the present invention is that the temperature and volume of the fluids extracted are sufficiently high and large enough as to allow the commercial production of electrical energy. The system is able to operate for at least N years before the extracted fluid falls below the minimum temperature needed for energy production. Additionally, fractures are separated from each other by a sufficiently large volume of rock (Vcrit) relative to the fractures surface area such that the ratio of the rate of heat extraction to the rate of heat supply controlled by the thermal conductivity of the rock is such that the intervening rock is cooled at a rate that is sufficiently slow to be economic.

Description

FIELD OF THE INVENTION

The present invention relates generally to energy production. More particularly, the present invention relates to a system and method for harvesting of geothermal energy from non-hydrothermal sources.

BACKGROUND OF THE INVENTION

Non-hydrothermal sources of thermal energy include hot dry rock (HDR) and hot sedimentary aquifers (HSA). Non-hydrothermal sources of thermal energy can only be harvested by what are called enhanced geothermal systems (EGS). EGS are essentially manufactured thermal reservoirs.

Extracting thermal energy from non-hydrothermal sources requires overcoming a major physical barrier, the low thermal conductivity of rock. To date no EGS systems have been able to harvest commercial quantities of thermal energy because there is currently no method for overcoming this problem. A purely conductive method for extracting geothermal energy is easily defeated by the very low thermal conductivity of rock. While natural hydrothermal systems represent a solution to this problem, because they ultimately depend on heat conduction for their source of thermal energy, there is not currently a method for implementing such a system. This suggests that an EGS emulating a hydrothermal system may offer a solution to the problem of the low thermal conductivity of rock and at the same time permit extraction of a sufficient volume of fluid that is of a temperature permitting commercial electrical generation.

Accordingly, there is a need in the art for a system and method to facilitate an EGS emulating a natural hydrothermal system in order to harvest geothermal energy at commercial levels.

SUMMARY OF THE INVENTION

The foregoing needs are met, to a great extent, by the present invention which provides a system for harvesting geothermal energy including a non-hydrothermal source of energy. The system includes an injector well and a production well. The system also includes an approximately vertical radiator vane located in a plane defined by the injector well and the production well. The term ‘radiator vane’ in essence emulates in overall form and performance a radiator heat exchanger in a common internal combustion engine.

In accordance with an aspect of the present invention, the injector well and the production well each include a vertical pilot hole and a lateral. The injector well and the production well are each connected to a pumping unit. The injector well and the production well are oriented to define a vertical plane that is parallel to a maximum horizontal stress axis. The injector well and the production well are drilled sufficiently deep such that the principal stress axis is vertical. The critical depth is greater than approximately 700 m. The injector well is drilled to a depth, which is at the T_max isotherm and which is deeper than the production well. The production well is at the depth of a T_min isotherm, where T_min is greater than the minimum ambient rock temperature for commercial production of energy. The radiator vane is created by successive controlled fracs, one above the other, in successively higher laterals located in the plane defined by the injector and production wells.

In accordance with another aspect of the present invention, a method for harvesting geothermal energy includes creating an EGS power unit in a non-hydrothermal source. The method includes drilling an injector well with a lateral oriented to parallel a maximum horizontal stress at a depth where a principal stress axis is vertical and the ambient rock temperature is greater than T_min. The method also includes drilling a production well with a lateral oriented parallel to a maximum horizontal stress at a depth where a principal stress axis is vertical and the ambient rock temperature is T_min. Additionally, the method includes generating a radiator vane with successive controlled fracs, one above the other, in successively higher laterals located in the vertical plane defined by the injector and production wells.

In accordance with yet another aspect of the present invention, the method includes extracting fluid with a sufficiently high temperature and volume large enough to allow the commercial production of energy. The method includes optimizing the system to operate for a predetermined number of years before extracted fluid falls below a minimum temperature needed for energy production. The method also includes separating fractures from each other by a sufficiently large volume of rock (V_crit) relative to a surface area of the fractures, such that the ratio of the rate of heat extraction to the rate of heat supply controlled by thermal conductivity of the non-hydrothermal source is such that the intervening rock is cooled at a rate that is sufficiently slow as to provide an economic source of energy.

In order to keep the “Radiator” cells size reasonable it is necessary to make their heat production transitory, i.e. yield commercial quantities of heat for a fixed period rather than steady state. However if the unit is to be commercial it must be able to generate heat for what amounts to a steady state. This is done by having a set of radiator cells which are successively harvested. The individual cells must be of sufficient size to be able to provide the required energy for commercial production for a period of years. To meet this condition the “Radiator” EGS unit must cycle production among a set of “radiator” cells. A schematic of this system is shown in FIG. 5.

Continuous energy production is maintained by cycling through the radiator cell system in the following fashion. Energy production is from one cell at a time, e.g. Cell 1. When the temperature of the produced fluid reaches the minimum temperature required for commercial energy production, the cell is shut in and moved to a fully restored cell, e.g. Cell 3. Cell 1 then is allowed to re-equilibrate to the ambient geothermal gradient, e.g. Cell 2.

The number of cells (n) required and their size, is set by the number of years for a complete cycle through all of the cells as to allow the i^th cell to return to its initial thermal state. Thus each volume would be allowed to recover for the duration of each cycle; e.g. Volume i produces for N years, then is left to recover while heat extraction shifts to Volume j for N years, etc. The number of cells being set by the amount of time to “draw down” each successive volume such that the total cycle time T_total=n N, is sufficient to allow each Volume i to recover completely.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings provide visual representations, which will be used to more fully describe the representative embodiments disclosed herein and can be used by those skilled in the art to better understand them and their inherent advantages. In these drawings, like reference numerals identify corresponding elements and:

FIG. 1 illustrates a schematic diagram of a Single “vane” unit of a Radiator EGS. A vane is a vertical manufactured fracture system.

FIG. 2 illustrates a schematic diagram of two “vane” units defining the intervening rock volume as V_crit=l₁×l₂×l₃.

FIG. 3 illustrates a schematic diagram of temperature distribution due to injection of fluid at a depth where rock temperature is >T_min.

FIG. 4 illustrates a schematic diagram of a strike section to schematically illustrate how cooling will be slowed due to increasing temperature of fluid as it moves along the injector lateral.

FIG. 5 illustrates a schematic diagram of a multi-cell radiator unit to allow permanent continuous generation of electrical energy.

DETAILED DESCRIPTION

The presently disclosed subject matter now will be described more fully hereinafter with reference to the accompanying Drawings, in which some, but not all embodiments of the inventions are shown. Like numbers refer to like elements throughout. The presently disclosed subject matter may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will satisfy applicable legal requirements. Indeed, many modifications and other embodiments of the presently disclosed subject matter set forth herein will come to mind to one skilled in the art to which the presently disclosed subject matter pertains having the benefit of the teachings presented in the foregoing descriptions and the associated Drawings. Therefore, it is to be understood that the presently disclosed subject matter is not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims.

An embodiment in accordance with the present invention includes an enhanced geothermal system (EGS) configured to allow the commercial production of electrical energy. One criteria of an EGS according to the present invention is that the temperature and volume of the fluids extracted are of sufficiently high temperatures and large enough volumes as to allow the commercial production of electrical energy. The system is able to operate for at least N years before the extracted fluid falls below the minimum temperature needed for energy production. Additionally, fractures are separated from each other by a sufficiently large volume of rock (V_crit) relative to the fractures' surface area, such that ratio of the rate of heat extraction to the rate of heat supply controlled by the thermal conductivity of the rock is such that the intervening rock is cooled at a rate that is sufficiently slow to be economic.

The basic structure of the proposed EGS is to create a set of “manufactured” vertical fractures that can circulate fluids through a rock volume such that the final fluid temperature is sufficient for commercial energy production. The key to creating such fractures is to essentially imitate the way that natural transmissive fracture systems are created, but to do so in a fashion that emulates the geometry of radiator vanes, so as to allow control of the system temperature. The geometry and orientation of the “Radiator” systems fractures is controlled by the ambient stress field including S₁, the maximum stress, and S_Hmax, the horizontal stress component. Below about 700 m, and depending on the specific geologic area, S₁ is vertical and the average strike of transmissive fractures parallels S_Hmax.

Creating fractures that include S₁ and S_Hmax requires drilling laterals that parallel S_Hmax. This is normal to the direction typically chosen for exploiting unconventional Oil and Gas which are drilled normal to S_Hmax. Fracking to “manufacture” the transmissive fracture system will require a highly controllable fracking tool such as energetics which can directionally enhance the rock permeability. For example, propellant based fracking could be used to manufacture the transmissive fracture system. Because S₁ is vertical, fracture orientations will tend towards Mode 1 which tends to limit permeability. However, there should be sufficient variability in orientation of fragment surfaces such that most will have a significant shear component and therefore provide permeability.

FIG. 1 illustrates a schematic view of a set of two wells including of an injector and production well. The system 10 includes the injector well 12 and the production well 14 as a vane unit. Each well includes a vertical pilot hole 16 and a lateral 18 with each well 12, 14 connected to a pumping unit (not illustrated). While one set of injector well 12 and production well 14 are illustrated in FIG. 1 any number of wells can be used, as is known to or conceivable by one of skill in the art. Additionally, examples of expanding the number of wells are also included herein, below. The wells 12, 14 are oriented to parallel the maximum horizontal stress (S_Hmax) 20 with both drilled to depths where the principal stress axis (S₁) 22 is vertical. In general this will be depths >700 m. The production lateral is drilled to parallel the isotherm whose temperature T_min is greater than the minimum temperature required for commercial energy production (T_e). While Tmin is > that reuired for commercial production we can allow the cell to go to Te, where Te is <Tmin. FIG. 1 illustrates a plane 24 parallel to the isotherm whose temperature is T_min. The injector well lateral is drilled to a depth >the depth of the T_min isotherm to a depth of an isotherm T_max. FIG. 1 also illustrates a plane 26 that is parallel to the isotherm whose temperature is T_max. The length (l₃) of the injector below the T_min isotherm plane 25 and the length (l₂) of the lateral 34 are determined by the relationship V_crit=l₁×l₂×l₃, where l₁ is the distance between successive vanes. Here V_crit is the volume necessary to maintain a temperature of the heated fluid produced at the production well ≧T_e for a sufficient amount of time that it will meet the economic criteria for commercial power generation (T_e). The volume V_crit is defined as a single “Radiator” cell.

FIG. 2 illustrates a schematic diagram of two “vane” units defining the intervening rock volume as V_crit=l₁×l₂×l₃. FIG. 2 also illustrates a second vane unit. The second vane unit includes a second injection well 28 and a second production well 30. l₁, 32 is defined as a distance between the laterals 18 of the production wells 14 and 30. l₂, 34 is defined as a length of the lateral 18 of the injection well 28. l₃, 36 is defined as a height between the lateral of the injection well 12 and the production well 14. V_crit is then the volume defined by l₁×l₂×l₃.

The “Radiator Vane” or “manufactured” fracture, is created by successive controlled fracs, one above the other in successively higher laterals located in the plane defined by the injector and production wells. The pressures created by the fracking agent, must be sufficient and rapid enough to exceed the fundamental strength of the rock and thereby open up fractures in the rock. Rocket propellant is an example of such an agent. Because the production and injection wells are directly above and parallel to one another and because the plane they so define includes both S₁ 22 and S_hmax 20 with S_hmm 27 normal to this plane, the successive lateral fracs will form a vertical zone of fractured rock.

FIG. 3 illustrates a schematic diagram of temperature distribution due to injection of fluid at a depth where rock temperature is >T_min, and FIG. 4 illustrates a schematic diagram of a strike section to schematically illustrate how cooling will be slowed due to increasing temperature of fluid as it moves along the injector lateral. FIGS. 3 and 4 show how the system functions to solve the problem of the low thermal conductivity of rock resulting in the too rapid cooling of the surrounding rock. The basic concept is to inject fluid at some depth l₃, 36, below the isotherm surface 24 whose temperature T_min is the minimum required for producing commercial quantities of electrical energy. By doing so as the fluid rises at some point its temperature will be greater than that of the surrounding rock, thus instead of cooling the rock through which it is now passing, it will heat it, as illustrated in FIG. 3. Because the temperature of the surrounding rock through which the fluid is rising is >T_min, the fluid emerging above the T_min isosurface 24 and available for energy production, will be >T_min for some period of time. The critical parameter l₃ 36 is a function of the vertical temperature gradient, dT/dz. The fluid temperature can be maintained to the surface by insulating the casing above the T_min isosurface. The direction of fluid flow in FIG. 3 is vertical.

A similar effect will occur for the “Manufactured” fracture along its strike, as illustrated in FIG. 4. As the fluid enters the fracture at the beginning of the injector lateral, it cools the surrounding rock but itself is gradually heated as it moves along the injector lateral. The result is that as the fluid at the end of the lateral rises its temperature will be closer to the ambient temperature at the depth of the T_max isotherm 26. Consequently there will be even less and therefore slower cooling of the surrounding rock above the lateral “toe” as fluid rises towards the production well. As illustrated in FIGS. 3 and 4, color scale shows relative temperatures. Dark is hottest and light is coolest. T_min isotherm 24 is at a depth where the ambient rock temperature is higher than the one required for commercial energy production. The direction of fluid flow in FIG. 4 is lateral.

In order to keep the “Radiator” cells size reasonable it is necessary to make their heat production transitory, i.e. yield commercial quantities of heat for a fixed period rather than steady state. However, if the entire EGS, including of multiple cells, is to be commercial it must be able to generate heat for what amounts to a steady state. This is done by having a set of radiator cells which are successively harvested. The individual cells must be of sufficient size to be able to provide the required energy for commercial production for a period of years. To meet this condition the “Radiator” EGS unit must cycle production among a set of “radiator” cells. A schematic of this system is shown in FIG. 5 and described by the following.

FIG. 5 illustrates a schematic diagram of a multi-cell radiator unit to allow permanent continuous generation of electrical energy. Continuous energy production is maintained by cycling through the radiator cell system 50 in the following fashion. Energy production is from one cell at a time, e.g. Cell 1, 52. When the temperature of the produced fluid drops below the minimum temperature required for commercial energy production, the cell is shut in and moved to a fully restored cell, e.g. Cell 3, 54. Cell 1 then is allowed to re-equilibrate to the ambient geothermal gradient, e.g. Cell 2, 56.

The number of cells required and their size, is set by the number of years for a complete cycle through all of the cells as to allow the i^th cell to return to its initial thermal state. Thus each volume would be allowed to recover for the duration of each cycle; e.g. Volume i produces for N years, then is left to recover while heat extraction shifts to Volume j for N years, etc. The number of volumes (n) being set by the amount of time to “draw down” each successive volume such that the total cycle time T_total=N, is sufficient to allow each Volume i harvested to recover completely.

The many features and advantages of the invention are apparent from the detailed specification, and thus, it is intended by the appended claims to cover all such features and advantages of the invention, which fall within the true spirit and scope of the invention. Further, since numerous modifications and variations will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation illustrated and described, and accordingly, all suitable modifications and equivalents may be resorted to, falling within the scope of the invention.

Claims

1. A system for harvesting geothermal energy comprising: a non-hydrothermal source of energy;an injector well;a production well; anda radiator vane heat exchanger located in a plane defined by the injector well and the production well that includes S1 and SHmax.

2. The system for harvesting geothermal energy of claim 1 wherein the injector well and the production well each includes a vertical pilot hole and a lateral.

3. The system for harvesting geothermal energy of claim 1 wherein the injector well and the production well are each connected to a pumping unit.

4. The system for harvesting geothermal energy of claim 1 wherein the injector well and the production well are oriented parallel to a maximum horizontal stress axis.

5. The system for harvesting geothermal energy of claim 1 wherein the injector well and the production well are drilled to a position (depth) where a principal stress axis is vertical.

6. The system for harvesting geothermal energy of claim 5 wherein the position is a depth of less than approximately 700 m.

7. The system for harvesting geothermal energy of claim 1 wherein the injector well is drilled from a depth that is greater than the depth of a Tmin isotherm to a depth that is approximately at a Tmax isotherm.

8. The system for harvesting geothermal energy of claim 1 wherein the radiator vane is created by successive controlled fracs, one above the other, in successively higher laterals located in the plane defined by the injector and production wells.

9. The system for harvesting geothermal energy of claim 1 wherein the vane is created using propellant based fracking.

10. A method for harvesting geothermal energy comprising: creating an EGS power unit in a non-hydrothermal source;drilling an injector well oriented to parallel a maximum horizontal stress to a depth where a principal stress axis is vertical;drilling a production well oriented to parallel a maximum horizontal stress to a depth where a principal stress axis is vertical; andgenerating a radiator vane with successive controlled fracs, one above the other, in successively higher laterals located in the plane defined by the injector and production wells.

11. The method of claim 10 further comprising extracting fluid with a temperature and volume that is sufficiently high and large enough to allow the commercial production of energy.

12. The method of claim 10 further comprising optimizing the system to operate for a predetermined number of years before extracted fluid falls below a minimum temperature needed for energy production.

13. The method of claim 10 further comprising separating fractures from each other by a sufficiently large volume of rock relative to a surface area of the fractures, such that a ratio of a rate of heat extraction to a rate of heat supply controlled by thermal conductivity of the non-hydrothermal source is such that intervening rock is cooled at a rate that is sufficiently slow.

14. The method of claim 10 further comprising creating multiple cells in the EGS.

15. The method of claim 14 further comprising cycling through the multiple cells to allow a previously used cell to recover.

16. The method of claim 10 further comprising creating the radiator vane with propellant based fracking.

17. The method of claim 10 further comprising creating the radiator vane with energetics.

18. The method of claim 14 further comprising changing cells when the temperature of the cell drops below a minimum temperature needed for energy production.

19. The method of claim 18 further comprising leaving a geothermal gradient cell between the previously used cell and the next cell identified for production.

20. The method of claim 10 further comprising controlling the geometry and orientation of the radiator vane by an ambient stress field.