Well system for recovery of geothermal energy

Abstract

A well system for recovery of geothermal energy, the system having an injector well having a lateral section extending through a formation between horizontally spaced heel and toe portions of the injector well, and a producer well having a lateral section extending through the formation between horizontally spaced heel and toe portions of the producer well. The lateral sections extend substantially parallel to each other. The formation contains a fracture zone centred on a longitudinal fracture plane which contains the lateral sections. The fracture zone places the lateral sections in fluid communication. The well system is configured to flow a heat recovery fluid into the lateral section of the injector well at the heel portion thereof, pass the flow through the fracture zone, and collect the flow from the lateral section of the producer well at the heel portion thereof.

Description

[0001] Well System for Recovery of Geothermal Energy

[0002] Field of the Invention

[0003] The present invention relates to a well system for recovery of geothermal energy, a method of forming the well system and a method of operating the well system.

[0004] Background

[0005] Enhanced Geothermal Systems (EGSs) represent a useful approach for harnessing geothermal energy, providing a sustainable and low-carbon source of power. Unlike shallow thermal wells that take advantage of permeable aquifers, EGSs make use of subsurface formations that lack fluid permeability by hydraulically fracturing the impermeable rock.

[0006] Figure 1 is a schematic diagram of a conventional EGS 100. Two wellbores are illustrated: an injector well 102 and a producer well 108, each well having a (vertical or off-vertical) down section 104, 110 descending from a wellhead, and a lateral section 106, 112 extending from the bottom of the down section 104, 110 and through the subsurface formation between horizontally spaced heel 106H, 112H and toe portions 106T, 112T. The lateral sections 106, 112 extend parallel to each other in a heel-to-heel arrangement with the heel portion 106H of the injector well 102 adjacent to the heel portion 112H of the producer well 108, and their respective toe portions 106T, 112T similarly adjacent. The formation contains a plurality of fractures on planes 114 which are substantially perpendicular to and distributed along the lateral sections 106, 112, the fractures placing the lateral sections 106, 112 in fluid communication. These fractures can be generated by hydraulic fracturing the formation through perforations on the wellbore wall of one or both of the lateral sections 106, 112.

[0007] EGS wells target ‘hot’ subsurface formations which typically have insufficient natural fluid or permeability for ‘shallow’ geothermal systems to work effectively. For example, the target formation may be granite, 3 to 5 km underground where temperatures can reach over 200°C. The target formation is usually identified and investigated via geophysical surveys. A geological model of the site can then be built to better understand important downhole parameters such as temperature gradients, formation permeabilities and in-situ stresses.

[0008] These stresses are primarily due to a combination of gravitational and tectonic loadings on the subsurface formation. At any point, the stress may be decomposed into three principal stresses along respective orthogonal directions in which the normal stress components are maximized and the shear stress components are zero. Estimating the magnitude and orientation of these principal stresses is important as they determine the planes on which hydraulic fractures will tend to propagate.

[0009] More particularly, one of the principal stress, ov, is often orientated along or close to the vertical direction, as it is associated with the weight exerted at a point by the overburden above it. The other two principal stresses are generally less than ovand may both be orientated along or close to horizontal directions, with one of these stresses, CJHmax, being greater than the other, OHmin.008772329

[0010] 2

[0011] In a conventional EGS 100, such as that shown in Figure 1 , the lateral sections 106, 112 of the wells are drilled along directions that are closely aligned with OHmin. As hydraulic fractures are energetically inclined to open up across planes of minimum compressive stress, the fracture planes 114 generated between the injector and producer wells 102, 108 orientate perpendicularly to the lateral sections 106, 112.

[0012] In operation, a heat recovery fluid is pumped from the down section 104 of the injector well 102 into its lateral section 106 via the heel portion 106H. The heat recovery fluid then passes through the fractures and extracts heat from the formation, before arriving in the lateral section 112 of the producer well 108. The heat recovery fluid then flows upwards through the down section 110 of the producer well 108 for collection at its wellhead. Some of the heat recovery fluid is typically lost during this process, such that the flow rate Qi of fluid into the wellhead of the injector well 102 is greater than the flow rate Qo of fluid out of the wellhead of the producer well 108.

[0013] The fractures produce a large contact area between the formation and the heat recovery fluid to improve the heat transfer effectiveness. However, as shown in Figure 2, there is a tendency for the heat recovery fluid to flow on a given fracture plane on the shortest path between lateral sections 106, 112, and not to use the entirety of the fracture area on that plane.

[0014] The total contact area can be increased by adding more fracture planes 114 to the system. However, having many fracture planes 114 can cause a further problem of poor conformance in which fluid preferentially and self-reinforcingly flows through only some of the fractures.

[0015] In more detail, Figure 3 shows schematically a top-down view of the lateral sections 106, 112, the arrows indicating flow of heat recovery fluid through fractures of approximately equally spaced fracture planes 114, the sizes of the arrows indicating the relative amount of flow at the respective fracture planes.

[0016] Some fracture planes 114 have a lower resistance to fluid flow than others, resulting in the fracture paths within these planes being preferential for fluid flow. As these preferential paths receive a greater proportion of the flow of heat recovery fluid, the surrounding formation cools down more quickly, which in turn, tends to expand the fractures of the preferential paths. Thus, making these preferential paths more likely to take even more heat recovery fluid over time. This self-reinforcing process reduces fluid flow on other fracture planes 114 and limits the heat that can be extracted from them. This is a persistent problem in conventional EGSs 100.

[0017] Localisation of the fluid flow through only a few dominant fractures reduces the overall heat transfer effectiveness. US 11808121 (herein incorporated herein by reference) proposes techniques to address this problem, but these involve numerous pre-characterisation tests and may require re-evaluation as fractures dynamically change over the operational lifetime of an EGS.

[0018] There is, therefore, a need for alternative approaches to increase the heat transfer efficiency of geothermal energy systems.

[0019] The present invention has been devised in light of the above considerations.008772329

[0020] 3

[0021] Summary of the Invention

[0022] The present invention is at least partly based on a recognition that a fracture zone centred on a longitudinal fracture plane and placing an injector well and a producer well in fluid communication can overcome problems of poor conformance commonly associated with conventional EGSs.

[0023] Thus, in a first aspect, the present invention provides a well system for recovery of geothermal energy, the system having:

[0024] an injector well having a lateral section extending through a formation between horizontally spaced heel and toe portions of the injector well, and

[0025] a producer well having a lateral section extending through the formation between horizontally spaced heel and toe portions of the producer well;

[0026] the lateral sections extending substantially parallel to each other,

[0027] the formation containing a fracture zone centred on a longitudinal fracture plane which contains the lateral sections, the fracture zone placing the lateral sections in fluid communication, and

[0028] the well system being configured to flow a heat recovery fluid into the lateral section of the injector well at the heel portion thereof, pass the flow through the fracture zone, and collect the flow from the lateral section of the producer well at the heel portion thereof.

[0029] Advantageously, forming a fracture zone centred on a longitudinal fracture plane can provide a dense network of interconnected fractures in fluid communication with substantially the entire lengths of the injector and producer well lateral sections. Moreover, the heat recovery fluid traversing the fracture zone is less likely to segregate to preferential paths than in a conventional EGS as the flow resistance across the fracture plane is generally quite uniform over the length of the lateral sections. The longitudinal fracture plane also provides a large effective contact area between the formation and the heat recovery fluid.

[0030] In addition, as the fracture zone is largely bounded by the lateral sections of the injector and producer wells, there is significantly less opportunity for heat recovery fluid to be lost into the formation than in a conventional EGS. Therefore, the flow rate Qoof fluid out of the wellhead of the producer well may be more closely matched to the flow rate Qi of fluid into the wellhead of the injector well.

[0031] The lateral sections of the well system may extend parallel to each other in a heel-to-toe arrangement with the heel portion of the injector well adjacent the toe portion of the producer well and the toe portion of the injector well adjacent the heel portion of the producer well. A heel-to-toe arrangement, in which the heel portions are maximally spaced from each other, reduces the chance of the heat recovery fluid shortcircuiting the majority of the fracture zone. However, this is not to exclude that the lateral sections of the well system may extend parallel to each other in a heel-to-heel arrangement. Such an arrangement can allow the injector and producer wells to share a common wellhead area, reducing the surface footprint of the system.

[0032] The longitudinal fracture plane, on which the fracture zone is centred, may be a vertical plane. This is generally a preferred orientation for the plane when one of the principal stresses, ov, is the largest, or at008772329

[0033] 4

[0034] least not the smallest, principal stress and is along or close to the vertical direction. However, this is not to exclude that other orientations of the longitudinal fracture plane are possible, according to circumstance. For example, when there is no principal stress in the vertical direction, the longitudinal fracture plane may not be a vertical plane. Indeed, according to another example, it can be preferred that the longitudinal fracture plane is a horizontal plane, e.g. when ov is the smallest principal stresses.

[0035] Ideally, the lateral sections are perfectly aligned relative to each other within the longitudinal fracture plane. In practice, however, due to the typically branching nature of fractures in the fracture zone a degree of misalignment of the lateral sections can be tolerated. For example, where the intention is to drill parallel lateral sections contained in a longitudinal fracture plane that is normal to the direction of the smallest of the three principal stresses in the formation, a drilling error of a few metres can typically be tolerated, where the drilling error is defined as the relative offset of the two lateral sections in the direction of the smallest of the three principal stresses.

[0036] On heating, the heat recovery fluid reduces in density and thus tends to segregate upwards in the fracture zone, under the influence of gravity, relative to colder fluid. To take advantage of this, the lateral section of the producer well may be above the lateral section of the injector well.

[0037] One of the injector well and producer well lateral sections may be cased and perforated, and the other of the injector well and producer well lateral sections may be open hole. Preferably the injector well lateral section is cased and perforated, and the producer well lateral section is open hole. The fracture zone is generally formed by hydraulic fracturing from cased and perforated lateral section, as this allows for control of the location and direction of high pressure fluid into the fracture. Leaving the other lateral section open hole, however, can increase the likelihood of extensive fluid communication being established between that lateral section and the fracture zone, and thus between the lateral sections of both wells. Alternatively, the injector well and the producer well lateral sections may both be cased and perforated. Having both lateral sections cased and perforated advantageously enables ‘simul-fracking’, in which simultaneous hydraulic fracking is performed from both the injector and producer wells. However, whether the fracking is performed simultaneously or sequentially, having the fractures emanate from both lateral sections and meet in the fracture zone again improves the likelihood of extensive fluid communication being established between the lateral sections.

[0038] In a second aspect, the present invention provides a method of forming the well system according to the first aspect, the method including steps of:

[0039] drilling the injector well,

[0040] drilling the producer well, and

[0041] hydraulically fracturing the formation to generate the fracture zone.

[0042] The step of fracturing the formation to generate the fracture zone may include simultaneously or sequentially fracturing from both lateral sections.

[0043] The drilling of the injector well can be performed before, simultaneously with, or after the drilling of the producer well. Typically both wells are drilled before performance of the hydraulic fracturing. However,008772329

[0044] 5

[0045] this is not to exclude that the fracturing can be performed after the drilling of one well and before the drilling of the other well (which then extends along the generated fracture zone).

[0046] The method may further include a preliminary step of:

[0047] determining the three principal stress directions in the formation;

[0048] wherein the injector and producer wells are drilled such that the normal to the longitudinal fracture plane is the direction of the smallest of the three principal stresses.

[0049] As hydraulic fractures are energetically inclined to open up across planes of minimum compressive stress, drilling the injector and producer wells in this way means that the fracture zone generated between the injector and producer wells can thus be centred on a longitudinal fracture plane which contains the lateral sections. The determination of principal stresses may be performed by forming a geological model of the formation e.g. based on geological and geomechanical surveys.

[0050] In a third aspect, the present invention provides a method of operating the well system according to the first aspect for the recovery of geothermal energy, the method including:

[0051] flowing a heat recovery fluid into the lateral section of the injector well at the heel portion thereof, and

[0052] collecting the flow from the lateral section of the producer well at the heel portion thereof; wherein the flow travels through the fracture zone between the lateral sections.

[0053] The method of operating the well system may include reversing the direction of fluid flow such that the heat recovery fluid is flowed into the lateral section of the producer well at the heel portion thereof and collected from the lateral section of the injector well at the heel portion thereof, the flow travelling in reverse direction through the fracture zone between the lateral sections. An EGS has a limited operational lifetime primarily due to the depletion of the available heat in the formation and decreasing fluid flow rates. This may be extended by reversing the direction of fluid flow.

[0054] The invention includes the combination of the aspects and preferred features described except where such a combination is clearly impermissible or expressly avoided.

[0055] Summary of the Figures

[0056] Embodiments and experiments illustrating the principles of the invention will now be discussed with reference to the accompanying figures in which:

[0057] Figure 1 shows a schematic diagram of a conventional EGS in a perspective view.

[0058] Figure 2 shows a schematic diagram of one fracture plane of the EGS of Figure 1 .

[0059] Figure 3 shows a schematic diagram of lateral sections of the conventional EGS in a top-down view. Figure 4 shows a schematic diagram of an EGS, connected via a longitudinal fracture plane, in a heel-to-toe formation in a perspective view.

[0060] Figure 5 shows a schematic diagram of a fracture zone of the EGS of Figure 4.008772329

[0061] 6

[0062] Figure 6A shows a schematic diagram of lateral sections of the EGS of Figure 4 after fracking from just one lateral section, and Figure 6B shows a schematic diagram of lateral sections of the EGS of Figure 4 after simultaneous or sequential fracking from both lateral sections.

[0063] Detailed Description of the Invention

[0064] Aspects and embodiments of the present invention will now be discussed with reference to the accompanying figures. Further aspects and embodiments will be apparent to those skilled in the art. Figure 4 shows schematically an injector well 302 and a producer well 308 of an EGS 300 according to the present invention. The injector well 302 has a down section 304, and a lateral section 306 extending from a heel portion 306H with the down section 304 to a toe portion 306T. Similarly, the producer well has a down section 310, and a lateral section 312 extending from a heel portion 312H with the down section 310 to a toe portion 312T. The lateral sections 306, 312 extend through a ‘hot’ subsurface formation, and are located one above the other with the producer well lateral section 312 above the injector well lateral section 306. The lateral sections 306, 312 are parallel to each other, and conveniently they are arranged heel-to-toe with the heel portion of the injector well 306H adjacent to the toe portion of the producer well 312T, and similarly the heel portion of the producer well 312H adjacent to the toe portion of the injector well 306T. However, this is not to exclude that in other EGSs the lateral sections could be in a heel-to-heel and toe-to-toe arrangement. As discussed in more detail below, the formation contains a fracture zone 314 centred on a longitudinal fracture plane which contains the lateral sections 306, 312, the fracture zone placing the lateral sections 306, 312 in fluid communication.

[0065] In operation, a heat recovery fluid is pumped from the down section 304 of the injector well 302 into its lateral section 306 via the heel portion 306H. The heat recovery fluid then passes through the fracture zone 314 and extracts heat from the formation, before arriving in the lateral section 312 of the producer well 308, as illustrated by the schematic diagram of the fracture zone 314 in Figure 5. The heat recovery fluid then flows upwards through the down section 310 of the producer well 308 for collection at its wellhead.

[0066] Preliminary to drilling the wells 302, 308, the principal stresses in the formation are determined (e.g. by geological and geomechanical surveys). Thereafter, the injector 302 and producer 308 wells are drilled with their lateral sections 306, 312 extending along directions such that the normal to the plane containing the lateral sections is closely aligned with OHmin. Hydraulic fracturing (“fracking”) is typically employed to produce the fracture zone 314. As hydraulic fractures are energetically inclined to open up across planes of minimum compressive stress, the fracture zone 314 generated between the injector and producer wells 302, 308 is thus centred on a longitudinal fracture plane which contains the lateral sections 302, 308. As previously mentioned, one of the principal stresses, ov, is often orientated along or close to the vertical direction. When ov is not the smallest principal stress, the longitudinal fracture plane is then typically a vertical plane, as shown in Figure 4. However, this is not to exclude that the longitudinal fracture plane can have other orientations. For example, it is possible in some circumstances for the smallest principal008772329

[0067] 7

[0068] stress to lie along or close to the vertical direction, in which case the lateral sections 306, 312 may be drilled at the same depth and the longitudinal fracture plane may be a horizontal plane.

[0069] The pressure difference between the lateral sections 306, 312 is primarily responsible for driving the flow through the fracture zone 314. However, on heating, the heat recovery fluid reduces in density and thus tends to segregate upwards in the fracture zone relative to colder fluid. Locating the lateral section 312 of the producer well 308 above the lateral section 306 of the injector well 302 takes advantage of this tendency.

[0070] As indicated in Figure 5, the injector lateral section 306 can be cased and perforated (‘C&P’). To perform the fracking, high pressure fluid is pumped down the injector well 302 to generate fractures that spread upwards and meet the producer lateral section 312. Proppants are then deployed to keep the fractures open. The producer lateral section 312 may be left open hole, as indicated in Figure 5 for connection to the fractures. However, in other arrangements, the producer lateral section 312 may be cased and perforated. A cased and perforated producer lateral section advantageously allows simultaneous or sequential fracking operations to be performed in which hydraulic fracking is performed from both the injector and producer wells, the fractures emanating from the respective lateral sections meeting in the fractures zone 314 and improving the connectivity between the wells. After fracking, the casing perforations act as outlets / inlets for the heat recovery fluid between the lateral sections and the fractures. Figure 6A shows a schematic diagram of the lateral sections 306, 312 of the EGS 300 and illustrates the resulting fracture zone 314 when fracking is performed from just the injector lateral section 306. The fracture zone 314 comprises branched fractures emanating from the injector lateral section. This branched nature provides a high likelihood that at least some fractures will arrive at the producer lateral section 312 and establish good fluid communication across the fracture zone 314, even if the producer lateral section is horizontally misaligned within an acceptable drilling error relative to the injector lateral section. To ensure the injector well lateral section 306 and producer well lateral section 312 intersect with the fracture zone 314 along most of their length, the drilling error should be limited to a few metres.

[0071] Figure 6B shows a corresponding schematic diagram illustrating the result of generating fractures from both wells (either simultaneously or sequentially). The fractures emanating from both lateral sections meet in the formation at locations between the lateral sections, the branched fractures from both sides further increasing the likelihood of good fluid communication being established across the fracture zone 314.

[0072] Advantageously, the longitudinal fracture plane of the EGS 300 provides a large effective contact area between the formation and the heat recovery fluid. It also allows the entire lengths of the injector and producer lateral sections 306, 312 to be used to input the fluid into and extract the fluid out of the formation. This is in contrast to the conventional EGS 100 shown in Figure 1 in which just the specific locations on the lateral sections 106, 112 that join to the perpendicular fractures 114 are used for fluid input and extraction.

[0073] In addition, the longitudinal fracture plane of the EGS 300 helps to overcome problems of poor conformance, in which fluid preferentially and self-reinforcing ly flows through only some of the fractures.008772329

[0074] 8

[0075] The fractures within the fracture zone 314 are dense and interconnected, producing a largely uniform flow across the entire fracture zone in which localised preferential flow paths are discouraged. This good conformance increases the heat transfer effectiveness of the EGS. It also allows, in contrast to the heel-to-toe arrangement illustrated in Figure 4, a heel-to-heel arrangement of the injector and producer wells to be adopted in which the wellheads of the wells are located side-by-side, e.g. in order to reduce the total surface footprint of the EGS. In a heel-to-heel arrangement there could be a concern that the heat recovery fluid may open a preferential path across the fracture zone close to the heel sections of both wells, such a path short-circuiting the majority of the fracture zone and reducing the overall heat transfer effectiveness. However, the good conformance reduces the tendency for the system to short-circuit in this way.

[0076] Next, Figures 4 and 5 show that the effective area of the fracture zone 314 is bounded at top and bottom by the injector and producer lateral sections 306, 312. This further enhances the heat transfer effectiveness of the EGS because, unlike the perpendicular fracture plane shown in Figure 2, the entirety of the fracture zone 314 is used for the flow of heat transfer fluid. In addition, there is a significantly reduced opportunity for heat recovery fluid to be lost into the formation, such that the flow rate Qo of fluid out of the wellhead of the producer well 308 is more closely matched to the flow rate Qi of fluid into the wellhead of the injector well 302.

[0077] To extend the operational life of the EGS 300, the direction of fluid flow may be reversed so that the heat recovery fluid is pumped into the producer well 308 and collected from the injector well 302.

[0078] ***

[0079] The features disclosed in the foregoing description, or in the following claims, or in the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for obtaining the disclosed results, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

[0080] While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

[0081] For the avoidance of any doubt, any theoretical explanations provided herein are provided for the purposes of improving the understanding of a reader. The inventors do not wish to be bound by any of these theoretical explanations.

[0082] Any section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described.

[0083] Throughout this specification, including the claims which follow, unless the context requires otherwise, the word “comprise” and “include”, and variations such as “comprises”, “comprising”, and “including” will be008772329

[0084] 9

[0085] understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

[0086] It must be noted that, as used in the specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Ranges may be expressed herein as from “about” one particular value, and / or to “about” another particular value. When such a range is expressed, another embodiment includes from the one particular value and / or to the other particular value. Similarly, when values are expressed as approximations, by the use of the antecedent “about,” it will be understood that the particular value forms another embodiment. The term “about” in relation to a numerical value is optional and means for example + / - 10%.

Claims

00877232910Claims:

1. A well system for recovery of geothermal energy, the system having:an injector well having a lateral section extending through a formation between horizontally spaced heel and toe portions of the injector well, anda producer well having a lateral section extending through the formation between horizontally spaced heel and toe portions of the producer well;the lateral sections extending substantially parallel to each other,the formation containing a fracture zone centred on a longitudinal fracture plane which contains the lateral sections, the fracture zone placing the lateral sections in fluid communication, andthe well system being configured to flow a heat recovery fluid into the lateral section of the injector well at the heel portion thereof, pass the flow through the fracture zone, and collect the flow from the lateral section of the producer well at the heel portion thereof.

2. The well system according to claim 1 , wherein the lateral sections extend parallel to each other in a heel-to-toe arrangement with the heel portion of the injector well adjacent the toe portion of the producer well and the toe portion of the injector well adjacent the heel portion of the producer well path.

3. The well system according to claim 1 or 2, wherein the longitudinal fracture plane is a vertical plane.

4. The well system according to any one of the previous claims, wherein the lateral section of the producer well is above the lateral section of the injector well.

5. The well system according to any one of the previous claims, wherein one of the injector well and producer well lateral sections is cased and perforated, and the other of the injector well and producer well lateral sections is open hole.

6. The well system according to any one of claims 1 to 4, wherein the injector well and the producer well lateral sections are both cased and perforated.

7. A method of forming the well system according to any one of the previous claims, the method including steps of:drilling the injector well,drilling the producer well, andhydraulically fracturing the formation to generate the fracture zone.

8. The method of claim 7, wherein the step of fracturing the formation includes simultaneous or sequential fracturing from both lateral sections.

9. The method of claim 7 or 8, further including a preliminary step of:determining the three principal stress directions in the formation;00877232911wherein the injector and producer wells are drilled such that the normal to the longitudinal fracture plane is the direction of the smallest of the three principal stresses.

10. A method of operating the well system according to any one of claims 1 to 6 for the recovery of geothermal energy, the method including:flowing a heat recovery fluid into the lateral section of the injector well at the heel portion thereof, andcollecting the flow from the lateral section of the producer well at the heel portion thereof; wherein the flow travels through the fracture zone between the lateral sections.

11. The method of claim 10, further including:reversing the direction of fluid flow such that the heat recovery fluid is flowed into the lateral section of the producer well at the heel portion thereof and collected from the lateral section of the injector well at the heel portion thereof, the flow travelling in reverse direction through the fracture zone between the lateral sections.

Images