Well system for recovery of geothermal energy

Abstract

Enhanced Geothermal System 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 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. The formation contains a plurality of fractures on planes which are substantially perpendicular to and distributed along the lateral sections, the perpendicular fractures placing the lateral sections in fluid communication. The well system is configured to receive a heat recovery fluid into the lateral section of the injector well at the heel portion thereof, pass the heat recovery fluid through the fractures, and collect the heat recovery fluid from the lateral section of the producer well at the heel portion thereof. The lateral section of the injector well contains an injector well plug and the lateral section of the producer well contains a producer well plug, each plug forms a local block to fluid flow through the respective lateral section, and the plugs are at staggered positions such that a flow of the heat recovery fluid follows a serpentine path in which, to circumvent first the injector well plug, the flow is forced from the lateral section of the injector well to the lateral section of the producer well, and then, to avoid the producer well plug, the flow is forced from the lateral section of the producer well to return to the lateral section of the injector well downstream of the injector well plug, thereafter the flow returns to the producer well downstream of the producer well plug.

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.008772295

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[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 102, 108. The pressure difference between the lateral sections 106, 112 is primarily responsible for driving the flow through the fractures.

[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 flows in parallel 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. 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. Such issues of poor conformance can be exacerbated in an EGS with long lateral sections due to pressure losses along the wells.

[0014] In more detail, Figure 2 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.

[0015] 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.

[0016] 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.

[0017] There is, therefore, a need for alternative approaches that address the problem of poor conformance to increase the heat transfer efficiency of geothermal energy systems.

[0018] The present invention has been devised in light of the above considerations.008772295

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[0020] Summary of the Invention

[0021] The present invention is at least partly based on a recognition that providing a serpentine flow path for the heat recovery fluid, in which the fluid flows back and forth between injector and producer lateral section, can overcome problems of poor conformance commonly associated with conventional EGSs.

[0022] Thus, in a first aspect, the present invention provides a well system for recovery of geothermal energy, the system having:

[0023] an injector well having a lateral section extending through a formation between horizontally spaced heel and toe portions of the injector well, and

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

[0025] the lateral sections extending substantially 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,

[0026] the formation containing a plurality of fractures on planes which are substantially perpendicular to and distributed along the lateral sections, the perpendicular fractures placing the lateral sections in fluid communication, and

[0027] the well system being configured to receive a heat recovery fluid into the lateral section of the injector well at the heel portion thereof, pass the heat recovery fluid through the fractures, and collect the heat recovery fluid from the lateral section of the producer well at the heel portion thereof;

[0028] wherein the lateral section of the injector well contains an injector well plug and the lateral section of the producer well contains a producer well plug, each plug forming a local block to fluid flowthrough the respective lateral section, and the plugs being at staggered positions such that a flow of the heat recovery fluid follows a serpentine path in which, to circumvent first the injector well plug, the flow is forced from the lateral section of the injector well to the lateral section of the producer well, and then, to avoid the producer well plug, the flow is forced from the lateral section of the producer well to return to the lateral section of the injector well downstream of the injector well plug, thereafter the flow returning to the producer well downstream of the producer well plug.

[0029] Advantageously, introducing well plugs in this way directs the heat recovery fluid such that its serpentine path traverses the perpendicular fracture planes in series, crossing back and forth between the lateral sections multiple times. Consequently, the heat recovery fluid cannot segregate to preferential fracture planes offering lower flow resistance, as in a conventional EGS. Moreover, as the flow on the serpentine path does not split into parallel flows on different fracture planes, increasing the number of fracture planes for better heat transfer effectiveness or increasing the length of the lateral sections does not worsen the well system conformance.

[0030] The lateral section of the injector well may contain plural of the injector well plugs and the lateral section of the producer well may contain plural of the producer well plugs, the injector well plugs and the producer well plugs being at staggered and alternating positions to extend the serpentine path by increasing the number of back and forth returns of the flow between the lateral sections. Extending the serpentine path008772295

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[0032] in this way increases the number of fractures the heat recovery fluid passes through, thereby increasing the temperature of the fluid. However, increasing the number of back and forth returns also increases the overall flow resistance of the serpentine path, thereby raising the pumping pressure needed to achieve a desired heat recovery fluid flow rate.

[0033] Preferably the injector well lateral section is cased and perforated, and the producer well lateral section is open hole. The fracture planes are generally formed by hydraulic fracturing (“fracking”) from a 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 can increase the likelihood of extensive fluid communication being established between that lateral section and the fracture planes, 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 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 plane, again improves the likelihood of extensive fluid communication being established between the lateral sections.

[0034] The well system may have two of the producer wells, the lateral sections of the producer wells flanking the lateral section of the injector well. Such a triplet well system may be employed to increase the amount of heat extracted from a formation. Indeed, more generally, the well system may have plural of the producer wells and plural of the injector wells, the lateral sections of the producer wells and the lateral sections of the injector wells forming an alternating array.

[0035] Each plug may form a complete local block to fluid flow through the respective lateral section, such that all the heat recovery fluid arriving at a plug is forced to follow the serpentine path. Alternatively, however, one, some or all of the plugs may form a partial local block to fluid flow through the respective lateral section, such that a portion of the heat recovery fluid arriving at a plug is forced to follow the serpentine path, but another portion of the flow arriving at that plug continues along the respective lateral section. Splitting the flow in this way enables control of the pressure differentials in the well system, i.e. between the injector and producer well lateral sections at a given fracture plane, and between spaced positions along a given lateral section.

[0036] In a second aspect, the present invention provides a procedure for assisting (e.g. increasing) energy production from a well system for recovery of geothermal energy, the well 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 extending 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, the formation containing a plurality of fractures on planes which are substantially perpendicular to and distributed along the lateral sections, the perpendicular fractures placing the lateral sections in fluid communication, and the well system being configured to receive a heat recovery fluid into the lateral008772295

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[0038] section of the injector well at the heel portion thereof, pass the heat recovery fluid through the fractures, and collect the heat recovery fluid from the lateral section of the producer well at the heel portion thereof;

[0039] wherein the procedure includes:

[0040] modelling characteristics of the well system to determine suitable locations of well plugs such that the lateral section of the injector well contains an injector well plug and the lateral section of the producer well contains a producer well plug, each plug forming a local block to fluid flow through the respective lateral section, and the plugs being at staggered positions along such that a flow of the heat recovery fluid follows a serpentine path in which, to circumvent first the injector well plug, the flow is forced from the lateral section of the injector well to the lateral section of the producer well, and then, to avoid the producer well plug, the flow is forced from the lateral section of the producer well to return to the lateral section of the injector well downstream of the injector well plug, thereafter the flow returning to the producer well downstream of the producer well plug, and

[0041] locating well plugs in the lateral sections of the injector well and the producer well at the determined suitable locations.

[0042] Thus the procedure of the second aspect may be used to transform an existing well system into a well system of the first aspect. This can be particularly useful if the existing well system is suffering from poor conformance. Alternatively, the procedure of the second aspect may be used to reset or reconfigure the well plugs of an existing well system of the first aspect (e.g. by moving one or more plugs, adding one or more plugs and / or removing one or more plugs). In this way the procedure can improve the performance or extend the life of the existing well system.

[0043] The location of the well plugs is typically determined by the intersections of the fracture planes with the injector and producer lateral sections. Each plug should generally be located on its respective lateral section between a pair of intersections with adjacent fracture planes so that the plug diverts flow out of its lateral section on the upstream intersection of the pair, and receives flow back from the other lateral section on the downstream intersection of the pair. Plugs suitably configured for use in cased and / or open hole wells may be used.

[0044] The modelling may include modelling characteristics of the well system to determine suitable locations of well plugs such that the lateral section of the injector well contains plural of the injector well plugs and the lateral section of the producer well contains plural of the producer well plugs, the injector well plugs and the producer well plugs being at staggered and alternating positions to extend the serpentine path by increasing the number of back and forth returns of the flow between the lateral sections.

[0045] Each plug may form a complete local block to fluid flow through the respective lateral section, such that all the heat recovery fluid arriving at a plug is forced to follow the serpentine path. Alternatively, however, one, some or all of the plugs may form a partial local block to fluid flow through the respective lateral section, such that a portion of the heat recovery fluid arriving at a plug is forced to follow the serpentine path, but another portion of the flow arriving at that plug continues along the respective lateral section. Accordingly, the modelling of the characteristics of the well system may also suitably determine whether each plug should form a complete block or a partial block. Indeed, in the case where plugs can be008772295

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[0047] configured to provide different degrees of blockage, the modelling may also suitably determine the degree of partial blockage to be provided at a given plug.

[0048] Preferably the injector well is cased and perforated, and the producer well is open hole. Alternatively, the injector well and the producer well lateral sections may both be cased and perforated.

[0049] The well system may comprise two of the producer wells, the lateral sections of the producer wells flanking the lateral section of the injector well. More generally, the well system may have plural of the producer wells and plural of the injector wells, the lateral sections of the producer wells and the lateral sections of the injector wells forming an alternating array.

[0050] In a third aspect, the present invention provides a method of forming the well system according to the first aspect, the method including steps of:

[0051] drilling the injector well,

[0052] drilling the producer well,

[0053] hydraulically fracturing the formation to generate the fractures, and

[0054] locating the well plugs.

[0055] The step of fracturing the formation to generate the fractures may include fracturing from just one of the lateral sections, or simultaneously or sequentially fracturing from both lateral sections.

[0056] 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, 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 intersects the generated fractures).

[0057] The step of locating the well plugs may include performing the procedure of the second aspect.

[0058] The method may further include a preliminary step of:

[0059] determining the three principal stress directions in the formation;

[0060] wherein the injector and producer wells are drilled such that the normal to the fracture planes is the direction of the smallest of the three principal stresses.

[0061] 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 planes generated between the injector and producer wells orientate substantially perpendicularly to the well 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.

[0062] In a fourth 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:

[0063] directing a heat recovery fluid into the lateral section of the injector well at the heel portion thereof, and

[0064] collecting the heat recovery fluid from the lateral section of the producer well at the heel portion008772295

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[0066] thereof;

[0067] wherein a flow of the heat recovery fluid travels on the serpentine path between the heel portions. The method of operating the well system may include reversing the direction of fluid flow such that the heat recovery fluid is directed 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 heat recovery fluid flowing in reverse direction on the serpentine path between the heel portions. 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.

[0068] The invention includes the combination of the aspects and preferred features described except where such a combination is clearly impermissible or expressly avoided.

[0069] Summary of the Figures

[0070] Embodiments and experiments illustrating the principles of the invention will now be discussed with reference to the accompanying figures in which:

[0071] Figure 1 shows a schematic diagram of a conventional EGS from a perspective view.

[0072] Figure 2 shows a schematic diagram of lateral sections of the conventional EGS system in a top-down view.

[0073] Figure 3 shows a schematic diagram of lateral sections of an EGS with injector and producer well plugs in a top-down view.

[0074] Figure 4 shows a schematic diagram of lateral sections of an EGS with injector and producer well plugs with a triplet well system in a top-down view.

[0075] Figure 5 shows a schematic diagram of lateral sections of a variant EGS with injector and producer well plugs with a triplet well system in a top-down view.

[0076] Detailed Description of the Invention

[0077] 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. All documents mentioned in this text are incorporated herein by reference.

[0078] Figure 3 shows schematically an injector well 202 and a producer well 208 of an EGS 200 according to the present invention. The injector well 202 has a down section (not shown), and a lateral section 206 extending from a heel portion 206H with the down section to a toe portion 206T. Similarly, the producer well has a down section (not shown) and a lateral section 212 extending from a heel portion 212H with the down section to a toe portion 212T. The lateral sections 206, 212 are parallel to each other, and are arranged heel-to-toe with the heel portion of the injector well 206H adjacent to the toe portion of the008772295

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[0080] producer well 212T, and similarly with the heel portion of the producer well 212H adjacent to the toe portion of the injector well 206T. The formation contains a plurality of fractures on planes 214 which are substantially perpendicular to and distributed along the lateral sections 206, 212, the fractures placing the lateral sections 206, 212 in fluid communication. These fractures can be generated by hydraulic fracturing (“fracking”) the formation through perforations in the cased wall of the injector lateral section 206, the producer lateral section 212 remaining open hole to receive the fractures 214, as discussed in more detail below.

[0081] The lateral section of the injector well contains injector well plugs 216 and the lateral section of the producer well contains producer well plugs 218. The plugs locally completely block fluid flow in the respective lateral section, and direct it instead along the fracture plane 214 immediately upstream of the plug. The injector well plugs 216 and the producer well plugs 218 are located at staggered and alternating positions along the lateral sections 206, 212, with each plug 216, 218 being placed between the intersections of two adjacent fracture planes 214 with the respective lateral section.

[0082] In operation, a heat recovery fluid is pumped from the down section of the injector well 202 at a rate Qi into its lateral section 206 via the heel portion 206H. The heat recovery fluid is then forced to follow a serpentine path back forth between the lateral sections 206, 212 over the fracture planes 214. In more detail, to circumvent a given injector well plug 216, the flow is forced from the lateral section of the injector well 206 through the fractures of the fracture plane 214 upstream of that plug to the lateral section of the producer well 212, and then, to circumvent the subsequent producer well plug 218, the flow is forced from the lateral section of the producer well 212 through the fractures of the next fracture plane 214 to return to the lateral section of the injector well 216 downstream of the original injector well plug 216. This process repeats along the lateral sections, with each plug 216, 218 being located on its respective lateral section between a pair of intersections of that lateral section with adjacent fracture planes 214. At the end of the serpentine path the flow returns to the producer lateral section 212, and then exits via the down section of the producer well 208 at a rate Qo for collection at its wellhead. The serpentine path of the heat recovery fluid is illustrated by the alternating directions of the arrows in Figure 3. The arrows are of equal size because, barring some inevitable losses into the formation, the amount of flow is substantially the same on all the fracture planes of the serpentine path. As the heat recovery fluid passes between the injector and producer lateral sections via the fractures of the fracture planes 214 it extracts heat from the formation.

[0083] Preliminary to drilling the wells 202, 208, the principal stresses in the formation are determined (e.g. by geological and geomechanical surveys). Thereafter, the injector 202 and producer 208 wells are drilled with their lateral sections 206, 212 extending along directions which are closely aligned with OHmin. As hydraulic fractures are energetically inclined to open up across planes of minimum compressive stress, the fracture planes 214 generated between the injector and producer wells 202, 208 are thus perpendicular to the lateral sections 206, 212.

[0084] To perform the fracking, the wall of the injector well lateral section 206 is first cased and perforated (‘C&P’) along its length. High pressure fluid is pumped down the injector well 202 through the008772295

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[0086] perforations to generate fractures that spread laterally and meet the open hole producer lateral section 212. Proppants are then deployed to keep the fractures open. However, another option is for the fracking to be performed from a cased and perforated producer lateral section and the injector lateral section left open hole. A further option is for both lateral sections 206, 212 to be cased and perforated. This allows simultaneous or sequential hydraulic fracking to be performed from both the injector and producer wells, the fractures emanating from the respective lateral sections meeting in the fracture planes 214 and improving the connectivity between the wells.

[0087] Well characterisation and / or modelling can be used to optimise well plug placement. As a minimum, the injector lateral section 206 and the producer lateral section 212 each have one well plug 216, 218.

[0088] However, increasing the number of well plugs 216, 218 increases the number of back and forth returns of the serpentine path between the lateral sections. This in turn increases the number of fracture planes 214 over which the heat recovery fluid flows in series, and thus the heat recovery effectiveness of the system. However, the serpentine path of Figure 3 may be combined with one or more regions of parallel flow between the lateral sections 206, 212. This can be beneficial if, for example, a longer serpentine path with more back and forth returns would require, for an adequate flowrate Qo of fluid out of the system, a pumping pressure that is too high.

[0089] Advantageously, an EGS 200, such as shown in Figures 3 and 4, by enforcing series flow over the fracture planes 214 helps to overcome the problems of poor conformance seen in conventional EGSs, in which fluid preferentially and self-reinforcingly flows through only some of the fractures planes because of the parallel flow arrangement of those planes.

[0090] Figure 4 shows schematically a top-down view of lateral sections 206, 212 of an alternative well arrangement in which the injector well 206 is positioned between two producer wells 212 to further increase the amount of heat extractable from the formation. In this arrangement, a respective serpentine flow path is provided between the central injector lateral portion 206 and each of the flanking producer lateral sections 212, each flow path being the mirror image of the other. More generally, the well system may have plural of the producer wells and plural of the injector wells, the lateral sections of the producer wells and the lateral sections of the injector wells forming an alternating array.

[0091] To extend the operational life of the EGS 200, the direction of fluid flow may be reversed so that the heat recovery fluid is pumped into the producer well 208 and collected from the injector well 202.

[0092] Figure 5 shows schematically a top-down view of lateral sections 206, 212 of a variant well arrangement As in the Figure 4 arrangement, the injector well 206 is positioned between two producer wells 212. However, in the variant, instead of all the plugs completely blocking fluid flow in the respective lateral sections, some of the plugs 216, 218 form only partial blocks to flow of fluid. Thus the first plug 216 encountered by the heat recovery fluid in the injector lateral section 206 is configured to divert a first portion of the fluid into the fracture plane 214 upstream of the first plug onto the serpentine flow paths, but allow a second portion to pass through the plug 216 and remain on the injector lateral section. The first plugs 218 encountered in the producer lateral sections 212 then divert the entire first portion back to the injector lateral section 206, where it recombines with the second portion. Next, the second plug 216008772295

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[0094] encountered by the fluid in the injector lateral section 206 diverts an increased first portion of the fluid into the fracture plane 214 upstream of the second plug 216 onto the serpentine flow paths, and allows a decreased second portion to pass through the plug 216 and remain on the injector lateral section. The second plugs 218 encountered in the producer lateral sections 212 then divert the entire increased first portion back to the injector lateral section 206, where it recombines with the decreased second portion. Finally, the third plug 216 encountered by the fluid in the injector lateral section 206 forms a complete block that diverts all of the fluid into the fracture plane 214 upstream of the third plug 216 onto the serpentine flow paths, and thereafter the third plugs 218 encountered in the producer lateral sections 212 return the fluid to the injector lateral section 206.

[0095] Thus by varying the amount of fluid that is allowed to pass through each plug 216 in the injector lateral section 206, the variant well arrangement controls the amount of flow on the serpentine paths at different well positions. This can be particularly beneficial if such control allows operational performance to be improved. For example, having higher serpentine flows at selected preferential locations in the well can allow overall pumping pressures to be reduced, while maintaining good levels of heat transfer. Thus different wells can benefit not only from different placements of well plugs, but also from different degrees of blockage provided by the plugs, as determined for example by well characterisation and / or modelling.

[0096] ***

[0097] 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.

[0098] 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.

[0099] 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.

[0100] Any section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described.

[0101] 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 be 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.

[0102] 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 expressed008772295

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[0104] 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

00877229512Claims:

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 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,the formation containing a plurality of fractures on planes which are substantially perpendicular to and distributed along the lateral sections, the perpendicular fractures placing the lateral sections in fluid communication, andthe well system being configured to receive a heat recovery fluid into the lateral section of the injector well at the heel portion thereof, pass the heat recovery fluid through the fractures, and collect the heat recovery fluid from the lateral section of the producer well at the heel portion thereof;wherein the lateral section of the injector well contains an injector well plug and the lateral section of the producer well contains a producer well plug, each plug forming a local block to fluid flowthrough the respective lateral section, and the plugs being at staggered positions such that a flow of the heat recovery fluid follows a serpentine path in which, to circumvent first the injector well plug, the flow is forced from the lateral section of the injector well to the lateral section of the producer well, and then, to avoid the producer well plug, the flow is forced from the lateral section of the producer well to return to the lateral section of the injector well downstream of the injector well plug, thereafter the flow returning to the producer well downstream of the producer well plug.

2. The well system according to claim 1 , wherein the lateral section of the injector well contains plural of the injector well plugs and the lateral section of the producer well contains plural of the producer well plugs, the injector well plugs and the producer well plugs being at staggered and alternating positions to extend the serpentine path by increasing the number of back and forth returns of the flow between the lateral sections.

3. The well system according to claim 1 or 2, wherein the injector well is cased and perforated, and the producer well is open hole.

4. The well system according to any one of the previous claims having two of the producer wells, the lateral sections of the producer wells flanking the lateral section of the injector well.

5. The well system according to any one of the previous claims, wherein each plug forms a complete local block to fluid flow through the respective lateral section, such that all the heat recovery fluid arriving at a plug is forced to follow the serpentine path.008772295136. The well system according to any one of claims 1 to 4, wherein one, some or all of the plugs forms a partial local block to fluid flow through the respective lateral section, such that a portion of the heat recovery fluid arriving at a plug is forced to follow the serpentine path, but another portion of the flow arriving at that plug continues along the respective lateral section7. A procedure for assisting energy production from a well system for recovery of geothermal energy, the well 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 extending 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, the formation containing a plurality of fractures on planes which are substantially perpendicular to and distributed along the lateral sections, the perpendicular fractures placing the lateral sections in fluid communication, and the well system being configured to receive a heat recovery fluid into the lateral section of the injector well at the heel portion thereof, pass the heat recovery fluid through the fractures, and collect the heat recovery fluid from the lateral section of the producer well at the heel portion thereof;wherein the procedure includes:modelling characteristics of the well system to determine suitable locations of well plugs such that the lateral section of the injector well contains an injector well plug and the lateral section of the producer well contains a producer well plug, each plug forming a local block to fluid flow through the respective lateral section, and the plugs being at staggered positions along such that a flow of the heat recovery fluid follows a serpentine path in which, to circumvent first the injector well plug, the flow is forced from the lateral section of the injector well to the lateral section of the producer well, and then, to avoid the producer well plug, the flow is forced from the lateral section of the producer well to return to the lateral section of the injector well downstream of the injector well plug, thereafter the flow returning to the producer well downstream of the producer well plug, andlocating well plugs in the lateral sections of the injector well and the producer well at the determined suitable locations.

8. The procedure for increasing energy production from a well system for recovery of geothermal energy according to claim 7, wherein the modelling includes modelling characteristics of the well system to determine suitable locations of well plugs such that the lateral section of the injector well contains plural of the injector well plugs and the lateral section of the producer well contains plural of the producer well plugs, the injector well plugs and the producer well plugs being at staggered and alternating positions to extend the serpentine path by increasing the number of back and forth returns of the flow between the lateral sections.

9. The procedure for increasing energy production from a well system for recovery of geothermal energy according to claim 7 or 8, wherein the modelling of the characteristics of the well system also00877229514determines whether each plug should form a complete block to fluid flow through the respective lateral section or a partial block to fluid flow through the respective lateral section.

10. The procedure for increasing energy production from a well system for recovery of geothermal energy according to any one of claims 7 to 9, wherein the injector well is cased and perforated, and the producer well is open hole.

11. A procedure for increasing energy production from a well system for recovery of geothermal energy according to any of claims 7 to 10, wherein the well system comprises two of the producer wells, the lateral sections of the producer wells flanking the lateral section of the injector well.

12. A method of forming the well system according to any one of claims 1 to 6, the method including steps of:drilling the injector well,drilling the producer well,hydraulically fracturing the formation to generate the fractures, andlocating the well plugs.

13. The method of claim 12, wherein the step of locating the well plugs includes performing the procedure of any of claims 7 to 11.

14. The method of claim 12 or 13, further including a preliminary step of:determining the three principal stress directions in the formation;wherein the injector and producer wells are drilled such that the normal to the fracture planes is the direction of the smallest of the three principal stresses.

15. 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:directing a heat recovery fluid into the lateral section of the injector well at the heel portion thereof, andcollecting the heat recovery fluid from the lateral section of the producer well at the heel portion thereof;wherein a flow of the heat recovery fluid travels on the serpentine path between the heel portions.

16. The method of claim 15 further including:reversing the direction of fluid flow such that the heat recovery fluid is directed 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 heat recovery fluid flowing in reverse direction on the serpentine path between the heel portions.

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