FR2470240A1 - Fracture area complex prodn. in rock formation - involves parallel bores intersecting fracturing planes at right angles with hydraulic equipment applying stress - Google Patents

Abstract

At least one fluid feed hole and at least one fluid drain hole are bored near enough vertically to the approx. zone in a crystalline rock formation having a temp. suited to the extraction of exploitable heat energy. The boreholes are kinked from their vertical alignment, into an alignment at right angles near enough to the compass direction of the probably fracture plane of the formation. The pipes are arranged so that the drain hole is above, parallel to and approx. in the same vertical plane as the fluid feed hole. Simultaneously the formation is broken out from the feed and drain holes so as to create a single large area fracture which connects the two bores hydraulically.

Description

 Process for the production of a geothermal reservoir in a hot dry rock formation for the recovery of geothermal energy.

 Geothermal energy is an important potential energy source. So far, the use of potential energy available from geothermal reservoirs has been limited to natural hydrothermal underground reservoirs. Hydrothermal tanks contain sufficient quantities of native fluid which can be brought to the surface by a well system for the production of electrical energy, so that they can among themselves be considered as economically viable sources of energy. Previously, the energy contained in the known hot dry rock formations was not extracted, since these formations do not contain sufficient quantities of fluid naturally contained in the soil, which can be brought to the surface, so that the electricity production is economical.

Until recently, the exploitation of hot dry rock reservoirs has not taken place, partly because of the difficulty and cost of drilling in hard hot crystalline rock, but mainly because of the fact that the low thermal conductivities of these formations have shown that thermal energy could not be extracted at a sufficient flow rate, in the absence of any type of drilling structure having a very large area. It was assumed that a surface heat transfer required value could not other created at the bottom of a borehole by existing methods
It was stated in United States patent 3,786,858 that a practical size heat transfer surface could be created in a hot dry rock formation by the technique of hydraulic fracturing, common in the quarrying industry. petroleum This patent describes a process for creating a structure at the bottom of a borehole for the extraction of thermal energy from a hot dry rock formation consisting in drilling a first well (injection) to the depth of the formation, to hydraulically fracture the formation from the injection well (to create a thin vertical disc having a large heat transfer surface), to drill a second well (sample) cutting the fracture, and to circulate a fluid d heat exchange inside the fracture by the injection well by bringing it towards the surface by the sampling well for the extraction of thermal energy.

 The process described in this US patent 3,786,858 has several drawbacks.

The first is that it uses only one fracture plane to form its reservoir, thus limiting the speed at which heat can be extracted
The second is the fact that the sampling well must be drilled precisely to cut the narrow fracture plane without, however, cutting it at a point which could short-circuit a significant proportion of the heat transfer surface thus created. The intersection of the fracture plane and the precision required for the point of intersection may require directional drilling techniques of a difficult and storytelling plume implementation.

US Patents 3,878,884 and 3,863,709 have described methods for creating a geothermal reservoir comprising several fracture planes, but significantly increasing the effective area available for heat extraction. According to the patent of the United States of America 3 878 884, a first well is drilled vertically until the hot dry rock formation, then is deflected with respect to the vertical in a geographical direction corresponding to the lines of least principal stress of the formation . Then, several parallel fracture planes are created in the formation, by hydraulic fracturing, in positions spaced from each other along the deviated well. A sampling well is then drilled above the first well and parallel to him to cut the majority of the fracture planes. According to US Patent 3,863,709, two wells are drilled and deflected from the vertical
through the hot dry rock formation.

Fracture planes are hydraulically induced from one of the wells and propagate through the formation to cut the second well. Then, the zone located along the second well, in which the intersection occurs, is localized by injection of a radioactive marker in the fracture plane, starting from the first well. The second well is perforated at this point to put in hydraulic communication with the other well.

 In both cases, the drawbacks of a correct location of the second well inside the fracturing complex still exist. In U.S. Patent 3,878,884, the second well must be formed to cut the fracturing complex without shorting a significant proportion of the effective heat transfer area of the fractures created. In US Patent 3,863,709, the second well must be drilled parallel to and at a predetermined distance from the first well, the maximum distance being the maximum radius according to which it can be assumed that a unitarily induced fracture will occur propagate.

Thus, the possibility exists of not obtaining any intersection if the distance is too large, or of short-circuiting the heat transfer surface if this distance is too small
The object of the invention is to create a process by which the injection and sampling wells can be placed in positive communication during the creation of a fracturing complex for thermal transfer, thus avoiding the problems no intersection or short circuit intersection of the previous processes. In addition, as will appear later, an object of the invention is to provide a method for creating fracturing complexes having larger heat transfer areas than those which can be obtained by the methods according to the prior art.

The creation of fracturing planes having greater heat transfer areas allows the heat between extracted at higher speeds per unit of drilling cost than is not the case with the prior methods.

 According to the invention, several vertical wells are drilled to the desired hot dry rock formation.

At a depth at which useful thermal energy can be extracted, the wells are deflected from the vertical in a geographic direction substantially perpendicular to the horizontal composition of the assumed fracture plane of the formation. Core samples are taken along the deviated drilled wells to determine the degree of verticality (the deviation from the absolute vertical of the vertical component) of the assumed fracture plane of the formation. Fracturing means are disposed in the deviated and aligned drilled holes or wells to lie within the single assumed or anticipated fracture plane.The formation is fractured from the various wells drilled simultaneously to produce a fracture d 'interconnection in single or double ellipsoid of large area, connecting the various drilled wells to establish a hydraulic communication. Thus, the drilled wells are subjected to positive communication during the formation of the fracture and a very large area fracture is created.

 The description which follows, made with reference to the appended drawing, given without limitation, will allow a better understanding of the invention.

 The single figure shows a hot dry geothermal rock formation up to which several wells have been drilled vertically then, when a desired temperature level has been reached, deviated from the vertical to extend through the formation in principle perpendicularly to the supposed or anticipated fracture plan of the formation. Vertical fractures laterally spaced from each other by a predetermined distance along the deviated wells have been shown.

The surface equipment for the conversion of thermal energy and the conduits or connecting pipes between the wells in the upper sedimentary layer, or cover for the conversion of the thermal energy, extracted by the various wells, into electrical energy are shown without details. The details of this surface equipment and of the connection system between the wells are well known and are illustrated diagrammatically in the drawing, but will not be described in detail.

Hot dry rock formations have permeabilities ranging from about 1 darcy lower to more than 10 milli-darcy
The temperature range in which useful thermal energy can be extracted goes from 100 ° C. for applications in space heating to 300 ° C. and more for the production of electricity. According to the invention, a hot dry rock formation of low permeability is preferred, in order to minimize the problems of loss of circulation fluid which are encountered in the case of formations with high permeability. Since the invention mainly relates to the creation of a fracturing complex of a hot dry rock formation for the production of electrical energy, a hot dry rock formation having a temperature of about 235 C or more is preferred.Electric energy can be produced from formations having slightly lower temperatures, and higher temperatures at around 3000 C can be used since the instruments and tools used at the bottom which can withstand such higher temperature levels are now available
The existence of many hot dry rock formations of this type at economically accessible depths is already known and other formations can be discovered by the application of geophysical techniques
Referring to the drawing, once a hot dry rock formation 1 having unstable characteristics has been selected, several wells 2,3 (ultimately at least one injection well 2 and a well of sample 3) are drilled vertically in the formation until a desired formation temperature T1, which is preferably about 235 C, is reached The well which will ultimately serve as an injection well 2 is extended vertically over an additional distance D which is equal to the separation distance between the wells when these are then deviated from the vertical and extend through the formation. This distance will be discussed later. Generally, at depths greater than several thousand feet (a thousand meters or more) at which most hot dry rock formations exist, the planes along which these fractures are oriented directionally and aligned in a substantially vertical plane. Although certain formations of this type have been studied, so that we know the geographic direction of the vertical plane according to which the probability of fracture of the formation is greater if this direction is not not known, or as an additional precaution, a geographically directed core can be taken from the bottom of one or at least the vertical wells and this core and the vacuum left by it can between analyzed to determine the granular orientation and the stress tectonics, which in combination with other geophysical data available about the formation allow to determine the direction of u plan where the probability of a vertical fracture is greatest. Other methods can be used to determine the direction of the fracture plane, for example by producing a test fracture whose direction can between determined by the injection of radioactive markers or by the use of a printing gland.

When the geographic direction of the most probable fracture plane for formation has been determined, the drilled wells 2a and 3a are deviated from the vertical in a direction approximately perpendicular to the
geographic direction of these plans. Although it is preferable to obtain a perpendicular relationship between the deviated wells and the assumed fracture plane, absolute perpendicularity is not essential. Deviated wells can intersect the assumed fracture planes at an angle that deviates from the perpendicular relationship of up to 45 ". The term" approximately perpendicular "is intended to cover such variation.

The angle of deviation from the vertical of the deviated parts of the wells can range from a value of not more than 50 to a value of up to 900, but angles between about 30 and 450 are preferred. The angle of deflection depends on a compromise determined by the temperature gradient of the formation and the cost of drilling the operation. Given that in general it is preferable to extend the drilled wells deflected through the hot dry rock formation until a temperature of at least 15 C higher is reached, the degree of additional drilling must be a function of the temperature gradient of the formation and the angle of deviation from the vertical No matter what the temperature gradient, or where the angle of deviation from the vertical is 90, the minimum distance over which the deviated wells extend through the hot dry rock formation must be sufficient to take into account the multiplicity of fracturing planes which subsequently go between created along the deviated wells. This minimum distance is a function of the number of fractures desired multiplied by the distance between the fractures
The wells or drilled holes 2a and 3a are extended through the formation in a relation such that one of them is t approximately parallel to the other and is located approximately in the same vertical plane. deviated are found inside a formation with low permeability, it currently seems that this part can be drilled without the need to use ser production tubes or casings. The elimination of production tubes will significantly reduce the cost of being bliss and is preferred. But in the event that the fracturing cannot be triggered correctly in the absence of production tubes, these tubes can be extended along the deviated parts When the drilled holes have been extended through the hot dry rock formation over the desired distance , another geographically oriented core is taken from the bottom of at least one deviated well and preferably from all wells. These carrots and / or the voids they leave are analyzed to determine the granular orientation of the rocks, tectonic stress and other physical properties, to determine the degree of verticality that can be expected for the fracturing plane, most likely the formation.

 The spacing of the various wells measured over their deviated lengths D is determined mainly by the distance over which a hydraulically induced fracture can be expected to propagate within the formation. If this distance is already known from field studies, the various wells can then be completed simultaneously-, the spacing measured on their deviated parts then being slightly less than twice the radius of the distance of propagation of a fracture unique. This distance can reach two and a half times the propagation distance, since the radial distance between the wells of the single interconnection fracture which will subsequently be produced by simultaneous fracturing from the various wells must exceed the sum of the radii of two fractures But to obtain the maximum possibility of interconnecting fractures with certainty and to have a margin of error, it is preferable to keep a value for the aforementioned distance not exceeding twice the propagation distance.

 In the case where the propagation distance cannot be predicted between by calculation, the various wells can then be completed successively, a test fracture being produced at the bottom of the first well completed. The propagation distance of a separately produced test fracture can be measured, and the other well (s) can then be completed so as to be at a distance from the first well not more than double the propagation distance.

 When the deviated wells have been completed, the formation is fractured from at least two wells 2a and 3a of baleen simultaneously. Simultaneous fracturing can be ensured by the use of a technology using remote cable glands with zone delimitation. The double insulated cable glands 4 and 5 (represented by accentuated lines for the wells in the drawing) are introduced into the various wells and are aligned respectively so that the plane in which these cable glands are found corresponds to the point direction view to the vertical direction of the most likely fracture plane as determined previously. Each zone with cable glands separated 4 and 5 is sealed below and above each cable gland jointly by hydraulic or mechanical means. The pressure is gradually increased and jointly by hydraulic systems. independent in each well, so that the rupture pressure of the rocks is reached within a few seconds (essentially simultaneously) in each well.

The pressure required to cause fracturing can be calculated based largely on the tensile strength of the rock in situ for each well. According to a variant and more precisely, a single test fracture can be produced in a well and the fracture pressure can be determined empirically.

 It is important that the hydraulic pressure inside each one delimited by cable glands is gradually increased jointly in approximately 10 to 30 minutes. A gradual increase in pressure exerts a stress on the rock exposed to it. location of the cable glands ê which increases the molecular pressure of the rock in these zones The stress radiates through the rock To allow the constraint to extend over a maximum distance from each zone delimited by the cable glands 5 de such that the stresses exerted from the wells overlap at a point between these wells, the pressure increase must take place gradually and jointly The increase in molecular pressure or the stress on the rock between the wells makes it more sensitive to fracturing Then, when the fracturing pressure applicable to each well has been reached jointly, fracturing occurs for each of th ux, essentially at an interval of a few seconds, and it propagates along the lines of maximum stress to produce a single large interconnection fracture.

 The gradual or gradual increase in pressure to create the zone of overlap of stresses between the wells before fracturing is important for another reason. All fractures produced hydraulically in a hot dry rock formation have very small widths. In the case of an isolated propagated fracture, the width of the fracture gradually decreases towards its outer perimeter and it tends towards zero.

If we tried to produce a single interconnection fracture by successive fracturing from each well, the single fracture would at best be the product of two fractures formed in isolation, intersecting and interconnecting on their respective outer perimeters. The interconnection section would thus be a surface of minimum fracture width, creating a zone of high impedance or resistance opposing the flow of the heat exchange fluid through such a fracture. This high impedance zone would significantly lower the hydraulic efficiency of subsequent thermal extraction operations. By creating a zone of overlapping stresses in the rock between the wells at the point where interconnection occurs during fracturing simultaneous, the width of the resulting fracture which is obtained in the overlap zone will be greater than that which would result from a successive fracturing.

 The fact of reaching the fracturing pressure simultaneously in each well causes by tinkling a simultaneous rupture of the rock, resulting in a maximum probability of obtaining a hydraulic interconnection between the wells. The induced fracture 8 has a very large section or area, with a minimum impedance in the interconnected zone, and must essentially form in a single plane. The interconnection fracture 8 may be slightly irregular and not literally in a single ideal plane, but it will be considered below as a fracture in a single plane.

 According to a preferred fracturing method, the hydraulic pressure in the zones delimited by cable glands of each well is gradually increased in a joint manner up to a point situated just below the calculated or measured point of rupture of the rock at each place occupied by a cable gland, and is kept constant. Subsequently, a sudden simultaneous increase in pressure, so that the total pressure in the gland areas of each well significantly exceeds the breaking pressure of the rock, is applied to the areas delimited by gland of all the wells considered. The simultaneous abrupt application of a high pressure markedly increases the probability of producing fracturing in a simultaneous manner and thus creates the single large interconnection fracture in a single plane which is desired.

 The sudden increase in pressure can be handled in a number of ways. For example, an auxiliary pump can be used on the surface to produce a tank filled with liquid under high pressure.

This high pressure tank will be interconnected to the hydraulic systems of the cable glands by quick opening valves. When the breaking pressure of the rock has almost been reached at the bottom using the primary pumps, the quick-opening valves are opened, allowing the high-pressure tank to empty suddenly in the hydraulic lines connected to the zones Cable glands Not only does this provide the sudden increase in pressure necessary9 but also the pressure will be maintained throughout the rock breaking operation.

A hydraulic ram or ram can be connected to the hydraulic conduits for each zone delimited by cable glands, in order to provide the sudden pressure increase. Alternatively, the hydraulic conduits can be connected to an explosion device provided for in the surfacfe, as shown in the state patent
United States of America 3 848 674, or else explosives can be used at the bottom of the well to simultaneously provide this sudden increase in pressure
Once the initial fracture has been created, the boundary glands are relieved of pressure or mechanically released and transferred to a new position in each well), and the process is repeated to create another fracture. Z1 is preferable that the initial fracture is caused in the lowest or deepest part of the deviated wells and that the process is repeated successively in positions located higher.

The recurrent process of producing simultaneous simple plane fractures 8 results in a massive fracturing complex 6 which can be called a hot rock furnace. For clarity, the drawing simply shows a complex comprising four fractures, but it will be understood that a complex comprising a significantly greater number of fractures can be created by the method according to the invention.

The individual fractures constituting the furnace must be spaced 20 to 50 meters apart, in order to have the certainty that each will work substantially in conditions of thermal insulation with respect to the others. In order to be certain that these fractures do not close, they will be filled with sand, gravel or artificial or similar support agents.

 The hot rock furnace thus created has hundreds of thousands of square meters of heat exchange surface for the extraction of heat by circulation of a heat exchange fluid such as water, this surface being sufficient to provide a adequate capacity and to give with certainty an economically acceptable lifetime for the furnace. The heated fluid which is conveyed to the surface will be used to supply thermal energy or to generate electrical energy by means of installations 7 comprising heat exchange surfaces, or for other applications. Since it is envisaged to carry out the heat exchange operation in the form of a loop or a closed circuit, the heat exchange fluid can undergo continuous recycling through the hot rock furnace for the extraction of heat. Such an oven must also ensure sufficient transmission for the circulation of the geothermal fluid, so as to present acceptable levels of impedance to the flow of the liquid through the required loop in the ground.

If extracting heat for an extended period of time causes the temperature of the hot rock oven to drop to an unacceptable level, a new oven complex can be created by extending all the wells and simultaneously fracturing a blank area of the formation, substantially as described previously The old furnace can be isolated from the geothermal circuit until its initial temperature is regenerated, then it can be put back into service and reincorporated into the underground circuit
Although the method has been described here for the creation of an underground oven complex for the purpose of extracting thermal energy, other applications of this method are possible. The method can also be used in formations other than formation of hot dry rock 9 to create underground storage areas for the burial or reception of waste materials such as solids or radioactive liquids In addition, the process can be used in order to create an underground complex for gasification of coal or for the retraction of minerals by injection of steam or chemicals into underground environments
Modifications can be made to the modes of implementation described, in the field of technical equivalences, without departing from the invention.

Claims (2)

CLAIMS 1.- Method for creating a fracturing complex in a hot dry rock formation, characterized in that: a) at least one fluid injection well and a fluid withdrawal well are drilled in approximately vertical directions to an approximate position inside a crystalline rock formation in which a temperature prevails allowing the extraction of useful thermal energy; b) these wells are deflected with respect to their vertical orientation in a direct-ion substantially perpendicular to the geographic direction of the most likely fracturing plane of this formation, so that the sampling well is located above the well d injection of the fluid, parallel to it, and is situated approximately in the same vertical plane, and  c) this formation is simultaneously fractured from the injection and sampling wells to produce a single large interconnection fracture connecting these wells for hydraulic communication.  2.- Method according to claim 1, characterized in that means are provided for orienting the fracturing of this formation inside the injection and sampling wells, in alignment with the vertical direction of the plane assumed or anticipated fracturing for this training.  3.- Method according to claim 2, characterized in that gradually and simultaneously subjecting the fracturing means to the effect of a pressure, up to the pressure of rupture of the rock, to cause the simultaneous fracturing of training from these wells. 4.- Method according to claim 3, characterized in that the pressures of the fracturing means are increased jointly for a period of time ranging from 10 to 30 minutes approximately.  5. Method according to claim 1, characterized in that the simultaneous fracturing operation is repeated in several parallel positions along the deviated parts of the injection and sampling wells, to create a fracturing complex.  6.- Method according to claim 5, characterized in that the distance between the various fractures is between 20 and 50 meters approximately. 7.- A method according to claim 1, characterized in that the injection and sampling wells are separated in their deviated parts from a distance slightly less than approximately twice the distance over which a fracture caused in isolation will propagate to inside the training.  8- Process for creating a fracturing complex in a hot dry rock formation9 characterized in that  a) at least one fluid injection well and a fluid withdrawal well are forced in approximately vertical directions to an approximate position: inside a crystalline rock formation in which a temperature prevails useful thermal energy extraction;  b) these wells are deflected with respect to their vertical orientation in a direction substantially perpendicular to the geographic direction of the most likely fracturing plane of this formation, so that the sampling well is located above the injection well fluid9 parallel to it, and is located approximately in the same vertical plane;  c) means are provided for orienting the fracturing of this formation inside the injection and sampling wells, in alignment with the vertical direction of the fracturing plane assumed or anticipated for this formation, and  d) the fracturing means are gradually subjected to the effect of a pressure up to a level slightly lower than the rupture pressure of the rock of this formation and the said formation is simultaneously fractured from the wells. injection and sampling by simultaneously exerting an abrupt pressure exceeding the rupture pressure of the rock in the fracturing means, to produce a single interconnection fracture of large area connecting the wells for hydraulic communication.  9. A method according to claim 8, characterized in that means acting by explosion are used to suddenly exert this high pressure in the fracturing means.  10.- Method according to claim 9, characterized in that a hydraulic cylinder or ram is used to suddenly exert this high pressure in the fracturing means.