US3613785A

US3613785A - Process for horizontally fracturing subsurface earth formations

Info

Publication number: US3613785A
Application number: US11710A
Authority: US
Inventors: Philip J Closmann; Dionysios M Phocas
Original assignee: Shell Oil Co
Current assignee: Shell USA Inc
Priority date: 1970-02-16
Filing date: 1970-02-16
Publication date: 1971-10-19
Anticipated expiration: 1988-10-19

Abstract

Horizontal fractures are formed in a subsurface earth formation which tends to fracture vertically at the naturally occurring formation temperature by a process which includes the steps of extending at least one well borehole into the formation, generating a vertical fracture by pressurizing said borehole, injecting hot fluid into at least one borehole to heat the formation, continuing the injection of hot fluid until thermal stressing of the formation matrix material causes the horizontal compressive stress in the formation to exceed the vertical compressive stress therein at a location selected for a second well, extending the borehole of the second well into the formation, and hydraulically fracturing the formation through this second well borehole to form a horizontal fracture extending therefrom into the formation.

Description

xx 88 00 BB 66 66 11 y at the natuperature by a process which ing at least one well borehole into y pressurizing ne borehole to eat the formation, continuing the injection of hot fluid until 3,303,883 2/1967 Slusser Dionysios M. Phoeas, both of Houston, Tex. 3,455,391 7/1969 Matthews et al..... "11,710 3,501,201 3/1970 Closmann et a1. Feb. 16, 1970 3,500,913 3/1970 Nordgren et Primary Examinerlan A. Calvert Shell 011 Company New York NY. Attorneys-J. H. McCarthy and T. E. Bleber ABSTRACT: Horizontal fractures are formed in a subsurface earth formation which tends to fracture verticall rally occurring formation tern 166/271, eludes the steps of extendin 1 166/308 the formation, generating a vertical fracture b Ezlb 43/24, said borehole, injecting hot fluid into at least 0 7 166/259, themtal stressing of the formation matrix material causes the 271, 303, 303 horizontal compressive stress in the formation to exceed the R f cud vertical compressive stress therein at a location selected for a e "cums second well, extending the borehole of the second well into the fonnation, and hydraulically fracturing the formation 166/308 UX through this second well borehole to form a horizontal frac- 166/271 X ture extending therefrom into the fonnation.

UNITED STATES PATENTS PROCESS FOR HORIZONTALLY FRACIURING SUBSURFACE EARTH FORMATIONS 10 Claims, 9 Drawing Figs.

Unlted States Patent [72] Inventors Philip J. Closmann;

[21] AppLNo. [22] Filed [45] Patented Oct. 19,1971 [73] Assignee 2,859,818 11/1958 Halletal...................... 3,129,761 4/1964 Staadt....

PATENTEUnm 19 I9?! 3. 6 1 3 785 SHEET 1 or 3 3 INVENTORS:

- P. J. CLOSMANN D.M. PHOCAS PAIENTEnnm 19 l97l SHEET 2 BF 3 I 1 Us FIG- llflll Ill 1 Will FIG. 5

INVENTORS P. J. CLOSMANN D. M- PHOCAS FIG. 6

PATENTEDHU 9 I97! SHEET 3 BF 3 Iv. I

FIG. 9

INVENTORS:

J- CLOSMANN D. M. PHOCAS PROCESS FOR HORIZONTALLY FRACTURING SUBSURFACE EARTH FORMATIONS BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a process for forming horizontally directed fractures in a subsurface earth formation which in its natural state tends to fracture vertically.

2. Description of the Prior Art Hydraulic fracturing is a conventional procedure that is often employed where the permeability of an earth formation is too low to pennit fluid to flow into or out of the formation at a rate which is economically suitable in respect to a recovery of petroleum or other material from the earth formation. In petroleum thermal recovery operations, hydraulic fractures are formed to permit heat to be injected over a wide area into the oil-bearing formation. Fracturing is also useful in recovering liquifiable components from essentially impermeable earth formations such as oil shale or bed deposits of cinnabar.

In a hydraulic fracturing operation, a fluid is confined in a region in which it is in contact with a subterranean earth for mation and the pressure on the fluid is increased until a fracture is formed within the earth formation. It is generally recognized that hydraulic fractures form along planes which are perpendicular to the least one of the three principal compressive stresses that exist along a vertical and two mutually perpendicular horizontal axes within any subterranean earth formation. Where the vertical stress is least, hydraulic fracturing produces a horizontal fracture. In such a situation the fracturing occurs when the pressure applied to the fracturing fluid exceeds a pressure that results from the weight of the overlying earth formations by an amount sufficient to overcome the tensile and/or shear strength of the earth formation, or rock.

The pressure which results from the weight of the overlying earth formations is commonly referred to as the overburden pressure. It is generally equal to, or slightly less than, about 1 pound per square inch per foot of depth. The pressure required to cause a failure of a subsurface earth formation in situ is commonly referred to as the fracturing pressure or formation breakdown pressure.

In respect to a horizontal fracture, the fracturing pressure is necessarily greater than the overburden pressure, since the overburden must be lifted in order to separate the layers of rock. In respect to a vertical fracture, the vertical compressive stress is greater than one or both of the principal compressive stresses that are perpendicular to each other within the horizontal plane. In the latter type of situation, the hydraulic fractures are vertical, and are oriented in a plane perpendicular to the weaker of the two horizontal principal compressive stresses.

Where a fracture is to be used in an oil-producing or fluidmining operation, it is generally advantageous to use a horizontal fracture. A horizontal fracture is better than a vertical fracture in respect to both the distributing of fluid over a region having a significant areal extent, and the interconnecting of a pair of wells. However, it has proven to be difficult to overcome the fracturing tendencies that are dictated by regional tectonics, and in many of the reservoirs in the United States the least principal stress is horizontal. In such reservoirs, in order to form a horizontal fracture, it is necessary to either increase the horizontal compressive stresses or decrease the vertical compressive stress, or do both, until the vertical stress becomes the least of the three principal stresses.

Since a confined material can be stressed by heat, the state of stress that exists in the rocks within a subterranean region can be changed by heating a subterranean zone of the appropriate size and shape. The resulting stress distribution is a function of temperature distribution, the elastic constants of the material, the coefficient of thermal expansion of the material, and the boundary conditions. One such method for thermally increasing the horizontal compressive stresses until the vertical stress becomes the least principal stress is taught by C. S. Matthews, P. Van Muers and C. W. Volek in US. Pat.

No. 3,455,39l. Therein, a well is fractured at the earth formation temperature and hot liquid is flowed into a resulting vertical fracture to heat the earth formation around the fracture. As the formation is heated it thermally expands and the walls of the fracture move toward each other until the fracture is closed. At this point, the rocks are left with unrelieved components of thermally induced horizontal stress. As the fracture becomes closed, the pressure required to cause fluid to flow into the rocks becomes higher and, when a second fracturing pressure is reached, another fracture is formed. Such a second fracture is apt to be another vertical fracture, and one that forms in a direction perpendicular to the first fracture at a pressure which is less than the overburden pressure. By continuing the injection of hot liquid, the above sequence of events is repeated and the injection pressure is further increased. Eventually, the pressure required to inject liquid exceeds the overburden pressure and a horizontal fracture is formed.

Tests and theory indicate that heating the face of a vertical fracture in an earth formation generally causes the vertical stress at the heated face to increase at a rate at least as great as the rate of increase of the horizontal stresses at that face; whereas, the horizontal stresses at some distance away from the heated surface increase more rapidly'than the vertical stress. The present invention provides a method for taking advantage of the differential rate of stress increase by heating the formation at one well bore and then fracturing the formation through another well bore at some distance from the first.

BRIEF SUMMARY OF THE INVENTION Horizontally directed fractures are formed in a subterranean earth formation that tends to fracture vertically at the naturally occurring formation temperature by a method which comprises the steps of extending at least one well borehole into the formation, preferably fracturing the formation to form vertically directed fractures which extend into the formation from the borehole, injecting a hot fluid having a temperature significantly higher than the surrounding earth formation, continuing the injection of hot fluid until thermal stressing of the formation matrix material causes the horizontal compressive stress in the formation to exceed the vertical compressive stress therein at a location selected for a second well, extending the borehole of this second well into the formation (if the borehole was not previously so extended), and hydraulically fracturing the formation through this second well borehole to form a horizontal fracture extending therefrom into the formation.

The first well is preferably located on the basis of knowledge of the trend along which vertical fractures are likely to be formed. The location of the second well may be selected by solving therrnoelastic equations to determine the compressive stress distribution in the reservoir rock after a planned period of heating. The second well is preferably placed at a location where the amount by which the horizontal compressive stress exceeds the vertical compressive stres is a minimum. If a specific location is desired for the second well, the thermoelastic equations may be solved to find the period of heating required to make the vertical compressive stres the least principal stress at the desired location. The second well may be extended into the formation of interest before or after heating has been initiated in the first well. However, the second well should not be fractured until heating of the first well has been continued for a period of time sufficient to reverse the relative magnitude of the horizontal and compressive stresses in the formation at the location of the second well.

In a modification of the proces two well boreholes may be extended into the subsurface formation which is then fractured through each well at the naturally occurring formation temperature to form at least a pair of parallel vertically directed fractures, one of the pair extending into the formation from each borehole. The fonnation is heated by injecting hot fluid into both of these parallel fractures for a time sufficient to reverse thermally the relative sizes of the horizontal and vertical compressive stresses in the reservoir at one or more locations (between the pair of parallel fractures) selected for one or more additional wells.- The necessary period of hot fluid injection may be determined by solving the thermoelastic equations to determine the heating period required to reverse thermally the relative sizes of the vertical and horizontal compressive stresses at a given location. When heating has continued for a period sufficient to make the vertical compressive stress the least principal stress at a selected location the formation may be hydraulically fractured through a well bore extending into the formation at that selected location to form a horizontal fracture. As in the two well case, the well bore at the selected location may be extended into the formation before or after heating is initiated in the first fractured two wells, however, the well at the selected location should not be fractured before the relative sizes of the vertical and horizontal compressive stresses in the formation are thermally reversed at that selected location.

If in the above modification of the process, the heating of the first fractured two wells from which the pair of parallel vertically directed fractures extends is continued for a sufficient time, the vertical compressive stress may become the least principal stress in the formation at the well bore of one or both of these two wells. Therefore, in a further modification of the process, the formation may be hydraulically fractured through one or both of the first fractured two wells after such a period of heating to form horizontal fractures extending from the borehole of the hydraulically fractured well into the earth formation.

In any of the above methods of practicing this invention, well bores in the subsurface earth formation may be interconnected by extending the fractures formed from well bores at the location of which the relative sizes of the horizontal and vertical compressive stresses have been thermally reversed until fluid communication with adjacent wells is achieved.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional view showing a completed well, a proposed well, and a surrounding earth formation.

FIG. 2 is a horizontal cross section taken along line 2-2 of FIG. 1 which shows the formation of interest after it has been treated in accordance with a process of the present invention.

FIG. 4 is a vertical cross section of the formation of interest taken along the line 4-4 of FIG. 3 after the formation has been treated according to a process of this invention.

FIG. 5 is a cross-sectional view showing two completed wells, a proposed well, and a surrounding earth formation.

FIG. 6 is a horizontal cross section taken along line 6-6 of FIG. 5 which shows the formation of interest after it has been treated with a process of this invention.

FIG. 7 is a plot of stress versus distance along the line 88 of FIG. 6 after a period of heating from

wells

12 and 13.

FIG. 8 is a vertical cross section of the formation of interest taken along the line 8-8 of FIG. 6 after the formation has been treated according to an embodiment of this invention.

FIG. 9 is a plot of stress versus distance along the line 8-8 of FIG. 6 after a long period of heating from

wells

12 and 13.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 shows a well which is provided with equipment suitable for use in practicing this invention. A string of casing 14 is installed to extend from a surface location into an earth formation 15 in which it is desired to create a horizontal fracture. The casing is provided with perforations 16 which open the well into fluid communication with the earth formation 15. A tubing string 17 carrying a packer 18 provides a means of conveying fluid from a surface location to a location adjacent to earth formation 15. The upper end of the tubing string is connected to a pump 19 which may be connected to heating and fluid-handling units (not shown) which units can be conventional types of such equipment.

In practicing one embodiment of this invention, the borehole of the first well 10 of a pair of wells 10 and I1 is extended into the earth formation I5 and the well is equipped as described above. This well 10 is preferably located on the basis of knowledge of the trend along which vertical fractures are likely to be fonned. Such knowledge is generally available from measurements of fracture orientation in other wells or from information on the regional tectonics. A preferred placement of the first well 10 is such that a line connecting this well and the proposed location of the second well 11 of the pair of wells is parallel to the direction 26 of the naturally occurring least principal stress in the earth formation 15.

Earth formation 15 is preferably fractured through well 10 at the naturally occurring formation temperature by a conventional means such as pumping liquid through the tubing 17 to a selected region of the casing 14 which is within the earth formation 15. In this region the liquid is confined by the packer l8 and the borehole bottom 29 and is conveyed by the perforations 16 into contact with the earth formation 15. The pressure is increased until the bottom hole liquid pressure exceeds the fracturing pressure and causes a fracture, such as fracture 31 (FIG. 2), to form in the earth formation. This fracture, most likely, is vertically oriented and extends from the well 10 in a direction perpendicular to the direction 26 of the naturally occurring least principal stress. This fracture is preferably propped open with a propping material 35 by procedures known in the art.

Hot fluid such as hot water or steam may then be flowed down tubing 17 into the well 10 and the fracture 31. This fluid should have a temperature greater than that of the surrounding earth formation. The injection should be continued for a time sufiicient to reverse thermally the relative sizes of the horizontal and vertical compressive stresses within the earth formation at a location selected for the second well 13. That is, hot fluid injection is continued until thermal stressing of the matrix material of the earth formation 15 causes the horizontal compressive stress to be greater than the vertical compressive stress at the location proposed for well 1 1.

At the time such a stress configuration is achieved in the earth formation 15 at the desired location, the borehole of well 11 may be extended into the earth formation 15 and the well 11 equipped in a manner similar to well 10. However, it should be noted that it is not critical to the practice of this invention that well 11 be extended only at this time; the formation 15 may be penetrated with well I1 before or after heating through the first well 10 is commenced.

Finally, the earth formation is fractured through well 11 by a conventional hydraulic fracturing procedure such as that described with respect to well 10. The result of this fracturing procedure may be a substantially horizontal fracture 30 (FIG. 4) extending from the well 11 into the earth formation 15. This fracture may be propped with a propping material 37 by procedures known in the art.

The required temperature of the hot fluid injected into fracture 31 and the extent of time injection must be continued are related to the thermal conductivity, expansion properties, and preexisting stress conditions of earth formation 15. The compressive stress distribution in the earth formation for a given period of heating from fracture 31 with a fluid having a given temperature may be determined by mathematical modeling. For example, for heating from a vertical fracture 31 in a transversely isotropic formation 15 a model may be constructed using the following assumptions:

1. The linear theory of thermoelasticity, involving only infinitesimal displacements, is applicable over the temperature and stress conditions to be encountered. Under these conditions the stresses due to the weight of the overburden are known and may be subtracted from the stress distribution calculated in the presence of overburden to obtain stress conditions independent of depth.

2. The elastic and thermal constants are not functions of temperature.

3. Temperature and deformations are independent of each other.

4. The temperature distribution within a plane normal to the fracture 31 may be computed independently of the temperature distribution along the fracture 31 once the temperature at the fracture face is known.

5. The elasticity problem may be treated as one of plane strain, i.e., no strain in the horizontal'direction in which the fracture 31 was originally propagated.

6. At sufficiently great distances from the fracture 31 the stresses, displacements, and temperature in the formation are undisturbed by the heating from the fracture.

7. The plane of transverse isotropy corresponds to horizontal bedding planes of the formation, and the plane of the fracture 31 is parallel to an axis of symmetry, perpendicular to this plane of isotropy.

8. Total stresses can be computed by adding those due to a uniform body force to the thermal stresses. Tectonic stresses, if known, can be added to the solution obtained.

If we take an xyz coordinate system having the x-y plane as the horizontal plane of isotropy, the x-direction perpendicular to the plane of the fracture, and the z-axis as an axis of symmetry parallel to the plane of the fracture, then the ther moelastic strain equations are as follows:

a 1 X y)%-n+a( r) (1) 1 Z= .-W.)%- .+a TTI (2) 1 a 1) 6w WWW 4) 1 bu Ow ,7 afia 5 1 .2(1+v) O z Q u E a 0a: (6)

where u=displacement in the x-direction v=displacement in the y-direction w=displacement in the Z-direction normal strain component in the X-direction 3; normal strain component in the y-direction i normal strain component in the z-direction I Tr, total normal stress in x-direction a, total normal stress in y-direction 0- total normal stress in z-direction r,,= shear stress in y-z plane 1,, shear stress in z-x plane 1' shear stress in x-y plane T= temperature T,.= initial fonnation temperature E Youngs modulus in plane of isotropy E Young's modulus normal to plane of isotropy v Poissons ratio for transverse reduction in the plane of isotropy for tension in the same plane.

v Poissons ratio for transverse reduction in the plane of 55 where p body force due to the weight of the overburden. From equations (1) through (8), and the assumption of plane strain in the y-direction, i.e., 511/ Oy=0, the following differential equations for displacements in the x-y plane may be obtained:

T=T (x,s) temperature distribution obtained by solving the heat conduction equation:

a T b T O T x=W+ i'o Pf f a t where K thermal conductivity parallel to plane of isotropy,

K thennal conductivity normal to plane of isotropy, and

p C volumetric heat capacity of fonnation.

Equations (9) and l0) can be solved for an appropriate set of boundary conditions by numerical methods known in the art such as the finite difference method. The results of such a solution may be used to compute the compressive stresses in the formation for a given temperature distribution from the following relationships:

FIG. 3 shows a stress distribution in the earth formation 15 after a period of heating from a single vertical fracture 3!. There is a region 33 near the walls of the fracture where the vertical compressive stress 0,, exceeds the horizontal compressive stress, 0-,. However, as one moves away from the fracture face, the vertical stress decreases more rapidly than the horizontal stress and eventually becomes the smaller stress throughout a zone 34 in which thermal stressing of the formation matrix material has created conditions favorable to the formation of horizontal fractures. The borehole of the second well 11 is preferably located near the midpoint of this zone 34 so that the horizontal fracture 30 formed fromthe well 11 has maximum arcal extent. For a given temperature of injected fluid, the heating time required to create a properly positioned zone 34 in which the vertical compressive stress is the least principal stress may be determined by solving equations (16) through 18) for various heating times and selecting that heating time which will create a zone of thermal stress reversal in which the amount by which the horizontal compressive stress exceeds the vertical compressive stress is a maximum at the desired location of the well bore of the second well 11.

In a second embodiment of this invention the boreholes of at least two wells such as boreholes of

wells

12 and 13 in FIG. may be extended into the earth formation 15 and equipped in a manner similar to that heretofore described for well 10. These wells are preferably located so that a straight line drawn between the two wells is parallel to the direction 26 of the naturally occurring least principal stress in the earth formation 15. The earth formation 15 may be fractured through each of the

wells

12 and 13 at the naturally occurring formation temperature by a conventional means. The resulting fractures 33 and 32 (FIG. 6) are most likely substantially parallel vertically oriented fractures, one of which extends from each of the

respective wells

12 and 13 in a direction perpendicular to the direction 26 of the naturally occurring least principal stress. These fractures are preferably propped with a propping

material

36 and 38 by procedures well known in the art. Hot fluid such as water or steam may then be flowed down the

tubing

22 and 23 of

wells

12 and 13 and into the propped

fractures

33 and 32. The injection of hot fluid is continued until thermal stressing of the matrix material of earth formation 15 causes the vertical compressive stress to become the least compressive stress throughout a zone 39 (FIG. 7) between the

parallel fractures

33 and 32 which zone includes the proposed location of one or more additional wells such as well 20. These additional well locations are preferably midway between

fractures

31 and 32. The borehole of an additional well may then be extended into the earth formation 15 (if it has not previously been so extended) and equipped in a manner similar to that described above with respect to well 10. The earth formation 15 may then be fractured through this well 20 by a conventional method to form a substantially horizontal fracture 24 (FIG. 8) extending from the well 20 into the formation. This fracture 24 is preferably propped with a propping material 25 by a procedure well known in the art.

In a modification of this second embodiment the injection of hot fluid through the

tubing

22 and 23 of the

wells

12 and 13 and into contact with the subsurface earth formation 15 at the faces of the

vertical fractures

33 and 32 may be continued until thermal stressing of the matrix material of earth formation 15 causes the vertical compressive stress to become the least compressive stress at the boreholes of the

wells

12 and 13 into which hot fluid is being injected (FIG. 9). When such a stress configuration is achieved, the subsurface earth formation 15 may be hydraulically fractured through well 12 or well 13 by conventional procedures, preferably with a heated liquid, to form a substantially horizontal fracture (not shown) extending from the borehole of the well into the formation. This substantially horizontal fracture may be propped by procedures known in the art.

In any of the above methods of practicing this invention, boreholes in the subsurface earth formation may be interconnected by extending the fractures formed from a well at the location of which the relative sizes of the horizontal and vertical compressive stresses have been thermally reversed until fluid communication with one or more adjacent wells is achieved.

In summary, this invention provides a process for forming substantially horizontal fractures in a subsurface earth formation in which fractures when formed at the naturally occurring formation temperature tend to be vertically directed. The

process comprises: extending into the subsurface earth formation at least a first well of a group of at least two wells, injecting through the first well and into contact with the subsurface earth formation a heated fluid having a temperature greater than that of the surrounding earth formation, continuing said injection of heated fluid for a time suflicient to heat the subsurface earth formation and thereby cause the vertical compressive stress to become the least principal stress within the subsurface earth formation at a location selected for a second well of the group of at least two wells, extending into the heated subsurface earth formation at said location a second well of the group of at least two wells, and hydraulically fracturing the heated subsurface earth formation at the location of said second well to form a substantially horizontal fracture extending from the second well into the subsurface earth formation. Prior to the step of injecting a hot fluid through the first well and into contact with the subsurface earth formation said subsurface earth formation may be fractured at the location of the first well to form a substantially vertically directed fracture extending from said first well into the subsurface earth formation. At least some of the wells in the group of wells may be interconnected by extending the substantially horizontal fractures formed in the subsurface earth formation at the location of the second well until fluid communication with at least one adjacent well is achieved.

To form substantially horizontal fractures in a subsurface earth formation penetrated by a plurality of wells in which formation fractures fonned at the naturally occurring formation temperature tend to be vertically directed the invention provides a modified process which comprises: injecting through at least one well and into contact with the subsurface earth formation a heated fluid having a temperature greater than the temperature of the surrounding earth formation, continuing said injection of heated fluid for a time sufficient to heat the subsurface earth formation and thereby cause the vertical compressive stress to become the least principal at at least one selected location in the subsurface earth formation at which it is desired to form a horizontal fracture, providing another well which well penetrates the heated subsurface earth formation at said selected location, and hydraulically fracturing the heated subsurface earth formation at the selected location to form a substantially horizontal fracture extending into the subsurface earth formation.

In one embodiment of this modified process, prior to the step of injecting a heated fluid through at least one well and into contact with the subsurface earth location, the subsurface earth formation may be fractured at the location of at least some of the wells through which said hot fluid is injected.

In a preferred embodiment of the modified process the subsurface earth formation is fractured prior to the step of injecting a heated fluid at the location of at least two wells through which wells said hot fluid is then injected. Preferably, at least some of the fractures formed when said wells are fractured prior to the step of injecting a heated fluid are substantially vertically directed fractures at least one fracture of which extends from each of said wells in a direction such that substantially parallel fractures are formed.

When practicing this invention according to this preferred embodiment of the modified process, at least one selected location in the subsurface earth formation at which it is desired to form a horizontal fracture may be the location of one of the two wells through which the hot fluid is injected.

In practicing this invention according to any of the above embodiments of the modified process at least some wells of the plurality of wells may be interconnected by extending at least some of the substantially horizontal fractures formed in the subsurface earth formation from wells at the selected locations until fluid communication with adjacent wells is achieved.

We claim as our invention:

1. A process for forming substantially horizontal fractures in a subsurface earth formation in which fractures when formed at the naturally occurring formation temperature tend to be vertically directed, which process comprises:

extending into the subsurface earth formation at least a first well of a group of at least two wells;

injecting through the first well and into contact with the subsurface earth formation a heated fluid having a temperature greater than that of the surrounding earth formation;

continuing said injection of heated fluid for a time sufficient to heat the subsurface earth formation and thereby cause the vertical compressive stress to become the least principal stress within the subsurface earth formation at a location selected for a second well of the group of at least two wells;

extending into the heated subsurface earth formation at said location a second well of the group of at least two wells; and

hydraulically fracturing the heated subsurface earth formation at the location of said second well by injecting fluid into said second well and increasing the pressure on said fluid to form a substantially horizontal fracture extending from the second well into the subsurface earth formation.

2. The process of claim I wherein prior to the step of injecting a hot fluid through the first well and into contact with the subsurface earth formation said subsurface earth formation is fractured at the location of the first well to form a substantially vertically directed fracture extending from said first well into the subsurface earth formation.

3. The process of claim 1 wherein at least some of the wells in the groups of wells are interconnected by extending the horizontal fractures formed in the subsurface earth formation at the location of the second well until fluid communication with at least one adjacent well is achieved.

4. A process for forming horizontal fractures in a subsurface earth formation penetrated by a plurality of wells in which formation fractures formed at the naturally occurring formation temperature tend to be vertically directed, which process comprises: 1

injecting through at least two wells and into contact with the subsurface earth formation a heated fluid having a temperature greater than the temperature of the surrounding earth formation;

continuing said injection of heated fluid for a time sufficient to heat the subsurface earth formation and thereby cause the vertical compressive stress to become the least principal stress within the earth fonnation at at least one selected location in the subsurface earth formation at which it is desired to form a horizontal fracture;

providing a well penetrating the heated subsurface earth formation at said selected location; and

hydraulically fracturing the heated subsurface earth formation at the selected location by injecting fluid into the well penetrating the subsurface earth formation at the selected location and increasing the pressure on said fluid to form a substantially horizontal fracture extending into the subsurface earth formation.

5. The process of claim 4 wherein prior to the step of injecting a heated fluid through at least one well and into contact with the subsurface earth formation said subsurface earth location is fractured at the location of at least some of the wells through which said hot fluid is injected.

6. The process of claim 5 wherein prior to the step of injecting a heated fluid, the subsurface earth formation is fractured at the location of at least two wells through which said hot fluid is then injected.

7. The process of claim 6 wherein at least some of the fractures formed when said two wells are fractured prior to the step of injecting a heated fluid are substantially vertically directed fractures at least one fracture of which extends from each of said wells in a direction such that substantially parallel fractures are formed, one of said parallel fractures extending from each of said wells.

8. The process of claim 7 wherein at least one selected location in the subsurface earth formation at which it is desired to form a horizontal fracture is a location between said substantially1parallel fractures.

9. he process of claim 6 wherein at least one selected location in the subsurface earth formation at which it is desired to form a horizontal fracture is the location of one of the at least two wells through which the hot fluid is injected.

10. The process of claim 4 wherein at least some of the plurality of wells are interconnected by extending at least some of the horizontal fractures formed in the subsurface earth formation from wells at the selected locations until fluid communication with adjacent wells is achieved.

Claims

1. A process for forming substantially horizontal fractures in a subsurface earth formation in which fractures when formed at the naturally occurring formation temperature tend to be vertically directed, which process comprises: extending into the subsurface earth formation at least a first well of a group of at least two wells; injecting through the first well and into contact with the subsurface earth formation a heated fluid having a temperature greater than that of the surrounding earth formation; continuing said injection of heated fluid for a time sufficient to heat the subsurface earth formation and thereby cause the vertical compressive stress to become the least principal stress within the subsurface earth formation at a Location selected for a second well of the group of at least two wells; extending into the heated subsurface earth formation at said location a second well of the group of at least two wells; and hydraulically fracturing the heated subsurface earth formation at the location of said second well by injecting fluid into said second well and increasing the pressure on said fluid to form a substantially horizontal fracture extending from the second well into the subsurface earth formation.

2. The process of claim 1 wherein prior to the step of injecting a hot fluid through the first well and into contact with the subsurface earth formation said subsurface earth formation is fractured at the location of the first well to form a substantially vertically directed fracture extending from said first well into the subsurface earth formation.

4. A process for forming horizontal fractures in a subsurface earth formation penetrated by a plurality of wells in which formation fractures formed at the naturally occurring formation temperature tend to be vertically directed, which process comprises: injecting through at least two wells and into contact with the subsurface earth formation a heated fluid having a temperature greater than the temperature of the surrounding earth formation; continuing said injection of heated fluid for a time sufficient to heat the subsurface earth formation and thereby cause the vertical compressive stress to become the least principal stress within the earth formation at at least one selected location in the subsurface earth formation at which it is desired to form a horizontal fracture; providing a well penetrating the heated subsurface earth formation at said selected location; and hydraulically fracturing the heated subsurface earth formation at the selected location by injecting fluid into the well penetrating the subsurface earth formation at the selected location and increasing the pressure on said fluid to form a substantially horizontal fracture extending into the subsurface earth formation.

8. The process of claim 7 wherein at least one selected location in the subsurface earth formation at which it is desired to form a horizontal fracture is a location between said substantially parallel fractures.

9. The process of claim 6 wherein at least one selected location in the subsurface earth formation at which it is desired to form a horizontal fracture is the location of one of the at least two wells through which the hot fluid is injected.