Formation Fracturing Method

Abstract

A method for producing multiple fractures in earth formations in which the lines of least principal stress deviate substantially from vertical is described. A generally vertical borehole is drilled into the formation, the formation is hydraulically fractured from the vertical borehole, the plane in which the fracture lies is determined, a slanted borehole is drilled out from the vertical borehole in a direction such that the azimuth of slanted borehole is generally perpendicular to the plane of the fracture and then the formation adjacent the slanted borehole is hydraulically fractured at a plurality of positions along the length of the slanted borehole. When the direction of lines of least principal stress is known for the formation, the steps of hydraulically fracturing from the vertical borehole and determining the plane in which the fracture lies may be omitted and the slanted borehole is drilled in the azimuth parallel to the known lines of least principal stress.

Description

BACKGROUND OF THE INVENTION

This invention relates to a method for producing a plurality of non-coplanar and approximately parallel fractures in an earth formation adjacent a borehole to facilitate the recovery of a resource contained in the formation.

Nearly forty years ago it was found that oil bearing formations could be fractured by introducing low penetration fluids into a borehole under hydraulic pressure sufficiently high to cause propagation of a fracture from the borehole and that fracturing was generally followed by an increase in oil production from the borehole. Studies of the hydraulic fracturing process have shown that the fractures are generally planar and oriented perpendicular to the direction of least principal stress (commonly designated S.sub.3) in the rock.

In a great many formations or formation zones, the direction of least principal stress is approximately horizontal and the planes of hydraulically produced fractures are generally vertical and perpendicular to the direction of least principal stress in the rock. In such formations or formation zones hydraulic pressure exerted in an approximately vertical well produces just a single approximately vertical planar fracture. While hydraulically induced fractures may propagate away from boreholes distances of 100 meters or so, the benefit obtained from a single fracture is limited and it would be desirable to produce a plurality of generally parallel fractures in the formation and so obtain greatly increased benefits from the fracturing technique.

BRIEF DESCRIPTION OF THE INVENTION

Pursuant to the present invention, a plurality of planar and approximately parallel fractures are produced along the length of a borehole permitting more efficient recovery of the contained resource from the formation than has heretofore been possible. The method involves first drilling an approximately vertical borehole in the formation to a depth either penetrating, or very close to the horizon of, the resource bearing zone of the formation. A slanted borehole is then drilled into the formation from the lower part of the vertical borehole. Optimally, the azimuth (i.e., the compass direction of the horizontal line defined by the intersection of the vertical plane containing the line of the slanted hole with the surface) of the slanted borehole would be the same as the direction of lines of least principal stress in the rock. To obtain the benefits of this invention, however, a slanted borehole need not be in precisely the same direction as the direction of the lines of least principal stress in the formation but may be in a direction such that the angle between the line of the slanted borehole and the lines of least principal stress in the formation is not more than 60.degree.. After the slanted borehole has been drilled to the desired depth, the slanted hole is completed and cased. Hydraulic fractures are then propagated from the slanted borehole and the fractures may be propped with sand in conventional manner to hold them open. The fractures are produced in conventional manner by packing off sections of the slanted borehole so that the injected fluid does not enter into existing perforations.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 of the appended drawings is a diagrammatic illustration of a cross-section of a formation in which a vertical hole and a slanted borehole have been drilled into the formation and a plurality of parallel planar fractures have been created adjacent the slanted borehole.

FIG. 2 of the appended drawings is a diagrammatic representation of a cross-section of a formation in which the method of the invention is adapted to recovery of geo-thermal energy from hot, impermeable formations by injection of water through one vertical and slanted bore hole and recovery of steam through a second slanted and vertical bore hole, the second slanted bore hole being drilled so that it communicates with at least the major proportion of the fractures propagated from the first slanted borehole.

Referring now to FIG. 1 of the appended drawings, vertical borehole 1 is drilled into the formation so that it bottoms either in the resource containing zone of a formation or close to the horizon of the resource containing zone. Slanted borehole 2 is then drilled into the resource containing zone of the formation from the point in the lower part of the vertical borehole. The formations chosen for the application of the method are those in which the direction of the lines of least principal stress are generally horizontal and in which the planes of hydraulic fractures, which are known to be generally perpendicular to the lines of least principal stress, are generally vertical.

The direction of the slanted borehole is such that the angle between the line of the slanted borehole and the lines of least principal stress is not greater than 60.degree., preferably not more than 45.degree. and optimally less than 30.degree.. Many formations have been drilled and studied to the point that the direction of least principal stress is already known and existing information permits the operator to direct the slanted borehole properly. There are, of course, many formations in which the direction of least principal stress is not known and must be determined before the slanted borehole is drilled. The direction of least principal stress of such formations may be determined by hydraulically fracturing the formation adjacent the lower part of the vertical borehole and then determining the plane in which the fracture lies. Methods for determining the position of the fracture plane are well-known and readily available as by the use of impression packers or by injecting radioactive tracers into the fracture and then determining the position of the plane from the pattern of the signals emitted by the radioactive material in the fracture. After the position of the fracture plane has been determined, the direction of the lines of least principal stress become known since they are perpendicular to the fracture plane.

When the position of the fracture plane is so determined, the direction of the slanted hole may be described either in terms of the angle it makes with the lines of least principal stress or in terms of the angle of incidence which the borehole makes with the fracture plane, the angle of incidence being the angle between the line of the slanted borehole and the perpendicular to the fracture plane at the point of intersection of the borehole and the plane. Thus, the direction of the slanted borehole may be described as a direction such that the angle between the line of the borehole and the lines of least principal stress in less than 60.degree. or as a direction such that the angle of incidence, i.e., the angle between the borehole and the perpendicular to the fracture plane, is less than 60.degree..

After slanted borehole 2 has been drilled to the desired depth, it is completed and cased. Fractures 3 and 9 inclusive are then made by perforating the casing at the shallowest practical depth, adjacent the area of fracture 3, and hydraulically fracturing the formation to maximum distance of fracture propagation consistant with economic considerations. This first fracture is propped with sand for gas and oil production and also may be propped for geo-thermal power production, if desired. At a few meter's greater depth, the casing is perforated again, packed off and the fracturing operation is repeated. The fracturing here and in all successive positions down the hole will be conducted in packed off sections of the bore hole so that the injected fluid does not enter existing perforations.

The fracturing method above described can be used to increase the recovery of gas or oil from low permeability formations. It can also be used for in-situ recovery of oil from oil shale, or solution mining or extracting geo-thermal energy from subterranean formations.

There are large areas on the earth where hot, impermeable rock is accessible to drilling. Geo-thermal energy may be extracted by drilling and fracturing the hot formation as described in connection with FIG. 1 of the drawings, then pumping water down the hole into contact with the fractured surfaces, permitting the water to reside in the formation for time sufficient to heat it and then permitting the water to reissue from the same hole as steam or super-heated water.

FIG. 2 of the appended drawings illustrates a modification of the invention which is particularly well adapted to either recovery of geo-thermal energy or solution mining. Vertical well 10 is drilled to appropriate depth and if the direction of the lines of least principal stress in the formation are not known, as they probably will not be, the formation adjacent the lower part of vertical well 10 is hydraulically fractured and the plane of fracture is determined. Slant borehole 11 is then drilled in a direction such that its angle of incidence, i.e., the angle between the borehole and the perpendicular to the plane of fracture, is less than 60.degree.. When the slant well has been drilled to appropriate depth, a plurality of non-coplanar approximately parallel fractures are produced adjacent the slanted well illustrated by fractures 12 through 18 inclusive on the drawing. A second approximately vertical borehole 20 is then drilled and a second slanted borehole is drilled from a point near the bottom of the second vertical borehole and in a direction approximately parallel to that of the first slanted borehole. The second slanted borehole is so spaced from the first slanted borehole that it intersects most of the fractures which were produced from the first slanted borehole. It is important that the second slanted borehole intersect the major portion of the fractures extending out from the first slanted borehole. In areas where the characteristics of the formations are well known, it is possible to space the separate second slanted borehole apart from the first slanted borehole by a distance such that the desired fracture penetration by the second borehole will be achieved. In areas where the characteristics of the formation are not well known the second slanted borehole can be drilled to a depth where some of the fractures should have been intersected. A radioactive tracer can then be injected into the first slanted borehole and into the fractures propagated from it and the second slanted borehole can be logged for the presence of radioactive material. Alternatively, fluid can be injected into the first vertical borehole and first slanted borehole at high pressure (in excess of the parting pressure) into the fractures extending from the first slanted borehole and, if intersection of the fractures has been accomplished, a pressure increase will be produced quickly in the second slanted borehole. Failure to intersect the fractures can be corrected by then altering the inclination of the second slanted borehole or by re-drilling at a deeper level. The second slanted borehole, when completed, may be cased and perforated at the points of intersection with the fractures. After the drilling and fracturing has been completed, water is pumped down borehole 10 into slanted borehole 11 and forced into the several fractures propagated from borehole 11. The water contacts the hot subterranean rock along the fracture surfaces producing steam and the steam flows into the second slanted borehole 19 and is recovered at the surface through vertical borehole 20.

In some situations it may be desired to avoid the expense of drilling two vertical and two slanted boreholes to recover geothermal energy. In this event the pattern shown in FIG. 1 may be used. Water is injected into the formation and held under pressure for a time sufficient to produce superheated water and steam or water above its critical temperature. The pressure is then released to permit flow of superheated water and steam to the surface. Successive cycles of injection and recovery are then used to remove geothermal energy.

Claims

I claim:

1. A method of opening an earth formation in which the lines of least principal stress deviate substantially from the vertical to facilitate recovery of a resource held in the formation which comprises the steps of:

a. drilling an approximately vertical borehole into the formation;

b. drilling a slanted borehole extending from the vertical borehole into the formation in a direction such that the angle between the slanted borehole and the lines of least principal stress in the formation is not more than 60.degree.; and

c. hydraulically fracturing the formation adjacent the slanted borehole at a plurality of positions along the length of the slanted borehole.

2. The method of opening an earth formation in which the lines of least principal stress deviate substantially from the vertical to permit recovery of a resource held in the formation which comprises the steps of:

a. drilling an approximately vertical borehole in the earth formation;

b. hydrofracturing the formation adjacent the lower portion of the borehole;

c. determining the plane in which the fracture lies;

d. drilling a slanted borehole extending from the vertical borehole into the formation in a direction such that the angle between the slanted borehole and the perpendicular to any plane parallel to the plane determined in step (c) which is intersected by the borehole is less than 60.degree.; and

e. hydrofracturing the formation adjacent the slanted borehole at a plurality of positions along the length of the slanted borehole.

3. The method defined in claim 2 wherein the angle between the borehole and the perpendicular to the plane is less than 30.degree..

4. The method defined in claim 1 characterized by a further step of drilling a second vertical borehole adjacent the first borehole and a second slanted borehole extending from the second vertical borehole into the formation generally parallel to the first slanted borehole and spaced from the first slanted borehole by a distance such that the second slanted borehole intersects at least a major proportion of the formation fractures produced in step (c) of claim 1.

5. The method of recovering heat from a subterranean geothermal zone which comprises the steps of:

a. drilling an approximately vertical borehole penetrating said geothermal zone;

b. hydrofracturing the formation adjacent the lower portion of the borehole;

c. determining the plane in which the fracture lies;

d. drilling a slanted borehole extending from the vertical borehole into the geothermal zone in a direction such that the angle between the slanted borehole and the perpendicular to any plane parallel to the plane determined in step (c) which is intersected by the borehole is less than 60.degree.;

e. hydrofracturing the formation adjacent the slanted borehole at a plurality of positions along the length of the slanted borehole;

f. injecting water into the borehole to contact the geothermal zone under pressure,

g. holding the injected water under pressure until it is heated to elevated temperature and,

h. releasing the pressure to permit flow of heated water and steam to the surface.