Title:
Formation of Fractures Within Horizontal Well
Kind Code:
A1


Abstract:
Producing transverse fractures in a horizontal well may be achieved at a relatively lower fracturing pressure by forming one or more tunnels extending from the horizontal wellbore. One or more tunnels may be formed at each location along the horizontal wellbore where a transverse fracture is desired. The tunnel(s) may be formed mechanically, optically, or hydraulically. Further, fracturing may be formed at a lower pressure than would otherwise be required to form transverse fractures from a horizontal wellbore. According to some implementations, the transverse fractures may be formed without isolating a portion of the horizontal wellbore.



Inventors:
Abass, Hazim H. (Cypress, SA)
Tahini, Ashraf M. (Al-Khobar, SA)
Soliman, Mohamed Y. (Cypress, TX, US)
Meadows, David (Marlow, OK, US)
Application Number:
12/558747
Publication Date:
03/17/2011
Filing Date:
09/14/2009
Assignee:
HALLIBURTON ENERGY SERVICES, INC. (Houston, TX, US)
Primary Class:
Other Classes:
166/50
International Classes:
E21B43/26; E03B3/11
View Patent Images:
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Primary Examiner:
ANDREWS, DAVID L
Attorney, Agent or Firm:
FISH & RICHARDSON P.C. (HALLIBURTON) (P.O. BOX 1022 MINNEAPOLIS MN 55440-1022)
Claims:
What is claimed is:

1. A method of forming transverse fractures extending from a horizontal wellbore comprising: forming a wellbore within a subterranean zone, the wellbore having a horizontal wellbore portion; forming a tunnel extending from the horizontal wellbore portion into the subterranean zone towards the overburden, the tunnel formed with a length adapted to initiate a fracturing extending from the tunnel along a longitudinal axis thereof being influenced insignificantly by the horizontal wellbore portion; applying fluid pressure to an interior of the horizontal wellbore portion at a location proximate the tunnel to form a fracture extending from the tunnel along a longitudinal axis thereof; and propagating the initiated fracture to encompass the horizontal wellbore portion.

2. The method of claim 1, wherein forming a tunnel extending from the horizontal wellbore portion into the subterranean zone towards the overburden comprises: inserting a tool in the horizontal wellbore portion; and orienting the tool into a desired orientation to form the tunnel.

3. The method of claim 1, wherein forming a tunnel extending from the horizontal wellbore portion into the subterranean zone towards the overburden comprises: forming a first tunnel extending from a first portion of the horizontal wellbore portion; and forming a second tunnel extending from a second portion of the horizontal wellbore portion opposite the first portion.

4. The method of claim 1, wherein forming a tunnel extending from the horizontal wellbore portion into the subterranean zone towards the overburden comprises forming the tunnel with one of a hydrajet, a laser, or a drilling tool.

5. The method of claim 4, wherein forming the tunnel with a hydrajet comprises: disposing a hydrajet into the horizontal wellbore portion at a desired location therein; orienting the hydrajet to form the tunnel; and operating the hydrajet to impinge a fluid flow onto a surface of the horizontal wellbore portion to form the tunnel.

6. The method of claim 4, wherein forming the tunnel with a laser comprises: disposing a laser into the substantially horizontal wellbore portion; orienting the laser to form the tunnel; and operating the laser to form the tunnel.

7. The method of claim 4, wherein forming the tunnel with a drilling tool comprises: disposing a drilling tool into the substantially horizontal wellbore portion; orienting the drilling tool to form the tunnel; and operating the drilling tool to form the tunnel.

8. The method of claim 1, wherein forming a tunnel extending from the horizontal wellbore portion into the subterranean zone towards the overburden comprises forming a tunnel extending from the horizontal wellbore portion into the subterranean zone towards the overburden at two or more different locations along an axial length of the horizontal wellbore portion.

9. The method of claim 1 further comprising isolating a portion of the horizontal wellbore at a location of the tunnel before applying the fluid pressure.

10. The method of claim 1, wherein forming a tunnel extending from the horizontal wellbore portion into the subterranean zone towards the overburden comprises forming the tunnel with a length of at least one and a half (1.5) times a radius of the horizontal wellbore portion.

11. The method of claim 1, wherein forming a tunnel extending from the horizontal wellbore portion into the subterranean zone towards the overburden comprises forming the tunnel with a length of at least three (3) times a radius of the horizontal wellbore portion.

12. The method of claim 1, wherein forming a tunnel extending from the horizontal wellbore portion into the subterranean zone towards the overburden comprises forming the tunnel with a length of at least six (6) times a radius of the horizontal wellbore portion.

13. A wellbore system comprising: a horizontal wellbore extending through a subterranean zone; and at least one tunnel extending from the horizontal wellbore into the subterranean zone towards the overburden, the at least one tunnel having a length adapted to form transverse fractures relative to the horizontal wellbore.

14. The wellbore system of claim 13, wherein at least a portion of the horizontal wellbore comprises a slanted portion and wherein the tunnel extends from the slanted portion of the horizontal wellbore.

15. The wellbore system of claim 13, wherein the at least one tunnel extending from the horizontal wellbore into the subterranean zone towards the overburden comprises: a first substantially vertical tunnel extending from a first portion of the horizontal wellbore; and a second substantially vertical tunnel extending from a second portion of the horizontal wellbore along a perimeter thereof opposite the first portion.

16. The wellbore system of claim 13, wherein the at least one tunnel having a length adapted to form transverse fractures relative to the horizontal wellbore comprises a tunnel having a length of at least one and a half (1.5) times a radius of the horizontal wellbore.

17. The wellbore system of claim 13, wherein the at least one tunnel having a length adapted to form transverse fractures relative to the horizontal wellbore comprises a tunnel having a length of at least three (3) times a radius of the horizontal wellbore.

18. The wellbore system of claim 13, wherein the at least one tunnel having a length adapted to form transverse fractures relative to the horizontal wellbore comprises a tunnel having a length of at least six (6) times a radius of the horizontal wellbore.

19. A method of forming fractures transverse to a horizontal wellbore comprising: forming a wellbore having a horizontal wellbore portion within a subterranean zone; forming a tunnel extending from the horizontal wellbore portion into the subterranean zone towards the overburden, the tunnel formed with a length such that the horizontal wellbore portion has insignificant effects on formation of a fracture extending from the tunnel along a longitudinal axis thereof; applying fluid pressure to an interior of the horizontal wellbore portion at a location proximate the tunnel to form the fracture extending from the tunnel along the longitudinal axis thereof; and propagating the initiated fracture to encompass the horizontal wellbore portion.

20. The method of claim 19, wherein forming a tunnel extending from the horizontal wellbore portion into the subterranean zone towards the overburden comprises forming the tunnel with a length of at least one and a half (1.5) times a radius of the horizontal wellbore portion.

21. The method of claim 19, wherein forming a tunnel extending from the horizontal wellbore portion into the subterranean zone towards the overburden comprises forming the tunnel with a length of at least three (3) times a radius of the horizontal wellbore portion.

22. The method of claim 19, wherein forming a tunnel extending from the horizontal wellbore portion into the subterranean zone towards the overburden comprises forming the tunnel with a length of at least six (6) times a radius of the horizontal wellbore portion.

23. The method of claim 19, wherein forming a tunnel extending from the horizontal wellbore portion into the subterranean zone towards the overburden comprises: inserting a tool in the horizontal wellbore portion; and orienting the tool into a desired orientation to form the tunnel.

24. The method of claim 19, wherein forming a tunnel extending from the horizontal wellbore portion into the subterranean zone towards the overburden comprises forming the tunnel with one of a hydrajet, a laser, or a drilling tool.

25. The method of claim 19, wherein forming a tunnel extending from the horizontal wellbore portion into the subterranean zone towards the overburden comprising forming a tunnel at two or more different locations along an axial length of the horizontal wellbore portion.

26. The method of claim 19 further comprising isolating a portion of the horizontal wellbore portion at a location of the tunnel before applying the fluid pressure.

Description:

TECHNICAL FIELD

This disclosure relates to forming transverse fractures into a subterranean zone from a horizontal well and more particularly to using a tunnel extending from the horizontal wellbore as a transverse fracture initiation location.

BACKGROUND

Reservoir stimulation may be used to enhance recovery of reservoir fluids from a subterranean reservoir or zone. An example reservoir stimulation is hydraulic fracturing (interchangeably referred to as “fracturing”) in which fluid is pumped into a wellbore at an elevated pressure to form one or more fractures in the subterranean reservoir bordering the wellbore. The fractures formed during fracturing provide flow conduits emanating from the wellbore, providing flowpaths for the reservoir fluid to collect in the wellbore and subsequently be produced to the surface.

SUMMARY

One aspect of the present disclosure is directed to a method of forming transverse fractures extending from a horizontal wellbore. The method may include forming a wellbore having a horizontal wellbore portion within a subterranean zone and forming a tunnel extending from the horizontal wellbore portion into the subterranean zone towards the overburden. The tunnel may be formed with a length adapted to initiate a fracturing extending from the tunnel along a longitudinal axis thereof being influenced insignificantly by the horizontal wellbore portion. The method may also include applying fluid pressure to an interior of the horizontal wellbore portion at a location proximate the tunnel to form a fracture extending from the tunnel along a longitudinal axis thereof and propagating the initiated fracture to encompass the horizontal wellbore portion.

A second aspect is directed to a wellbore system including a horizontal wellbore extending through a subterranean zone and at least one tunnel extending from the horizontal wellbore into the subterranean zone towards the overburden. The at least one tunnel may have a length adapted to form transverse fractures relative to the horizontal wellbore.

A third aspect is directed to a method of forming fractures transverse to a horizontal wellbore including forming a wellbore having a horizontal wellbore portion within a subterranean zone and forming a tunnel extending from the horizontal wellbore portion into the subterranean zone towards the overburden. The tunnel may be formed with a length such that the horizontal wellbore portion has insignificant effects on formation of a fracture extending from the tunnel along a longitudinal axis thereof The method may also include applying fluid pressure to an interior of the horizontal wellbore portion at a location proximate the tunnel to form the fracture extending from the tunnel along the longitudinal axis thereof and propagating the initiated fracture to encompass the horizontal wellbore portion.

One or more of the aspects may include one or more of the following features. Forming a tunnel extending from the horizontal wellbore portion into the subterranean zone towards the overburden may include inserting a tool in the horizontal wellbore portion and orienting the tool into a desired orientation to form the tunnel. Forming a tunnel extending from the horizontal wellbore portion into the subterranean zone towards the overburden may include forming a first tunnel extending from a first portion of the horizontal wellbore portion and forming a second tunnel extending from a second portion of the horizontal wellbore portion opposite the first portion. Forming a tunnel extending from the horizontal wellbore portion into the subterranean zone towards the overburden may include forming the tunnel with one of a hydrajet, a laser, or a drilling tool. Forming the tunnel with a hydrajet may include disposing a hydrajet into the horizontal wellbore portion at a desired location therein, orienting the hydrajet to form the tunnel, and operating the hydrajet to impinge a fluid flow onto a surface of the horizontal wellbore portion to form the tunnel. Forming the tunnel with a laser may include disposing a laser into the substantially horizontal wellbore portion, orienting the laser to form the tunnel, and operating the laser to form the tunnel. Forming the tunnel with a drilling tool may include disposing a drilling tool into the substantially horizontal wellbore portion, orienting the drilling tool to form the tunnel, and operating the drilling tool to form the tunnel.

One or more of the aspects may also include one or more of the following features. Forming a tunnel extending from the horizontal wellbore portion into the subterranean zone towards the overburden may include forming a tunnel extending from the horizontal wellbore portion into the subterranean zone towards the overburden at two or more different locations along an axial length of the horizontal wellbore portion. A portion of the horizontal wellbore may be isolated at a location of the tunnel before applying the fluid pressure. Forming a tunnel extending from the horizontal wellbore portion into the subterranean zone towards the overburden may include forming the tunnel with a length of at least one and a half (1.5) times a radius of the horizontal wellbore portion. Forming a tunnel extending from the horizontal wellbore portion into the subterranean zone towards the overburden may include forming the tunnel with a length of at least three (3) times a radius of the horizontal wellbore portion. Forming a tunnel extending from the horizontal wellbore portion into the subterranean zone towards the overburden may include forming the tunnel with a length of at least six (6) times a radius of the horizontal wellbore portion.

One or more of the aspects may additionally include one or more of the following features. At least a portion of the horizontal wellbore may include a slanted portion, and the tunnel may extend from the slanted portion of the horizontal wellbore. The at least one tunnel extending from the horizontal wellbore into the subterranean zone towards the overburden may include a first substantially vertical tunnel extending from a first portion of the horizontal wellbore and a second substantially vertical tunnel extending from a second portion of the horizontal wellbore along a perimeter thereof opposite the first portion. The at least one tunnel having a length adapted to form transverse fractures relative to the horizontal wellbore may include a tunnel having a length of at least one and a half (1.5) times a radius of the horizontal wellbore, a tunnel having a length of at least three (3) times a radius of the horizontal wellbore, or a tunnel having a length of at least six (6) times a radius of the horizontal wellbore.

One or more of the aspects may further include one or more of the following features. Forming a tunnel extending from the horizontal wellbore portion into the subterranean zone towards the overburden may include forming the tunnel with a length of at least one and a half (1.5) times a radius of the horizontal wellbore portion, forming the tunnel with a length of at least three (3) times a radius of the horizontal wellbore portion, or forming the tunnel with a length of at least six (6) times a radius of the horizontal wellbore portion. Forming a tunnel extending from the horizontal wellbore portion into the subterranean zone towards the overburden may include inserting a tool in the horizontal wellbore portion and orienting the tool into a desired orientation to form the tunnel. Forming a tunnel extending from the horizontal wellbore portion into the subterranean zone towards the overburden may include forming the tunnel with one of a hydrajet, a laser, or a drilling tool. Forming a tunnel extending from the horizontal wellbore portion into the subterranean zone towards the overburden may include forming a tunnel at two or more different locations along an axial length of the horizontal wellbore portion. A portion of the horizontal wellbore portion may be isolated at a location of the tunnel before applying the fluid pressure.

The details of one or more implementations are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 shows a wellbore extending from a terranean surface into a subterranean zone.

FIG. 2 shows a longitudinal fracture extending from a horizontal portion of a wellbore.

FIG. 3 shows a longitudinal fracture extending from a horizontal wellbore.

FIG. 4 shows transverse fractures extending from a horizontal portion of a wellbore.

FIG. 5 shows a transverse fracture extending from a horizontal wellbore.

FIG. 6 is a view along an axis of a horizontal wellbore in which a transverse fracture extends from the horizontal wellbore.

FIG. 7 shows a longitudinal fracture extending from a vertical wellbore.

FIG. 8 shows a tunnel extending from a portion of a horizontal wellbore.

FIG. 9 shows first and second tunnels extending vertically from opposite locations along a perimeter of a horizontal wellbore.

FIG. 10 is a cross-sectional view along A-A in FIG. 8 showing a transverse fracture surrounding the horizontal wellbore that was initiated from the tunnel.

FIG. 11 is a cross-sectional view along B-B in FIG. 9 showing a transverse fracture surrounding the horizontal wellbore that was initiated from the tunnel.

FIGS. 12-14 illustrate the formation of a transverse fracture relative to a horizontal wellbore.

FIG. 15 is a summary table of experimentation data.

FIGS. 16-22 are schematic diagrams illustrating the configuration of the bores extending through various test samples.

DETAILED DESCRIPTION

Producing transverse fractures in a horizontal well is described. FIG. 1 shows a wellbore 10 having a substantially horizontal portion (hereinafter referred to as “horizontal wellbore”) 20. The wellbore 10 extends from a terranean surface 30 and extends into a subterranean zone 40. During the producing life of the wellbore 10, such as after formation of the wellbore 10 or at one or more occasions after the wellbore 10 has been producing reservoir fluids, the subterranean zone 40 may be subjected to a fracturing operation to enhance production of the reservoir fluids.

Previously, fracturing was performed, for example, by isolating a relatively small section of the wellbore 10 (such as with one or more packers) and injecting a fluid into the isolated section at high pressure. The high pressure fluid increased the stress state of the subterranean zone 40 resulting in the formation of fractures extending into the subterranean zone. However, controlling the orientation of the produced fracturing with respect to the wellbore 10 using this fracturing method was difficult, resulting in high friction pressure and sometimes creating axial fractures (also referred to herein as longitudinal fractures). In some instances, as the axially fractures propagated, the axial fractures would become re-oriented so as to be perpendicular to the minimum stress of the subterranean zone 40. The re-orientation of these fractures may lead to a sand out. That is, the fracture is unable to accept additional proppant during the fracturing operation and only the carrier fluid is injected into the formation through these fractures. FIGS. 2 and 3 illustrate longitudinal fractures 50 extending longitudinally along an axis 60 of the horizontal wellbore 20. Longitudinal fractures, though, are not optimum and generally result in reduced production in comparison to transverse fractures formed in a horizontal wellbore. FIGS. 4-6 illustrate transverse fractures 70 formed in the subterranean zone 40 bordering the horizontal wellbore 20. Further, longitudinal fractures are generally more likely to result when fracturing a horizontal wellbore.

Longitudinal fractures are also more likely to be formed in vertical wellbores at lower fluid pressures. That is, longitudinal fractures are formed from a vertical wellbore at a lower breakdown pressure. FIG. 7 illustrates longitudinal fractures 50 extending from a vertical well. This characteristic can be utilized to promote formation of transverse fractures in a horizontal well. Particularly, one or more bores or tunnels 80 may be extended from a horizontal wellbore 20 and used to promote the formation of a transverse fracture about the horizontal wellbore 20. The one or more tunnels 80 may extend towards the overburden. Generally, this means that the one or more tunnels 80 extend vertically or substantially vertically from the horizontal wellbore 20. For the purposes of this disclosure, forming the one or more tunnels 80 towards the overburden is described as being formed vertical or substantially vertical. However, it is understood that the tunnels 80 may be formed in a direction other than vertical or substantially vertical in situations where the overburden is not at a location vertically offset from the horizontal wellbore 20. Further, the one or more tunnels 80 may deviate from vertical or substantial vertical by 15°. The tunnels 80 promote the initiation and propagation of fractures that are independent of influences associated with horizontal and vertical orientations aspects of the well.

FIG. 8 shows a single tunnel 80 extending substantially vertically from a first portion 82 of the horizontal wellbore 20, while FIG. 9 shows a pair of tunnels 80 extending from the horizontal wellbore 20. In FIG. 9, one of the tunnels 80 extends from the first portion 82 of the horizontal wellbore 20, and the second tunnel 80 extends from a second portion 84 of the horizontal wellbore 20, opposite the first portion 82. Further, both tunnels 80 are oriented vertically or substantially vertically so as to promote the formation of the transverse fracture relative to the horizontal wellbore 20.

The horizontal wellbore 20 may also include numerous tunnels 80 formed along the length of the horizontal wellbore 20. Particularly, a tunnel 80 may be included on the horizontal wellbore 20 at any location where a transverse fracture is desired. Thus, the number of tunnels 80 formed into the subterranean zone 40 from the horizontal wellbore 20 may be dependent upon the number of transverse fractures 70 desired. Consequently, the number of tunnels may be determined according to the design of the stimulation activity.

FIGS. 10 and 11 show cross-sectional views of the horizontal wellbore 20 along lines A-A and B-B, respectively. FIGS. 10 and 11 show example transverse fractures 70 extending into the subterranean zone 40 that were initiated at the tunnels 80.

The tunnels 80 may be formed in any number of different ways. For example, one or more of the tunnels 80 may be formed mechanically, such as by drilling into the reservoir from the horizontal wellbore 20. According to other implementations, one or more of the tunnels 80 may be formed using one or more lasers. A laser device may be included on a tubing string extending into the horizontal wellbore 20 and used to form the tunnels 80 therefrom. According to still other implementations, one or more of the tunnels 80 may be formed with a stream of pressurized fluid, e.g., by hydrajetting, which forces a concentrated jet of fluid at elevated pressures towards a point within a wellbore. Example hydrajets that may be used are described in U.S. Pat. No. 5,361,856 and U.S. Pat. No. 5,494,103, each of which is incorporated herein by reference in their entirety. A pressurized fluid is then introduced into the horizontal well 20 to form the transverse fracture 70.

Unlike perforations formed in a wellbore, the tunnel 80 has a better defined elongated shape with less damage to the surrounding subterranean zone 40. This damage provides leak-off paths for the fracturing fluid to flow off into the subterranean zone 40, thereby reducing the effective pressure exerted on the subterranean zone 40 to form the fractures therein, i.e., the damage to the surrounding subterranean zone 40 may cause an increase in the breakdown pressure required to fracture the subterranean zone 40. Further, during a perforating operation, a plurality of perforations are formed in the subterranean zone 40. These multiple perforations also act to lessen the effect of the pressurized fluid, because the multiple perforations require more pressure and fluid flow.

Additionally, perforating a wellbore with a hydrajet expels a plurality of fluid streams through respective nozzles. The fluid streams form a plurality of openings into the subterranean formation from the wellbore. However, the effect of using the plurality of fluid streams results in enlarging the openings into an enlarged cavity formed in the subterranean zone surrounding the wellbore. Thus, when the pressurized fluid is introduced into the wellbore for fracturing, the enlarged cavity reduces the effectiveness of concentrating the pressurized fluid to initiate and propagate a fracture in a controlled manner. Further, present hydrajets for perforating a subterranean zone are also deficient in that the nozzles expelling the fluid streams are not capable of being aligned with a particular orientation within the wellbore and are, thus, incapable of aligning openings formed by the hydrajet with a desired orientation.

Once the one or more tunnels 80 are formed, the subterranean zone 40 may then be fractured. According to some implementations, the pressurized fluid may be introduced into the horizontal wellbore 20 via a concentrated stream at or near the location of the tunnel(s) 80. Alternately, a portion of the horizontal wellbore 20 including the tunnel(s) 80 is isolated according to any desired manner, and the pressurized fluid is introduced into the isolated portion of the horizontal wellbore 20 to form the transverse fracture 70.

It is believed that the introduced pressurized fluid works on the tunnel 80 to form a longitudinal fracture extending therefrom. As this longitudinal fracture extends, the fracture encompasses the horizontal wellbore 20, resulting in a transverse fracture with respect to the horizontal wellbore 20. FIGS. 12-14 illustrate the progression of the fracture believed to occur at a location along a horizontal wellbore 20 having a tunnel 80. In FIG. 12, the pressurized fluid (represented by the plurality of arrows 90) is introduced into the horizontal wellbore 20. In FIG. 13, the longitudinal fracture 50 is formed extending from the tunnel 80. The initiated longitudinal fracture 50 extends and expands to encompass the horizontal wellbore 20, thereby resulting in a transverse fracture 70 extending into the subterranean zone 40, as shown in FIG. 14.

The one or more tunnels 80 may have any desired length L. However, as the length L of the tunnel 80 increases, influences from the horizontal wellbore 20 during fracturing are reduced, resulting in a greater likelihood that a transverse fracture with respect to the horizontal wellbore 20 will result. These influences include how the horizontal wellbore 20 affects the stress state of the subterranean zone 40 surrounding the tunnels 80 during fracturing. Moreover, for a tunnel 80 having a length L of three (3) times the diameter D or six (6) times the radius of the horizontal wellbore, the influences from the horizontal wellbore 20 are negligible. In fact, the influences from the horizontal wellbore 20 are also small with respect to tunnels 80 having lengths L smaller than three times the diameter D of the horizontal wellbore 20. For example, a horizontal wellbore 20 may have substantially inconsequential effects on a tunnel 80 having a length of three times the radius or more (e.g., three, three and a half, four, four and a half, five, and five and half times the radius of the horizontal wellbore). A tunnel 80 having a length less than three times the radius of the horizontal wellbore 20, such as two and a half, two, and even one and a half times the radius of the horizontal wellbore 20, may also form transverse fractures notwithstanding the larger, though non-detrimental, effects on the formation of the transverse fractures associated with these smaller lengths.

A further benefit of using one or more tunnels 80 is that the size of any isolated portion of the wellbore that may be used can be larger than conventionally isolated portions. In still other implementations, the pressurized fluid may be introduced into the horizontal wellbore 20 at or near the tunnel(s) 80 without isolating a portion of the horizontal wellbore 20. The manner of injecting the pressurized fluid into the horizontal wellbore 20 may be selected based on conditions associated with the wellbore 10, the subterranean zone 40, and/or any number of different considerations. For example, porosity of the subterranean zone 40, the stress condition of the subterranean zone 40, properties of the reservoir fluids, and/or any other considerations may affect the manner chosen for introducing the pressurized fluid into the horizontal wellbore 20.

As mentioned above, the tunnel 80 represents a vertical well, and, during fracturing of a vertical well, a longitudinal fracture more readily forms at a lower pressure. A longitudinal fracture extending from a vertical wellbore more readily occurs because of the stress state of the subterranean zone. Fractures propagate perpendicular to the minimum principal stress in the subterranean zone. Generally, the minimum principal stress is oriented horizontally. Thus, for a vertical wellbore, longitudinal fractures are more likely to form and form more readily at lower breakdown pressures. Thus, it is believed that by including the tunnel 80 along the horizontal wellbore 20, the tunnel 80 acts as a fracture initiation location for a longitudinal fracture with respect to the tunnel 80. The fracture propagates to the horizontal wellbore perpendicular to the minimum principal stress of the subterranean zone.

Further, it is believed that the initiated fracture intersects the horizontal wellbore 20 irrespective of the orientation thereof. That is, the horizontal wellbore 20 may be oriented horizontally or substantially horizontally, or may be slanted within the subterranean zone 40, and the fracture initiated at the tunnel 80 still extends to the horizontal wellbore 20 to form a transverse fracture relative thereto. For example, some horizontal wellbores may be slanted at one or more locations so as to follow a particular formation within a subterranean reservoir. A wellbore extending through a subterranean zone, such as subterranean zone 40, that is horizontal, substantially horizontal, or that is at least partially slanted is considered horizontal within the scope of this disclosure. Thus, the longitudinal fracture 50 formed from the tunnel 80 represents a transverse fracture with respect to the horizontal wellbore 20. Consequently, forming the tunnel 80 permits the formation of a transverse fracture along the horizontal wellbore 20 using fluid at a lower fluid pressure than would otherwise be required to form a transverse fracture along a horizontal wellbore. Use of the tunnel 80 also allows consistent formation of a transverse fracture 70 relative to the horizontal wellbore 20. Further, depending on the downhole conditions, the pressurized fluid may be introduced without the need for isolating one or more portions of the well. Therefore, use of the tunnel 80 has lower associated fracturing costs. Moreover, the tunnel 80 is also believed to essentially eliminate the formation of multiple fractures and fracture tortuosity that may result during a fracturing operation.

Experimentation, described below, has been performed demonstrating the effectiveness of a tunnel extending from a horizontal wellbore in forming a fracture transverse to the horizontal wellbore at a relatively low fracturing pressure. FIG. 15 shows test summary data for six test samples. Each of the test samples were performed by casting a bore and, in some of the experiments, a vertical or substantially vertical tunnel extending therefrom in hydrostone, a gypsum cement. The hydrostone was prepared having a ratio of 30 parts of water per 100 parts of hydrostone. FIGS. 16-22 show schematic diagrams of the configuration of the bores and, optionally, the tunnels within the hydrostone. Each of the test samples were subjected to a 3000 psi pressure on a top surface (as shown in the figures), which resulted in the following stress state: vertical stress=3000 psi, minimum horizontal stress=1800 psi, and maximum horizontal stress=2500 psi. It is noted that, although some of the tests described in FIGS. 15-22 include a wellbore slanted relative to horizontal (e.g., some wellbores have a slant of 5° relative to horizontal), the wellbores may have a slant of greater than or less than 5° and still be within the scope of the disclosure. For example, in some instances, the wellbore may have a slant of 15° or greater and a tunnel extending therefrom may still be operable to produce a transverse fracture at a relatively low fracture pressure.

FIG. 16 is an elevation view of a schematic of test sample 1. Test sample 1 was formed having a bore 100 having a casing 110. A tunnel 120 extends vertically or substantially vertically from the bore 100. (Dimensions of the bore 100 and tunnel 120 are provided in the table of FIG. 15.) The bore 100 was formed at approximately 5° from horizontal. An interior of the tunnel was in communication with an interior of the bore via an opening formed in the casing 110. As a result of the casing 110, fluid pressure introduced into the bore 100 was exerted on the hydrostone (formation 130) via the tunnel 120. As a result, a fracture initiated at a fluid pressure of 3323 psi transverse to the bore 100. The fracture is believed to have initiated from the tunnel 120 and extended to encompass the bore 100. The fracture extended transverse to the minimum horizontal stress.

FIGS. 17 and 18 are schematic plan and elevation views, respectively, of test sample 2. Test sample 2 included an uncased bore 100 formed at approximately 5° from horizontal. The bore 100 was also formed at approximately 45° within a horizontal plane, as shown in the plan view of FIG. 17. The bore 100 of test sample 2 was not cased but did include a tunnel 120 extending vertically or substantially vertically from the bore 100. A fracture transverse to the bore 100 was initiated in the test sample at 2889 psi. The fracture is believed to have initiated at the tunnel 120 and extended to encompass the bore 100. The resulting fracture extended past the bore 100 without causing multiple fractures.

FIG. 19 shows a schematic view of test sample 3. Test sample 3 included a bore 100 that was not cased and did not include a tunnel, and the bore 100 was formed at an angle of 5° from horizontal. A fracture extending longitudinally along the bore 100 was formed at a fluid pressure of 3903 psi introduced into the bore 100.

FIG. 20 shows a schematic elevation view of test sample 4. Test sample 4 included an uncased bore 100 formed at an angle of 5° from horizontal. A vertical or substantially vertical tunnel 120 extended from the bore 100. Fluid pressure was introduced into the interior of the bore 100 and the tunnel 120, which caused a fracture transverse to the bore 100 at a fluid pressure of 3596 psi. The fracture is believed to have initiated in the tunnel and propagated to encompass the bore 100.

FIG. 21 shows a schematic elevation view of test sample 5. Test sample 5 included an uncased bore 100 formed at an angle of 5° from horizontal. The bore 100 did not include a tunnel extending therefrom. The bore 100 was subjected to an internal fluid pressure which, at a fluid pressure of 3525 psi, caused a fracture extending longitudinally along the bore 100.

FIG. 22 shows a schematic elevation view of test sample 6, which includes a vertical or substantially vertical uncased bore 100. Fluid pressure was introduced into the bore 100, resulting in a fracture extending longitudinally along the bore 100 at a fluid pressure of 2726 psi. Test sample 6 illustrates the tendency to forming fractures extending longitudinally along a vertical bore under stress conditions similar to those in an earth formation.

In each of the experiments, the resulting fractures propagated perpendicular to the minimum stress state. Further, the results show that, for the bores including vertical or substantially vertical tunnels extending therefrom, a fracture transverse to the bore was formed at a fluid pressure approximately the same as or lower than pressures forming a fracture longitudinal to those bores that did not include a vertical or substantially vertical tunnel extending therefrom.

A number of implementations have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the disclosure. Accordingly, other implementations are within the scope of the following claims.