FRACTURE INITIATION WITH AUXILIARY NOTCHES
Improved notching techniques are described for transverse fracture initiation from the wellbore (cased or open). One or more auxiliary notches are formed that are sealed from borehole pressure increase, yet still deformable under the fracturing pressure in the wellbore. Through the use of the auxiliary notches, lower fracturing pressure and/or shallower notches can be used when compared to known notching techniques without auxiliary notching. The described approaches can also provide greater control over the initial direction of the fracture.
The subject disclosure generally relates to hydraulic fracturing subterranean rock formations. More particularly, the disclosure relates to hydraulic fracture initiation in a main notch facilitated by one or more auxiliary notches.
BACKGROUNDWhen a borehole is drilled in the direction of minimal far-field stress, the preferred hydraulic fracture propagation direction is in a plane perpendicular (or transverse) to the borehole axis. This assumes a rock where fracture propagation is driven by stresses, whereas effects of rock fabric are negligible. In this case, techniques to induce transverse hydraulic fracturing from the borehole are well-known in petroleum and environmental industries. One known technique includes cutting a circumferential notch (or slot) in a wellbore wall into the formation. For both cased and open holes, when wellbore pressure is increasing during fracturing operation, the tip of the notch concentrates axial tensile stress longitudinal to the wellbore axis at the predetermined location along the wellbore. A transverse fracture is thus created, extending from an extremity of the notch. In addition to controlling the position and orientation of induced fracture, tensile stress concentration developed at the notch tip also results in much lower wellbore pressure used to initiate the fracture compared to the case of an un-notched wellbore. Note that a non-intervened, i.e., un-notched and un-perforated, open hole with an ideal wellbore surface free of natural flaws would fracture longitudinally regardless of the orientation of the wellbore with respect to far-field stress direction.
SUMMARYThis summary is provided to introduce a selection of concepts that are further described below in the detailed description. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in limiting the scope of the claimed subject matter.
According to some embodiments, a method is described for fracturing a subterranean rock formation from a borehole penetrating the formation. The method includes: creating a fracture initiation notch extending from a wall of the borehole into the formation; creating at least one auxiliary notch extending from the borehole wall into the formation positioned and configured to facilitate fracture initiation from the fracture initiation notch; protecting the auxiliary notch(es) from direct fluid communication with the borehole; and increasing fluid pressure within the borehole thereby initiating a fracture from the fracture initiation notch. The auxiliary notch(es) are protected from the fluid pressure increase and are deformable such that the fracture initiation occurs at a lower pressure than if the auxiliary notch(es) did not exist.
According to some embodiments, borehole can be an open hole borehole or a cased hole borehole in the location of the fracturing. The notches can be formed using various notch cutting tools (e.g. jet cutting tool) and can have various cross section shapes such as V-shaped, or round-ended. According to some embodiments, the auxiliary notch(es) are protected from direct fluid communication with the borehole by sealing their openings with a sleeve made of elastomer material, by sealing their openings using a polymer resin material, or by sealing their openings with an expandable metallic sleeve which may contain a swelling elastomer external coating which serves to seal against the wellbore wall. According to some embodiments, the auxiliary notch(es) are spaced from the fracture initiation notch by less than six times the radius of the borehole.
According to some embodiments, the fracture initiation notch is created having a depth that is less than would have been needed for fracture initiation at an equivalent pressure if the auxiliary notch(es) did not exist. The notches can be planar and approximately perpendicular to a central borehole axis. In cases where there are two auxiliary notches, they can be positioned with the fracture initiation notch between the two auxiliary notches. The two auxiliary notches can be spaced away from the fracture initiation notch by less than about 3 times the radius of the borehole. According to some embodiments, the two auxiliary notches are spaced away from the fracture initiation notch by less than about 2 times the radius of the borehole. In some cases more than two auxiliary notches are formed to aid in the fracture initiation.
According to some embodiments, creating of the fracture initiation notch and creating and protecting of the auxiliary notch(es) is repeated in multiple locations within a target region of the borehole such that fracturing can be sequentially or simultaneously initiated from a plurality of fracturing initiation notches. According to some embodiments, fractures are not initiated from any of the auxiliary notches.
According to some embodiments, a system is described for fracturing a subterranean rock formation from a borehole penetrating the formation. The system includes: a notch forming tool configured to form a fracture initiation notch extending from a wall of the borehole into the formation, and at least one auxiliary notch extending from the borehole wall into the formation. The at least one auxiliary notch is positioned and configured to facilitate fracture initiation from the fracture initiation notch. The system further includes a notch sealing system configured to seal and protect the at least one auxiliary notch from direct fluid communication with the borehole; a fluid pressurizing system configured to increase pressure in the borehole; and a control system programmed and configured to cause the pressurizing system to increase the pressure in the borehole to a fracture initiation pressure, which is calculated to initiate a fracture from the fracture initiation notch, and wherein the fracture initiation pressure is lower than a pressure that would be needed to initiate fracturing if the auxiliary notch(es) did not exist.
Further features and advantages of the subject disclosure will become more readily apparent from the following detailed description when taken in conjunction with the accompanying drawings.
To further clarify the above and other advantages and features of the subject matter of this patent specification, specific examples of embodiments thereof are illustrated in the appended drawings. It should be appreciated that these drawings depict only illustrative embodiments, and are therefore not to be considered limiting of the scope of this patent specification or the appended claims. The subject matter hereof will be described and explained with additional specificity and detail through the use of the accompanying drawings in which:
The particulars shown herein are by way of example and for purposes of illustrative discussion of the examples of the subject disclosure only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the subject disclosure. In this regard, no attempt is made to show structural details in more detail than is necessary; the description taken with the drawings, making apparent to those skilled in the art how the several forms of the subject disclosure may be embodied in practice. Furthermore, like reference numbers and designations in the various drawings indicate like elements.
When using a circumferential notch formed in a wellbore wall, axial tensile stress concentrates at the location of the notch tip to create transverse fracture extending from the notch tip. See, e.g. U.S. Pat. No. 3,313,348; U.S. Pat. No. 4,974,675 and U.S. Pat. Appl. Publ. No. US 2014/0069653.
According to some embodiments, an improved notching approach to transverse fracture initiation from the wellbore (cased or open) is described. The described techniques, which use one or more auxiliary notches, can use lower fracturing pressure and/or use shallower notches compared to known notching techniques without auxiliary notching. The described approaches also allow greater control over the initial direction of the fracture.
The two auxiliary notches 124 and 126 are in fluid isolation from the wellbore 110, and are thus kept under lower pressure during fracturing treatment. This allows a larger opening of the fracture initiation notch 120 and thus higher tensile stress concentration developed at its tip compared to the single notch setup at the same borehole pressure. The mechanism is demonstrated in
By forming the sealed auxiliary notches a number of benefits can be gained, for example: (1) the fracture initiation pressure can be decreased while keeping the same dimensions for the fracture initiation notch; (2) shallower dimensions can be used for the fracture initiation notch(es) while using the same pressure; or (3) a combination of lower pressure and shallower dimensions for the fracture initiation notch can be used. Both lower pressure and shallow notch dimension have positive impacts in cases such as (1) hard rock formations under high stress where building up sufficient breakdown pressure is a challenge; and (2) situations when it is impractical or impossible to cut a fracture initiation notch sufficiently deep so as to ensure desired transverse fracture initiation. According to some embodiments, isolation of the auxiliary notches from the wellbore can be attained by sealing means, such as rubber or metallic sleeves. According to some embodiments, by varying the depth of the auxiliary notches (e.g. forming un-equally, so as to be asymmetrical) it is possible to adjust the initial direction of the fracture.
Further details are now provided for numerical simulations of stress concentration at the tip of a fracture initiation notch facilitated by auxiliary sealed notches, according to some embodiments. For simplicity, results presented are of simulations for open hole boreholes.
In what follows, the distances were normalized with respect to wellbore radius Rw. The stresses, pressures, Young's modulus, and tensile strength of rock were normalized with respect to absolute value of the far-field stress acting along the wellbore |σz∞|. This is convenient scaling to study the fracture that opens against σz∞. The base configuration of dimensionless parameters is shown in Table 1. Note that hereinafter, the sign convention of compressive stresses as negative, and tensile as positive, is used.
Thus, for typical in-situ stress magnitude of 2000 psi, one gets Young's modulus E=4 Mpsi and tensile strength T0=1000 psi which, with Poisson ratio ν=0.2, correspond to Indiana limestone.
For the rock that behaves linear elastically, which is assumed in our simulations, tensile fracture is expected to initiate at the surface of the borehole or notch. Thus, in the case of un-notched borehole, when longitudinal fracture initiates along the borehole, it is controlled by the maximum hoop stress value at the wellbore wall which gives the well-known Hubert-Willis estimation for fracture initiation pressure: PinitHW=T0−2σr∞ (see, e.g., E. Detournay, R. Carbonell, “Fracture-Mechanics Analysis of the Breakdown Process in Minifracture or Leakoff Test”, SPE Production & Facilities, 1997). In case of notched wellbore, initiation of transverse fracture is controlled by the axial tensile stress, which reaches its maximum at the tip of the notch: σz|tip. Note that for the linear problem statement, this value depends linearly on borehole pressure and far-field stresses with coefficients which can be found numerically with any precision required. For the base set of problem parameters shown in Table 1, tensile axial stress at the tip of fracture initiation notch, σz|tip, will depend solely on the borehole pressure Pw as expressed in Table 2 and shown in
From Table 3 it is apparent that hydraulic fracturing initiation pressure in three-notch configurations is lower than in one-notch configurations, i.e.: Pi,3<Pi,1 and particularly for T0=0.5 (which corresponds to 1000 psi) we have Pi,1=1.3 (2600 psi) and Pi,3=0.55 (1100 psi). In
It is apparent that the difference between single-notch and three-notch configurations diminishes quickly with increasing distances between notches S. Thus in order to benefit from the effect, according to some embodiments, the auxiliary notches should not be further than 3-4 wellbore radii from the central fracture initiation notch. In practice, the distance S should be balanced with other considerations, such as the technology used for forming the notches, etc.
Further, a comparison is made between (1) a three-notch configuration where notches are formed at half of the depth (1 wellbore radius deep); and (2) a one-notch configuration with a notch that is 2 wellbore radii deep.
Tensile stress at the tip of central notch 420 and the hydraulic fracturing initiation pressure for three-notch configuration is calculated as shown in Table 5.
It is apparent from the simulations that, for the same borehole pressures, a higher tension will be developed at the notch tip in a three-notch configuration with notches of one bore hole radius deep than in a single notch configuration with notch of two bore hole radii deep. Thus, the hydraulic fracturing initiation pressures will also be smaller in a three-notch configuration. In particular, T0=0.5, Pi,3 (Dn=1, S=2)=0.94 and Pi,3 (Dn=1, S=3)=1.17 which both are less than Pi,1(Dn=2)=1.3.
According to some embodiments, the described auxiliary-notch-assisted transverse fracturing technique is repeated multiple times within a given wellbore. The fracturing is carried out either sequentially over multiple fracturing stages or simultaneously from several auxiliary-notch assisted fracture initiations notches. According to some embodiments, the described pattern of fracture initiation notch surrounded by two auxiliary notches isolated from the borehole pressure is repeated for each “sweet spot” along a wellbore where transverse fracturing is desired.
Although some embodiments have been described herein with respect to hydrocarbon-bearing formations, the techniques described are applicable to other types of subterranean formations. According to some embodiments, the hydraulic fracturing facilitation techniques are applied to other types of formations including: geothermal formations, CO2 storage formations, and/or water-bearing formations.
The foregoing outlines features of several embodiments so that those skilled in the art may better understand the aspects of the present disclosure. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions and alterations herein without departing from the spirit and scope of the present disclosure.
Claims
1. A method of fracturing a subterranean formation from a borehole penetrating the formation, the method comprising:
- creating a fracture initiation notch extending from a wall of the borehole into the formation;
- creating at least one auxiliary notch extending from the borehole wall into the formation positioned and configured to facilitate fracture initiation from said fracture initiation notch;
- protecting said at least one auxiliary notch from direct fluid communication with said borehole; and
- increasing fluid pressure within the borehole and said fracture initiation notch thereby initiating a fracture from said fracture initiation notch, wherein said at least one auxiliary notch being protected from said fluid pressure increase and being deformable such that said fracture initiation occurs at a lower pressure than if the auxiliary notch did not exist.
2. The method according to claim 1 wherein said borehole is an open-hole or cased borehole in the location of the fracture initiation notch and the at least one auxiliary notch.
3. (canceled)
4. The method according to claim 1 wherein said fracture initiation notch and said at least one auxiliary notch are shaped.
5. The method according to claim 4 wherein said fracture initiation notch and said at least one auxiliary notch have a shape selected from a group consisting of a U-shape and a V-shape.
6. The method according to claim 1 wherein protecting said at least one auxiliary notch from direct fluid communication with said borehole includes sealing an opening to said at least one auxiliary notch.
7. The method according to claim 6 wherein protecting said at least one auxiliary notch from direct fluid communication with said borehole includes sealing an opening to said at least one auxiliary notch with a polymer resin material.
8. The method according to claim 1 wherein said at least one auxiliary notch is spaced from said fracture initiation notch by less than six times a radius of the borehole.
9. The method according to claim 1 wherein said fracture initiation notch is created having a depth that is less than would have been needed for fracture initiation at an equivalent pressure if said auxiliary notch did not exist.
10. The method according to claim 1 wherein said fracture initiation notch is planar and approximately perpendicular to a central borehole axis.
11. The method according to claim 1 wherein said fracture initiation notch is planar and non-perpendicular to a central borehole axis.
12. The method according to claim 1 where said at least one auxiliary notch includes two auxiliary notches positioned with the fracture initiation notch between the two auxiliary notches.
13. The method according to claim 12 wherein each of the two auxiliary notches is spaced away from the fracture initiation notch by less than about 3 times a radius of the borehole.
14. The method according to claim 12 wherein the two auxiliary notches are spaced away from the fracture initiation notch by non-equal distances.
15. The method according to claim 1 where said at least one auxiliary notch includes at least three auxiliary notches.
16. The method according to claim 1 wherein said fracture initiation notch and said at least one auxiliary notch are created using a jet cutting tool deployed via coiled tubing.
17. The method according to claim 1 wherein protecting said at least one auxiliary notch from direct fluid communication with said borehole includes at least partially filling said at least one auxiliary notch with a degradable plugging material.
18. The method according to claim 1 wherein the subterranean formation is a hydrocarbon-bearing formation.
19. The method according to claim 1 further comprising repeating said creating a fracture initiation notch, creating at least one auxiliary notch, and protecting, such that said increasing fluid pressure initiates a plurality of fractures from a plurality of fracturing initiation notches.
20. The method according to claim 1 further comprising repeating said creating a fracture initiation notch, creating at least one auxiliary notch, protecting, and increasing fluid pressure, thereby sequentially fracturing the formation from each of several fracturing initiating notches.
21. The method according to claim 1 wherein fractures are not initiated from said at least one auxiliary notch.
22. A system for fracturing a subterranean formation from a borehole penetrating the formation, the system comprising:
- a notch forming tool configured to form a fracture initiation notch extending from a wall of the borehole into the formation, and at least one auxiliary notch extending from the borehole wall into the formation, the at least one auxiliary notch being positioned and configured to facilitate fracture initiation from said fracture initiation notch;
- a notch sealing system configured to sealingly protect said at least one auxiliary notch from direct fluid communication with said borehole;
- a fluid pressurizing system configured to increase pressure in the borehole; and
- a control system programmed and configured to cause the pressurizing system to increase the pressure in the borehole to a fracture initiation pressure which is calculated to initiate a fracture from said fracture initiation notch and wherein said fracture initiation pressure is lower than a pressure that would be needed to initiate fracturing if the at least one auxiliary notch did not exist.
23. The system according to claim 22 wherein said control system and said notch forming tool are configured to form the at least one auxiliary notch and the fracture initiation notch such they are spaced apart by no more than six times a radius of the borehole.
24. The system according to claim 22 wherein said control system and said notch forming tool are configured to form the at least one auxiliary notch and the fracture initiation notch such they are spaced apart by no more than three times a radius of the borehole.
25. The system according to claim 22 wherein said control system and said notch forming tool are configured to form the fracture initiation notch having a depth that is less than would have been needed for fracture initiation at an equivalent pressure if said at least one auxiliary notch did not exist.
26. The system according to claim 22 where said at least one auxiliary notch includes two auxiliary notches positioned with the fracture initiation notch between the two auxiliary notches.
27. The system according to claim 22 wherein said notch forming tool is a jet cutting tool deployable via coiled tubing.
Type: Application
Filed: Jan 13, 2015
Publication Date: Jul 14, 2016
Inventors: Gallyam Aidagulov (Dammam), Mikhail Stukan (Al-Khobar), Stephen Dyer (Katy, TX)
Application Number: 14/595,983