METHOD FOR FRACTURING ROCK MULTI-DIRECTIONALLY AND DEVICE FOR FRACTURING ROCK MULTI-DIRECTIONALLY

Abstract

A method for fracturing a rock multi-directionally includes: a borehole formation step of forming a borehole in a rock; a first fluid introduction step of introducing a first fluid to the borehole; a second fluid introduction pressurization fracturing step of introducing and pressurizing a second fluid to the borehole to generate a fractured portion around the borehole, and allowing the first fluid to close the fractured portion; and a pressurization fracturing step of continuously introducing and pressurizing the second fluid to the borehole even after the fractured portion is generated, and generating a fractured portion in a direction different from the fractured portion of the rock.

Claims

1. A method for fracturing a rock multi-directionally, the method comprising: a borehole formation step of forming a borehole in a rock; a first fluid introduction step of introducing a first fluid to the borehole; a second fluid introduction pressurization fracturing step of introducing and pressurizing a second fluid to the borehole to generate a fractured portion around the borehole, and allowing the first fluid to close the fractured portion; and a pressurization fracturing step of continuously introducing and pressurizing the second fluid to the borehole even after the fractured portion is generated, and generating a fractured portion in a direction different from the fractured portion of the rock.

2. The method for fracturing a rock multi-directionally according to claim 1, wherein the first fluid is a functional fluid.

3. The method for fracturing a rock multi-directionally according to claim 1, wherein the pressurization fracturing step is repeated a plurality of times.

4. A device for fracturing a rock multi-directionally, the device comprising: a drilling unit configured to drill a borehole in the rock; a fluid introduction pressurization unit configured to introduce and pressurize a first fluid and a second fluid into the borehole; and a control unit for the fluid introduction pressurization unit configured to continuously operate the fluid introduction pressurization unit even after a fractured portion is generated around the borehole of the rock by the fluid introduction pressurization unit and perform control to repeat fracturing around the borehole of the rock.

5. The device for fracturing a rock multi-directionally according to claim 4, wherein the first fluid is a functional fluid.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0012] FIG. 1 is a schematic diagram showing a multi-directional fracturing device according to an embodiment of the invention.

[0013] FIG. 2 is a diagram showing a fracturing method in the related art,

[0014] FIG. 3 is a flowchart showing a multi-directional fracturing method according to the embodiment of the invention.

[0015] FIG. 4 is a diagram showing the multi directional fracturing method according to the embodiment of the invention.

[0016] FIG. 5 is a diagram showing a result of an experiment using the multi-directional fracturing method according to the embodiment of the invention.

[0017] FIG. 6 is a diagram showing a result of an experiment using the multi-directional fracturing method according to the embodiment of the invention.

DESCRIPTION OF EMBODIMENTS

[0018] Hereinafter, a method for fracturing rock multi-directionally and a. device for fracturing rock multi-directionally according to an embodiment of the invention will be described with reference to the drawings. However, the embodiment described below is merely an example, and is not intended to exclude various modifications and application of techniques not explicitly described in the embodiment. That is, the embodiment can be variously modified without departing from the gist thereof.

[0019] As shown in FIG. 1, a device 1 for fracturing rock multi-directionally according to the embodiment (hereinafter, simply referred to as multi-directional fracturing device 1) includes a drilling part 2, an introduction pressurization part 3, and a control unit 4.

[0020] The drilling part 2 (drilling unit) bores (forms) a borehole H (well) in a rock B. The borehole H is formed in the ground or rock by a well-known boring device or the like, which is an example of the drilling part 2.

[0021] The introduction pressurization part 3 (fluid introduction pressurization unit) is a pressurization unit that introduces and pressurizes the first fluid and the second fluid into the borehole H.

[0022] The first fluid and the second fluid may be different fluids or the same fluid. Each of the first fluid and the second fluid may be held in a storage tank (not shown) or the like, or a fluid in the vicinity of a drilling site may be used, and a thickener described later may be mixed with the fluid to be used as a functional fluid.

[0023] The control unit 4 is a known CPU or the like that comprehensively controls the operation of the multi-directional fracturing device 1. For example, the control unit 4 (the control unit for the fluid introduction pressurization unit) continuously operates the introduction pressurization part 3 even after a fractured portion C is generated around the borehole H of the rock B by the introduction pressurization part 3, and controls to repeat fracturing around the borehole H of the rock B.

[0024] Next, a method for fracturing rock multi-directionally according to the embodiment will be described in comparison with the fracturing method in the related art.

[0025] In the related art, a hydraulic fracturing method for fracturing rock by drilling the borehole H in the rock B to several kilometers underground and injecting a high-pressure fluid (for example, water) into the borehole H has been performed. Such a fracturing method is used for the purpose of, for example, recovering resources such as petroleum buried in the rock B or developing underground resources such as geothermal development.

[0026] FIG. 2 shows the fracturing method in the related art. as described above. (a) and (b) in FIG. 2 are diagrams of the borehole H as viewed from a drilling direction. In the fracturing method in the related art, after the borehole H is formed, a fluid is introduced into the borehole H, and as shown in (a) and (c) of FIG. 2, a water pressure Pp of the borehole H increases. Then, when the water pressure Pp in the borehole H exceeds a threshold Pb at a certain time t1, as shown in (b) of FIG. 2, the fractured portion C is generated in a direction in which a maximum principal stress acts around the borehole H. As shown in (c) of FIG. 2, when the fractured portion C is generated, the water pressure Pp in the borehole H rapidly decreases because the fluid leaks into the rock through the fractured portion C. In addition, even when the introduction of the fluid is continued after the fractured portion C is generated, the fluid continues to leak from the fractured portion C to the outside of the borehole H, so that the water pressure Pp in the borehole H does not substantially increase, and other fractured portions C are not generated.

[0027] In the fracturing method in the related art described above, the direction in which the fractured portion C is generated is determined by a stress distribution in the rock B, Specifically, the fractured portion C is generated in a direction in which the maximum principal stress acts (maximum principal stress direction SHmax) in a plane orthogonal to the borehole H. In other words, the fractured portion C is generated in a direction orthogonal to the direction in which a minimum principal stress acts (minimum principal stress direction SHmin) in the plane orthogonal to the borehole H. Therefore, when a direction in which resources such as petroleum exist is different from the maximum principal stress direction when viewed from the borehole H, the fractured portion C may not come into contact with the resources, and it may be difficult to recover the resources. In this case, it is conceivable to perform the drilling of the borehole H and the injection of the high-pressure fluid again at another place, but the drilling of a new borehole H is expensive.

[0028] In view of the above problems, as a result of intensive studies, the inventors of the invention have found a method for generating fractured portions in multiple directions of the rock B as shown in FIG. 1. With this method, even when a crack is generated in a direction other than the direction in which the resources exist, a fractured portion can be formed also in the direction in which the resources exist by forming the fractured portion in another direction again. It is possible to increase the number of fractured portions that can be simply connected in a stratum in which resources exist in multiple directions. That is, as shown in FIG. 3, the multi-directional fracturing method according to the embodiment includes a borehole formation step S1, a first fluid introduction step S2, a second fluid introduction pressurization fracturing step S3, and a pressurization fracturing step S4.

[0029] In the borehole formation step S1, for example, the drilling part 2 of the multi-directional fracturing device 1 forms the borehole H in the rock B.

[0030] After the borehole formation step S1 is performed, the first fluid introduction step S2 is performed. In the first fluid introduction step S2, the first fluid is introduced into the borehole H. In the first fluid introduction step S2, for example, the introduction pressurization part 3 of the multi-directional fracturing device 1 introduces the first fluid into the borehole H.

[0031] The first fluid is a functional fluid, for example, a dilatancy fluid (shear thickening fluid). The dilatancy fluid is a mixture of powder particles (for example, SiO.sub.2 nanoparticles) and a liquid as the thickener, and is a type of non-Newtonian fluid. The dilatancy fluid has a thickening property that a viscosity rapidly increases when a shear rate exceeds a predetermined value. Due to this thickening property, the dilatancy fluid temporarily behaves as a solid when an external force is applied. When the external force is removed, the thickening property is lost, and the viscosity of the fluid returns to the original state.

[0032] The powder particles and the liquid constituting the dilatancy fluid can be appropriately changed, and the powder particles are not limited to the SiOz nanoparticles. For example, a fluid, in which particles of about 50 nm to 10 m are dispersed in a liquid, which has a shear thickening effect, and which is used in a corn starch aqueous solution or the like, can be used. In addition, a dilatancy fluid reinforced by nanowires may be used as the first fluid. The fluid is not limited to water, and for example, an organic solvent such as ethylene glycol can be used. As described above, a thickening property of the first fluid can be freely changed by changing the powder particles and the liquid constituting the dilatancy fluid or changing a content of the nanowires or the like.

[0033] A particle diameter of the powder particles constituting the dilatancy fluid is preferably a particle diameter of about 10 m at which Brownian motion can be ignored.

[0034] The combination of the solvent and the solute can greatly vary the thickening property.

[0035] After the first fluid introduction step S2 is performed, the second fluid introduction pressurization fracturing step S3 is performed. In the second fluid introduction pressurization fracturing step S3, the second fluid is introduced into the borehole H, and the borehole H is pressurized. In the second fluid introduction pressurization fracturing step S3, for example, the introduction pressurization part 3 of the multi-directional fracturing device 1 introduces and pressurizes the second fluid into the borehole H. The second fluid is, for example, water. As the second fluid, a dilatancy fluid may be used similarly to the first fluid. As the dilatancy fluid used as the second fluid, dilatancy fluid in which the component of the dilatancy fluid used as the first fluid is changed, for example, a dilatancy fluid in which a concentration of the powder particles is reduced may be used.

[0036] As shown in (a) and (c) in FIG. 4, when the water pressure Pp in the borehole H increases due to the introduction of the second fluid, the water pressure P exceeds the threshold Pb at a certain time t1 as in the fracturing method in the related art, and the fractured portion C (first fractured portion C1) is generated in a direction in which the maximum principal stress of the borehole H acts (hereinafter, sometimes referred to as first pressurization fracturing). At this time, the fractured portion C (first fractured portion C1) is generated in the maximum principal stress direction SHmax in the plane orthogonal to the borehole H. A mechanism until the first pressurization fracturing in which the first fractured portion C (first fractured portion C1) is generated is the same as that of the fracturing method in the related art.

[0037] On the other hand, in the multi-directional fracturing method according to the embodiment, after the fractured portion C (first fractured portion C1) is generated, the functional fluid introduced as the first fluid behaves as a solid due to the thickening effect caused by the pressurization of the second fluid in a process of flowing into the first fractured portion C1. Accordingly, as shown in (a) of FIG. 4, the functional fluid, which is the first fluid, constitutes a closing body A at the fractured portion C (first fractured portion C1) to close the fractured portion C.

[0038] After the second fluid introduction pressurization fracturing step S3 is performed, the pressurization fracturing step S4 is performed. In the pressurization fracturing step S4, the borehole H is pressurized by continuously introducing and pressurizing the second fluid into the borehole H even after the fractured portion C (first fractured portion C1) is generated. The control unit 4 continuously operates the introduction pressurization part 3, and the introduction pressurization part 3 continuously introduces and pressurizes the second fluid into the borehole H.

[0039] As described above, in the fracturing method in the related art, the water pressure Pp in the borehole H does not increase once the fractured portion C is generated, or the fractured portion C cannot be generated again in the borehole H without continuously pressurizing the borehole H even once the fractured portion C is generated. On the other hand, in the fracturing method according to the embodiment, since the functional fluid introduced as the first fluid closes the fractured portion C (first fractured portion C1) with the closing body A, the water pressure Pp in the borehole H turns to increase again by continuously pressurizing the borehole H even after the fractured portion C (first fractured portion C1) is generated.

[0040] As shown in (b) and (c) of FIG. 4, when the water pressure Pp in the borehole H increases again, the water pressure Pp exceeds a threshold Pb at a certain time t2, and the fractured portion C (second fractured portion C2) is generated again around the borehole H (hereinafter, sometimes referred to as second pressurization fracturing). Here, the fractured portion C (first fractured portion C1) generated in the second fluid introduction pressurization fracturing step S3 and the fractured portion C (second fractured portion C2) generated in the pressurization fracturing step S4 are generated in different directions. This is considered to be because the stress around the borehole H is redistributed when the first fractured portion C (first fractured portion C1) is generated.

[0041] After performing the second pressurization fracturing, the pressurization fracturing step S4 may be repeated a plurality of times as necessary to perform the third and fourth pressurization fracturing. By repeating the pressurization fracturing step S4 a plurality of times, it is possible to increase a plurality of the fractured portions C having different generation directions, and the pressurization fracturing step is stopped at a stage where the fractured portions C are generated in desired directions.

[0042] In the method according to the embodiment of the invention, details of the cause of the multi-directional fracturing of the rock are under study, but high reproducibility is obtained.

[0043] The following mechanism is considered as the reason why the multi-directional fracturing occurs. First, as described above, since the first fluid is a functional fluid, when the first fluid is indirectly pressurized by the second fluid, the first fluid forms a surface that behaves as a solid on an inner peripheral surface of the borehole H to pressurize the surface of the borehole H. Accordingly, fracturing occurs. Then, the first fluid around the surface of the borehole H at a portion where the fracturing has occurred flows into the fractured portion. In this state, the second fluid is introduced and the pressurization is continued, so that the first fluid is solidified at a predetermined site of the fractured portion (a fracturing surface of the borehole H, a narrow portion of a crack, or the like), thereby forming the closing body A. Then, the closing body A closes the fractured portion. Accordingly, the water pressure in the borehole H can be increased again. By continuously introducing the second fluid in this state, the pressure is continuously applied to a solidified region of the first fluid, the solidification is maintained, and the borehole H can be smoothly pressurized. Accordingly, it is considered that fracturing can occur in another portion of the borehole H.

[0044] Different compositions of the first fluid and the second fluid are suitable for multi-directional fracturing due to interaction between fluids, but multi-directional fracturing of the rock is sufficiently possible even when the first fluid and the second fluid are the same fluid.

[0045] As described above, the method for fracturing the rock multi-directionally according to the embodiment performs: the borehole formation step S1 of forming the borehole H in the rock B; the first fluid introduction step S2 of introducing the first functional fluid into the borehole H; the second fluid introduction pressurization fracturing step S3 of introducing and pressurizing the second fluid into the borehole H to generate the fractured portion c (first fractured portion C1) around the borehole H; and then the pressurization fracturing step S4 of continuing the second fluid introduction, pressurizing the borehole H, and generating the fractured portion C (second fractured portion C2) in a direction different from the fractured portion C (first fractured portion C1) of the rock B, so that the first functional fluid constitutes the closing body A to close the fractured portion C (first fractured portion C1).

[0046] With this configuration, the fractured portions C can be generated in a plurality of directions in the plane orthogonal to the borehole H. Accordingly, it is possible to increase the possibility that the fractured portions C reach resources such as petroleum buried in the rock B from the drilled borehole H. Since the fracturing occurs in multiple directions, a fracturing area in contact with the resource storage stratum can be simply increased.

[0047] The pressurization fracturing step S4 may be repeated a plurality of times, such as third and fourth times. According to this configuration, the fractured portions C can be generated in more directions in the plane orthogonal to the borehole H.

[0048] The device for fracturing the rock multi-directionally according to the embodiment includes: the drilling part 2 (drilling unit) for boring the borehole H in the rock B; the introduction pressurization part 3 (fluid introduction pressurization unit) for introducing and pressurizing the first fluid and the second fluid into the borehole H; and the control unit 4 (control unit for the fluid introduction pressurization unit) for continuously operating the introduction pressurization part 3 even after the fractured portions C are generated around the borehole H of the rock B by the introduction pressurization part 3 and performing control to repeat fracturing for forming a plurality of the fractured portions C around the borehole H of the rock B.

[0049] According to the multi-directional fracturing device 1, the multi-directional fracturing method in the rock B described above can be easily executed only by continuous control of the second fluid introduction and the pressurization fracturing for forming the functional fluid and the plurality of fractured portions C without performing a large investment.

EXAMPLE

[0050] Hereinafter, the above embodiment will be described using specific examples. The invention is not limited to the following examples.

[0051] A real triaxial tester was prepared as the multi-directional fracturing device 1, and an experiment was performed in which the multi-directional fracturing method according to the above embodiment was applied to a granite test piece that is a cube having a 10 cm side and a tensile strength of about 6 MPa. A dilatancy fluid (water+SiO.sub.2 (particle diameter: about 100 nm), mass percent concentration: 25%) was used as the functional fluid. As a procedure, after a borehole was formed in the test piece, the borehole was filled with the functional fluid, and then water was pressurized at a constant flow rate of 1 ml/sec under injection conditions. An axial pressure during pressurization was set to 50 MPa. FIGS. 5 and 6 are diagrams showing results of the experiment. (a) of FIG. 5 shows a temporal change of the axial pressure, (b) of FIG. 5 shows a displacement of a compression axis of the tester, (c) of FIG. 5 shows a temporal change of the water pressure in the borehole H, and (d) of FIG. 5 shows a temporal change of vibration energy released from the test piece. FIG. 6 is a photograph of the test piece after the multi-directional fracturing method according to the embodiment is applied.

[0052] As shown in (c) of FIG. 5, in the experiment, a rapid decrease and a re-increase of the water pressure in the borehole H were observed a plurality of times. This means that fracturing has occurred a plurality of times. As shown in (d) of FIG. 5, in the experiment, large vibration energy was detected twice (about 440 seconds, about 740 seconds). This means that large fracturing has occurred twice. As shown in (c) and (d) of FIG. 5, a timing at which the large vibration energy is detected is synchronized with a timing at which the water pressure in the borehole H rapidly decreases.

[0053] As shown in FIG. 6, one fractured portion C (first fractured portion C1) extending from the borehole H and four fractured portions C extending in a direction different from the fractured portion C are formed in the test piece. As described above, according to the multi-directional fracturing method of the embodiment, it was experimentally confirmed that the fractured portions C can be generated in the plurality of directions.

[0054] The technical scope of the invention is not limited to the above-described embodiment, and various modifications can be made without departing from the gist of the invention.

[0055] For example, although the dilatancy fluid is used as the first fluid in the above embodiment, the first fluid may not be the dilatancy fluid. A type of the first fluid is not limited to the dilatancy fluid as long as the fluid can constitute the closing body A by applying an external force. The second fluid is not limited to water and can be changed as appropriate.

[0056] The first fluid may be introduced again after the second fluid pressurization fracturing step or after the pressurization fracturing step, and then the second fluid may be introduced again. Accordingly, the thickening property can be enhanced again, and it can be expected that fracturing in multiple directions occurs.

[0057] In addition, it is possible to appropriately replace the components in the above embodiment with well-known components without departing from the gist of the invention, and the above embodiment and modifications may be appropriately combined.

[0058] Among the components of the multi-directional fracturing device 1, for example, the function of the control unit 4 may be implemented by a hardware processor such as a central processing unit (CPU) executing a program (software). A part or all of these components may be implemented by hardware (circuit unit; including circuitry) such as a large scale integration (LSI), an application specific integrated circuit (ASIC), a field-programmable gate array (FPGA), and a graphics processing unit (GPU), or may be implemented by cooperation of software and hardware. The program may be stored in advance in a storage device (a storage device including a non-transitory storage medium) such as a hard disk drive (HDD) or a flash memory, or may be stored in a removable storage medium (a non-transitory storage medium) such as a DVD or a CD-ROM and installed by attaching the storage medium to a drive device.

REFERENCE SIGNS LIST

[0059] 1: multi-directional fracturing device (device for fracturing rock multi-directionally) [0060] 2: drilling part (drilling unit) [0061] 3: introduction pressurization part (fluid introduction pressurization unit) [0062] 4: control unit (control unit for fluid introduction pressurization unit) [0063] A: closing body [0064] B: rock [0065] C: fractured portion [0066] H: borehole