CN115577656B - Rock directional fracturing method utilizing crack disturbance stress - Google Patents
Rock directional fracturing method utilizing crack disturbance stress Download PDFInfo
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Abstract
The invention discloses a rock directional fracturing method utilizing fracture disturbance stress, which comprises the steps of arranging three hydraulic fractures which are linearly arranged along the fracture direction of a top plate design and guiding the fracturing fractures to expand along the extension direction of the design; carrying out hydraulic fracturing on the middle crack to ensure that the middle crack turns to spread after cracking, and the direction of the surrounding minimum main stress deflects; continuously pumping to drive the middle hydraulic fracture to expand until the middle hydraulic fracture turns to the direction of the maximum main stress and exceeds the upper fracture and the lower fracture, and the minimum main stress around the upper fracture and the lower fracture is influenced by the middle hydraulic fracture and is deflected to be almost perpendicular to the designed directional fracturing direction; the upper hydraulic fracture and the lower hydraulic fracture are pumped to crack, and the hydraulic fracture is expanded approximately along the design fracturing direction under the control of the disturbance stress field of the middle hydraulic fracture. The method can realize directional fracturing of the hard top plate in any indeterminate stress field, eliminates dynamic disasters caused by the existence of the hard top plate, and further provides reference for directional fracturing control of oil gas and geothermal reservoirs.
Description
Technical Field
The invention relates to the field of hydraulic fracturing directional fracturing, in particular to a rock directional fracturing method utilizing crack disturbance stress.
Background
Fracturing refers to a process of injecting high-pressure fluid (water, gas and the like) through a drilling (hole) pump, and breaking rock and driving crack propagation under the action of fluid-solid coupling. For coal mining, directional drilling and fracturing are effective means for weakening a hard roof of a well and mining industry and inhibiting dynamic disasters caused by sudden collapse of the roof. However, as the mining and tunneling of the roadway are carried out on the working face of the coal in the well and mining industry, the roof is influenced by strong mining stress due to the change of stratum structure, the local stress distribution of the roof is complex, the direction of the roof is difficult to determine, the design extension direction of the directional drilling fracturing crack is often oblique to the main direction of the ground stress, the fracturing crack is spatially and rotationally extended and has extremely large deviation from the design extension track, and the fracturing weakening effect of the hard roof is restricted. Therefore, the directional expansion of the fracturing cracks in the rock with complex stress state is effectively controlled, and the directional expansion control method is the basis of directional fracturing of the hard top plate.
At present, students at home and abroad develop the research on the steering expansion rule of the hydraulic fracture through research methods such as true triaxial fracturing physical simulation test, theoretical model analysis, numerical simulation and the like successively. The results of the study show that the steering expansion of the hydraulic fracture is mainly due to two factors: (1) Because of mining stress, stress shadow of multi-section fracturing cracks and special crack design extension tracks, the simulated extension direction of the cracks is oblique to the main stress direction of the rock; (2) The bedding and weakness in the rock (e.g., the bedding surface of shale and coal-rock cutting) result in an anisotropic fracture propagation resistance.
Based on the research results of the fracture steering and expansion, the methods for inhibiting the fracture steering and further controlling the fracture directional expansion mainly comprise three types: (1) The pumping displacement of the fracturing pump is improved, and the steering radius of the fracturing fracture is increased; (2) Changing the local stress field of the rock by adopting dense linear drilling so as to realize directional expansion of cracks during the fracturing of the drilling; (3) Drilling or drilling holes in a target extension area of the fracturing fracture, injecting water, and controlling directional expansion of the fracturing fracture by inhibiting steering of the fracturing fracture in a mode of improving local pore pressure. The method provides an extremely important reference for controlling the directional expansion of the fracturing fracture. However, the above method still has three limitations: (1) The lifting displacement can only release the steering of the fracturing cracks (the design extending direction is oblique to the main stress direction), but the directional expansion of the fracturing cracks can not be truly realized, (2) the range of the local stress field (namely the stress concentration area) changed by the round hole is limited, and the area with the diameter of about 3 times of the drilling diameter is limited, so that the extremely dense drilling holes are needed to be drilled for the directional drilling fracturing, and the complexity of engineering is increased; (3) For low-permeability and ultra-low-permeability rocks, the local pore pressure field is difficult to change due to the low permeability of the rocks, and the change of the local pore pressure field of the rocks is a long-time process, so that the engineering efficiency is reduced.
Disclosure of Invention
Aiming at the technical defects, the invention mainly aims at providing a method for sequentially fracturing a hard roof of a directional fracturing well based on a stress shadow theory of fracturing cracks, which adopts 3 linearly arranged hydraulic cracks, and utilizes disturbance stress formed by primary fracturing cracks to change a stress field around subsequent fracturing cracks, so that on one hand, the initial two-way main stress difference is reduced, and on the other hand, the minimum main stress direction around the subsequent fracturing cracks is deflected to be more approximate to the vertical direction perpendicular to the directional fracturing direction, thereby achieving the effects of inhibiting the steering of the subsequent fracturing cracks and enhancing the directional expansion of the subsequent fracturing cracks, realizing the directional fracturing of the hard roof in any difficult-to-confirm stress field, eliminating power disasters caused by the existence of the hard roof, and further providing references for the directional fracturing control of oil gas and geothermal reservoirs.
In order to achieve the above purpose, the present invention adopts the following technical scheme:
a rock directional fracturing method utilizing fracture disturbance stress, comprising the steps of:
s1, arranging three directional cracking guide slits along a design direction; three initial cracks which are linearly arranged are prefabricated along the fracture direction of the top plate through drilling and slotting, the middle hydraulic crack in the three cracks is the initial crack for first fracturing, the upper hydraulic crack and the lower hydraulic crack are the initial crack for subsequent fracturing, and the slotting aims at guiding the expansion of the fracturing crack along the design extension direction;
s2, cracking the middle hydraulic fracture; pumping high-pressure fluid into the middle hydraulic fracture to carry out hydraulic fracturing, wherein the initial fracture is normally oblique to the main direction of the ground stress, the middle hydraulic fracture turns to expand after being cracked, the expansion of the fracture surface can apply extrusion acting force to rocks at two sides, and the fracture has stress disturbance effect, so that the minimal main stress direction around the middle hydraulic fracture deflects;
s3, continuously steering and expanding the middle hydraulic fracture; continuously pumping to drive the middle hydraulic fracture to expand until the middle hydraulic fracture turns to be parallel to the direction of the maximum main stress and exceeds the upper hydraulic fracture and the lower hydraulic fracture in the horizontal direction, and the minimum main stress around the upper hydraulic fracture and the lower hydraulic fracture is influenced by the middle hydraulic fracture and deflected to be almost perpendicular to the designed directional fracturing direction;
s4, cracking and nearly directionally expanding an upper hydraulic fracture and a lower hydraulic fracture; the net pressure in the middle hydraulic fracture is maintained by extremely small pumping displacement (about one ten thousandth of the pumping displacement of the middle hydraulic fracture), and simultaneously, the high-pressure fluid is pumped to the upper hydraulic fracture and the lower hydraulic fracture to drive the fracture to expand, and the locally reduced stress difference and the direction of the minimum main stress of deflection can inhibit the steering expansion of the upper hydraulic fracture and the lower hydraulic fracture. Finally, the three hydraulic fracture directional expansion sections form a breaking track of the hard top plate, and the breaking track is basically consistent with the design fracturing direction of the top plate.
Preferably, the three hydraulic fractures are distributed linearly, in the same plane and in the area of the plane the hydraulic fractures are a line (the plane characterizes the roof rock transverse to the hydraulic fracture face).
Preferably, as the disturbance stress of the well and the mine is prominent, the direction and the size of the main stress of the local rock mass are difficult to determine, the minimum main stress and the maximum main stress of the three hydraulic cracks in the plane only represent the stress characteristics of the local rock, the ground stress direction of the rock mass in an actual layer is not represented, and the minimum main stress is uniformly distributed along the vertical direction before fracturing.
Preferably, in the first step, the included angle between the designed breaking direction of the top plate and the minimum main stress direction is often θ, and because the size and direction of the stress field of the hard top plate of the well and mining are difficult to determine, θ is a variable, and changing the θ value can represent that directional hydraulic fracturing is performed in a random stress field in an undetermined direction.
Preferably, in step S1, the stress field at the tip of the fracture in the formation and the significant compression of the rock on both sides by fracture opening will create stress disturbances, i.e. "stress shadows". The stress disturbance of the plane tensile fracture mainly causes the increase of the extrusion stress at two sides of the fracture, the local ground stress difference and the minimum ground stress direction of the fracture stratum are changed, the stress disturbance of a single fracture in a hard top plate with a limited size is more remarkable, and the stress disturbance among the fracture is more prominent.
Preferably, in step S1, stress disturbance (stress shadow) generated by the fracture crack in a direction perpendicular to the expansion direction gradually weakens from the symmetrical center to the expansion direction, so that the subsequent fracture initiation crack can more effectively initiate and expand in the disturbance stress field of the initial fracture crack, and the arrangement positions of the upper hydraulic fracture and the lower hydraulic fracture deviate from the designed directional fracture direction.
Preferably, in step S2, the formation of the middle hydraulic fracture increases the stress of the upper and lower hydraulic fractures in the vertical direction, and reduces the difference between the stresses in the horizontal direction and the vertical direction around the upper and lower hydraulic fractures.
Preferably, in step S2, the extrusion effect of the middle hydraulic fracture generated perpendicular to the expansion direction is gradually reduced from the initiation point to the expansion method.
Preferably, in step S3, the middle hydraulic fracture is expanded until the expansion length affecting the stress disturbance of the upper and lower hydraulic fractures is 1.5-2 times the distance from the center of the middle hydraulic fracture to the center of either one of the upper and lower hydraulic fractures, and the disturbance stress of the middle hydraulic fracture is uneven along the expansion direction.
Preferably, in step S3, the extension length of the middle hydraulic fracture is monitored by adopting methods such as drilling monitoring or microseismic, so as to determine that the extension length of the middle hydraulic fracture reaches the requirement for the next work.
Preferably, in step S4, the net intra-fracture pressure of the middle hydraulic fracture is maintained at a minimum pumping displacement (about one ten thousandth of the pumping displacement of the middle hydraulic fracture), so that the middle hydraulic fracture continuously produces stress disturbance to the rock surrounding the upper and lower hydraulic fractures, the ground stress difference between the horizontal direction and the vertical direction of the second fracture zone is reduced, and the minimum main stress direction is deflected perpendicular to the designed fracture direction.
The invention has the beneficial effects that:
(1) The method for sequentially fracturing the hard roof of the well and the mine by adopting 3 linearly arranged hydraulic cracks is adopted, and only 3 times of directional drilling-hydraulic cutting and 2 times of fracturing are carried out on the roof, so that the process flow is relatively simple.
(2) According to the invention, the stress field around the subsequent fracturing fracture is changed by using the disturbance stress formed by the primary fracturing fracture, so that the initial two-way main stress difference is reduced, and the minimum main stress direction around the subsequent fracturing fracture is deflected to the vertical direction which is more nearly perpendicular to the directional fracturing direction, so that the effect of inhibiting the steering of the subsequent fracturing fracture and further enhancing the directional expansion of the subsequent fracturing fracture is achieved, the stress disturbance range of the primary fracturing fracture is wide, and the directional fracturing and expansion control effect on the subsequent fracturing fracture is good.
(3) The invention realizes the dual advantages of directional fracturing of the hard top plate in any difficult-to-ascertain stress field, low limit degree of the size and direction of the ground stress field, enhanced directional expansion performance of the fracturing crack in the hard top plate of the well and mining and convenient implementation of engineering site, and in addition, the directional fracturing method provided by the invention can also provide reference for directional fracturing control of oil gas and geothermal reservoirs.
Drawings
In order to more clearly illustrate the embodiments of the invention or the technical solutions in the prior art, the drawings that are required in the embodiments or the description of the prior art will be briefly described, it being obvious that the drawings in the following description are only some embodiments of the invention, and that other drawings may be obtained according to these drawings without inventive effort for a person skilled in the art.
FIG. 1 is a flow chart of a rock directional fracturing method utilizing fracture disturbance stress provided in one embodiment of the present application;
FIG. 2a is a schematic representation of the distribution of three initial hydraulic fractures;
FIG. 2B is a schematic illustration of the local stresses around hydraulic fractures B1 and B2 under stress disturbance of hydraulic fracture A (first fracture);
fig. 2c is a schematic representation of the directional propagation of hydraulic fractures B1 and B2 in a stress field disturbed by hydraulic fracture a (secondary fracturing).
Detailed Description
The following description of the embodiments of the present invention will be made clearly and completely with reference to the accompanying drawings, in which it is apparent that the embodiments described are only some embodiments of the present invention, but not all embodiments. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.
Referring to fig. 1, which shows a main flow of a method for directionally fracturing a hard roof of a well by using a crack disturbance stress, the embodiment provides a rock directional fracturing method by using the crack disturbance stress.
2a-2c, a two-dimensional graph characterizes roof rock transverse to the hydraulic fracture plane, in which area the hydraulic fracture is a line;
because the disturbance stress of the well engineering and the mine is prominent, the direction and the size of the principal stress of the local rock mass are difficult to determine, so the minimum principal stress (sigma) in the two-dimensional plane shown in figure 2 min ) And the maximum principal stress (sigma) max ) Only the stress characteristics of local rock are represented, and the ground stress direction (such as vertical direction and horizontal direction) of the rock body in an actual stratum is not represented;
σ min the included angle between the hydraulic fracturing device and the designed directional hydraulic fracturing direction is theta, and the change of the value of theta can represent the development of directional hydraulic fracturing in a random stress field in an undetermined direction.
As shown in fig. 1 to 2c, an embodiment of the present invention provides a rock directional fracturing method using fracture disturbance stress, including the steps of:
(1) Three directional initial fractures are arranged along the design direction, including hydraulic fractures A, B1, B2. Three initial cracks arranged linearly along the design breaking direction of the top plate are preformed by drilling-slotting, as shown in figure 2, sigma min The hydraulic fracture A is the first fracture initiation fracture, and the hydraulic fractures B1 and B2 are the subsequent fracture initiation fractures. The purpose of the slots is to guide the propagation of the fracture along the design extension direction. The design fracture direction of the top plate tends to be aligned with the minimum principal stress (sigma min ) The included angle of the direction is theta, and the theta is a variable because the size and the direction of the stress field of the hard roof of the well and the mine are difficult to determine. In particular, the arrangement position of the hydraulic cracks B1 and B2 deviates from the design orientation fracturing direction, as shown in fig. 2, in order to enable the subsequent fracturing initial cracks to more effectively initiate and expand in the primary fracturing crack disturbance stress field because the stress disturbance (stress shadow) generated by the fracturing cracks in the direction perpendicular to the expanding direction gradually weakens from the symmetrical center to the expanding direction 1 And d h Shown; wherein d is 1 The actual horizontal distance between the center of the middle hydraulic fracture and the centers of the upper hydraulic fracture and the lower hydraulic fracture; d, d h Is centered between the middle hydraulic fracture and the upper and lower hydraulic fracturesDesigning a horizontal distance in the fracturing direction; d, d v Is the vertical distance between the center of the middle hydraulic fracture and the centers of the upper hydraulic fracture and the lower hydraulic fracture.
(2) The middle hydraulic fracture a initiated the fracture. First, pumping high-pressure fluid into the hydraulic fracture A positioned in the middle to carry out hydraulic fracturing. Since the initial fracture is typically diagonal to the principal direction of ground stress, the hydraulic fracture will turn to propagate as shown in fig. 2 b. Although fracture a is directionally propagated, the opening of the fracture face applies compressive forces to the rock on both sides, as shown by the fracture microelements in fig. 2. Taking the example of the fracturing fracture of fig. 2B, the formation of the hydraulic fracture a increases the stress of the hydraulic fractures B1 and B2 along the y direction, and reduces the stress difference between the x direction and the y direction around the hydraulic fractures B1 and B2.
(3) The middle slit a continues to steer to spread. The pumping is continued to drive the slot A to expand until the slot A is turned to be parallel to the far-field main stress direction and exceeds the upper slot B1 and the lower slot B2 in the horizontal direction, and the minimum main stress around the slots B1 and B2 is influenced by the A and deflected to be almost perpendicular to the designed directional fracturing direction. Based on stress shadow theory, the extrusion effect generated by the fracture in the direction perpendicular to the expansion direction gradually weakens from the starting point to the expansion method, and the primary fracture A expands to about twice d in the x direction 1 The minimum principal stress around the initial fractures B1 and B2 of the subsequent fractures will deflect perpendicular to the design fracture direction (fig. 2B) due to the non-uniformity of the fracture a disturbance stress along the propagation direction. Thus, the disturbance stress field created by the fracture A will inhibit the steering expansion of the fractures B1 and B2 to enhance its directional expansion capability.
(4) The upper and lower hydraulic fractures B1 and B2 initiate and propagate nearly directionally. The primary fracture expansion in the x direction is monitored to about twice d by drilling a monitoring hole or micro-vibration 1 And (3) maintaining the net intra-fracture pressure of the hydraulic fracture a at a minimum pumping displacement (about one ten thousandth of the pumping displacement of the middle hydraulic fracture a) so that the hydraulic fracture continuously generates stress disturbance to the rock surrounding the hydraulic fractures B1 and B2, reducing the differential x-directional and y-directional ground stress of the secondary fracture region and deflecting the minimum principal stress perpendicular to the designed fracture direction. Simultaneously pumping high-pressure fluid into the hydraulic cracks B1 and B2 to drive the crackingThe locally reduced stress differences (x-direction and y-direction) and the minimal principal stress direction of deflection will inhibit the directional propagation of the fracture B1 and B2, i.e. contribute to the directional propagation of the fracture. The directional expansion of the final fracture A, B1 and B2 as shown in fig. 2c will make up the fracture trajectory of the hard roof, substantially coincident with the roof design fracture direction.
The theory of "stress shading" described in this example refers to stress disturbance generated by significant compression of the rock on both sides by fracture tip stress field and fracture opening in the formation. Stress disturbance of a plane tensile fracture mainly causes the increase of extrusion stress at two sides of the fracture, and changes the local ground stress difference and the minimum ground stress direction of the fracture stratum. Stress disturbance of individual fracture will be more pronounced in a hard top plate of limited size and stress disturbance between fracture will be more pronounced.
The method for sequentially fracturing the hard roof of the well and the mine by using 3 hydraulic cracks which are linearly arranged is adopted in the embodiment, and only 3 times of directional drilling-hydraulic cutting and 2 times of fracturing are carried out on the roof, so that the process flow is relatively simple.
The stress field around the subsequent fracturing fracture is changed by using disturbance stress formed by the primary fracturing fracture, so that the initial two-way main stress difference is reduced on one hand, and the minimum main stress direction around the subsequent fracturing fracture is deflected to be more close to the vertical direction perpendicular to the directional fracturing direction on the other hand, so that the effect of inhibiting the subsequent fracturing fracture from turning and further enhancing the directional expansion of the subsequent fracturing fracture is achieved, the stress disturbance range of the primary fracturing fracture is wide, and the directional fracturing and expansion control effect on the subsequent fracturing fracture is good.
The method for directional fracturing of the hard top plate in the stress field, which is difficult to ascertain, is realized, the degree of restriction of the size and the direction of the ground stress field is low, the directional expansion performance of the fracturing crack in the hard top plate in the well and the mine is enhanced, and the method for directional fracturing is convenient for the implementation of engineering sites.
It will be apparent to those skilled in the art that various modifications and variations can be made to the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention also include such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.
Claims (8)
1. The rock directional fracturing method utilizing the fracture disturbance stress utilizes the principle that the fracture disturbance stress changes the local stress field of the rock stratum in a large range, and inhibits the fracture steering to realize the directional expansion of the fracture by designing the spatial distribution and the sequential fracturing sequence of the combined fracture, and is characterized by comprising the following steps:
s1, arranging three directional cracking cracks along a design direction: three initial hydraulic cracks which are linearly arranged are prefabricated along the fracture direction of the top plate through drilling and slotting, the middle hydraulic crack in the three cracks is the initial crack for first fracturing, the upper hydraulic crack and the lower hydraulic crack are the initial crack for subsequent fracturing, and the purpose of slotting is to guide the expansion of the fracturing crack along the design extension direction;
s3, cracking the middle hydraulic fracture: pumping high-pressure fluid into the middle hydraulic fracture to carry out hydraulic fracturing, wherein the initial fracture is normally oblique to the main direction of the ground stress, the middle hydraulic fracture turns to expand after being cracked, the expansion of the fracture surface can apply extrusion acting force to rocks at two sides, and the fracture has stress disturbance effect, so that the minimal main stress direction around the middle hydraulic fracture deflects;
s3, continuous steering expansion of the middle hydraulic fracture: continuously pumping to drive the middle hydraulic fracture to expand until the middle hydraulic fracture turns to be parallel to the direction of the maximum main stress and exceeds the upper hydraulic fracture and the lower hydraulic fracture in the horizontal direction, and the minimum main stress around the upper hydraulic fracture and the lower hydraulic fracture is influenced by the middle hydraulic fracture and deflected to be perpendicular to the designed directional fracturing direction;
s4, cracking and directional expansion of an upper hydraulic fracture and a lower hydraulic fracture: the method comprises the steps of maintaining the intra-seam net pressure of the middle hydraulic fracture by using the minimum pumping displacement, wherein the minimum pumping displacement is one ten thousandth of the pumping displacement of the middle hydraulic fracture, pumping high-pressure fluid to the upper hydraulic fracture and the lower hydraulic fracture to drive the fracture to expand, locally reducing the stress difference and the direction of the minimum main stress for deflection to inhibit the steering expansion of the upper hydraulic fracture and the lower hydraulic fracture, and finally, forming a breaking track of a hard top plate by using three hydraulic fracture directional expansion sections, wherein the breaking track is basically consistent with the design fracturing direction of the top plate.
2. The rock directional fracturing method utilizing fracture disturbance stress according to claim 1, wherein: the three hydraulic fractures are distributed linearly, and the hydraulic fractures are in the same plane and in the area of the plane, and the plane represents roof rock crossing the hydraulic fracture surface.
3. The rock directional fracturing method utilizing fracture disturbance stress according to claim 1, wherein: because the disturbance stress of the well engineering and the mine is prominent, the direction and the size of the main stress of the local rock mass are difficult to determine, the minimum main stress and the maximum main stress of the three hydraulic cracks in the plane only represent the stress characteristics of the local rock, the ground stress direction of the rock mass in an actual layer is not represented, and the minimum main stress is uniformly distributed along the vertical direction before fracturing.
4. The rock directional fracturing method utilizing fracture disturbance stress according to claim 1, wherein: in step S1, it is assumed that the included angle between the designed breaking direction of the top plate and the minimum main stress direction is θ, θ is a variable, and changing the θ value can characterize that directional hydraulic fracturing is performed in a random stress field in an undetermined direction.
5. The rock directional fracturing method utilizing fracture disturbance stress according to claim 1, wherein: in the step S1, stress disturbance generated by the fracturing crack in a direction perpendicular to the expansion direction gradually weakens from the symmetrical center to the expansion direction, so that the subsequent fracturing initial crack can more effectively crack and expand in the disturbance stress field of the primary fracturing crack, and the arrangement positions of the upper hydraulic fracture and the lower hydraulic fracture deviate from the designed directional fracturing direction.
6. The method of directional fracturing of rock using fracture disturbance stress according to claim 1, wherein in step S3, the middle hydraulic fracture is expanded until the expansion length affecting the stress disturbance of the upper and lower hydraulic fractures is 1.5-2 times the distance from the center of the middle hydraulic fracture to the center of either one of the upper and lower hydraulic fractures.
7. The rock directional fracturing method utilizing fracture disturbance stress according to claim 1, wherein: in step S3, the expansion length of the middle hydraulic fracture is monitored by adopting a drilling monitoring or microseismic method, so that the expansion length of the middle hydraulic fracture is determined to reach the requirement for the next work.
8. The rock directional fracturing method utilizing fracture disturbance stress according to claim 1, wherein: in step S4, the minimum pumping displacement maintains the net pressure in the middle hydraulic fracture, so that the middle hydraulic fracture continuously generates stress disturbance on rocks around the upper hydraulic fracture and the lower hydraulic fracture, the ground stress difference between the horizontal direction and the vertical direction of the second fracturing area is reduced, and the minimum main stress direction is deflected perpendicular to the design fracturing direction.
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