CN106650100B - Alternate volume fracturing method for horizontal well of experimental shale reservoir - Google Patents
Alternate volume fracturing method for horizontal well of experimental shale reservoir Download PDFInfo
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Abstract
The invention discloses an alternate volume fracturing method for a horizontal well of an experimental shale reservoir, which comprises the steps of collecting stratum parameters, natural fracture parameters and hydraulic fracture basic parameters of the reservoir; establishing a fracture criterion of opening, shearing and penetrating of the natural fracture when the hydraulic fracture and the natural fracture interact, and quantitatively analyzing the influence of horizontal main stress difference on the fracture of the natural fracture; establishing a hydraulic fracture induced stress calculation model, and calculating the influence of the interval of the fractures of different perforation clusters and the extension length of the fractures on the induced stress of the extension area of the fractures of the middle perforation cluster; the interval of the fracturing perforation clusters in the horizontal well volume of the shale reservoir and the extension length of the fractures of the perforation clusters at two ends are optimized, so that the hydraulic fracturing fracture extension of the middle perforation cluster and the natural fractures are interactively generated at the same time to open, shear and penetrate through a failure mode to form complex fractures. The method organically combines the induced stress field generated by hydraulic fracture expansion, the perforation interval and the fracture expansion length in the same fracturing section, and better perfects the horizontal well volume fracturing technology.
Description
Technical Field
The invention belongs to the technical field of mineral resources, and particularly relates to an alternate volume fracturing method for horizontal wells of experimental shale reservoir reservoirs.
Background
The shale reservoir has the characteristics of low porosity, low permeability and natural fracture development. A large-scale complex fracture network zone is formed through volume fracturing modification, sufficient channels are provided for shale gas flow, and economically recoverable yield and recovery ratio can be obtained. At present, the shale reservoir horizontal well volume fracturing technology mainly comprises two technologies:
(1) the synchronous fracturing technology is used for implementing synchronous staged fracturing on two or more horizontal wells, and utilizes hydraulic fractures formed by fracturing among different horizontal wells to induce stress interference, so that the fracture density and the fracture degree of a fractured horizontal shaft region are increased, and a reconstruction region is increased to the maximum extent.
(2) The staged fracturing technology of the horizontal well is characterized in that staged multi-cluster perforation fracturing is adopted for the horizontal well section of the same shale reservoir, an effective fracture network can be formed in a stress interference area, the well drilling and completion cost is greatly reduced under the condition of the same modification volume, and the yield increasing efficiency is high.
The shale reservoir horizontal well volume fracturing scheme (1) needs to be implemented in two or more horizontal wells; in the fracturing scheme (2), as the commonly used staged fracturing technology of the horizontal well is that a plurality of perforation clusters in the same fracturing section of the same fractured horizontal well extend and expand simultaneously, the induced stress generated when the multiple fractures are formed in the inner section of the same fracturing section is not fully utilized.
The shale reservoir horizontal well volume fracturing schemes do not organically combine an induced stress field generated by hydraulic fracture expansion, perforation intervals in the same fracturing section and fracture expansion length.
Disclosure of Invention
In view of the above, the present application provides an alternate volume fracturing method for horizontal wells of an experimental shale reservoir.
In order to solve the technical problem, the application discloses an alternate volume fracturing method for horizontal wells of experimental shale reservoir, which mainly comprises the following steps:
1) collecting stratum parameters, natural fracture parameters and hydraulic fracture basic parameters of a reservoir;
2) establishing a fracture opening, shearing and passing failure criterion of the natural fracture when the hydraulic fracture interacts with the natural fracture, and quantitatively analyzing the influence of horizontal principal stress difference (difference between maximum horizontal principal stress and minimum horizontal principal stress) on the failure of the natural fracture;
3) establishing a hydraulic fracture induced stress calculation model, and calculating the influence of the interval of the fractures of different perforation clusters and the extension length of the fractures on the induced stress of the extension area of the fractures of the middle perforation cluster;
4) the interval of the fracturing perforation clusters in the horizontal well volume of the shale reservoir and the extension length of the fractures of the perforation clusters at two ends are optimized, so that the hydraulic fracturing fracture extension of the middle perforation cluster and the natural fractures are interactively generated at the same time to open, shear and penetrate through a failure mode to form complex fractures.
Further, the parameters specifically include: the maximum and minimum horizontal principal stress of the formation, the rock Young's modulus, Poisson's ratio, cohesion and tensile strength, natural fracture length, wall friction coefficient, and angle of approach to hydraulic fracture and natural fracture, pore pressure, fluid net pressure in fracture, hydraulic fracture length and height.
Further, the specific implementation process of the step 2) is as follows:
(1) the hydraulic fracture can generate induced stress to the stratum in the process of extending in the stratum, so the stress of the natural fracture is actually the superposition of in-situ stress and the induced stress generated by the hydraulic fracture, and if the approach angle of the hydraulic fracture and the natural fracture is beta, and the specified tensile stress is positive and the specified compressive stress is negative, the following steps are provided:
in the formula: sigmax、σy、τxy-the normal and shear stress components, MPa, at x and y coordinates, respectively;
σH、σh-maximum and minimum horizontal principal stresses, MPa, of the formation, respectively;
KI-is a stress intensity factor having a value of
Wherein p isnetIs the net pressure of the fluid in the fracture,/1Is half-long of hydraulic crack, MPa.m1/2；
r is the distance from any point on the wall surface of the natural fracture to the tip O of the hydraulic fracture, m;
theta is the included angle between the connecting line of any point on the wall surface of the natural fracture and the tip of the hydraulic fracture and the direction of the maximum horizontal main stress, rad;
converting the stresses in the formulae (1) to (3) to the coordinate βxAnd betayThen, the normal stress and the shear stress distribution of the natural fracture wall surface are obtained, and the normal stress of the natural fracture surface is as follows:
(2) when the hydraulic fracture interacts with the natural fracture, the fracture extension path of the hydraulic fracture can be greatly influenced by the natural fracture which can be opened, sheared and penetrated, the natural fracture can be damaged by the expansion of the hydraulic fracture, and a fracture criterion model of the natural fracture which is opened, sheared and penetrated when the hydraulic fracture interacts with the natural fracture is established respectively;
① natural fracture initiation opening failure criterion
When the fluid pressure p in the hydraulic fracture is greater than the positive stress sigmaβyWhen the fracture is closed, the original closed natural fracture is opened:
p＝σβy(7)
according to fracture propagation theory, under the same other conditions, the fluid pressure required for linear fracture propagation is minimal, and the fluid pressure in the hydraulic fracture is expressed as:
p＝σh+pnet(8)
in a similar way, based on the theory of elastic mechanics, when the natural crack is opened and damaged, the opening width of the crack is as follows:
in the formula: w-the opening width of the natural fracture, m;
v-Poisson's ratio, dimensionless;
Hf-natural fracture height, m;
e-is Young's modulus, MPa;
substituting the formulas (7) to (8) into the formula (9) to obtain:
② shear failure criterion for natural fracture
Since shear slip is likely to occur in a natural fracture when shear stress applied to a wall surface of the natural fracture is too large, a critical state for determining whether shear failure occurs in the natural fracture is represented as:
|τβ|＝s0-μσβy(11)
when is tauβ|＞s0-μσβyIn the process, natural cracks can generate shear slip, and according to a Westergaard function in fracture mechanics, a shear displacement expression of a II-type crack surface (single-side) in an infinite medium is as follows:
in the formula: s0The cohesive force of the wall surface of the natural crack is MPa;
us-shear displacement, m;
k-Kolosov constant, k-3-4 ν;
g-shear modulus, G ═ E/2(1+ ν), MPa;
x is the coordinate of any point on the crack surface, m;
l1-half the length of a natural crack, m;
③ failure criteria for hydraulic fracture crossing natural fracture
When the hydraulic fracture intersects with the natural fracture, when the maximum principal stress applied to the wall surface of the natural fracture reaches the tensile strength of the rock and the natural fracture does not shear and slide, the hydraulic fracture can penetrate through the natural fracture.
Critical transit time sigma1To achieve tensile strength T0：
σ1＝T0(14)
In addition to satisfying the formula (14), it must be satisfied that the crack does not undergo shear failure, i.e. | τβ|＜s0-μσβyWhen both conditions are met, the hydraulic fracture will continue to extend through the natural fracture;
the following discusses the crossing threshold distance and the initial steering angle:
order to
And substituting the formulas (1) to (3) and (13) into the formula (14) to obtain:
equation (15) has two solutions, one is the solution when the maximum principal stress equals the tensile strength of the rock, and the other is the solution when the minimum principal stress equals the tensile strength of the rock, the former is the desired solution, and the corresponding critical distance r iscAnd steering angle γ:
critical distance rc：
The steering angle γ is:
in the formula: gamma-the steering angle, defines the angle with the direction of maximum horizontal principal stress, being "positive" counterclockwise, i.e. passing up through the natural fracture.
Further, the hydraulic fracture induced stress calculation model building process in the step 3) is as follows:
the hydraulic fracture generates induced stress at the position point A of two fracture tips O1And O2Superposition of induced stresses generated:
in the formula: sigmaxx、σyy-hydraulic fracture is induced stress in x and y directions, respectively, MPa;
r1、r2from any point A in the formation to two tips O of the hydraulic fracture1And O2M;
if n hydraulic fractures are generated in the fracturing, the induced stress at the point A is the superposition of the induced stresses generated by the n hydraulic fractures:
in the formula: sigmaxx(i)、σyy(i) -induced stress in x and y directions, respectively, for the ith crack, MPa.
Further, in the step 4), by specifically combining stratum basic parameters and analyzing the influence of the main stress difference on the opening, shearing and passing failure modes of the natural fractures and quantitative analysis, the shale reservoir horizontal well volume fracturing effect is better improved under any condition, and therefore the interval of perforation clusters of the shale reservoir horizontal well alternate volume fracturing and the fracture extension length of perforation clusters at two ends are optimized.
Compared with the prior art, the application can obtain the following technical effects:
the method can be used for better perfecting the existing horizontal well volume fracturing technology, and according to the stratum parameters, the natural fracture parameters and the hydraulic fracture parameters of the reservoir, the breaking criteria of opening, shearing and passing of natural fractures during interaction of the hydraulic fractures and the natural fractures are established, and the influence of horizontal principal stress difference on the natural fractures is quantitatively analyzed; and calculating the influence of the interval and the extension length of the cracks of different perforation clusters on the induced stress of the extension area of the cracks of the middle perforation cluster by using a hydraulic crack induced stress model, aiming at promoting the natural cracks to simultaneously generate a plurality of failure modes, and preferably optimizing the interval of the perforation clusters and the extension length of the previous cracks.
Of course, it is not necessary for any one product to achieve all of the above-described technical effects simultaneously.
Drawings
The accompanying drawings, which are included to provide a further understanding of the application and are incorporated in and constitute a part of this application, illustrate embodiment(s) of the application and together with the description serve to explain the application and not to limit the application. In the drawings:
FIG. 1 is a graph of stress distribution at the face of a natural fracture when a hydraulic fracture of the present application is proximate to the natural fracture;
FIG. 2 is a hydraulic fracture induced stress field model of the present application;
FIG. 3 is a physical model of hydraulic fracture distribution around a wellbore of an embodiment;
FIG. 4 is the effect of different stress differences on normal stress;
FIG. 5 is a graph of the effect of different stress differential fractures on the opening width;
FIG. 6 is the effect of different stress differences on shear stress;
FIG. 7 is an illustration of the effect of different stress differences on shear displacement;
FIG. 8 is the effect of different stress differences on the maximum principal stress;
FIG. 9 is an illustration of the effect of different stress differences on critical distance-through;
FIG. 10 is an illustration of the effect of different stress differences on the initial penetration angle;
FIG. 11 is a result of a pass through at different angles of approach;
FIG. 12 is a schematic illustration of alternate fracturing construction within the same fracture zone of a horizontal well of a shale reservoir;
FIG. 13 is an illustration of the effect of fracture spacing on the equi-induced stress difference lines;
FIG. 14 is a graph of the effect of fracture length on the line of equal induced stress differences.
Detailed Description
Embodiments of the present application will be described in detail with reference to the drawings and examples, so that how to implement technical means to solve technical problems and achieve technical effects of the present application can be fully understood and implemented.
An alternate volume fracturing method for horizontal wells of experimental shale reservoir mainly comprises the following steps:
1) collecting stratum parameters, natural fracture parameters and basic parameters of hydraulic fractures of a reservoir;
the method specifically comprises the following steps: maximum and minimum horizontal principal stress of the stratum, Young's modulus, Poisson's ratio, cohesion and tensile strength of the rock, natural fracture length, wall friction coefficient, and angle of approach to hydraulic fracture and natural fracture, pore pressure, net pressure of fluid in fracture, hydraulic fracture length and height;
2) establishing a fracture opening, shearing and passing failure criterion of the natural fracture when the hydraulic fracture interacts with the natural fracture, and quantitatively analyzing the influence of horizontal principal stress difference (difference between maximum horizontal principal stress and minimum horizontal principal stress) on the failure of the natural fracture;
the specific implementation process of the step 2) is as follows:
(1) the hydraulic fracture induces stress on the formation during the propagation process in the formation, so the stress of the natural fracture is actually the superposition of the in-situ stress and the induced stress generated by the hydraulic fracture, and the stress distribution on the natural fracture surface when the hydraulic fracture approaches the natural fracture is described in fig. 1. Assuming that the approach angle of the hydraulic fracture and the natural fracture is beta, and the tensile stress is specified to be positive and the compressive stress is specified to be negative, then:
in the formula: sigmax、σy、τxy-the normal and shear stress components, MPa, at x and y coordinates, respectively;
σH、σh-maximum and minimum horizontal principal stresses, MPa, of the formation, respectively;
KI-is a stress intensity factor having a value of
(wherein p isnetIs the net pressure of the fluid in the fracture,/1Half-length of hydraulic crack), MPa.m1/2；
r is the distance from any point on the wall surface of the natural fracture to the tip O of the hydraulic fracture, m;
theta is the included angle between the connecting line of any point on the wall surface of the natural fracture and the tip of the hydraulic fracture and the direction of the maximum horizontal main stress, rad;
converting the stresses in the formulae (1) to (3) to the coordinate βxAnd betayThen, the normal stress and the shear stress distribution of the natural fracture wall surface are obtained, and the normal stress of the natural fracture surface is as follows:
(2) when the hydraulic fracture interacts with the natural fracture, the fracture extension path of the hydraulic fracture can be greatly influenced by the fact that the natural fracture can be opened, sheared and penetrated, the natural fracture can be damaged by the fact that the natural fracture is opened, sheared and penetrated through due to the expansion of the hydraulic fracture, and the fracture opening, shearing and penetrating criteria of the natural fracture when the hydraulic fracture interacts with the natural fracture are respectively modeled.
① natural fracture initiation opening failure criterion
When the fluid pressure p in the hydraulic fracture is greater than the positive stress sigmaβyWhen the fracture is closed, the original closed natural fracture is opened:
p＝σβy(7)
according to fracture propagation theory, under the same other conditions, the fluid pressure required for linear fracture propagation is minimal, and the fluid pressure in the hydraulic fracture is expressed as:
p＝σh+pnet(8)
in a similar way, based on the theory of elastic mechanics, when the natural crack is opened and damaged, the opening width of the crack is as follows:
in the formula: w-the opening width of the natural fracture, m;
v-Poisson's ratio, dimensionless;
Hf-natural fracture height, m;
e-is Young's modulus, MPa;
substituting the formulas (7) to (8) into the formula (9) to obtain:
② shear failure criterion for natural fracture
Since shear slip is likely to occur in a natural fracture when shear stress applied to a wall surface of the natural fracture is too large, a critical state for determining whether shear failure occurs in the natural fracture is represented as:
|τβ|＝s0-μσβy(11)
when is tauβ|＞s0-μσβyIn the process, natural cracks can generate shear slip, and according to a Westergaard function in fracture mechanics, a shear displacement expression of a II-type crack surface (single-side) in an infinite medium is as follows:
in the formula: s0The cohesive force of the wall surface of the natural crack is MPa;
us-shear displacement, m;
k-Kolosov constant, k-3-4 ν;
g-shear modulus, G ═ E/2(1+ ν), MPa;
x is the coordinate of any point on the crack surface, m;
l1the natural fracture is half-length, m.
③ failure criteria for hydraulic fracture crossing natural fracture
When the hydraulic fracture intersects with the natural fracture, when the maximum principal stress applied to the wall surface of the natural fracture reaches the tensile strength of the rock and the natural fracture does not shear and slide, the hydraulic fracture can penetrate through the natural fracture.
Critical transit time sigma1To achieve tensile strength T0：
σ1＝T0(14)
In addition to satisfying the formula (14), it must be satisfied that the crack does not undergo shear failure, i.e. | τβ|＜s0-μσβyWhen both conditions are satisfied at the same time,the hydraulic fracture will continue to extend through the natural fracture.
The following discusses the crossing threshold distance and the initial steering angle:
order to
And substituting the formulas (1) to (3) and (13) into the formula (14) to obtain:
equation (15) has two solutions, one is the solution when the maximum principal stress equals the tensile strength of the rock, and the other is the solution when the minimum principal stress equals the tensile strength of the rock, the former is the desired solution, and the corresponding critical distance r iscAnd steering angle γ:
critical distance rc：
The steering angle γ is:
in the formula: gamma-the steering angle, defines the angle with the direction of maximum horizontal principal stress, being "positive" counterclockwise, i.e. passing up through the natural fracture.
3) Establishing a hydraulic fracture induced stress calculation model, and calculating the influence of the interval of the fractures of different perforation clusters and the extension length of the fractures on the induced stress of the extension area of the fractures of the middle perforation cluster;
as shown in FIG. 2, the hydraulic fracture generates induced stresses at location point A of two fracture tips O1And O2Superposition of induced stresses generated:
in the formula: sigmaxx、σyy-hydraulic fracture is induced stress in x and y directions, respectively, MPa;
r1、r2from any point A in the formation to two tips O of the hydraulic fracture1And O2M, m.
If n hydraulic fractures are generated in the fracturing, the induced stress at the point A is the superposition of the induced stresses generated by the n hydraulic fractures:
in the formula: sigmaxx(i)、σyy(i) -induced stress in x and y directions, respectively, for the ith crack, MPa.
4) The interval of the fracturing perforation clusters in the horizontal well volume of the shale reservoir and the extension length of the fractures of the perforation clusters at two ends are optimized, so that the hydraulic fracturing fracture extension of the middle perforation cluster and the natural fractures are interactively generated at the same time to open, shear and penetrate through a failure mode to form complex fractures.
Specifically, by combining stratum basic parameters and analyzing the influence and quantitative analysis of the main stress difference on the opening, shearing and passing failure modes of the natural fractures, the shale reservoir horizontal well volume fracturing effect is better improved under any condition, and therefore the interval of perforation clusters for shale reservoir horizontal well alternate volume fracturing and the fracture extension length of perforation clusters at two ends are optimized.
Example (b):
the method for improving the volume fracturing effect of the horizontal well of the shale reservoir comprises the following specific steps:
some shale reservoir base parameters are as follows: maximum and minimum horizontal principal stresses of 50MPa and 45MPa, respectively, Poisson's ratio of 0.25, and Young's modulus of 2.0 × 104MPa, rock resistanceThe tensile strength is 3MPa, the rock cohesion is 10MPa, the friction coefficient of the wall surface of the natural fracture is 0.9, the length of the natural fracture is 10m, the approach angle is 60 degrees, the length of the hydraulic fracture is 60m, the net pressure of fluid is 5MPa, and fig. 3 is a physical model of the hydraulic fracture distributed around the shaft in the embodiment.
1. Effect of Primary stress Difference on Natural fracture opening failure
FIG. 4 is a graph representing the normal stress effect of different stress differences on the wall of a natural fracture, and it can be seen from FIG. 4 that: the smaller the stress difference, the larger the normal stress; under the same stress difference, the normal stress of the wall surface of the right wing of the natural crack is larger than that of the left wing, and the maximum value of the normal stress appears near the right side of the interaction point.
FIG. 5 depicts the effect of different stress differences on the opening width of the natural fracture wall, as can be seen in FIG. 5: the smaller the stress difference is, the larger the opening width of the crack is; the opening width of the wall surface of the left wing of the natural crack is smaller than that of the right wing, and the width of the wall surface of the left wing of the natural crack reaches the maximum value near the right side of the intersection point.
In summary, the smaller the stress difference, the more favorable the opening failure of the wall surface of the natural fracture, and the opening failure of the right wing of the natural fracture is more likely to occur.
2. Effect of Primary stress Difference on Natural fracture shear failure
Fig. 6 represents the effect of different stress differences on the shear stress of the wall of a natural fracture, as can be seen from fig. 6: the smaller the stress difference, the larger the shear stress, and the shear stress peak was obtained at the right side of the interaction point.
FIG. 7 is a graph representing the effect of different stress differences on the shear displacement of the wall of a natural fracture, as can be seen in FIG. 7: the smaller the stress difference is, the larger the shearing displacement is, and the more the position points where shearing occurs are; in the left wing of the natural crack, as the total shear stress (absolute value) is smaller than that of the right wing, the shearing is not easy to occur in the region, and the shearing is easy to occur only in the vicinity of the interaction point; the shear displacement reaches a peak (3.7mm) due to the maximum shear stress (absolute value) of the attachment at the point of interaction.
In general, smaller stress differences are more favorable for shear failure of the natural fracture wall.
3. Effect of Primary stress Difference on Natural fracture failure by Hydraulic fracture penetration
FIG. 8 is a graph representing the effect of different principal stress differences on the maximum principal stress of a natural fracture wall. As can be seen in fig. 8: the smaller the stress difference, the greater the maximum principal stress, and the maximum principal stress appears as a "tensile" stress only in the region immediately to the right of the point of interaction, and it can be seen from equation (14) that if a hydraulic fracture failure occurs through a natural fracture, its point of penetration should be in the region immediately to the right of the point of interaction.
FIG. 9 depicts different principal stress differences versus the critical distance-through rcThe effect of (c) can be seen in fig. 9: the smaller the stress difference, the larger the critical passing distance, and the peak was taken at an approach angle of 60 °, indicating that the point of passing occurred at an approach angle of 60 ° was farthest from the point of interaction.
While fig. 10 characterizes the effect of different primary stress differences on the initial penetration angle γ, it can be seen from fig. 10 that: when the stress difference is-5 MPa, the initial penetrating angle gamma is positive within the range of an approach angle of 0-15 degrees, which indicates that the hydraulic fracture upwards penetrates through the natural fracture, and the initial penetrating angle gamma is negative within the range of an approach angle of 15-60 degrees, which indicates that the hydraulic fracture downwards penetrates through the natural fracture, and when the approach angle is more than 60 degrees, the hydraulic fracture upwards penetrates through the natural fracture; when the stress difference is 0MPa, 5MPa and 10MPa, the hydraulic fracture downwards passes through the natural fracture within the range of an approach angle of 15-60 degrees, and when the approach angle is more than 60 degrees, the hydraulic fracture upwards passes through the natural fracture; the smaller the stress difference, the larger the initial penetration angle (absolute value).
FIG. 11 is a graph of the crossing criteria for stress ratios (ratio of maximum horizontal principal stress to minimum horizontal principal stress) greater than 0.1 at different approach angles, with the area to the right of each curve representing the hydraulic fracture crossing a natural fracture. From fig. 11, it can be derived that: when the approach angle is reduced from 90 degrees to 15 degrees, and the stress ratio is more than 1, the corresponding passing area is sharply reduced, namely the difficulty of the hydraulic fracture passing through the natural fracture is increased, and the hydraulic fracture extends along the natural fracture more; when the stress ratio is less than 1, the passing area increases as the approach angle decreases. Considering that the friction coefficient of the wall surface of the natural fracture is larger, in order to enable the hydraulic fracture to pass through the natural fracture under different approaching angles, the stress ratio is more favorable, namely the difference between the maximum level principal stress and the minimum level principal stress is reduced.
The simulation calculation analysis of the opening, shearing and penetrating damage of the natural fracture wall surface is obtained as follows: the smaller the primary stress difference, the larger the opening width of the natural fracture, the more locations where shearing occurs, the larger the shear displacement, and the larger the critical penetration distance and initial penetration angle. Therefore, the smaller the stress difference is, the more beneficial the complex seam net is formed, thereby achieving the ideal construction effect.
4. The interval of the horizontal well volume fracturing perforation clusters of the shale reservoir and the extension length of the fractures of the perforation clusters at two ends are optimized, so that the hydraulic fracturing fractures of the middle perforation cluster are interactively opened, sheared and penetrated and damaged as much as possible after being extended with the natural fractures, and complex fractures are formed.
Fig. 12 is a schematic diagram of alternate fracturing construction within the same fracture zone of a horizontal well of a shale reservoir. In the same fracturing segment, the initiation and the expansion of the perforation clusters 1 and 2 are controlled, preferably, the distance between the perforation clusters 1 and 2 and the extension lengths of the perforation clusters 1 and 2 are adjusted through construction parameters to promote the initiation and the extension of the cracks of the perforation clusters 3.
To reduce the stress difference, the induced stress difference (sigma) is increasedyy-σxx) As an important means, FIG. 13 simulates the equi-induced stress differential lines (σ) of the crack spacing of different perforation clusters under the condition that the length of the cracks at two ends is 60myy-σxx5MPa), i.e. points in the xy plane with an induced stress difference of 5MPa are connected to form a line of equal induced stress difference. Under the condition of 5MPa of the equal induced stress difference line, the area of the 5MPa equal induced stress difference line is larger along with the increase of the crack spacing, but the region included in the range of the induced stress can be generated in the region near the horizontal well bore, namely, complex cracks can not be generated near the near well bore, complex cracks are comprehensively considered to be formed near the near well bore and the far well bore, and finally, the crack spacing is preferably 80 m.
FIG. 14 simulates the isoinduction stress differential lines (. sigma.) for different perforation cluster fracture spacing and fracture propagation length for a fracture spacing of 60myy-σxx5MPa) as seen in fig. 14: when the crack distance is constant (d is 60m), the equal induced stress difference control area expands towards the crack extension direction (the maximum horizontal main stress direction) and gradually retracts towards the middle area of the crack along with the increase of the length of the crack; considering that complex fractures in the near wellbore region and the far wellbore region are formed favorably, the fracture extension lengths at two ends are preferably 60m finally, namely after the extension lengths of two fractures are 60m, the fracture in the middle is promoted to initiate and expand, so that the final complex fracture is formed.
And in consideration of fully utilizing the induced stress formed by hydraulic fracture extension and expansion and reducing the ground stress difference as much as possible by combining the graphs 13 and 14, the fracture length at two ends is preferably 60m and the interval between the fractures at two ends is preferably 80m, so that the complex fracture is formed most favorably in the situation, and the construction of the shale reservoir horizontal well alternate volume fracturing is realized.
While the foregoing description shows and describes several preferred embodiments of the invention, it is to be understood, as noted above, that the invention is not limited to the forms disclosed herein, but is not to be construed as excluding other embodiments and is capable of use in various other combinations, modifications, and environments and is capable of changes within the scope of the inventive concept as expressed herein, commensurate with the above teachings, or the skill or knowledge of the relevant art. And that modifications and variations may be effected by those skilled in the art without departing from the spirit and scope of the invention as defined by the appended claims.
Claims (4)
1. The alternate volume fracturing method for the horizontal well of the experimental shale reservoir is characterized by mainly comprising the following steps of:
1) collecting stratum parameters, natural fracture parameters and hydraulic fracture basic parameters of a reservoir;
2) establishing a fracture criterion of opening, shearing and penetrating of the natural fracture when the hydraulic fracture and the natural fracture interact, and quantitatively analyzing the influence of horizontal main stress difference on the fracture of the natural fracture;
3) establishing a hydraulic fracture induced stress calculation model, and calculating the influence of the interval of the fractures of different perforation clusters and the extension length of the fractures on the induced stress of the extension area of the fractures of the middle perforation cluster;
4) selecting the interval of the volume fracturing perforation clusters of the horizontal well of the shale reservoir and the extension lengths of the fractures of the perforation clusters at two ends, and promoting the extension of the hydraulic fracturing fracture of the middle perforation cluster and the natural fracture to interactively generate opening, shearing and penetrating damage modes at the same time to form complex fractures;
step 3) the establishment process of the hydraulic fracture induced stress calculation model is as follows:
the hydraulic fracture generates induced stress at the position point A of two fracture tips O1And O2Superposition of induced stresses generated:
in the formula: sigmaxx、σyy-hydraulic fracture is induced stress in x and y directions, respectively, MPa;
r1、r2from any point A in the formation to two tips O of the hydraulic fracture1And O2M;
if n hydraulic fractures are generated in the fracturing, the induced stress at the point A is the superposition of the induced stresses generated by the n hydraulic fractures:
in the formula: sigmaxx(i)、σyy(i) -induced stress in x and y directions, respectively, for the ith crack, MPa.
2. The alternate volume fracturing method of horizontal well of experimental shale reservoir according to claim 1, wherein the parameters specifically comprise: the maximum and minimum horizontal principal stress of the formation, the rock Young's modulus, Poisson's ratio, cohesion and tensile strength, natural fracture length, wall friction coefficient, and angle of approach to hydraulic fracture and natural fracture, pore pressure, fluid net pressure in fracture, hydraulic fracture length and height.
3. The alternate volume fracturing method for horizontal wells of experimental shale reservoir according to claim 1, wherein the specific implementation process of the step 2) is as follows:
(1) the hydraulic fracture can generate induced stress to the stratum in the process of extending in the stratum, so the stress of the natural fracture is actually the superposition of in-situ stress and the induced stress generated by the hydraulic fracture, and if the approach angle of the hydraulic fracture and the natural fracture is beta, and the specified tensile stress is positive and the specified compressive stress is negative, the following steps are provided:
in the formula: sigmax、σy、τxy-the normal and shear stress components, MPa, at x and y coordinates, respectively;
σH、σh-maximum and minimum horizontal principal stresses, MPa, of the formation, respectively;
KI-is a stress intensity factor having a value of
Wherein p isnetIs the net pressure of the fluid in the fracture,/1Is half-long of hydraulic crack, MPa.m1/2；
r is the distance from any point on the wall surface of the natural fracture to the tip O of the hydraulic fracture, m;
theta is the included angle between the connecting line of any point on the wall surface of the natural fracture and the tip of the hydraulic fracture and the direction of the maximum horizontal main stress, rad;
converting the stresses in the formulae (1) to (3) to the coordinate βxAnd betayThen, the normal stress and the shear stress distribution of the natural fracture wall surface are obtained, and the normal stress of the natural fracture surface is as follows:
(2) when the hydraulic fracture interacts with the natural fracture, the natural fracture can be opened, sheared and penetrated, so that the extending path of the hydraulic fracture is greatly influenced, the natural fracture can be opened, sheared and penetrated through a failure mode due to the expansion of the hydraulic fracture, and a failure criterion model of the opening, shearing and penetrating of the natural fracture when the hydraulic fracture interacts with the natural fracture is established respectively;
① natural fracture initiation opening failure criterion
When the fluid pressure p in the hydraulic fracture is greater than the positive stress sigmaβyWhen the fracture is closed, the original closed natural fracture is opened:
p＝σβy(7)
according to fracture propagation theory, under the same other conditions, the fluid pressure required for linear fracture propagation is minimal, and the fluid pressure in the hydraulic fracture is expressed as:
p＝σh+pnet(8)
in a similar way, based on the theory of elastic mechanics, when the natural crack is opened and damaged, the opening width of the crack is as follows:
in the formula: w-the opening width of the natural fracture, m;
v-Poisson's ratio, dimensionless;
Hf-natural fracture height, m;
e-is Young's modulus, MPa;
substituting the formulas (7) to (8) into the formula (9) to obtain:
② shear failure criterion for natural fracture
Since shear slip is likely to occur in a natural fracture when shear stress applied to a wall surface of the natural fracture is too large, a critical state for determining whether shear failure occurs in the natural fracture is represented as:
|τβ|＝s0-μσβy(11)
when is tauβ|＞s0-μσβyWhen the fracture surface is in a single-sided state, the natural fracture can generate shear slip, and according to a Westergaard function in fracture mechanics, a shear displacement expression of a II-type fracture surface in an infinite medium is as follows:
in the formula: s0The cohesive force of the wall surface of the natural crack is MPa;
us-shear displacement, m;
k-Kolosov constant, k-3-4 ν;
g-shear modulus, G ═ E/2(1+ ν), MPa;
x is the coordinate of any point on the crack surface, m;
l1-half the length of a natural crack, m;
③ failure criteria for hydraulic fracture crossing natural fracture
When the hydraulic fracture is intersected with the natural fracture, when the maximum principal stress applied to the wall surface of the natural fracture reaches the tensile strength of the rock and the natural fracture does not shear and slide, the hydraulic fracture can penetrate through the natural fracture;
critical transit time sigma1To achieve tensile strength T0：
σ1＝T0(14)
In addition to satisfying the formula (14), it must be satisfied that the crack does not undergo shear failure, i.e. | τβ|＜s0-μσβyWhen both conditions are met, the hydraulic fracture will continue to extend through the natural fracture;
the following discusses the crossing threshold distance and the initial steering angle:
order to
And substituting the formulas (1) to (3) and (13) into the formula (14) to obtain:
equation (15) has two solutions, one is the solution when the maximum principal stress equals the tensile strength of the rock, and the other is the solution when the minimum principal stress equals the tensile strength of the rock, the former is the desired solution, and the corresponding critical distance r iscAnd steering angle γ:
critical distance rc：
The steering angle γ is:
in the formula: gamma-the steering angle, defines the angle with the direction of maximum horizontal principal stress, being "positive" counterclockwise, i.e. passing up through the natural fracture.
4. The experimental shale reservoir horizontal well alternate volume fracturing method according to claim 1, wherein the condition more beneficial to improving the shale reservoir horizontal well volume fracturing effect is judged by analyzing the influence and quantitative analysis of the main stress difference on the natural fracture opening, shearing and passing failure modes through specifically combining with the stratum basic parameters in the step 4), so that the perforation cluster interval of the shale reservoir horizontal well alternate volume fracturing and the fracture extension length of the perforation clusters at two ends are selected.
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