Method and device for determining crack initiation pressure of unconventional low-permeability reservoir

Abstract

The invention discloses a method and a device for determining crack initiation pressure of an unconventional low-permeability reservoir. The method comprises the following steps: collecting crustal stress data of an oil-gas well of the unconventional low-permeability reservoir; converting the crustal stress data under the rectangular coordinate system into crustal stress data under a shaft local coordinate system according to the stress conversion matrix; calculating a first pressure value of crack initiation pressure according to the crustal stress data under the local coordinate system of the shaft; calculating a second pressure value of crack initiation pressure according to the crustal stress data under the rectangular coordinate system; and determining the initial fracture pressure range value of the unconventional low-permeability reservoir according to the first pressure value and the second pressure value.

Description

Technical Field

[0001] The present invention relates to the technical field of unconventional reservoir oil and gas development, and in particular to a method and device for determining the fracturing initiation pressure of multi-cluster temporary plugging fracturing in horizontal wells. Background Art

[0002] The permeability of unconventional low-permeability oil and gas reservoirs is generally very low, and they do not have natural production capacity after drilling and completion. Only hydraulic fracturing can effectively improve the recovery rate and meet the standards of industrial production. In order to effectively develop unconventional low-permeability oil and gas reservoirs, it is necessary to induce the formation of complex fractures through large-scale volume fracturing and increase the transformation volume. However, the conventional fracture hydraulic fracturing theory is no longer well applicable to unconventional low-permeability oil and gas reservoirs. The reservoir rocks of unconventional low-permeability oil and gas reservoirs are typical brittle materials. From a microscopic point of view, after billions of years of geological evolution and multiple complex tectonic movements, these rocks generally develop randomly distributed microscopic pores and cracks of different orders. From a macroscopic point of view, they may contain a variety of geological structural planes (joints, faults, and weak planes, etc.). The mechanical properties and strength of these microcracks and structural weak planes such as natural fractures are significantly different from those of the rock matrix. When the fracturing fluid is pumped in to apply energy and load to it, the existing microcracks will initiate, expand, and evolve under the load, and the macroscopic structural plane will be fractured and damaged, which will eventually have a significant impact on the initiation behavior of hydraulic fractures. Only by studying and understanding the fracture initiation mechanism of volume fracturing in unconventional low-permeability oil and gas reservoirs can we effectively predict and control the fracture initiation behavior and optimize the fracturing process design. Summary of the invention

[0003] In some technologies, systematic research has been conducted on the fracturing technology of unconventional low-permeability oil and gas reservoirs, and many fracturing models have been established. These existing fracturing models mainly consider: (1) the stress concentration effect of the wellbore wall and the hole wall; (2) the seepage of high-pressure fluid in the wellbore or hole; (3) arbitrary azimuth or well inclination, so they are applicable to vertical wells, inclined wells or horizontal wells. However, the existing models rarely consider the influence of structural weak surfaces in the reservoir on the fracture initiation behavior.

[0004] Since unconventional low-permeability reservoirs generally have structural weak surfaces such as microcracks and natural fractures, these structural weak surfaces may intersect with the perforations and significantly affect the initiation behavior of hydraulic fractures, causing the prediction results of the initiation pressure to deviate significantly from the actual situation.

[0005] In order to solve this problem, the inventors took into account the characteristics of widespread development of weak planes in unconventional low-permeability reservoir rocks and established a method for calculating the initiation pressure of volume fracturing in unconventional low-permeability reservoirs to provide technical support for fracturing optimization in related well areas.

[0006] The present application provides a method and device for determining the initiation pressure of unconventional low-permeability reservoirs. The method comprehensively considers different initiation modes of hydraulic fractures under the influence of structural weak surfaces such as microcracks and cracks, studies the initiation criteria of different initiation modes, and finally establishes a corresponding fracture pressure calculation model to form a method for calculating the initiation pressure of volume fracturing in unconventional low-permeability reservoirs.

[0007] In a first aspect, the present application provides a method for determining the fracturing pressure of an unconventional low permeability reservoir, characterized in that the method comprises:

[0008] Collect geostress data of oil and gas wells in unconventional low-permeability reservoirs;

[0009] The ground stress data in the rectangular coordinate system is converted into the ground stress data in the local coordinate system of the wellbore according to the stress conversion matrix;

[0010] Calculating a first pressure value of the fracturing pressure according to the ground stress data in the local coordinate system of the wellbore;

[0011] Calculating a second pressure value of the fracture initiation pressure according to the ground stress data in the rectangular coordinate system;

[0012] The fracturing pressure range value of the unconventional low permeability reservoir is determined according to the first pressure value and the second pressure value.

[0013] In a second aspect, an embodiment of the present invention further provides a device for determining the fracturing pressure of an unconventional low permeability reservoir, characterized in that the device comprises: a memory and a processor; the memory is used to store a program for determining the fracturing pressure of an unconventional low permeability reservoir, and the processor is used to read and execute the program for determining the fracturing pressure of an unconventional low permeability reservoir, and execute any one of the methods described in the above embodiments.

[0014] In a third aspect, an embodiment of the present invention further provides a computer-readable storage medium having a data processing program stored thereon, and the data processing program is executed by a processor to perform the method for determining the initiation pressure of an unconventional low-permeability reservoir as described in any one of the above embodiments.

[0015] Compared with the related art, the present application provides a method and device for determining the initiation pressure of unconventional low-permeability reservoirs, the method comprising: collecting geostress data of oil and gas wells in unconventional low-permeability reservoirs; converting geostress data in a rectangular coordinate system into geostress data in a local coordinate system of the wellbore according to a stress conversion matrix; calculating a first pressure value of the initiation pressure according to the geostress data in the local coordinate system of the wellbore; calculating a second pressure value of the initiation pressure according to the geostress data in the rectangular coordinate system; determining the initiation pressure range value of the unconventional low-permeability reservoir according to the first pressure value and the second pressure value. The present application comprehensively considers the different initiation modes of hydraulic fractures under the influence of structural weak surfaces such as microcracks and cracks, studies the initiation criteria of different initiation modes, and finally establishes a corresponding fracture pressure calculation model to form a method for calculating the initiation pressure of volume fracturing of unconventional low-permeability reservoirs.

[0016] Other features and advantages of the present application will be described in the following description, and partly become apparent from the description, or be understood by implementing the present application. Other advantages of the present application can be realized and obtained by the schemes described in the description and the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The accompanying drawings are used to provide an understanding of the technical solution of the present application and constitute a part of the specification. Together with the embodiments of the present application, they are used to explain the technical solution of the present application and do not constitute a limitation on the technical solution of the present application.

[0018] Figure 1 A flow chart of a method for determining the fracturing pressure of an unconventional low-permeability reservoir according to an embodiment of the present application;

[0019] Figure 2 A schematic diagram of a device for determining the fracturing pressure of an unconventional low-permeability reservoir according to an embodiment of the present application;

[0020] Figure 3 Schematic diagram of the variation of the fracture pressure with the perforation angle and Poisson's ratio when the X well is fractured without encountering a structural weak surface in some exemplary embodiments;

[0021] Figure 4 A schematic diagram of the variation of the fracturing pressure when the structural weak surface intersects the top of the perforation when the X-well encounters the structural weak surface in some exemplary embodiments;

[0022] Figure 5 It is a schematic diagram of the variation law of the fracturing pressure when the structural weak plane intersects the perforation top and the X-well encounters the structural weak plane at a position rotated 45 degrees in some exemplary embodiments;

[0023] Figure 6 Schematic diagram of the variation of the fracturing pressure when the structural weak surface intersects the perforation side and the X-well encounters the structural weak surface in some exemplary embodiments. DETAILED DESCRIPTION

[0024] The present application describes multiple embodiments, but the description is exemplary rather than restrictive, and it is obvious to those skilled in the art that there may be more embodiments and implementations within the scope of the embodiments described in the present application. Although many possible feature combinations are shown in the drawings and discussed in the specific embodiments, many other combinations of the disclosed features are also possible. Unless specifically limited, any feature or element of any embodiment may be used in combination with any other feature or element in any other embodiment, or may replace any other feature or element in any other embodiment.

[0025] The present application includes and contemplates combinations of features and elements known to those of ordinary skill in the art. The embodiments, features and elements disclosed in the present application may also be combined with any conventional features or elements to form a unique invention scheme defined by the claims. Any features or elements of any embodiment may also be combined with features or elements from other invention schemes to form another unique invention scheme defined by the claims. Therefore, it should be understood that any feature shown and / or discussed in the present application may be implemented individually or in any appropriate combination. Therefore, except for the limitations made according to the attached claims and their equivalents, the embodiments are not subject to other restrictions. In addition, various modifications and changes may be made within the scope of protection of the attached claims.

[0026] In addition, when describing representative embodiments, the specification may have presented the method and / or process as a specific sequence of steps. However, to the extent that the method or process does not rely on the specific order of the steps described herein, the method or process should not be limited to the steps of the specific order described. As will be understood by those of ordinary skill in the art, other sequences of steps are also possible. Therefore, the specific sequence of the steps set forth in the specification should not be interpreted as a limitation to the claims. In addition, the claims for the method and / or process should not be limited to the steps of performing them in the order written, and those skilled in the art can easily understand that these sequences can be changed and still remain within the spirit and scope of the embodiments of the present application.

[0027] The embodiment of the present invention provides a method for determining the fracture initiation pressure of an unconventional low permeability reservoir. Figure 1 As shown, the method includes steps S100-S140:

[0028] S100: Collect geostress data of oil and gas wells in unconventional low permeability reservoirs;

[0029] S110: converting the geostress data in the rectangular coordinate system into geostress data in the wellbore local coordinate system according to the stress conversion matrix;

[0030] S120: Calculating a first pressure value of a fracturing initiation pressure according to the ground stress data in the local coordinate system of the wellbore;

[0031] S130: Calculating a second pressure value of the fracturing initiation pressure according to the ground stress data in the rectangular coordinate system;

[0032] S140: Determine a fracturing pressure range value of an unconventional low permeability reservoir according to the first pressure value and the second pressure value.

[0033] In an exemplary embodiment, the geostress data includes: vertical reservoir stress, minimum reservoir horizontal principal stress, and maximum reservoir horizontal principal stress.

[0034] In an exemplary embodiment, the vertical stress of the reservoir is set to σ z , the minimum principal stress at reservoir level is σ x , the maximum horizontal principal stress of the reservoir is σ y , and establish a coordinate system (x, y, z) based on the three principal stress directions. Let α be the angle between the wellbore projection on the horizontal plane and the horizontal minimum principal stress direction, β be the wellbore inclination angle, and transform the reservoir principal stress into the stress field distribution under the wellbore local coordinate system (xx, yy, vv) according to the stress conversion matrix; where the stress conversion matrix is:

[0035]

[0036] In the above formula, σ xx , σ yy , σ vv are the principal stresses corresponding to the local coordinate systems xx, yy, and vv of the wellbore respectively; τ yv , τ xv , τ xy are the shear stress in the local coordinate system of the wellbore; z is the vertical stress of the reservoir, σ x is the minimum principal stress at reservoir level, σ y is the maximum horizontal principal stress of the reservoir; α is the angle between the projection of the wellbore on the horizontal plane and the direction of the horizontal minimum principal stress; β is the wellbore inclination angle.

[0037] In an exemplary embodiment, calculating a first pressure value of the fracturing pressure according to the ground stress data in the local coordinate system of the wellbore includes:

[0038] Calculate the principal stress and shear stress in the perforation polar coordinate system according to the in-situ stress data in the local coordinate system of the wellbore;

[0039] Determine the maximum tensile stress on the wall according to the principal stress and shear stress in the hole polar coordinate system;

[0040] According to the determined maximum tensile stress and in combination with a first crack initiation judgment criterion, a first pressure value of the crack initiation pressure is determined.

[0041] In an exemplary embodiment, a polar coordinate system (r, θ, z) with the center of the perforation hole as the origin is established, and the compressive stress is also set to be positive and the tensile stress is set to be negative. Considering the stress concentration generated by the hole and the filtration effect of the fracturing fluid, the stress field of the wellbore surrounding rock is obtained; the principal stress and shear stress in the perforation polar coordinate system are:

[0042]

[0043]

[0044] In the above formula, σ r , σ θ , σ z is the principal stress of the perforation polar coordinate system xx, yy, vv; τ rθ , τ rz , τ θz is the shear stress of the perforation polar coordinates; ω is any radial direction and stress σ θ The polar coordinate angle of the counterclockwise direction, α is the effective stress coefficient; p w is the fluid pressure; p p is the original formation pore pressure; r w is the borehole radius, v is the Poisson's ratio, φ is the formation porosity, and r is the polar coordinate radius of the fracture initiation point.

[0045] In an exemplary embodiment, when the reservoir has no microcracks and natural fractures, or the perforation holes do not encounter microcracks and natural fracture surfaces, hydraulic fractures are initiated along the rock matrix. In this case, the hydraulic fracture initiation model is similar to the conventional case, and the maximum tensile fracture criterion is usually used as a criterion. That is, when the effective tensile stress on the rock reaches the tensile strength of the rock, the rock will be tensilely damaged in the direction perpendicular to the tensile stress to form hydraulic fractures.

[0046] Based on the theory of elasticity, the maximum tensile stress σ mo for:

[0047]

[0048] Considering the influence of rock pore pressure, σ t is the tensile strength, then the first crack judgment criterion is:

[0049] σ mo -γp p >σ t

[0050] In the above formula, γ is the effective stress coefficient, σ tis the tensile strength of rock.

[0051] In an exemplary embodiment, when the perforation holes encounter natural weak surfaces such as fissures and natural fractures, hydraulic fractures may be affected and initiate along the fissures and fractures. At this time, the initiation of fractures requires that the fluid pressure at the intersection is greater than the effective normal stress acting on the natural weak surface. That is, when the fluid pressure is greater than the effective normal stress in the natural normal direction, the fracture will initiate.

[0052] The three principal stresses and the normal stress σ on the crack surface n The calculation relationship between them is as follows:

[0053] σ n =cos 2 η x σ x +cos 2 η y σ y +cos 2 η z η z

[0054] In the above formula, σ n is the normal stress on the crack surface, η x , η y , η z It is the angle between the principal stress direction (x, y, z) and the normal of the natural fracture surface.

[0055] The hydraulic fracture along the natural weak plane requires that the fluid pressure at the intersection is greater than or equal to the effective compressive stress on the front of the fracture surface, that is, the second pressure value of the fracture initiation pressure is:

[0056] p s =σ n -γp p

[0057] In the above formula, p s is the second pressure value of the fracture initiation pressure.

[0058] In an exemplary embodiment, the fracturing pressure range of the unconventional low permeability reservoir is determined according to the first pressure value and the second pressure value. In this embodiment, for unconventional low permeability reservoirs with developed fractures, considering the uncertainty of encountering fractures, the fracturing pressure may have a pressure range with fluctuations. In this regard, the inclination and strike of the main development zone of structural weak surfaces such as fractures are estimated according to exploration data or nearby fracturing pressure data, and the fracturing pressures of not encountering and encountering structural weak surfaces are calculated respectively as the fracturing pressure range.

[0059] The method for calculating the fracturing initiation pressure of staged multi-cluster temporary plugging fracturing in a shale gas reservoir horizontal well in this embodiment has the following beneficial effects:

[0060] (1) The existing fracturing model only considers the hydraulic fracture initiation along the rock matrix. The method of the present application comprehensively considers the influence of natural weak surfaces such as microcracks and natural fractures commonly developed in unconventional low-permeability oil and gas reservoirs on the hydraulic fracture initiation;

[0061] (2) The method of the present application can be applied based on exploration, design data or adjacent well fitting, and is easy to use and has high calculation accuracy;

[0062] (3) The method of the present application is not only applicable to unconventional low-permeability oil and gas reservoirs, but also to other oil and gas reservoirs with or without developed structural weak surfaces, and can be applied to various well types such as vertical wells, inclined wells, and horizontal wells, and has a very wide range of applications.

[0063] In a second aspect, an embodiment of the present invention further provides a device for determining the fracturing pressure of an unconventional low permeability reservoir, characterized in that the device comprises: a memory 200 and a processor 210; the memory is used to store a program for determining the fracturing pressure of an unconventional low permeability reservoir, and the processor is used to read and execute the program for determining the fracturing pressure of an unconventional low permeability reservoir, and execute any one of the methods described in the above embodiments.

[0064] In a third aspect, an embodiment of the present invention further provides a computer-readable storage medium having a data processing program stored thereon, and the data processing program is executed by a processor to perform the method for determining the initiation pressure of an unconventional low-permeability reservoir as described in any one of the above embodiments.

[0065] Example 1

[0066] Based on the X-well in the H1 block of the unconventional low permeability reservoir, the process of determining the fracture initiation pressure is as follows:

[0067] S1: Based on the unconventional low permeability reservoir H1 block X well, the case application was carried out, and the geological and engineering key parameters of the H1 block X well were collected. See Table 1

[0068] Table 1 Key parameters of Well X in Block H1

[0069] Maximum horizontal principal stress (MPa) 42 Horizontal minimum principal stress (MPa) 34 Vertical principal stress (MPa) 49 Pore ​​fluid pressure (MPa) 21 Well radius (m) 0.08 Hole depth (m) 0.5 Formation porosity 0.1 Poisson's ratio 0.22 Rock tensile strength (MPa) 3.6 Natural fracture cohesion (MPa) 1 Natural fracture friction coefficient 0.2 Effective stress coefficient 0.6

[0070] S2: Set the vertical stress of the reservoir to σ z , the minimum principal stress at reservoir level is σ x , the maximum horizontal principal stress of the reservoir is σ y , and establish a coordinate system (x, y, z) based on the three principal stress directions. Let α be the angle between the wellbore projection on the horizontal plane and the horizontal minimum principal stress direction, and β be the wellbore inclination angle. According to the stress conversion matrix, the reservoir principal stress is converted into the stress field distribution in the local wellbore coordinate system (xx, yy, vv):

[0071]

[0072] In the formula, σ xx , σ yy , σ v are the principal stresses in the local wellbore coordinate system (xx, yy, vv), MPa;

[0073] τ yv , τ xv , τ xy are the shear stresses in the local coordinate system of the wellbore, MPa.

[0074] S3: Establish a polar coordinate system (r, θ, z) with the center of the perforation hole as the origin, and also set the compressive stress to be positive and the tensile stress to be negative. Considering the stress concentration generated by the hole and the percolation effect of the fracturing fluid, the stress field of the surrounding rock of the wellbore is obtained as follows:

[0075]

[0076]

[0077] In the formula, σ xx , σ yy , σ vv are the principal stresses of the local wellbore coordinate system (xx, yy, vv), MPa; τ yv , τ xv , τ xy are the shear stress of the local coordinate system of the wellbore, MPa; ω is the shear stress of any radial direction and stress σ θ The polar coordinate angle of the counterclockwise rotation direction, °; ​​α is the effective stress coefficient, dimensionless; p s is the fluid pressure in the perforation, MPa; p p is the original formation pore pressure, MPa; σ r , σ θ , σ z is the principal stress of the perforation polar coordinate system (xx, yy, vv), MPa; τ rθ , τ rz , τ θz are the shear stress of perforation polar coordinates, MPa.

[0078] S4: When the reservoir has no microcracks or natural fractures, or the perforation holes do not encounter microcracks or natural fracture surfaces, hydraulic fractures initiate along the rock matrix. In this case, the fracturing fracture initiation model is similar to the conventional case, and the maximum tensile fracture criterion is usually used as the criterion. That is, when the effective tensile stress on the rock reaches the tensile strength of the rock, the rock will be tensilely damaged in the direction perpendicular to the tensile stress to form hydraulic fractures.

[0079] Based on the theory of elasticity, the maximum tensile stress σ mo for:

[0080]

[0081] Considering the influence of rock pore pressure, σ t is the tensile strength, and the first cracking judgment criterion is:

[0082] σ mo -γp p >σ t (5)

[0083] In the above formula, γ is the effective stress coefficient, σ t is the tensile strength of rock.

[0084] like Figure 3 As shown in the figure, the calculation results show that the fracture pressure is the lowest when the perforation hole of Well X in Block H1 is facing the horizontal maximum principal stress. As the angle increases, the fracture pressure gradually increases. It is the highest when the hole faces the horizontal minimum principal stress, which is more than 10MPa higher than the lowest point. As the Poisson's ratio of the rock increases, the fracture pressure will increase. In the case of low perforation angles, the fracture pressure does not increase significantly; while in the case of high perforation angles, the fracture pressure increases significantly. The analysis shows that poor perforation angles will amplify the adverse effects of rock properties on fracture pressure. Calculation at the optimal perforation angle shows that when the fracture of Well X in Block H1 is initiated, when the perforation hole does not encounter microcracks or natural fracture surfaces, the fracture pressure is about 41MPa.

[0085] S5: When the perforation holes of Well X in Block H1 encounter structural weak surfaces such as fissures and natural fractures, hydraulic fractures are affected and initiate along the fissures and fractures. At this time, the fracture initiation requires that the fluid pressure at the intersection is greater than the effective normal stress acting on the structural weak surface. That is, when the fluid pressure is greater than the effective normal stress in the normal direction of the weak surface, the fracture will initiate.

[0086] The calculation relationship between the three principal stresses and the normal stress σn on the crack surface is as follows:

[0087] σ n =cos 2 η x σ x +cos 2 η y σ y +cos 2 η z σ z (6)

[0088] Where η x , η y , η zare the angles between the principal stress directions (x, y, z) and the normal of the natural weak surface, degrees.

[0089] The hydraulic fracture along the natural weak plane requires that the fluid pressure at the intersection is greater than or equal to the effective compressive stress on the front of the fracture surface. The second pressure value of the fracture initiation pressure is:

[0090] p s =σ n -γp p (7) In the above formula, p s is the second pressure value of the fracture initiation pressure.

[0091] The maximum horizontal principal stress in the H1 block is in the X well in the near east direction, and the perforations of the X well are located in the direction of the minimum horizontal principal stress. Figure 4 The relationship between the direction and inclination of the structural weak surface and the fracture pressure is shown when the structural weak surface intersects the hole at the top position of the hole end. Figure 5 The relationship between the strike direction, dip angle and fracture pressure of the structural weak surface when the structural weak surface intersects the hole at a position 45° clockwise from the top of the hole end is shown. Figure 6 The relationship between the direction and dip of the structural weak surface and the fracture pressure is shown when the structural weak surface intersects the hole at both sides of the hole end.

[0092] S6: According to the exploration data, the reservoir is determined to have a main fracture zone with a similar trend and inclination, and the normal fracturing pressure of Well X is estimated to be about 41MPa; when the perforation holes accidentally encounter structural weak surfaces such as cracks and natural fractures, the fracturing pressure is about 37MPa. In the final actual construction, the average fracturing pressure of the fracturing section that did not encounter natural fractures was 42.8MPa; the fracturing pressures of the two fracturing sections were 35.7 and 36.2MPa. It can be seen that the prediction results of the method for determining the fracturing pressure of unconventional low-permeability reservoirs in the embodiment of this application have a high prediction accuracy.

[0093] It will be appreciated by those skilled in the art that all or some of the steps, systems, and functional modules / units in the methods disclosed above may be implemented as software, firmware, hardware, and appropriate combinations thereof. In hardware implementations, the division between the functional modules / units mentioned in the above description does not necessarily correspond to the division of physical components; for example, a physical component may have multiple functions, or a function or step may be performed by several physical components in cooperation. Some or all components may be implemented as software executed by a processor, such as a digital signal processor or a microprocessor, or implemented as hardware, or implemented as an integrated circuit, such as an application-specific integrated circuit. Such software may be distributed on a computer-readable medium, which may include a computer storage medium (or non-transitory medium) and a communication medium (or temporary medium). As known to those skilled in the art, the term computer storage medium includes volatile and non-volatile, removable and non-removable media implemented in any method or technology for storing information (such as computer-readable instructions, data structures, program modules, or other data). Computer storage media include, but are not limited to, RAM, ROM, EEPROM, flash memory or other memory technology, CD-ROM, digital versatile disks (DVD) or other optical disk storage, magnetic cassettes, magnetic tapes, magnetic disk storage or other magnetic storage devices, or any other medium that can be used to store the desired information and can be accessed by a computer. In addition, it is well known to those of ordinary skill in the art that communication media typically contain computer-readable instructions, data structures, program modules, or other data in a modulated data signal such as a carrier wave or other transport mechanism, and may include any information delivery media.

Claims

1. A method for determining the fracturing pressure of an unconventional low permeability reservoir, characterized in that: The method comprises: Collect geostress data of oil and gas wells in unconventional low-permeability reservoirs; The ground stress data in the rectangular coordinate system is converted into the ground stress data in the local coordinate system of the wellbore according to the stress conversion matrix; Calculating a first pressure value of the fracturing pressure according to the ground stress data in the local coordinate system of the wellbore; Calculating a second pressure value of the fracture initiation pressure according to the ground stress data in the rectangular coordinate system; The fracturing pressure range value of the unconventional low permeability reservoir is determined according to the first pressure value and the second pressure value.

2. The method for determining the fracturing pressure of an unconventional low permeability reservoir according to claim 1, characterized in that: The geostress data include: reservoir vertical stress, reservoir horizontal minimum principal stress and reservoir horizontal maximum principal stress.

3. The method for determining the fracturing pressure of an unconventional low permeability reservoir according to claim 2, characterized in that: The stress conversion matrix is: In the above formula, σ xx , σ yy , σ vv are the principal stresses corresponding to the local coordinate systems xx, yy, and vv of the wellbore respectively; τ yv , τ xv , τ xy are the shear stress in the local coordinate system of the wellbore; z is the vertical stress of the reservoir, σ x is the minimum principal stress at reservoir level, σ y is the maximum horizontal principal stress of the reservoir; α is the angle between the projection of the wellbore on the horizontal plane and the direction of the horizontal minimum principal stress; β is the wellbore inclination angle.

4. The method for determining the fracturing pressure of an unconventional low permeability reservoir according to claim 3, characterized in that: The step of calculating the first pressure value of the fracturing pressure according to the ground stress data in the local coordinate system of the wellbore comprises: Calculate the principal stress and shear stress in the perforation polar coordinate system according to the in-situ stress data in the local coordinate system of the wellbore; Determine the maximum tensile stress on the wall according to the principal stress and shear stress in the hole polar coordinate system; According to the maximum tensile stress on the wall surface and in combination with a first crack initiation judgment criterion, a first pressure value of the crack initiation pressure is determined.

5. The method for determining the fracturing pressure of unconventional low permeability reservoirs according to claim 4, characterized in that: The determination process of the principal stress and shear stress in the perforation polar coordinate system is as follows: In the above formula, σ r , σ θ , σ z is the principal stress of the perforation polar coordinate system xx, yy, vv; τ rθ , τ rz , τ θz is the shear stress of the perforation polar coordinates; ω is any radial direction and stress σ θ The polar coordinate angle of the counterclockwise direction, α is the effective stress coefficient; p w is the fluid pressure; p p is the original formation pore pressure; r w is the borehole radius, r is the polar coordinate radius of the fracture initiation point, v is the Poisson's ratio; φ is the formation porosity.

6. The method for determining the fracturing pressure of unconventional low permeability reservoirs according to claim 5, characterized in that: The maximum tensile stress on the wall is: In the above formula, σ mo is the maximum tensile stress on the wall.

7. The method for determining the fracturing pressure of an unconventional low permeability reservoir according to claim 6, characterized in that: The first crack initiation judgment criterion is: s mo -γp p >s t In the above formula, γ is the effective stress coefficient, σ t is the tensile strength of rock.

8. The method for determining the fracturing pressure of unconventional low permeability reservoirs according to claim 1, characterized in that: Calculating a second pressure value of the fracture initiation pressure according to the principal stress data; Calculate the normal stress on the natural fracture surface based on the principal stress data; A second pressure value of the fracture initiation pressure is calculated based on the determined normal stress.

9. The method for determining the fracturing pressure of an unconventional low permeability reservoir according to claim 8, characterized in that: The normal stress on the crack surface is: s n =cos 2 or x s x +cos 2 or y s y +cos 2 or z s z In the above formula, σ n is the normal stress on the crack surface, η x , η y , η z It is the angle between the principal stress direction and the normal line of the natural fracture surface.

10. The method for determining the fracturing pressure of an unconventional low permeability reservoir according to claim 9, characterized in that: The second pressure value of the crack initiation pressure is: p s =s n -γp p In the above formula, p s is the second pressure value of the fracture initiation pressure.

11. A device for determining the fracture initiation pressure of an unconventional low-permeability reservoir, characterized in that: The device comprises: a memory and a processor; the memory is used to store a program for determining the fracturing pressure of an unconventional low-permeability reservoir, and the processor is used to read and execute the program for determining the fracturing pressure of an unconventional low-permeability reservoir, and execute the method described in any one of claims 1-10.

12. A computer-readable storage medium having a data processing program stored thereon, wherein the data processing program is executed by a processor to implement the method for determining the initiation pressure of an unconventional low-permeability reservoir as claimed in any one of claims 1 to 10.

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