Method for evaluating the fracturing capacity of shale reservoirs

Abstract

This invention belongs to the field of oil and gas field development technology and discloses a method for evaluating the fracture formation capacity of shale reservoirs. The method includes: S1, determining the contribution coefficient ζ of clay in the experimental sample; S2, calculating the mineral evaluation value L of the experimental sample. mineral S3. Determine the fracture propagation capacity value L after the nth repeated fracturing according to the formula. Fn Among them, L Fn‑1 Let P be the fracture propagation capacity value after the (n-1)th fracturing operation, where e is the natural base and P is the fracture propagation capacity value. max ρ is the peak pressure reached during fracturing, σ is the tensile strength of the experimental sample, and v is the peak pressure reached during fracturing. p T is the propagation speed of the stress wave. d S4. Where BI is the pressure decay time, and R is the radius of the pre-drilled hole. Fn· L mineral The stimulation efficiency evaluation value BI of the experimental sample under n repeated fracturing cycles was calculated. This evaluation method takes into account the dynamic effects and pressure changes during the phase change fracturing process, making the evaluation method more comprehensive and accurate, and guiding the precise stimulation and efficient development of shale oil reservoirs.

Description

Technical Field

[0001] This invention relates to the field of oil and gas field development technology, and in particular to a method for evaluating the fracture formation capacity of shale reservoirs. Background Technology

[0002] Reservoir stimulation refers to engineering techniques employed to improve the production of oil and gas wells or the water injection rate of injection wells. For shale reservoirs with low matrix permeability, reservoir stimulation can significantly enhance their productivity. Carbon dioxide phase change fracturing technology, as an emerging reservoir stimulation method, can effectively improve the sweep efficiency of carbon dioxide in the nanoscale pore throats of shale. By relying on artificial fractures induced by carbon dioxide phase change, the effective range of carbon dioxide can be greatly increased, thereby enhancing oil recovery and carbon sequestration.

[0003] Carbon dioxide phase change fracturing can generate complex fracture networks. To maximize the extension of these networks, continuous carbon dioxide phase change fracturing is required for repeated fracturing of the reservoir. Traditional fracture-forming capacity assessments primarily utilize mineral content and reservoir mechanical parameters, such as elastic modulus. However, when evaluating the effectiveness of continuous carbon dioxide phase change fracturing, its dynamic effects and pressure variations (such as peak pressure and decay time) significantly influence fracture development and propagation. Therefore, traditional fracture-forming capacity assessment methods cannot fully meet the requirements of continuous carbon dioxide phase change fracturing. Summary of the Invention

[0004] The purpose of this invention is to provide a method for evaluating the fracture formation capability of shale reservoirs, which can take into account the dynamic effects and pressure changes of the phase transformation fracturing process, so as to improve the accuracy and comprehensiveness of the evaluation of fracture formation effect.

[0005] To achieve this objective, the present invention adopts the following technical solution:

[0006] Methods for evaluating the fracture-forming capacity of shale reservoirs include:

[0007] S1. Determine the contribution coefficient ζ of clay in the experimental sample;

[0008] S2, according to formula L mineral =(V 石英 +V 长石 ) / (ζ·V 黏土 +V 石英 +V TOC +V 长石 +V 其他矿物 ), calculate the mineral evaluation value L of the experimental sample. mineral ;

[0009] Among them, V 石英 V is the volume of quartz. 长石 V is the volume of feldspar. 黏土V is the volume of the clay. TOC V is the volume of organic matter. 其他矿物 For the volume of other minerals;

[0010] S3, according to the formula Determine the fracture propagation capacity value L after the nth repeated fracturing. Fn ;

[0011] Among them, L Fn-1 Let P be the fracture propagation capacity value after the (n-1)th fracturing operation, e be the natural base, and P be the fracture propagation capacity value. max This represents the peak pressure reached during fracturing. v represents the tensile strength of the experimental sample. p T is the propagation speed of the stress wave. d R is the pressure decay time, and R is the radius of the pre-made hole.

[0012] S4. According to the formula BI=L Fn· L mineral The evaluation value BI of the transformation efficiency under n repeated fracturing of the experimental sample was calculated.

[0013] Preferably, in step S3, according to the formula Calculate the fracture propagation capacity L after a single fracturing operation. F1 .

[0014] Preferably, according to the formula ζ=1.2726+0.0964φ+0.63687φ 2 Calculate the contribution coefficient ζ of the clay;

[0015] Wherein, φ is the percentage of the clay content in the volume of the experimental sample.

[0016] Preferably, it is characterized in that, ;

[0017] Where E is Young's modulus, v is Poisson's ratio, and ρ is shale density.

[0018] As a preferred option, P max =54.47d-1.308r+3.7P0-20.43;

[0019] Where P0 is the initial filling pressure, d is the rupture disc thickness, and r is the release port size.

[0020] As a preferred option, T d =-21.08d-3.0833r+20P0-42.33.

[0021] The beneficial effects of this invention are as follows:

[0022] This invention provides a method for evaluating the fracture-forming capacity of shale reservoirs, using the fracture propagation capacity value L of the experimental sample after the nth fracturing operation. Fn and mineral evaluation value L mineral The calculated stimulation efficiency evaluation value BI fully considers the dynamic effects and pressure changes (peak pressure, decay time) during the phase transformation fracturing process, making the evaluation method more comprehensive and accurate. This provides predictive values ​​and theoretical basis for reservoir stimulation construction technology, guiding the precise stimulation and efficient development of shale oil reservoirs. Attached Figure Description

[0023] Figure 1 This is a cross-sectional view of a sample after fracturing, provided in a specific embodiment of the present invention;

[0024] Figure 2 This is a cross-sectional view of the sample after fracturing, provided in a specific embodiment of the present invention;

[0025] Figure 3 This is a cross-sectional view of the sample after triple fracturing, provided in a specific embodiment of the present invention. Detailed Implementation

[0026] The present invention will now be described in further detail with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and not intended to limit it. Furthermore, it should be noted that, for ease of description, the accompanying drawings show only the parts relevant to the present invention, and not all of the structures.

[0027] In the description of this invention, unless otherwise explicitly specified and limited, the terms "connected," "linked," and "fixed" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0028] In this invention, unless otherwise explicitly specified and limited, "above" or "below" the second feature can include direct contact between the first and second features, or contact between the first and second features through another feature between them. Furthermore, "above," "over," and "on top" of the second feature includes the first feature directly above or diagonally above the second feature, or simply indicates that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature includes the first feature directly below or diagonally below the second feature, or simply indicates that the first feature is at a lower horizontal level than the second feature.

[0029] In the description of this embodiment, the terms "upper," "lower," "right," and "left," etc., refer to the orientation or positional relationship shown in the accompanying drawings. They are used only for ease of description and simplification of operation, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the present invention. In addition, the terms "first" and "second" are used only for distinction in description and have no special meaning.

[0030] This invention provides a method for evaluating the fracture formation capacity of shale reservoirs, the method comprising the following steps:

[0031] S1. Determine the contribution coefficient ζ of clay in the experimental sample;

[0032] S2, according to formula L mineral =(V 石英 +V 长石 ) / (ζ·V 黏土 +V 石英 +V TOC +V 长石 +V 其他矿物 ), calculate the mineral evaluation value L of the experimental sample. mineral ;

[0033] Among them, V 石英 V is the volume of quartz. 长石 V is the volume of feldspar. 黏土 V is the volume of the clay. TOC V is the volume of organic matter. 其他矿物 For the volume of other minerals;

[0034] S3, according to the formula Determine the fracture propagation capacity value L after the nth repeated fracturing. Fn ;

[0035] Among them, L Fn-1 Let P be the fracture propagation capacity value after the (n-1)th fracturing operation, e be the natural base, and P be the fracture propagation capacity value. max This represents the peak pressure reached during fracturing. v represents the tensile strength of the experimental sample. p T is the propagation speed of the stress wave. d R is the pressure decay time, and R is the radius of the pre-made hole.

[0036] S4. According to the formula BI=L Fn· L mineral The evaluation value BI of the fracturing efficiency under n repeated fracturing cycles of the experimental sample was calculated.

[0037] Specifically, the propagation capacity value L of the experimental sample after the nth fracturing is used. Fn and mineral evaluation value Lmineral The calculated stimulation efficiency evaluation value BI fully considers the dynamic effects and pressure changes (peak pressure, decay time) during the phase transformation fracturing process, making the evaluation method more comprehensive and accurate. This provides predictive values ​​and theoretical basis for reservoir stimulation construction technology, guiding the precise stimulation and efficient development of shale oil reservoirs.

[0038] In this embodiment, a simulated fracturing experiment was conducted on the shale experimental sample using the shale reservoir fracture formation capacity evaluation method, and the experiment was performed using a carbon dioxide phase change fracturing generator commonly used in the field. This generator mainly includes a storage tank, an injection module, a heating module, and an energy release module. The specific structure and working principle of the carbon dioxide phase change fracturing generator are not detailed here. Before the experiment, pre-drilled holes were drilled in the shale experimental sample, and carbon dioxide was injected into these holes to achieve the fracturing effect. After the experiment, the fracturing capacity was evaluated using the stimulation efficiency evaluation value (BI). A higher BI value indicates higher stimulation efficiency and better fracturing effect.

[0039] Furthermore, in step S3, according to the formula Calculate the fracture propagation capacity L after a single fracturing operation. F1 In this embodiment, e is the natural base, and P max This represents the peak pressure reached during fracturing. v represents the tensile strength of the experimental sample. p T is the propagation speed of the stress wave. d L is the pressure decay time, and R is the radius of the pre-made hole. In this embodiment, L F1 L represents the fracture propagation capacity after a single fracturing operation. Fn This represents the fracture propagation capacity value after n consecutive fracturing operations. When evaluating the effect of multiple consecutive phase change fracturing operations, it is necessary to first calculate the fracture propagation capacity value after each of the previous phase change fracturing operations; for example, after three consecutive phase change fracturing operations, the operator needs to first use the formula... Calculate the fracture propagation capacity L after the first hydraulic fracturing. F1 Subsequently, based on L F1 Calculate the fracture propagation capacity L after the second hydraulic fracturing. F2 Finally, based on L F2 Calculate the fracture propagation capacity L after the third hydraulic fracturing. F3 Thus, the evaluation value of the fracture propagation capacity after three consecutive phase transformation fracturings was finally obtained.

[0040] Specifically, according to the formula ζ=1.2726+0.0964φ+0.63687φ 2The contribution coefficient ζ of clay is calculated; where φ is the percentage of clay in the volume of the experimental sample. In this embodiment, the shale contains a certain amount of clay, which is used in calculating the mineral evaluation value L. mineral First, the percentage of clay in the volume of the experimental sample needs to be determined. Then, the formula ζ=1.2726+0.0964φ+0.63687φ is used. 2 The contribution coefficient ζ of clay was calculated, and then substituted into the mineral evaluation value L. mineral The calculation formula is used to obtain the mineral evaluation value L. mineral .

[0041] When determining the content of each component in the shale experimental sample, X-ray diffraction analysis, a method commonly used in this field, was employed to accurately obtain Vt. 石英 V 长石 V 黏土 V TOC and V 其他矿物 .

[0042] In this embodiment, when determining the stress wave propagation speed v p When using the formula The calculations are performed, where E is Young's modulus, v is Poisson's ratio, and ρ is shale density; the peak pressure P reached during the fracturing experiment is determined. max The fitting formula P is used. max The calculation is performed using the formula = 54.47d - 1.308r + 3.7P0 - 20.43, where P0 is the initial filling pressure of the carbon dioxide phase change fracturing generator, d is the thickness of the rupture disc of the carbon dioxide phase change fracturing generator, and r is the size of the release port of the carbon dioxide phase change fracturing generator. It is understood that this fitting formula is a dimensionless formula, so when using this formula for calculation, the dimensions of each physical quantity must be removed and only its numerical value is used.

[0043] Specifically, in determining the pressure decay time T d When, the fitting formula T is used. d The formula =-21.08d-3.0833r+20P0-42.33 is used for calculation, and the peak pressure P is calculated. max The formula is the same, where P0 is the initial charging pressure of the carbon dioxide phase change fracturing generator, d is the thickness of the rupture disc of the carbon dioxide phase change fracturing generator, and r is the size of the release port of the carbon dioxide phase change fracturing generator; it can be understood that this fitting formula is the same as the above-mentioned calculation of the peak pressure P. max All formulas are dimensionless formulas. When calculating, the dimensions of each physical quantity must be removed and only its numerical value is used.

[0044] Tensile strength of the experimental sample The following formulas, commonly used in this field, are used for calculation:

[0045]

[0046]

[0047]

[0048]

[0049]

[0050]

[0051]

[0052]

[0053]

[0054]

[0055] In the formula, S is the contact stiffness; p is the load; h is the indentation depth; h c h represents the contact depth. max P represents the maximum depth. max The maximum load is ε; ε is a geometry-related parameter, a constant with a value of 0.75; A c U is the contact area; H is the contact hardness; β is a constant related to the geometry of the indenter; U pp Energy lost during the pure plastic phase; U e h represents the pure elastic energy during the indentation process. f Residual depth; U t U represents the total energy during the compression process. c K represents the energy at which the crack fractures; c Gc represents the rock fracture toughness; Gc represents the fracture energy; and E represents Young's modulus. In this embodiment, the data for the above physical quantities were obtained through nanoindentation experiments.

[0056] The following experiment uses three sets of experimental samples as examples to demonstrate the method for evaluating the fracture formation capacity of this shale reservoir. The three sets of samples are Sample 1, Sample 2, and Sample 3. Two phase transformation fracturing experiments were conducted on each of these three sets of samples. X-ray diffraction analysis revealed that Sample 1 contained zero clay and had a feldspar volume of 70 cm³. 3 The volume of other minerals is 30 cm³. 3 In sample two, the clay content was zero, and the feldspar volume was 70 cm³. 3 Other mineral content is 30cm3 In sample three, the volume of clay was 37.9 cm³. 3 The volume of quartz is 36 cm³. 3 The feldspar has a volume of 10 cm. 3 The volume of other minerals is 16.1 cm³. 3 .

[0057] Furthermore, for the experiment of Sample 1, the initial filling pressure P0 was set to 10 MPa, the rupture disc thickness d of the carbon dioxide phase change fracturing device was 2 mm, and the release port diameter r of the carbon dioxide phase change fracturing device was 24 mm; for the experiment of Sample 2, the initial filling pressure P0 was set to 10 MPa, the rupture disc thickness d of the carbon dioxide phase change fracturing device was 2 mm, and the release port diameter r of the carbon dioxide phase change fracturing device was 12 mm; for the experiment of Sample 3, the initial filling pressure P0 was set to 12 MPa, the rupture disc thickness d of the carbon dioxide phase change fracturing device was 1 mm, and the release port diameter r of the carbon dioxide phase change fracturing device was 24 mm.

[0058] After the experiment began, the peak pressure P applied to sample one for the first time was... max The pressure is 400 MPa, and the pressure decay time T d The peak pressure P applied for the second time was 200 ms. max The pressure is 50 MPa, and the pressure decay time T d The time was 3000 ms. The calculated fracture propagation capacity L after a single fracturing of sample one was obtained. F1 The value of L, representing the fracture propagation capacity after the second fracturing, is 0.363. F2 Its mineral evaluation value is 3.32, and its L value is 3.32. mineral The value was 0.7, and the final calculated modification efficiency evaluation value BI was 2.324. Its crack propagation effect was as follows: Figure 1 As shown.

[0059] Subsequently, fracturing operations were performed on sample two, with the first applied peak pressure P max The pressure is 400 MPa, and the pressure decay time T d The peak pressure P applied for the second time was 200 ms. max The pressure is 50 MPa, and the pressure decay time T d The time frame was 2000 ms. The calculated fracture propagation capacity L after a single fracturing of sample one was obtained. F1 The value of L, representing the fracture propagation capacity after the second fracturing, is 0.363. F2 Its mineral evaluation value L is 2.28. mineral The value was 0.7, and the final calculated evaluation value of the modification efficiency, BI, was 1.596. Its crack propagation effect was as follows: Figure 2 As shown.

[0060] Finally, fracturing was performed on sample three, with the first applied peak pressure P... max The pressure is 400 MPa, and the pressure decay time T d The peak pressure P applied for the second time was 200 ms. max The pressure is 50 MPa, and the pressure decay time T d The time was 3000 ms. The calculated fracture propagation capacity L after a single fracturing of sample one was obtained. F1 The value of L, representing the fracture propagation capacity after the second fracturing, is 0.363. F2 Its mineral evaluation value is 3.32, and its L value is 3.32. mineral The value was 0.3994, and the final calculated evaluation value of the modification efficiency, BI, was 1.326. Its crack propagation effect was as follows: Figure 3 As shown.

[0061] The evaluation method for fracture formation capacity of shale reservoirs shows that, for the stimulation efficiency evaluation value BI, Sample 1 > Sample 2 > Sample 3. Therefore, Sample 1 exhibits the best fracture formation effect, while Sample 3 shows the worst. Figures 1 to 3 As shown.

[0062] To verify the validity of the above conclusions, the staff adopted the fracture numerical simulation method, imported the conditions of the three groups of samples into the simulation software for calculation, and obtained the same conclusions as above, namely: for the evaluation value BI of the stimulation efficiency, Sample 1 > Sample 2 > Sample 3; thus proving the true validity of the shale reservoir fracture formation capacity evaluation method.

[0063] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the implementation of the present invention. Those skilled in the art will be able to make various obvious changes, readjustments, and substitutions without departing from the scope of protection of the present invention. It is neither necessary nor possible to exhaustively describe all embodiments here. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the claims of the present invention.

Claims

1. A method for evaluating the fracture-forming capacity of shale reservoirs, characterized in that, include: S1. Determine the contribution coefficient ζ of clay in the experimental sample; S2, according to formula L mineral =(V 石英 +V 长石 ) / (ζ·V 黏土 +V 石英 +V TOC +V 长石 +V 其他矿物 ), calculate the mineral evaluation value L of the experimental sample. mineral ; Among them, V 石英 V is the volume of quartz. 长石 V is the volume of feldspar. 黏土 V is the volume of the clay. TOC V is the volume of organic matter. 其他矿物 For the volume of other minerals; S3, according to the formula Determine the fracture propagation capacity value L after the nth repeated fracturing. Fn ; Among them, L Fn-1 Let P be the fracture propagation capacity value after the (n-1)th fracturing operation, e be the natural base, and P be the fracture propagation capacity value. max This represents the peak pressure reached during fracturing. v represents the tensile strength of the experimental sample. p T is the propagation speed of the stress wave. d R is the pressure decay time, and R is the radius of the pre-made hole; S4. According to the formula BI=L Fn· L mineral The evaluation value BI of the transformation efficiency under n repeated fracturing of the experimental sample was calculated.

2. The method for evaluating the fracture formation capacity of shale reservoirs according to claim 1, characterized in that, In step S3, according to the formula Calculate the fracture propagation capacity L after a single fracturing operation. F1 .

3. The method for evaluating the fracture formation capacity of shale reservoirs according to claim 1, characterized in that, According to the formula ζ=1.2726+0.0964φ+0.63687φ 2 Calculate the contribution coefficient ζ of the clay; Wherein, φ is the percentage of the clay content in the volume of the experimental sample.

4. The method for evaluating the fracture formation capacity of shale reservoirs according to claim 1, characterized in that, ； Where E is Young's modulus, v is Poisson's ratio, and ρ is shale density.

5. The method for evaluating the fracture formation capacity of shale reservoirs according to claim 1, characterized in that, P max =54.47d-1.308r+3.7P0-20.43; Where P0 is the initial filling pressure, d is the rupture disc thickness, and r is the release port size.

6. The method for evaluating the fracture formation capacity of shale reservoirs according to claim 5, characterized in that, T d =-21.08d-3.0833r+20P0-42.33。

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