A shale reservoir compressibility evaluation method and system

Abstract

The present application provides a shale reservoir compressibility evaluation method and system, which analyzes the water content of different depths of the shale reservoir according to the capillary force function and the model of drilling fluid invading the shale reservoir during the reservoir operation process, determines the Young's modulus and Poisson's ratio of the reservoir after hydration based thereon, calculates the shale brittleness index according to the Young's modulus and Poisson's ratio, obtains the horizontal differential stress coefficient of the shale, obtains the fracture toughness index based on the corresponding fracture toughness type, and then establishes a reservoir compressibility operation model according to the brittleness index, the horizontal differential stress coefficient of the shale and the fracture toughness index of the shale, solves the compressibility operation model to determine the compressibility index of the shale reservoir, and evaluates the compressibility of different depths of the target shale reservoir. The scheme overcomes the problem of incomplete consideration and insufficient accuracy of the prior art, introduces the influence of hydration on the performance of shale into the compressibility operation, and obtains more realistic reservoir compressibility evaluation data.

Description

Technical Field

[0001] This invention relates to the field of unconventional oil exploration, development and evaluation technology, and in particular to a method and system for evaluating the compressibility of shale reservoirs. Background Technology

[0002] Shale oil is generated and stored in organic-rich shale layers, significantly different from conventional oil and gas. Furthermore, shale layers have extremely low permeability. Shale oil development typically requires staged fracturing to connect the pores and microfractures within the shale reservoir, thereby extracting the crude oil. Ideally, fracturing in shale reservoirs should create a complex fracture network while simultaneously achieving a large reservoir stimulation volume, connecting a larger area of ​​the shale reservoir to maximize economic benefits. However, the fracturing effect of shale reservoirs is not only related to the fracturing process but also significantly influenced by reservoir geological conditions. This relationship between reservoir geological conditions and fracturing effectiveness is known as the compressibility of shale reservoirs.

[0003] Existing technologies have included studies on the compressibility of shale reservoirs. For example, Rickma et al. proposed using the normalized average of Young's modulus and Poisson's ratio as a brittleness index to characterize rock compressibility. Mike J. Mullen believes that compressibility should comprehensively consider reservoir sedimentary characteristics and mineral composition. Sui discussed in detail the influence of clay mineral content, cohesion, and internal friction angle on compressibility. Patent document CN106869911A discloses an evaluation method for describing the compressibility of shale reservoirs, which evaluates the compressibility of shale reservoirs through the formation pore fluid pressure gradient, the density of the overlying strata, the maximum horizontal stress, and the minimum horizontal stress.

[0004] However, existing methods only consider a single factor or a simple superposition of multiple factors, resulting in relatively poor accuracy of the final prediction results. At the same time, since shale formations usually contain a lot of clay minerals, when water-based fracturing fluid is used for fracturing, the clay will absorb water and hydrate. The mechanical properties of the hydrated shale will change, and creep will occur, causing changes in the fracture structure of the shale reservoir. Existing technologies do not take this problem into account, resulting in errors between the calculated data and the actual situation, which cannot meet the needs of actual engineering.

[0005] The information disclosed in the background section of this invention is intended only to enhance the understanding of the general background of this invention, and should not be construed as an admission or in any way implying that such information constitutes prior art known to those skilled in the art. Summary of the Invention

[0006] To address the aforementioned problems, this invention provides a method for evaluating the compressibility of shale reservoirs. This method obtains the brittleness index of shale using Young's modulus and Poisson's ratio after hydration; obtains the horizontal differential stress coefficient of shale; obtains the fracture toughness index of shale; establishes a compressibility calculation model based on the brittleness index, the horizontal differential stress coefficient, and the fracture toughness index of shale, and then solves the compressibility calculation model. This invention considers the influence of hydration on shale properties and incorporates this influence into the compressibility calculation, making the final evaluation results more consistent with actual formation conditions. In a preferred embodiment, the method includes: Step S1: Analyze the water content at different depths in the shale reservoir based on the capillary force function and drilling fluid intrusion model during reservoir operation. Based on the variation of elastic modulus and Poisson's ratio with shale water content, determine the Young's modulus and Poisson's ratio of the shale reservoir after hydration at the corresponding depth. Step S2: Calculate the brittleness index of shale based on Young's modulus and Poisson's ratio after shale reservoir hydration; Step S3: Considering the mechanical principles of reservoir opening and extension to form complex fracture networks, obtain the horizontal differential stress coefficient of shale; Step S4: Analyze the fracture toughness type of the reservoir, and obtain the fracture toughness index that characterizes the ease of fracturing the shale reservoir based on the fracture toughness type. Step S5: Establish a reservoir compressibility calculation model based on the brittleness index, shale horizontal differential stress coefficient, and shale fracture toughness index, and solve the reservoir compressibility calculation model to determine the compressibility index of the shale reservoir at different depths; Step S6: Analyze the compressibility assessment results of the target shale reservoir within the set depth range based on the compressibility index.

[0007] In one embodiment, the process of analyzing the water content in the shale reservoir in step S1 includes: The model of drilling fluid intrusion into shale reservoirs was analyzed by combining assumptions and constraints. The difference equation of the water-based drilling fluid intrusion model into shale reservoirs was then determined, and the water content in shale reservoirs was analyzed based on it.

[0008] Preferably, in one embodiment, in step S1, the model of drilling fluid intrusion into shale reservoirs is subjected to double implicit difference using staggered grids to obtain the following difference equation for the water-based drilling fluid intrusion into shale reservoir model:

[0009] In the formula, Radial distance, in meters (m); Permeability, mD; μ w The viscosity of the aqueous phase is expressed in mPa·s. pw Indicates the pressure in the aqueous phase, in Pa; K ro This represents the relative permeability of the oil phase in shale oil reservoirs, and is dimensionless. v o Indicates the oil phase flow rate, in m / s; μ o Indicates the oil phase viscosity, mPa·s; S w Indicates water saturation; K o The water phase permeability of shale oil reservoirs is expressed in mD. S o This indicates the oil saturation level. Further, in one embodiment, in step S1, the water content in the shale reservoir is determined according to the following formula. :

[0010] In the formula, Density of drilling fluid, g / cm³ 3 ; The density of the shale skeleton, in g / cm³ 3 ; The volume of water contained in the rock, in cm. 3 ; The volume of the shale skeleton in the rock is expressed in cm³. 3 .

[0011] In an optional embodiment, the elastic modulus of shale is used as the Young's modulus, and the Young's modulus and Poisson's ratio after hydration of the shale reservoir are determined according to the following formula:

[0012]

[0013] In the formula, The elastic modulus of shale after water absorption, in MPa; and They are coefficients, respectively. and - These represent the percentage of water adsorbed by the shale and the percentage of water content in the original shale, respectively. The Poisson's ratio for shale.

[0014] Preferably, in one embodiment, in step S2, the shale brittleness index is obtained according to the following formula. :

[0015] In the formula, The Young's modulus of the target layer after hydration is to be evaluated. The normalized Young's modulus of the target layer after hydration is given. and These are the minimum and maximum Young's moduli for the entire region, respectively. The Poisson's ratio after hydration of the target layer to be evaluated. The normalized Poisson's ratio after hydration of the target layer to be evaluated. and These are the minimum and maximum Poisson ratios for the entire region, respectively.

[0016] In an optional embodiment, in step S4, the shale reservoir is classified into Type I fracture toughness and Type II fracture toughness, and the fracture toughness index is determined accordingly as follows:

[0017]

[0018] In the formula, K Ⅰc Type I fracture toughness index; K ⅡC It is a type II fracture toughness index; The confining pressure on the crack surface is in MPa. denoted as uniaxial tensile strength of rock, MPa.

[0019] Furthermore, in one embodiment, in step S5, a reservoir compressibility calculation model is established based on the brittleness index, the shale horizontal differential stress coefficient, and the shale fracture toughness index: , In the formula, It is the compressibility index; b are operational coefficients; The brittleness index of shale; The horizontal differential stress coefficient is dimensionless. This represents the fracture toughness index of shale.

[0020] Based on other aspects of the methods described in any one or more of the foregoing embodiments, the present invention also provides a storage medium storing program code that can implement the methods described in any one or more of the foregoing embodiments.

[0021] Based on the application aspects of the methods described in any one or more of the above embodiments, the present invention also provides a shale reservoir compressibility evaluation system, which performs the methods described in any one or more of the above embodiments.

[0022] Compared with the closest prior art, the present invention also has the following beneficial effects: This invention provides a method and system for evaluating the compressibility of shale reservoirs. The method analyzes the water content at different depths of the shale reservoir based on the capillary force function during reservoir operation and the model of drilling fluid intrusion into the shale reservoir. Based on this, it determines the Young's modulus and Poisson's ratio of the shale reservoir after hydration. It calculates the shale brittleness index based on the Young's modulus and Poisson's ratio of the shale reservoir after hydration; obtains the horizontal differential stress coefficient of the shale; obtains the fracture toughness index based on the fracture toughness type; furthermore, it establishes a reservoir compressibility calculation model based on the brittleness index, the shale horizontal differential stress coefficient, and the shale fracture toughness index, and solves the reservoir compressibility calculation model to determine the compressibility index of the shale reservoir, thus evaluating the compressibility of the target shale reservoir at different depths. This scheme incorporates the impact of hydration on shale properties into compressibility calculations. It also considers the mechanical principles of reservoir opening and extension leading to complex fracture networks, combined with the horizontal differential stress coefficient. Furthermore, it determines the fracture toughness index, characterizing the ease of fracturing shale reservoirs. By comprehensively calculating the reservoir's compressibility index as an evaluation basis, it can effectively obtain reservoir compressibility assessment data that more closely reflects real-world conditions, providing reliable data support for actual shale reservoir fracturing.

[0023] Other features and advantages of the invention will be set forth in the description which follows, and will be apparent in part from the description, or may be learned by practicing the invention. The objects and other advantages of the invention may be realized and obtained by means of the structures particularly pointed out in the description, claims, and drawings. Attached Figure Description

[0024] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used in conjunction with the embodiments of the invention to explain the invention and do not constitute a limitation thereof. In the drawings: Figure 1 This is a schematic flowchart of a shale reservoir compressibility evaluation method provided in an embodiment of the present invention; Figure 2 This is an example diagram of well logging data from the Da'anzhai section of the Sichuan oil shale formation in the shale reservoir compressibility evaluation method provided in this embodiment of the invention. Figure 3 This is an example diagram illustrating the variation of shale water absorption with dimensionless distance and time in the shale reservoir compressibility evaluation method provided in another embodiment of the present invention; Figure 4 This is an example diagram showing the variation of the compressibility index of the shale reservoir compressibility evaluation method provided in this embodiment of the invention with depth, time, and dimensionless distance of the wellbore axis; Figure 5 This is a schematic diagram of the structure of a shale reservoir compressibility evaluation system provided in an embodiment of the present invention. Detailed Implementation

[0025] The embodiments of the present invention will be described in detail below with reference to the accompanying drawings and examples. Those skilled in the art will then fully understand how the present invention uses technical means to solve technical problems and achieve technical effects, and will be able to implement the present invention specifically based on the above-described implementation process. It should be noted that, as long as there is no conflict, the various embodiments and features of the present invention can be combined with each other, and the resulting technical solutions are all within the protection scope of the present invention.

[0026] Although the flowchart describes the operations as sequential processes, many of these operations can be performed in parallel, concurrently, or simultaneously. The order of the operations can be rearranged. A process can terminate when its operation is complete, but it may also have additional steps not included in the diagram. A process can correspond to a method, function, procedure, subroutine, subroutine, etc.

[0027] Computer equipment includes user equipment and network equipment. User equipment or clients include, but are not limited to, computers, smartphones, PDAs, etc.; network equipment includes, but is not limited to, a single network server, a server group consisting of multiple network servers, or a cloud based on cloud computing consisting of a large number of computers or network servers. Computer equipment can operate independently to implement this invention, or it can connect to a network and implement this invention through interaction with other computer equipment in the network. The network in which the computer equipment is located includes, but is not limited to, the Internet, wide area network, metropolitan area network, local area network, VPN network, etc.

[0028] The terms “first,” “second,” etc., may be used herein to describe various units, but these units should not be limited by these terms; they are used merely to distinguish one unit from another. The term “and / or” as used herein includes any and all combinations of one or more of the associated listed items. When a unit is referred to as “connected” or “coupled” to another unit, it may be directly connected or coupled to said other unit, or there may be intermediate units present.

[0029] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the exemplary embodiments. Unless the context clearly indicates otherwise, the singular forms “a” and “an” as used herein are also intended to include the plural. It should also be understood that the terms “comprising” and / or “including” as used herein specify the presence of the stated features, integers, steps, operations, units, and / or components, without excluding the presence or addition of one or more other features, integers, steps, operations, units, components, and / or combinations thereof.

[0030] Shale oil is generated and stored in organic-rich shale formations, which is significantly different from conventional oil and gas. At the same time, shale formations have extremely low permeability, so shale oil development typically requires staged fracturing to connect the pores and microfractures within the shale reservoir, thereby extracting the crude oil.

[0031] The ideal fracturing effect for shale reservoirs should satisfy the conditions of forming a complex fracture network while simultaneously achieving a large reservoir stimulation volume, enabling communication across a larger area of ​​shale reservoirs to effectively obtain higher economic benefits. However, the fracturing effect of shale reservoirs is not only related to the fracturing technology but also has a significant relationship with the reservoir geological conditions. The relationship between reservoir geological conditions and fracturing effect is known as the compressibility of shale reservoirs.

[0032] Existing technologies have included studies on the compressibility of shale reservoirs. For example, Rickma et al. proposed using the normalized average of Young's modulus and Poisson's ratio as a brittleness index to characterize rock compressibility. Mike J. Mullen believes that compressibility should comprehensively consider reservoir sedimentary characteristics and mineral composition. Sui discussed in detail the influence of clay mineral content, cohesion, and internal friction angle on compressibility. Patent document CN106869911A discloses an evaluation method for describing the compressibility of shale reservoirs, which evaluates the compressibility of shale reservoirs through the formation pore fluid pressure gradient, the density of the overlying strata, the maximum horizontal stress, and the minimum horizontal stress.

[0033] However, existing methods only consider a single factor or a simple superposition of multiple factors, resulting in relatively poor accuracy of the final prediction results, which cannot meet the needs of actual engineering. Furthermore, since shale formations typically contain a large amount of clay minerals, the clay absorbs water and hydrates during fracturing with water-based fracturing fluids. The mechanical properties of the hydrated shale change, and creep occurs, altering the fracture structure of the shale reservoir. Current technologies do not consider this issue, leading to errors between the calculated data and the actual situation.

[0034] To address the shortcomings of existing technologies, this invention provides a method for evaluating the compressibility of shale reservoirs. This method evaluates the compressibility of shale reservoirs by taking into account shale hydration, which is closer to the actual situation and yields more accurate evaluation results.

[0035] The method provided by this invention includes the following steps: obtaining the Young's modulus and Poisson's ratio after hydration; obtaining the brittleness index of shale using the Young's modulus and Poisson's ratio after hydration; obtaining the horizontal differential stress coefficient of shale; obtaining the fracture toughness index of shale; establishing a compressibility calculation model based on the brittleness index, the horizontal differential stress coefficient of shale, and the fracture toughness index of shale, and solving the compressibility calculation model. This invention considers the influence of hydration on shale properties and incorporates this influence into the compressibility calculation, making the final evaluation results more consistent with actual formation conditions and more accurate, thus providing reliable data support for the fracturing of actual shale reservoirs.

[0036] The following describes the detailed flow of the method according to an embodiment of the present invention with reference to the accompanying drawings, the steps of which can be executed in a computer system containing, for example, a set of computer-executable instructions. Although the logical order of the steps is shown in the flowchart, in some cases, the steps shown or described may be performed in a different order than that shown here.

[0037] Example 1 Figure 1 This diagram illustrates a flowchart of the shale reservoir compressibility evaluation method provided in Embodiment 1 of the present invention. (Refer to...) Figure 1 As can be seen, the method includes the following steps.

[0038] Young's modulus and Poisson's ratio analysis step S1: Based on the capillary force function and drilling fluid invasion model of the shale reservoir during reservoir operation, analyze the water content of the shale reservoir at different depths. Based on the variation law of elastic modulus and Poisson's ratio with shale water content, determine the Young's modulus and Poisson's ratio of the shale reservoir after hydration at the corresponding depth. Step S2 for determining the brittleness index: Calculate the brittleness index of shale based on Young's modulus and Poisson's ratio after hydration of the shale reservoir; Step S3 of horizontal differential stress analysis: Considering the mechanical principle of reservoir opening and extension to form a complex fracture network, obtain the horizontal differential stress coefficient of shale; Fracture toughness analysis step S4: Analyze the fracture toughness type of the reservoir, and obtain the fracture toughness index that characterizes the fracturing difficulty of the shale reservoir based on the fracture toughness type. Step S5 of establishing the compressibility model: Establish a compressibility calculation model based on the brittleness index, the horizontal differential stress coefficient of shale and the fracture toughness index of shale, and solve the compressibility calculation model to determine the compressibility index of shale reservoir at different depths; Evaluation Implementation Step S6: Analyze the compressibility assessment results of the target shale reservoir within a set depth range based on the compressibility index.

[0039] By employing the computational logic described in the above embodiments of the present invention, the compressibility of shale reservoirs is evaluated considering shale hydration, resulting in a more accurate assessment closer to actual conditions. In a preferred embodiment, the researchers of this invention considered that during drilling in shale reservoirs, the effects of pressure differential and capillary force jointly influence water entry into the reservoir; once water enters the reservoir, it alters the water saturation of the shale.

[0040] Based on this, different characteristic curves will be obtained for different shale reservoirs, resulting in different J0 functions, as shown in Equation 1 below: (1) In the formula, Represents the surface tension at the oil-water interface, in N·m. Let Pc(S) represent the oil-water interface contact angle in °, and let Pc(S) represent the capillary force as a function of saturation. This represents the porosity of shale oil reservoirs, %; S represents the degree of saturation. The value represents the fracture permeability. The subscript W indicates the aqueous phase, the subscript O indicates the oil phase, the subscript OW indicates the oil-water interface, and J0 represents the capillary force, which is dimensionless. The capillary force is a force that acts as an attraction in a liquid within a capillary or small pore. Its magnitude is usually described by the parameter J0, which is a dimensionless parameter derived from the Young-Laplace equation that can be used to represent the magnitude of the capillary force. The saturation can be represented by two methods: water content and saturation content. Sw + So = 1.

[0041] Furthermore, the capillary force function can be obtained from the J0 function, as shown in Equation 2 below: (2) In practical applications, the following assumptions are established during the calculation process: (1) Shale oil reservoirs are water-wet formations and are isotropic; (2) During the process of drilling fluid invading the shale oil layer, the fluid viscosity and volume coefficient remain unchanged; (3) Gravity is not considered during the entire two-phase flow process, and the oil and water do not dissolve in each other when they come into contact.

[0042] Based on the above assumptions, and according to Darcy's law for oil-water two-phase flow, we can obtain the following equation 3: (3) In the formula, v w The velocity of the water phase is expressed in m / s. K rw This represents the relative permeability of the water phase in shale oil reservoirs; μ wThe viscosity of the aqueous phase is expressed in mPa·s. p w The pressure in the aqueous phase is expressed in Pa; r represents the radial distance. K ro This represents the relative permeability of the oil phase in shale oil reservoirs, and is dimensionless. v o Indicates the oil phase flow rate, in m / s; μ o Indicates the oil phase viscosity, mPa·s; p o This indicates the oil phase pressure, in Pa.

[0043] During the process of drilling fluid invading the reservoir, the oil-water two-phase continuity equation is shown in Equation 4 below: (4) In the formula, ▽ represents the Laplace operator; K w The water phase permeability of shale oil reservoirs is expressed in mD. S w Indicates water saturation; K o The water phase permeability of shale oil reservoirs is expressed in mD. S o Indicates oil saturation; t This indicates the time of oil-water two-phase flow, in seconds.

[0044] Considering the volume compressibility coefficients of the oil and water phases, and combining Equations 3 and 4, the model for drilling fluid intrusion into shale reservoirs can be obtained, as shown in Equation 5 below: (5) In the formula, Radial distance, in meters (m); Where D is the permeability, mD; B is the liquid volume coefficient in shale. is the volume factor of the oil phase in shale. is the volume coefficient of the aqueous phase; μ w The viscosity of the aqueous phase is expressed in mPa·s. p w Indicates the pressure in the aqueous phase, in Pa; K ro This represents the relative permeability of the oil phase in shale oil reservoirs, and is dimensionless. v o Indicates the oil phase flow rate, in m / s; μ o This indicates the viscosity of the oil phase, in mPa·s.

[0045] In addition, the following constraints also exist: (6) (7) (8) In the above formula, A, B, and c are operation coefficients, all of which are constants. They are usually obtained experimentally based on the characteristics of the reservoir. In this embodiment, empirical values ​​are taken: A=0.9, B=0.8, c=4. S or and S wi These represent the residual oil saturation and the original water saturation of the reservoir, respectively.

[0046] The initial and boundary conditions for the above model are as follows: Initial moment: ; Shale reservoir internal boundary conditions:

[0047] Shale reservoir external boundary conditions:

[0048] Formulas 5 to 8 above together constitute a model of drilling fluid intrusion into shale reservoirs, which can then be solved by combining boundary conditions.

[0049] In the formula, Represents the wellbore radius, in meters (m). Represents the radial distance corresponding to the outer boundary of the reservoir, in meters (m). This represents the fluid column pressure at the wellbore, in Pa. This represents the formation pressure at the outer boundary of the reservoir, which is equal to the original formation pressure, in Pa.

[0050] In this embodiment, the above model is subjected to double implicit difference using staggered grids, and the difference equation of the water-based drilling fluid intrusion shale reservoir model is finally obtained as shown in Equation 9 below: (9) In the formula, i represents the i-th spatial node and j represents the j-th time node. This model is essentially a function of time and space. It can be understood that the grid has x and y axes, which represent space and time respectively. i and j are the corresponding xy coordinates, used to characterize the relative positions of the parameters. Indicates the time step increment, in seconds; This represents the water saturation at node ij; This represents the oil saturation at node ij.

[0051] Based on the law of conservation of mass, and combined with the above model and equations, the formula for water content in shale reservoirs is derived, as shown in Equation 10: (10) In the formula, W represents the water content; Density of drilling fluid, g / cm³ 3 ; The density of the shale skeleton, in g / cm³ 3 ; The volume of water contained in the rock, in cm. 3 ; The volume of the shale skeleton in the rock is expressed in cm³. 3 .

[0052] Subsequently, by combining the existing laws governing the variation of elastic modulus and Poisson's ratio with shale water content, the Young's modulus and Poisson's ratio of shale after hydration can be obtained. In this embodiment, the elastic modulus of shale is used to represent the Young's modulus. The final results are shown in Equations 11 and 12 below.

[0053] (11) (12) In the formula, The elastic modulus of shale after water absorption, in MPa; and These are the coefficients, and their empirical values ​​are 4 × 10. 4 and -11; and - These represent the percentage of water adsorbed by the shale and the percentage of water content in the original shale, respectively. The Poisson's ratio for shale.

[0054] In summary, the Young's modulus and Poisson's ratio after water absorption and hydration can be obtained using equations 10, 11, and 12.

[0055] Further, step S2 is performed to calculate the shale brittleness index based on the Young's modulus and Poisson's ratio after hydration of the shale reservoir; the shale brittleness index is obtained through the Young's modulus and Poisson's ratio after hydration. In an optional embodiment, it can be obtained by the formula shown in Equation 13 below: (13) In the formula, in the formula, The Young's modulus of the target layer after hydration is to be evaluated. The normalized Young's modulus of the target layer after hydration is given. and These are the minimum and maximum Young's moduli for the entire region, respectively. The Poisson's ratio after hydration of the target layer to be evaluated. The normalized Poisson's ratio after hydration of the target layer to be evaluated. and These are the minimum and maximum Poisson ratios for the entire region, respectively. Next, proceed to step S3: considering the mechanical principles of reservoir opening and extension to form a complex fracture network, obtain the horizontal differential stress coefficient of the shale.

[0056] For crack formation, the net pressure within the crack must overcome not only the tensile strength of the rock mass but also the horizontal stress difference for the crack to open and extend, forming a complex crack network. Therefore, it is necessary to calculate the horizontal stress difference coefficient, which can be obtained from the formula shown in Equation 14: (14) In the formula, The horizontal differential stress coefficient is dimensionless. The maximum horizontal principal stress is expressed in MPa. The minimum horizontal principal stress is , MPa.

[0057] Further, step S4 is performed: the fracture toughness type of the reservoir is analyzed, and a fracture toughness index characterizing the fracturing difficulty of the shale reservoir is obtained based on the fracture toughness type.

[0058] For some rocks, even if their Poisson's ratio and Young's modulus are similar, their brittleness varies greatly. A key factor contributing to this difference is the rock's fracture toughness. Therefore, fracture toughness is a crucial factor characterizing the ease of fracturing a reservoir. For this reason, the fracture toughness of the rock also needs to be considered in this invention. Regarding fracture toughness, it is divided into Type I fracture toughness K. Ⅰc and Type II fracture toughness K ⅡC Its calculation yields the shale fracture toughness index. The formulas are shown in Equations 15 and 16: (15) (16) In the formula, The confining pressure on the crack surface is in MPa. denoted as uniaxial tensile strength of rock, in MPa.

[0059] The confining pressure on the fracture surface during shale fracturing is shown in Equation 17: (17) In the formula, n =[cosβsinθ,sinβcosθ,sinβ], representing the unit normal vector of the three-dimensional fracture surface in space; β is the fracture dip angle; θ is the angle between the fracture direction and the well axis; The pressure of the overlying strata is MPa; The maximum horizontal ground stress is expressed in MPa. The minimum horizontal stress is given in MPa, and T represents the transpose matrix.

[0060] The uniaxial tensile strength of rock can be obtained from well logging data, and the calculation method is shown in Equation 18: (18) In the formula, The content of clay is dimensionless. Indicates the uniaxial compressive strength of rock; This represents the uniaxial tensile strength of the rock; C is a calculation coefficient, a constant, which can be set to 12.26 in practical applications. To facilitate a comprehensive consideration of two different fracture toughnesses, the fracture toughness index is obtained through Equation 19: (19) In the formula, It represents the maximum Type I fracture toughness index for the entire region. It is the minimum type I fracture toughness index for the entire region. It is the normalized index of the type I fracture toughness index. It represents the maximum Type II fracture toughness index for the entire region. It is the minimum type II fracture toughness index for the entire region. It is the normalized index of the type II fracture toughness index. This represents the fracture toughness index of shale.

[0061] Establish the compressibility calculation model as described in the following formula: (20) In the formula, The compressibility index, b are coefficients, with an empirical value of 0.5.

[0062] Solve the compressibility computational model; The larger the value, the better the compressibility of the target layer being evaluated. The smaller the index, the worse the compressibility. In practical applications, the compressibility index can be compared at each depth within a set depth range to accurately recommend layers with higher compressibility indices for fracturing.

[0063] The present invention will be further described below with reference to specific embodiments. The scope of the present invention is not limited to the embodiments, but is defined in the claims.

[0064] The Da'anzhai section of the Sichuan Basin's oil shale formation possesses the conditions for lacustrine shale hydrocarbon accumulation. Its high-quality shale is mainly developed in the Da'anzhai sub-section, exhibiting favorable conditions such as high organic carbon content, good physical properties, stable distribution, and good hydrocarbon potential. Taking NC2H as an example, it is the first horizontal shale oil well in the Sichuan Basin, with a depth of 2598-2703m in the Da'anzhai section. Its specific logging data is as follows: Figure 2 As shown, Figure 2 The example provided illustrates well logging data used in computational analysis, including Young's modulus, tensile strength, clay content, Poisson's ratio, and geostress coefficient.

[0065] Analysis of well logging data reveals that the Young's modulus of the Da'anzhai section in well NC2H ranges from 18.23 to 23.70 GPa, the tensile strength from 6.27 to 8.18 MPa, the clay content from 0.39 to 0.65%, the Poisson's ratio from 0.18 to 0.24, the maximum horizontal principal stress σH coefficient from 2.51 to 3.32, and the minimum horizontal principal stress σh from 2.20 to 2.41. Related data are shown in Table 1 below.

[0066] Table 1 shows some of the calculation data.

[0067] Based on the data in Table 1, the variation of shale water saturation with dimensionless distance and time can be calculated. The calculation results are as follows: Figure 3 As shown. Then combined with Figure 3 The calculation results Figure 2 Based on the logging data in this embodiment, the compressibility index of the Da'anzhai section was calculated using the method described in this embodiment. The variation of the compressibility index with depth, time, and dimensionless distance from the wellbore axis was obtained. The final calculation results are as follows: Figure 4 As shown.

[0068] Since the basic parameters change with depth, in practical applications, we first use the data in Table 1 to substitute the drilling fluid invasion formation model (Formula 1-10) for a certain depth point to calculate the variation law of water saturation. Because the depth points are different, the water saturation / water content distribution at each depth point will also be different.

[0069] Then, by substituting the obtained water content data into formulas 11 and 12, the change in the elastic modulus Poisson's ratio can be calculated, the brittleness index at the corresponding depth can be calculated, and the fracture toughness can be calculated using formulas 15-19.

[0070] Furthermore, the brittleness index and fracture toughness at various depths from 2598 to 2703 m were calculated using the same method. Then, the brittleness index, fracture toughness, and geostress parameters at each depth were normalized, and the compressibility index was calculated using the normalized three indices through the compressibility calculation model (Formula 20).

[0071] Finally, by comparing the compressibility index at each depth from 2598 to 2703 m, fracturing is recommended for layers with higher compressibility indices.

[0072] The shale reservoir compressibility evaluation method provided in this embodiment of the invention considers the impact of hydration on shale properties and incorporates this impact into the compressibility calculation model, making the final evaluation results more consistent with actual formation conditions and more accurate. It can provide reliable data support for the fracturing of actual shale reservoirs.

[0073] For the foregoing method embodiments, in order to simplify the description, they are all described as a series of actions. However, those skilled in the art should understand that the present invention is not limited to the described order of actions, because according to the present invention, some steps can be performed in other orders or simultaneously. Furthermore, those skilled in the art should also understand that the embodiments described in the specification are preferred embodiments, and the actions and modules involved are not necessarily essential to the present invention.

[0074] It should be noted that, in other embodiments of the present invention, the method can also combine one or more of the above embodiments to obtain a new method for evaluating the compressibility of shale reservoirs, so as to achieve refined analysis and optimized application of the compressibility of shale reservoirs.

[0075] It should be noted that, based on the methods in any one or more embodiments of the present invention described above, the present invention also provides a storage medium storing program code that can implement the methods described in any one or more embodiments, and when the code is executed by the operating system, it can implement the shale reservoir compressibility evaluation method as described above.

[0076] Those skilled in the art will understand that the storage medium includes a readable storage medium, which is a storage device commonly used in the art, including mass storage devices, removable storage devices, etc. Exemplary mass storage devices include disks, optical disks, hard disks, etc., and exemplary removable storage devices include floppy disks, memory cards, etc. Of course, the above examples are merely illustrative of readable storage media and are not intended to limit the scope of the invention. Those skilled in the art can use other storage devices that are applicable to this embodiment.

[0077] It should be noted that, based on the methods in any one or more embodiments of the present invention described above, the present invention also provides an apparatus, which includes: a processor, an information acquisition module, a memory, and an output module. The information acquisition module is used to acquire initial data required for shale reservoir compressibility evaluation; the memory stores a program for shale reservoir compressibility evaluation that can run on the processor, and when the shale reservoir compressibility evaluation program is executed by the processor, it implements the methods described in any one or more embodiments described above; the output module is used to output the evaluation results.

[0078] Example 2: The methods described in detail in the above-disclosed embodiments of the present invention can be implemented using various forms of devices or systems. Therefore, based on other aspects of the methods described in any one or more of the above embodiments, the present invention also provides a shale reservoir compressibility evaluation system, which is used to perform the shale reservoir compressibility evaluation method described in any one or more of the above embodiments. Specific embodiments are given below for detailed description.

[0079] Specifically, Figure 5 The diagram shows a schematic representation of the shale reservoir compressibility evaluation system provided in an embodiment of the present invention. Figure 5 As shown, the system includes: The Young's modulus and Poisson's ratio analysis module is configured to analyze the water content of shale reservoirs at different depths based on the capillary force function and drilling fluid intrusion model during reservoir operation. Based on this, combined with the variation law of elastic modulus and Poisson's ratio with shale water content, the Young's modulus and Poisson's ratio of the shale reservoir after hydration at the corresponding depth are determined. The brittleness index determination module is configured to calculate the brittleness index of shale based on Young's modulus and Poisson's ratio after shale reservoir hydration. The horizontal differential stress analysis module is configured to take into account the mechanical principles of reservoir opening and extension to form a complex fracture network, and obtain the horizontal differential stress coefficient of shale. The fracture toughness analysis module is configured to analyze the fracture toughness type of the reservoir and obtain the fracture toughness index, which characterizes the fracturing difficulty of the shale reservoir, based on the fracture toughness type. The compressibility model building module is configured to establish a compressibility calculation model based on the brittleness index, the horizontal differential stress coefficient of shale, and the fracture toughness index of shale, and to solve the compressibility calculation model to determine the compressibility index of shale reservoirs at different depths. The assessment implementation module is configured to analyze the compressibility assessment results of the target shale reservoir within a set depth range based on the compressibility index.

[0080] Preferably, in one embodiment, the Young's modulus and Poisson's ratio analysis module analyzes the water content in the shale reservoir as follows: The model of drilling fluid intrusion into shale reservoirs was analyzed by combining assumptions and constraints. The difference equation of the water-based drilling fluid intrusion model into shale reservoirs was then determined, and the water content in shale reservoirs was analyzed based on it.

[0081] Furthermore, in one embodiment, the Young's modulus and Poisson's ratio analysis module is configured to perform double implicit difference on the model of drilling fluid intrusion into shale reservoirs using staggered grids, to obtain the following difference equation for the water-based drilling fluid intrusion into shale reservoir model:

[0082] In the formula, Radial distance, in meters (m); Permeability, mD; μ w The viscosity of the aqueous phase is expressed in mPa·s. p w Indicates the pressure in the aqueous phase, in Pa; K ro This represents the relative permeability of the oil phase in shale oil reservoirs, and is dimensionless. v o Indicates the oil phase flow rate, in m / s; μ o Indicates the oil phase viscosity, mPa·s; S w Indicates water saturation; K o The water phase permeability of shale oil reservoirs is expressed in mD. S o This indicates the degree of oil saturation.

[0083] In an optional embodiment, the Young's modulus and Poisson's ratio analysis module is configured to determine the water content in the shale reservoir according to the following formula. :

[0084] In the formula, Density of drilling fluid, g / cm³ 3 ; The density of the shale skeleton, in g / cm³ 3 ; The volume of water contained in the rock, in cm. 3 ; The volume of the shale skeleton in the rock is expressed in cm³. 3 .

[0085] Further, in an optional embodiment, the Young's modulus and Poisson's ratio analysis module uses the elastic modulus of shale as the Young's modulus and determines the Young's modulus and Poisson's ratio of the shale reservoir after hydration according to the following formula:

[0086]

[0087] In the formula, The elastic modulus of shale after water absorption, in MPa; and These are the coefficients, and their empirical values ​​are 4 × 10. 4 and -11; and - These represent the percentage of water adsorbed by the shale and the percentage of water content in the original shale, respectively. The Poisson's ratio for shale.

[0088] In a preferred embodiment, the brittleness index determination module is configured to obtain the shale brittleness index according to the following formula. :

[0089] In the formula, in the formula, The Young's modulus of the target layer after hydration is to be evaluated. The normalized Young's modulus of the target layer after hydration is given. and These are the minimum and maximum Young's moduli for the entire region, respectively. The Poisson's ratio after hydration of the target layer to be evaluated. The normalized Poisson's ratio after hydration of the target layer to be evaluated. and These are the minimum and maximum Poisson ratios for the entire region, respectively.

[0090] In practical applications, in a preferred embodiment, the fracture toughness analysis module is configured to: classify shale reservoirs into Type I and Type II fracture toughness, and determine the fracture toughness index for each according to the following targeted approach:

[0091]

[0092] In the formula, K Ⅰc Type I fracture toughness index; K ⅡC It is a type II fracture toughness index; The confining pressure on the crack surface is in MPa. denoted as uniaxial tensile strength of rock, MPa.

[0093] Furthermore, in one embodiment, the compressibility model building module establishes the following reservoir compressibility calculation model based on the brittleness index, the shale horizontal differential stress coefficient, and the shale fracture toughness index: , In the formula, It is the compressibility index; b are operational coefficients; The brittleness index of shale; The horizontal differential stress coefficient is dimensionless. This represents the fracture toughness index of shale.

[0094] In the shale reservoir compressibility evaluation system provided in this embodiment of the invention, each module or unit structure can operate independently or in combination according to actual parameter analysis requirements and model calculation requirements to achieve the corresponding technical effects.

[0095] It should be understood that the embodiments disclosed herein are not limited to the specific structures, processing steps, or materials disclosed herein, but should be extended to equivalent substitutions of these features as understood by those skilled in the art. It should also be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting.

[0096] The phrase "an embodiment" in the specification means that a specific feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. Therefore, the phrase "an embodiment" appearing in various places throughout the specification does not necessarily refer to the same embodiment.

[0097] While the embodiments disclosed in this invention are as described above, the content is merely for the purpose of facilitating understanding of the invention and is not intended to limit the invention. Any person skilled in the art to which this invention pertains may make any modifications and variations in form and detail of the implementation without departing from the spirit and scope disclosed herein; however, the scope of patent protection for this invention shall still be determined by the scope defined in the appended claims.

Claims

1. A method for evaluating the compressibility of shale reservoirs, characterized in that, The method includes: Step S1: Analyze the water content of shale reservoirs at different depths based on the capillary force function and drilling fluid intrusion model during reservoir operation. Based on the variation law of elastic modulus and Poisson's ratio with shale water content, determine the Young's modulus and Poisson's ratio of the shale reservoir after hydration at the corresponding depth. Step S2: Calculate the brittleness index of shale based on Young's modulus and Poisson's ratio after shale reservoir hydration; Step S3: Considering the mechanical principles of reservoir opening and extension to form complex fracture networks, obtain the horizontal differential stress coefficient of shale; Step S4: Analyze the fracture toughness type of the reservoir, and obtain the fracture toughness index that characterizes the ease of fracturing the shale reservoir based on the fracture toughness type. Step S5: Establish a reservoir compressibility calculation model based on the brittleness index, shale horizontal differential stress coefficient, and shale fracture toughness index, and solve the reservoir compressibility calculation model to determine the compressibility index of the shale reservoir at different depths; Step S6: Analyze the compressibility assessment results of the target shale reservoir within a set depth range based on the compressibility index; In step S1, the corresponding J0 function is obtained based on the shale reservoir characteristic curve, as shown in the following formula: The capillary force function is obtained from the J0 function, as shown in the following formula: In the formula, Represents the surface tension at the oil-water interface, in N·m. Let Pc(S) represent the oil-water interface contact angle in °, and let Pc(S) represent the capillary force as a function of saturation. The % represents the porosity of the shale reservoir, and S represents the degree of saturation. The value represents the fracture permeability. The subscript W indicates the aqueous phase, O indicates the oil phase, OW indicates the oil-water interface, and J0 indicates the capillary force. S w Indicates water saturation; A model of drilling fluid intrusion into shale reservoirs is analyzed by combining assumptions and constraints, and then the difference equation of the water-based drilling fluid intrusion model into shale reservoirs is determined. Based on this equation, the water content in the shale reservoir is analyzed. The assumptions include: the shale reservoir is a water-wet formation and isotropic; the fluid viscosity and volume coefficient remain constant during the drilling fluid intrusion process; gravity is not considered throughout the two-phase flow process, and oil and water are immiscible at contact. The constraints include: In the formula, A, B, and c are operational coefficients. p w Indicates the pressure in the water phase. p o Indicates oil phase pressure. K ro This represents the relative permeability of the oil phase in shale reservoirs. K rw Indicates the relative permeability of the water phase in shale reservoirs. S o Indicates oil saturation. S or and S wi These represent the residual oil saturation and the original water saturation of the reservoir, respectively. A double implicit difference model was used to model drilling fluid intrusion into shale reservoirs using staggered grids, resulting in the following difference equations for the water-based drilling fluid intrusion model into shale reservoirs: In the formula, i represents the i-th spatial node, and j represents the j-th time node. Represents radial distance, in meters (m). Permeability, expressed in mD; μ w The viscosity of the aqueous phase is expressed in mPa·s. p w Indicates the pressure in the aqueous phase, in Pa; K ro This represents the relative permeability of the oil phase in shale reservoirs, and is dimensionless. v o Indicates the oil phase flow rate, in m / s; μ o Indicates the oil phase viscosity, mPa·s; K o The water phase permeability of shale reservoirs is expressed in mD. S o The value represents the oil saturation, and Δt represents the time step increment. This represents the water saturation at node ij; This represents the oil saturation at node ij; Using the elastic modulus of shale as Young's modulus, the Young's modulus and Poisson's ratio of the shale reservoir after hydration are determined according to the following formula: In the formula, The elastic modulus of shale after water absorption, in MPa; and They are coefficients, respectively. and These represent the percentage of water adsorbed by shale and the percentage of water content in the original shale, respectively. The Poisson's ratio for shale.

2. The method according to claim 1, characterized in that, In step S1, the water content in the shale reservoir is determined according to the following formula. : In the formula, Density of drilling fluid, g / cm³ 3 ; The density of the shale skeleton, in g / cm³ 3 ; The volume of water contained in the rock, in cm. 3 ; The volume of the shale skeleton in the rock is expressed in cm³. 3 .

3. The method according to claim 1, characterized in that, In step S2, the shale brittleness index is obtained according to the following formula. : In the formula, The Young's modulus of the target layer after hydration is to be evaluated. The normalized Young's modulus of the target layer after hydration is given. and These are the minimum and maximum Young's moduli for the entire region, respectively. The Poisson's ratio after hydration of the target layer to be evaluated. The normalized Poisson's ratio after hydration of the target layer to be evaluated. and These are the minimum and maximum Poisson ratios for the entire region, respectively.

4. The method according to claim 1, characterized in that, In step S4, the shale reservoir is classified into Type I fracture toughness and Type II fracture toughness, and the fracture toughness index is determined accordingly as follows: In the formula, K Ⅰc Type I fracture toughness index; K ⅡC It is a type II fracture toughness index; The confining pressure on the crack surface is in MPa. denoted as uniaxial tensile strength of rock, MPa.

5. The method according to claim 1, characterized in that, In step S5, the following reservoir compressibility calculation model is established based on the brittleness index, the shale horizontal differential stress coefficient, and the shale fracture toughness index: ， In the formula, It is the compressibility index; b are operational coefficients; The brittleness index of shale; The horizontal differential stress coefficient is dimensionless. This represents the fracture toughness index of shale.

6. A storage medium, characterized in that, The storage medium stores program code capable of implementing the method as described in any one of claims 1 to 5.

7. A shale reservoir compressibility evaluation system, characterized in that, The system performs the method as described in any one of claims 1 to 5.

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