Shale reservoir compressibility evaluation method and system

Abstract

The invention provides a shale reservoir compressibility evaluation method and system, and the method comprises the steps: analyzing the water content of different depths of a shale reservoir according to a capillary force function in a reservoir operation process and a model of drilling fluid invading the shale reservoir, and determining the Young modulus and Poisson's ratio of the hydrated reservoir based on the water content; calculating a shale brittleness index according to the Young modulus and the Poisson's ratio; acquiring a horizontal difference stress coefficient of the shale; correspondingly acquiring a fracture toughness index based on the fracture toughness type; according to the brittleness index, the shale level difference stress coefficient and the shale fracture toughness index, establishing a reservoir compressibility operation model, solving the compressibility operation model to determine the compressibility index of the shale reservoir, and evaluating the compressibility of the target shale reservoir at different depths. By the adoption of the scheme, the problems that in the prior art, factors are not comprehensively considered, and accuracy is insufficient are solved, the influence of the hydration effect on the shale performance is introduced into compressibility calculation, and reservoir compressibility evaluation data better conforming to the real situation are obtained.

Description

Technical Field

[0001] The present invention relates to the technical field of unconventional oil exploration, development and evaluation, and in particular to a shale reservoir compressibility evaluation method and system. Background Art

[0002] Shale oil is self-generated and self-stored in shale layers rich in organic matter, which is significantly different from conventional oil and gas. In addition, the permeability of shale layers is extremely low. When developing shale oil, it is usually necessary to perform segmented fracturing to connect the pores and microcracks in the shale reservoir, so as to extract the crude oil in the shale reservoir. The ideal fracturing effect of shale reservoirs should satisfy the formation of a complex fracture network, while obtaining a larger reservoir transformation volume, and connecting 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 process, but also has an important relationship with the geological conditions of the reservoir. The relationship between the geological conditions of the reservoir and the fracturing effect is called the compressibility of the shale reservoir.

[0003] In the prior art, some researchers have studied the compressibility of shale reservoirs. For example, Rickma et al. proposed to use the normalized average value of Young's modulus and Poisson's ratio as a brittleness index to characterize the compressibility of rocks. Mike J. Mullen believed that the compressibility should comprehensively consider the reservoir sedimentary characteristics and mineral content composition. Sui discussed in depth the effects 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 of the shale reservoir, the rock density of the overlying formation of the shale reservoir, the maximum horizontal ground stress, and the minimum horizontal ground stress.

[0004] However, the existing methods only consider a single factor or the simple superposition of multiple factors, resulting in relatively poor accuracy of the final prediction results. At the same time, since shale formations usually contain more 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. The existing technology does not take this problem into consideration, 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 technology section of the present invention is only intended to deepen the understanding of the general background technology of the present invention, and should not be regarded as acknowledging or suggesting in any form that the information constitutes the prior art known to those skilled in the art. Summary of the invention

[0006] In order to solve the above problems, the present invention provides a method for evaluating the compressibility of shale reservoirs, which obtains the shale brittleness index through the Young's modulus and Poisson's ratio after hydration; obtains the horizontal differential stress coefficient of the shale; obtains the shale fracture toughness index; establishes a compressibility calculation model based on the brittleness index, the horizontal differential stress coefficient of the shale and the shale fracture toughness index, and solves the compressibility calculation model. The present invention takes into account the influence of hydration on shale performance and introduces this influence into the compressibility calculation, so that the final evaluation result is more in line with the actual formation conditions; in a preferred embodiment, the method includes:

[0007] Step S1: analyzing the water content at different depths in the shale reservoir according to the capillary force function during the reservoir operation and the model of drilling fluid invading the shale reservoir, and determining the Young's modulus and Poisson's ratio of the shale reservoir after hydration at the corresponding depth based on the variation law of the elastic modulus and Poisson's ratio with the shale water content;

[0008] Step S2: calculating the shale brittleness index according to the Young's modulus and Poisson's ratio of the hydrated shale reservoir;

[0009] Step S3: Considering the mechanical principle of reservoir opening and extension to form a complex fracture network, the horizontal differential stress coefficient of shale is obtained;

[0010] Step S4: analyzing the fracture toughness type of the reservoir, and obtaining a fracture toughness index characterizing the difficulty of fracturing the shale reservoir based on the fracture toughness type;

[0011] Step S5: establishing a reservoir compressibility calculation model according to the brittleness index, the shale horizontal differential stress coefficient and the shale fracture toughness index, and solving the reservoir compressibility calculation model to determine the compressibility index of the shale reservoir at different depths;

[0012] Step S6: Analyze the compressibility assessment results of the target shale reservoir in the set depth range according to the compressibility index.

[0013] In one embodiment, in step S1, the process of analyzing the water content in the shale reservoir includes:

[0014] The shale analysis combines the assumptions and constraints to analyze the model of drilling fluid invading shale reservoirs, and then determines the differential equation of the water-based drilling fluid invading shale reservoir model, based on which the water content in the shale reservoir is analyzed.

[0015] Preferably, in one embodiment, in step S1, the model of drilling fluid invading shale reservoir is subjected to double implicit difference using staggered grids to obtain the following differential equation of the model of water-based drilling fluid invading shale reservoir:

[0016]

[0017] Where r is the radial distance, m; K is the permeability, mD; μ w represents the viscosity of the water phase, mPa·s; p w Indicates water phase pressure, Pa; K ro represents the relative permeability of the oil phase in the shale oil reservoir, dimensionless; v o Indicates oil phase flow rate, m / s; μ o Indicates the viscosity of the oil phase, mPa·s; S w Indicates water saturation; K o represents the water phase permeability of shale oil reservoir, mD; S o In one embodiment, in step S1, the water content W(r,t) in the shale reservoir is determined according to the following formula:

[0018]

[0019] In the formula, ρ wf is the drilling fluid density, g / cm 3 ρ s is the shale skeleton density, g / cm 3 ; V w is the volume of water in the rock, cm 3 ; V s is the volume of shale skeleton in rock, cm 3 .

[0020] In an optional embodiment, the elastic modulus of shale is used as Young's modulus, and the Young's modulus and Poisson's ratio of the hydrated shale reservoir are determined according to the following formula:

[0021]

[0022]

[0023] Where, E is the elastic modulus of shale after water absorption, MPa; E 1 and E 2 are coefficients; W and W a - are the mass percentage of shale adsorbed water and the mass percentage of shale original water content; ν is the Poisson's ratio of shale.

[0024] Preferably, in one embodiment, in step S2, the shale brittleness index B is obtained according to the following formula: rit :

[0025]

[0026] In the formula, E i is the Young's modulus of the target layer after hydration, E min and E maxare the minimum and maximum Young's modulus of the entire region, ν i is the Poisson's ratio of the target layer to be evaluated after hydration, ν min and ν max are the minimum and maximum Poisson's ratios of the entire area, respectively.

[0027] In an optional embodiment, in step S4, the shale reservoir is divided into type I fracture toughness and type II fracture toughness, and the fracture toughness index is determined according to the following specificity:

[0028]

[0029] K IIC =0.0956σ n +0.1383S t -0.0820

[0030] In the formula, K Ⅰc is the type I fracture toughness index; K ⅡC is the type II fracture toughness index; σ n is the confining pressure on the crack surface, MPa; S t is the uniaxial tensile strength of rock, MPa.

[0031] Furthermore, in one embodiment, 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:

[0032]

[0033] In the formula, FI is the compressibility index; a and b are operation coefficients; B rit is the shale brittleness index; K h is the horizontal stress coefficient, dimensionless; K n is the shale fracture toughness index.

[0034] Based on other aspects of the method described in any one or more of the above embodiments, the present invention further provides a storage medium storing program codes that can implement the method described in any one or more of the above embodiments.

[0035] Based on the application aspects of the method described in any one or more of the above embodiments, the present invention also provides a shale reservoir compressibility evaluation system, which executes the method described in any one or more of the above embodiments.

[0036] Compared with the closest prior art, the present invention also has the following beneficial effects:

[0037] The present invention provides a shale reservoir compressibility evaluation method and system. The method analyzes the water content of the shale reservoir at different depths according to the capillary force function in the reservoir operation process and the model of drilling fluid invading the shale reservoir, and determines the Young's modulus and Poisson's ratio of the hydrated shale reservoir based on the water content; calculates the shale brittleness index according to the Young's modulus and Poisson's ratio of the hydrated shale reservoir; obtains the horizontal differential stress coefficient of the shale; obtains the fracture toughness index based on the fracture toughness type; and then establishes a reservoir compressibility calculation model according to 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, so as to evaluate the compressibility of the target shale reservoir at different depths. This scheme is used to introduce the influence of hydration on shale performance into the compressibility calculation. At the same time, the mechanical principle of reservoir opening and extension to form a complex fracture network is considered in combination with the horizontal differential stress coefficient. In addition, the fracture toughness index that characterizes the difficulty of shale reservoir fracturing is determined. The reservoir compressibility index is comprehensively calculated as the basis for evaluation, which can effectively obtain reservoir compressibility evaluation data that is more in line with the actual situation, and provide reliable data support for the fracturing of actual shale reservoirs.

[0038] Other features and advantages of the present invention will be described in the following description, and partly become apparent from the description, or understood by practicing the present invention. The purpose and other advantages of the present invention can be realized and obtained by the structures particularly pointed out in the description, claims and drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] The accompanying drawings are used to provide a further understanding of the present invention and constitute a part of the specification. Together with the embodiments of the present invention, they are used to explain the present invention and do not constitute a limitation of the present invention. In the accompanying drawings:

[0040] Figure 1 It is a schematic flow chart of a shale reservoir compressibility evaluation method provided by one embodiment of the present invention;

[0041] Figure 2 This is an example diagram of well logging data of the oil shale formation in Da'anzhai section of central Sichuan in the shale reservoir compressibility evaluation method provided in the embodiment of the present invention;

[0042] Figure 3 is an example diagram of changes in shale water absorption with dimensionless distance and time in a shale reservoir compressibility evaluation method provided in another embodiment of the present invention;

[0043] Figure 4 This is an example diagram of changes in the compressibility index of the shale reservoir compressibility evaluation method provided by the embodiment of the present invention with depth, time and dimensionless distance from the wellbore axis;

[0044] Figure 5It is a structural schematic diagram of a shale reservoir compressibility evaluation system provided in one embodiment of the present invention. DETAILED DESCRIPTION

[0045] The following will describe the implementation methods of the present invention in detail in conjunction with the accompanying drawings and embodiments, so that the implementers of the present invention can fully understand how the present invention applies technical means to solve technical problems and achieve the implementation process of technical effects and implement the present invention specifically according to the above implementation process. It should be noted that as long as there is no conflict, the various embodiments and various features of the embodiments in the present invention can be combined with each other, and the technical solutions formed are all within the protection scope of the present invention.

[0046] Although the flowcharts describe the operations as sequential processes, many of the operations may be performed in parallel, concurrently, or simultaneously. The order of the operations may be rearranged. A process may be terminated when its operations are completed, but may also have additional steps not included in the accompanying drawings. A process may correspond to a method, function, procedure, subroutine, subprogram, etc.

[0047] Computer devices include user devices and network devices. Among them, user devices or clients include but are not limited to computers, smart phones, PDAs, etc.; network devices include but are not limited to a single network server, a server group consisting of multiple network servers, or a cloud composed of a large number of computers or network servers based on cloud computing. Computer devices can be operated alone to implement the present invention, or they can be connected to the network and implement the present invention through interactive operations with other computer devices in the network. The network where the computer device is located includes but is not limited to the Internet, wide area network, metropolitan area network, local area network, VPN network, etc.

[0048] The terms "first", "second", etc. may be used herein to describe various units, but these units should not be limited by these terms, and these terms are used only to distinguish one unit from another unit. The term "and / or" used herein includes any and all combinations of one or more of the listed associated items. When a unit is referred to as being "connected" or "coupled" to another unit, it can be directly connected or coupled to the other unit, or there can be intermediate units.

[0049] The terms used herein are only for describing specific embodiments and are not intended to limit exemplary embodiments. Unless the context clearly indicates otherwise, the singular forms "one", "one" and "item" used herein are also intended to include plural numbers. It should also be understood that the terms "include" and / or "comprise" used herein specify the existence of stated features, integers, steps, operations, units and / or components, without excluding the existence or addition of one or more other features, integers, steps, operations, units, components and / or combinations thereof.

[0050] Shale oil is self-generated and self-stored in shale layers rich in organic matter, which is significantly different from conventional oil and gas. At the same time, the permeability of shale layers is extremely low. When developing shale oil, it is usually necessary to perform segmented fracturing to connect the pores and micro-cracks in the shale reservoir, so as to extract the crude oil in the shale reservoir.

[0051] The ideal fracturing effect of shale reservoirs should satisfy the requirements of forming a complex fracture network, obtaining a larger reservoir transformation volume, and communicating with 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 process, but also has an important relationship with the reservoir geological conditions. The relationship between reservoir geological conditions and fracturing effect is called the compressibility of shale reservoirs.

[0052] In the prior art, some researchers have studied the compressibility of shale reservoirs. For example, Rickma et al. proposed to use the normalized average value of Young's modulus and Poisson's ratio as a brittleness index to characterize the compressibility of rocks. Mike J. Mullen believed that the compressibility should comprehensively consider the reservoir sedimentary characteristics and mineral content composition. Sui discussed in depth the effects 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 of the shale reservoir, the rock density of the overlying formation of the shale reservoir, the maximum horizontal ground stress, and the minimum horizontal ground stress.

[0053] However, the 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. At the same time, since shale formations usually contain more clay minerals, when water-based fracturing fluid is used for fracturing, the clay will absorb water and hydrate, and the mechanical properties of the hydrated shale will change. At the same time, creep will occur, causing changes in the shale reservoir's crevices. The existing technology does not take this problem into account, resulting in errors between the calculated data and the actual situation.

[0054] In view of the defects of the prior art, the present invention provides a method for evaluating the compressibility of shale reservoirs, which evaluates the compressibility of shale reservoirs while taking shale hydration into consideration, and is closer to the actual situation and has more accurate evaluation results.

[0055] The method provided by the present invention comprises the following steps: obtaining the Young's modulus and Poisson's ratio after hydration; obtaining the shale brittleness index through the Young's modulus and Poisson's ratio after hydration; obtaining the horizontal differential stress coefficient of the shale; obtaining the shale fracture toughness index; establishing a compressibility calculation model according to the brittleness index, the horizontal differential stress coefficient of the shale and the shale fracture toughness index, and solving the compressibility calculation model. The present invention takes into account the influence of hydration on shale performance and introduces the influence into the compressibility calculation, so that the final evaluation result is more in line with the actual formation conditions, more accurate, and can provide reliable data support for the fracturing of actual shale reservoirs.

[0056] Next, the detailed process of the method of the embodiment of the present invention is described in detail based on the accompanying drawings. The steps shown in the flowchart of the accompanying drawings can be executed in a computer system including a set of computer executable instructions. Although the logical order of each step is shown in the flowchart, in some cases, the steps shown or described can be executed in a different order than here.

[0057] Embodiment 1

[0058] Figure 1 The schematic diagram of the flow chart of the shale reservoir compressibility evaluation method provided in the first embodiment of the present invention is shown. Figure 1 It can be seen that the method includes the following steps.

[0059] Young's modulus and Poisson's ratio analysis step S1: analyzing the water content of the shale reservoir at different depths according to the capillary force function during the reservoir operation and the model of drilling fluid invading the shale reservoir, and determining the Young's modulus and Poisson's ratio of the shale reservoir after hydration at the corresponding depth based on the change law of the elastic modulus and Poisson's ratio with the shale water content;

[0060] Brittleness index determination step S2: calculating the shale brittleness index according to Young's modulus and Poisson's ratio after hydration of the shale reservoir;

[0061] Horizontal differential stress analysis step S3: considering the mechanical principle of complex fracture network formed by reservoir opening and extension, the horizontal differential stress coefficient of shale is obtained;

[0062] Fracture toughness analysis step S4: analyzing the fracture toughness type of the reservoir, and obtaining a fracture toughness index characterizing the difficulty of fracturing the shale reservoir based on the fracture toughness type;

[0063] Compressibility model establishment step S5: establishing a compressibility calculation model according to the brittleness index, the shale horizontal differential stress coefficient and the shale fracture toughness index, and solving the compressibility calculation model to determine the compressibility index of the shale reservoir at different depths;

[0064] Evaluation implementation step S6: analyzing the compressibility evaluation results of the target shale reservoir in the set depth range according to the compressibility index.

[0065] By using the operation logic in the above embodiment of the present invention, the compressibility of the shale reservoir is evaluated under the condition of considering shale hydration, which is closer to the actual situation and the evaluation result is more accurate. In the preferred embodiment, the researchers of the present invention consider that during the drilling process of the shale reservoir, the pressure difference and the capillary force will jointly affect the situation of water entering the reservoir; when water enters the reservoir, it will change the water saturation of the shale.

[0066] Based on this, different characteristic curves will be obtained for different shale reservoirs, thus obtaining different J 0 Function, as shown in Equation 1:

[0067]

[0068] In the formula, σ ow represents the surface tension of the oil-water interface, N·m, θ OW represents the contact angle of the oil-water interface, °, Pc(S) represents the capillary force as a function of saturation, φ represents the porosity of the shale oil reservoir, %, S represents the saturation, K f is the fracture permeability, subscript W represents the water phase, subscript O represents the oil phase, subscript OW represents the oil-water contact surface, and J 0 It represents the capillary force, dimensionless. The capillary force is a force that is generated by the liquid in the capillary or small pores, which is an attractive force. Its magnitude is usually expressed by J 0 Parameters to describe, J 0 It is a dimensionless parameter derived from the Young-Laplace equation that can be used to represent the magnitude of capillary force; among them, saturation can be expressed by two methods: water content and saturation content, Sw+So=1.

[0069] Furthermore, according to J 0 The capillary force function can be obtained by the function, as shown in the following formula 2:

[0070]

[0071] In actual application, the following assumptions are established during the calculation process:

[0072] (1) Shale oil reservoirs are water-wet and isotropic;

[0073] (2) During the process of drilling fluid invading the shale oil layer, the fluid viscosity and volume coefficient remain unchanged;

[0074] (3) During the entire two-phase flow process, gravity is not considered, and oil and water are immiscible when in contact.

[0075] Based on the above assumptions, the Darcy law of oil-water two-phase flow can be obtained as follows:

[0076]

[0077] In the formula, v w Indicates the water phase flow rate, m / s; K rw Represents the relative permeability of water phase in shale oil reservoir; μ w represents the viscosity of the water phase, mPa·s; p w represents the water phase pressure, Pa; r represents the radial distance; K ro represents the relative permeability of the oil phase in the shale oil reservoir, dimensionless; v o Indicates oil phase flow rate, m / s; μ o Indicates oil phase viscosity, mPa·s; p o Indicates oil phase pressure, Pa.

[0078] During the process of drilling fluid invading the reservoir, the oil-water two-phase continuity equation is shown in Equation 4:

[0079]

[0080] In the formula, represents the Laplace operator; K w represents the water phase permeability of shale oil reservoir, mD; S w Indicates water saturation; K o represents the water phase permeability of shale oil reservoir, mD; S o represents oil saturation; t represents the oil-water two-phase flow time, s.

[0081] Considering the volume compressibility coefficients of the oil phase and the water phase, the model of drilling fluid invading the shale reservoir can be obtained by combining Equation 3 and Equation 4, as shown in Equation 5 below:

[0082]

[0083] Where r is the radial distance, m; K is the permeability, mD; B is the liquid volume coefficient in shale, B O is the volume coefficient of the oil phase in shale, B W is the volume coefficient of the water phase; μ w represents the viscosity of the water phase, mPa·s; p w Indicates water phase pressure, Pa; K ro represents the relative permeability of the oil phase in the shale oil reservoir, dimensionless; v o Indicates oil phase flow rate, m / s; μ o Indicates the viscosity of the oil phase, mPa·s.

[0084] At the same time, the following constraints exist:

[0085] p c (S w )=pw -p o (6)

[0086] S w +S o =1 (7)

[0087]

[0088] In the above formula, A, B, and c are operation coefficients, which are all constants and are usually obtained by experiments based on the characteristics of the reservoir. In this embodiment, the empirical values ​​are: A = 0.9, B = 0.8, c = 4; S or and S wi are the residual oil saturation and initial water saturation of the reservoir, respectively.

[0089] For the above model, its initial conditions and boundary conditions are as follows:

[0090] Initial time: t = 0, r w <r<r e ,S w =S wi ;

[0091] Boundary conditions in shale reservoir:

[0092] External boundary conditions of shale reservoir:

[0093] The above formulas 5 to 8 together constitute a model of drilling fluid intrusion into shale reservoirs, which can be solved in combination with boundary conditions.

[0094] In the formula, r w represents the wellbore radius, m; r e represents the radial distance corresponding to the outer boundary of the reservoir, m; p b Indicates the liquid column pressure at the well wall, Pa; p e Represents the formation pressure at the outer boundary of the reservoir, which is equal to the original formation pressure, Pa.

[0095] In this embodiment, the above model is subjected to double implicit difference using staggered grids, and finally the differential equation of the water-based drilling fluid invasion shale reservoir model shown in the following equation 9 is obtained:

[0096]

[0097] In the formula, i represents the i-th spatial node, 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, representing space and time respectively. i and j are the corresponding xy coordinates, which are used to characterize the relative positions of the parameters. △t represents the time step increment, s. represents the water saturation at the ij node; Represents the oil saturation at the ij node.

[0098] According to the mass conservation theory, combined with the above models and equations, the water content formula in the shale reservoir is obtained, as shown in Formula 10:

[0099]

[0100] Where W represents the water content; ρ wf is the drilling fluid density, g / cm 3 ρ s is the shale skeleton density, g / cm 3 ; V w is the volume of water in the rock, cm 3 ; V s is the volume of shale skeleton in rock, cm 3 .

[0101] Then, by combining the existing elastic modulus and Poisson's ratio with the water content of shale, 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 refer to the Young's modulus. The final results are shown in the following equations 11 and 12.

[0102]

[0103]

[0104] Where, E is the elastic modulus of shale after water absorption, MPa; E 1 and E 2 are coefficients, and their empirical values ​​are 4×10 4 and -11; W and W a - are the mass percentage of shale adsorbed water and the mass percentage of shale original water content; ν is the Poisson's ratio of shale.

[0105] In summary, through equations 10, 11, and 12, the Young's modulus and Poisson's ratio after water absorption and hydration can be obtained.

[0106] Further, step S2 is performed to calculate the shale brittleness index according to the Young's modulus and Poisson's ratio of the shale reservoir after hydration; the shale brittleness index B is obtained through the Young's modulus and Poisson's ratio after hydration. rit , in an optional embodiment, can be obtained by the formula shown in the following formula 13:

[0107]

[0108] In the formula, E A is the Young's modulus of the target layer after hydration, E min and E maxare the minimum and maximum Young’s modulus of the entire region, v A is the Poisson's ratio of the target layer to be evaluated after hydration, ν min and ν max are the minimum and maximum Poisson's ratios of the entire area, respectively.

[0109] Next, step S3 is performed: considering the mechanical principle of reservoir opening and extension to form a complex fracture network, the horizontal differential stress coefficient of shale is obtained.

[0110] For the generation of cracks, the net pressure in the cracks needs to overcome the horizontal stress difference in addition to the tensile strength of the rock body, so that the cracks can open and extend to form a complex fracture network. Therefore, the horizontal stress difference coefficient needs to be calculated. The horizontal stress difference coefficient of shale can be obtained by the formula shown in the following formula 14:

[0111]

[0112] In the formula, K h is the horizontal stress coefficient, dimensionless; σ H is the maximum horizontal principal stress, MPa; σ h is the minimum horizontal principal stress, MPa.

[0113] Further, step S4 is performed: analyzing the fracture toughness type of the reservoir, and obtaining a fracture toughness index characterizing the difficulty of fracturing the shale reservoir based on the fracture toughness type.

[0114] For some rocks, even if their Poisson's ratio and Young's modulus are similar, their brittleness is very different. The important factor causing this effect is the fracture toughness of the rock. Therefore, fracture toughness is an important factor in characterizing the difficulty of reservoir fracturing. For this reason, the fracture toughness of the rock also needs to be considered in the present invention;

[0115] As for fracture toughness, it is divided into type I fracture toughness K Ⅰc and mode II fracture toughness K ⅡC , which calculates the shale fracture toughness index K n The formulas are shown in Equation 15 and Equation 16:

[0116]

[0117] K IIC =0.0956σ n +0.1383S t -0.0820 (16)

[0118] In the formula, σ n is the confining pressure on the crack surface, MPa; S t is the uniaxial tensile strength of rock, MPa.

[0119] Among them, the confining pressure on the fracture surface during shale fracturing is shown in formula 17:

[0120]

[0121] Where n = [cosβsinθ, sinβcosθ, sinβ], which represents the unit normal vector of the three-dimensional fracture surface; β is the fracture inclination; θ is the angle between the fracture strike and the well axis; σ v is the overburden pressure, MPa; σ H is the maximum horizontal ground stress, MPa; σ h is the horizontal minimum geostress, MPa, and T represents the transposed matrix.

[0122] The uniaxial tensile strength of rock can be obtained through logging data, and the calculation method is shown in formula 18:

[0123]

[0124] Where V cl is the mud content, dimensionless; σ c Represents the uniaxial compressive strength of rock; S t represents the uniaxial tensile strength of rock; C is the operation coefficient, which is a constant. In practical application, C can be set to 12.26. In order to comprehensively consider the two different fracture toughness, the fracture toughness index is obtained through formula 19:

[0125]

[0126] In the formula, K ICmax is the maximum mode I fracture toughness index of the entire region, K ICmin is the minimum mode I fracture toughness index of the entire region, K ICn is the normalized index of mode I fracture toughness index, K IICmax is the maximum mode II fracture toughness index of the entire region, K IICmin is the minimum mode II fracture toughness index of the entire region, K IICn is the normalized index of the type II fracture toughness index, K n is the shale fracture toughness index.

[0127] The compressibility calculation model described below is established:

[0128]

[0129] In the formula, FI is the compressibility index, a and b are coefficients, and the empirical value is 0.5.

[0130] Solve the compressibility calculation model; the larger the FI, the better the compressibility of the target layer to be evaluated, and the smaller the FI, the worse its compressibility. In practical application, the compressibility index of each depth within the set depth range can be compared to accurately recommend the layer with a higher compressibility index for fracturing.

[0131] The present invention is further described below in conjunction with examples of implementation. The scope of the present invention is not limited by the examples, but is set forth in the claims.

[0132] The oil shale formation in the Da'anzhai section of central Sichuan has the conditions for lacustrine shale oil and gas accumulation. Its high-quality shale is mainly developed in the second subsection, with favorable conditions such as high organic carbon content, good physical properties, stable distribution, and good oil and gas content. Taking NC2H as an example, it is the first shale oil horizontal well in the Sichuan Basin. The depth of the Da'anzhai section is 2598-2703m. Its specific logging data are as follows: Figure 2 As shown, Figure 2 , shows an example of the well logging data analyzed computationally, including its Young's modulus, tensile strength, shale content, Poisson's ratio, and geostress coefficient.

[0133] Combined with the analysis of logging data, it can be found that the Young's modulus of the Da'anzhai section of Well NC2H is within 18.23-23.70GPa, the tensile strength is 6.27-8.18MPa, the shale content is 0.39-0.65, the Poisson's ratio is 0.18-0.24, the maximum horizontal principal stress σH coefficient is 2.51-3.32, and the minimum horizontal principal stress σh is 2.20-2.41. At the same time, some relevant data values ​​are shown in Table 1 below.

[0134] Table 1 Part of the calculation data

[0135]

[0136] According to the data in Table 1, the variation law of shale water saturation with dimensionless distance and time can be calculated. The calculation results are as follows: Figure 3 Then combined Figure 3 The calculation results of Figure 2 The logging data in the embodiment are used to calculate the variation law of the compressibility index of the Da'anzhai section with depth, time and dimensionless distance from the wellbore axis. The final calculation result is as follows: Figure 4 shown.

[0137] Since the basic parameters change with depth, in actual application, first substitute the data in Table 1 for a certain depth point into the drilling fluid invasion formation model (Formula 1-10) to calculate the change law of water saturation. Because the depth of each point is different, the water saturation / water content distribution calculated at each depth point is also different.

[0138] Then, the obtained moisture content data is substituted into formulas 11 and 12 to calculate the change of the elastic modulus Poisson's ratio, the brittleness index of the corresponding depth can be calculated, and the fracture toughness can be calculated using formulas 15-19.

[0139] Furthermore, the brittleness index and fracture toughness at each depth of 2598-2703 m were calculated in the same way. The brittleness index, fracture toughness and ground stress parameters at each depth were normalized, and the compressibility index was calculated using the compressibility calculation model (Formula 20) using the three normalized indices.

[0140] Finally, by comparing the compressibility index at each depth of 2598-2703m, it is recommended to perform fracturing in the layers with higher compressibility index.

[0141] The shale reservoir compressibility evaluation method provided in the embodiment of the present invention takes into account the influence of hydration on shale performance and introduces this influence into the compressibility calculation model, so that the final evaluation result is more consistent with the actual formation conditions and more accurate, and can provide reliable data support for the fracturing of actual shale reservoirs.

[0142] For the above-mentioned method embodiments, for the sake of simplicity, they are all described as a series of action combinations, but those skilled in the art should know that the present invention is not limited by the order of the actions described, because according to the present invention, some steps can be performed in other orders or simultaneously. Secondly, those skilled in the art should also know that the embodiments described in the specification are all preferred embodiments, and the actions and modules involved are not necessarily required by the present invention.

[0143] It should be pointed out that in other embodiments of the present invention, the method can also obtain a new shale reservoir compressibility evaluation method by combining one or several of the above embodiments to achieve refined analysis and optimized application of the shale reservoir compressibility.

[0144] It should be noted that, based on the method in any one or more of the above embodiments of the present invention, the present invention also provides a storage medium, on which is stored a program code that can implement the method as described in any one or more of the above embodiments, and when the code is executed by the operating system, the shale reservoir compressibility evaluation method as described above can be implemented.

[0145] Those skilled in the art can understand that the storage medium includes a readable storage medium, which is a storage device commonly used in the art, including a large-capacity memory, a removable memory, etc., and exemplary large-capacity memories include magnetic disks, optical disks, hard disks, etc., and exemplary removable memories include floppy disks, memory cards, etc. Of course, the above examples are only illustrations of readable storage media and are not limitations, and those skilled in the art can use other storage devices that can be applied to this embodiment.

[0146] It should be noted that, based on the method in any one or more of the above embodiments of the present invention, the present invention also provides a device, which includes: a processor, an information acquisition module, a memory, and an output module. The information acquisition module is used to obtain initial data required for shale reservoir compressibility evaluation; the memory stores a program for shale reservoir compressibility evaluation that can be run on the processor, and the program for shale reservoir compressibility evaluation implements the method in any one or more of the above embodiments when executed by the processor; the output module is used to output the evaluation result.

[0147] Embodiment 2:

[0148] The method is described in detail in the embodiments disclosed in the present invention. The method of the present invention can be implemented by various devices or systems. Therefore, based on other aspects of the method described in any one or more embodiments, the present invention also provides a shale reservoir compressibility evaluation system, which is used to execute the shale reservoir compressibility evaluation method described in any one or more embodiments. Specific embodiments are given below for detailed description.

[0149] Specifically, Figure 5 FIG. 4 is a schematic diagram showing the structure of a shale reservoir compressibility evaluation system provided in an embodiment of the present invention. Figure 5 As shown, the system includes:

[0150] A Young's modulus and Poisson's ratio analysis module, which is configured to analyze the water content of the shale reservoir at different depths according to the capillary force function during the reservoir operation and the model of drilling fluid invading the shale reservoir, and determine the Young's modulus and Poisson's ratio of the shale reservoir after hydration at the corresponding depth based on the change law of the elastic modulus and Poisson's ratio with the shale water content;

[0151] a brittleness index determination module configured to calculate a shale brittleness index based on Young's modulus and Poisson's ratio of the hydrated shale reservoir;

[0152] The horizontal differential stress analysis module is configured to consider the mechanical principle of complex fracture networks formed by reservoir opening and extension, and obtain the horizontal differential stress coefficient of shale;

[0153] A fracture toughness analysis module configured to analyze the fracture toughness type of the reservoir and obtain a fracture toughness index characterizing the difficulty of fracturing the shale reservoir based on the fracture toughness type;

[0154] A compressibility model building module is configured to build a compressibility calculation model based on a brittleness index, a shale horizontal differential stress coefficient, and a shale fracture toughness index, and solve the compressibility calculation model to determine the compressibility index of the shale reservoir at different depths;

[0155] An evaluation implementation module is configured to analyze the compressibility evaluation results of the target shale reservoir in a set depth interval according to the compressibility index.

[0156] Preferably, in one embodiment, the Young's modulus and Poisson's ratio analysis module analyzes the water content in the shale reservoir according to the following operations:

[0157] The shale analysis combines the assumptions and constraints to analyze the model of drilling fluid invading shale reservoirs, and then determines the differential equation of the water-based drilling fluid invading shale reservoir model, based on which the water content in the shale reservoir is analyzed.

[0158] 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 invading shale reservoir using staggered grids to obtain the following differential equation of the water-based drilling fluid invading shale reservoir model:

[0159]

[0160] Where r is the radial distance, m; K is the permeability, mD; μ w represents the viscosity of the water phase, mPa·s; p w Indicates water phase pressure, Pa; K ro represents the relative permeability of the oil phase in the shale oil reservoir, dimensionless; v o Indicates oil phase flow rate, m / s; μ o Indicates the viscosity of the oil phase, mPa·s; S w Indicates water saturation; K o represents the water phase permeability of shale oil reservoir, mD; S o Indicates oil saturation.

[0161] In an optional embodiment, the Young's modulus and Poisson's ratio analysis module is configured to determine the water content W(r, t) in the shale reservoir according to the following formula:

[0162]

[0163] In the formula, ρ wf is the drilling fluid density, g / cm 3 ρ sis the shale skeleton density, g / cm 3 ; V w is the volume of water in the rock, cm 3 ; V s is the volume of shale skeleton in rock, cm 3 .

[0164] 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 hydrated shale reservoir according to the following formula:

[0165]

[0166]

[0167] Where, E is the elastic modulus of shale after water absorption, MPa; E 1 and E 2 are coefficients, and their empirical values ​​are 4×10 4 and -11; W and W a - are the mass percentage of shale adsorbed water and the mass percentage of shale original water content; ν is the Poisson's ratio of shale.

[0168] In a preferred embodiment, the brittleness index determination module is configured to obtain the shale brittleness index B according to the following formula: rit :

[0169]

[0170] In the formula, E i is the Young's modulus of the target layer after hydration, E min and E max are the minimum and maximum Young's modulus of the entire region, ν i is the Poisson's ratio of the target layer to be evaluated after hydration, ν min and ν max are the minimum and maximum Poisson's ratios of the entire area, respectively.

[0171] In practical application, in a preferred embodiment, the fracture toughness analysis module is configured to divide the shale reservoir into type I fracture toughness and type II fracture toughness, and determine the fracture toughness index according to the following specificity:

[0172] K IC =0.2176σ n +0.0059S t 3 +0.0923S t 2 +0.517S t -0.3322

[0173] K IIC =0.0956σ n +0.1383S t -0.0820

[0174] In the formula, K Ⅰc is the type I fracture toughness index; K ⅡC is the type II fracture toughness index; σ n is the confining pressure on the crack surface, MPa; S t is the uniaxial tensile strength of rock, MPa.

[0175] Furthermore, in one embodiment, the compressibility model building module builds the following reservoir compressibility calculation model based on the brittleness index, the shale horizontal differential stress coefficient and the shale fracture toughness index:

[0176]

[0177] In the formula, FI is the compressibility index; a and b are operation coefficients; B rit is the shale brittleness index; K h is the horizontal stress coefficient, dimensionless; K n is the shale fracture toughness index.

[0178] In the shale reservoir compressibility evaluation system provided by the embodiment of the present invention, each module or unit structure can be operated independently or in combination according to the actual parameter analysis requirements and model calculation requirements to achieve corresponding technical effects.

[0179] It should be understood that the embodiments disclosed in the present invention are not limited to the specific structures, processing steps or materials disclosed herein, but should be extended to equivalent substitutions of these features understood by ordinary technicians in the relevant field. It should also be understood that the terms used herein are only used for the purpose of describing specific embodiments and are not meant to be limiting.

[0180] The "one embodiment" mentioned in the specification means that a particular feature, structure or characteristic described in conjunction with the embodiment is included in at least one embodiment of the present invention. Therefore, the phrase "one embodiment" appearing in various places throughout the specification does not necessarily refer to the same embodiment.

[0181] Although the embodiments disclosed in the present invention are as above, the contents described are only embodiments adopted for facilitating the understanding of the present invention and are not intended to limit the present invention. Any technician in the technical field to which the present invention belongs can make any modifications and changes in the form and details of the implementation without departing from the spirit and scope disclosed in the present invention, but the patent protection scope of the present invention shall still be subject to the scope defined in the attached claims.

Claims

1. A method for evaluating the compressibility of a shale reservoir, characterized in that: The method comprises: Step S1: analyzing the water content of the shale reservoir at different depths according to the capillary force function during the reservoir operation and the model of drilling fluid invading the shale reservoir, and determining the Young's modulus and Poisson's ratio of the shale reservoir after hydration at the corresponding depth based on the variation law of the elastic modulus and Poisson's ratio with the shale water content; Step S2: calculating the shale brittleness index according to the Young's modulus and Poisson's ratio of the hydrated shale reservoir; Step S3: Considering the mechanical principle of reservoir opening and extension to form a complex fracture network, the horizontal differential stress coefficient of shale is obtained; Step S4: analyzing the fracture toughness type of the reservoir, and obtaining a fracture toughness index characterizing the difficulty of fracturing the shale reservoir based on the fracture toughness type; Step S5: establishing a reservoir compressibility calculation model according to the brittleness index, the shale horizontal differential stress coefficient and the shale fracture toughness index, and solving 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 in the set depth range according to the compressibility index.

2. The method according to claim 1, characterized in that: In step S1, the process of analyzing the water content in the shale reservoir includes: The shale analysis combines the assumptions and constraints to analyze the model of drilling fluid invading shale reservoirs, and then determines the differential equation of the water-based drilling fluid invading shale reservoir model, based on which the water content in the shale reservoir is analyzed.

3. The method according to claim 2, characterized in that In step S1, a double implicit difference is performed on the model of drilling fluid invading shale reservoir using staggered grids to obtain the following differential equation of the model of water-based drilling fluid invading shale reservoir: Where r is the radial distance, m; K is the permeability, mD; μ w represents the viscosity of the water phase, mPa·s; p w Indicates water phase pressure, Pa; K ro represents the relative permeability of the oil phase in the shale oil reservoir, dimensionless; v o Indicates oil phase flow rate, m / s; μ o Indicates the viscosity of the oil phase, mPa·s; S w Indicates water saturation; K o represents the water phase permeability of shale oil reservoir, mD; S o Indicates oil saturation.

4. The method according to claim 2, characterized in that: In step S1, the water content W(r,t) in the shale reservoir is determined according to the following formula: In the formula, ρ wf is the drilling fluid density, g / cm 3 ρ s is the shale skeleton density, g / cm 3 ; V w is the volume of water in the rock, cm 3 ; V s is the volume of shale skeleton in rock, cm 3 .

5. The method according to claim 1, characterized in that Taking the elastic modulus of shale as Young's modulus, the Young's modulus and Poisson's ratio of the hydrated shale reservoir are determined according to the following formula: Where, E is the elastic modulus of shale after water absorption, MPa; E1 and E2 are coefficients; W and W are a - are the mass percentage of shale adsorbed water and the mass percentage of shale original water content; ν is the Poisson's ratio of shale.

6. The method according to claim 1, characterized in that In step S2, the shale brittleness index B is obtained according to the following formula: rit : In the formula, E i is the Young's modulus of the target layer after hydration, E min and E max are the minimum and maximum Young's modulus of the entire region, ν i is the Poisson's ratio of the target layer to be evaluated after hydration, ν min and ν max are the minimum and maximum Poisson's ratios of the entire area, respectively.

7. The method according to claim 1, characterized in that In step S4, the shale reservoir is divided into type I fracture toughness and type II fracture toughness, and the fracture toughness index is determined according to the following pertinence: K IIC =0.0956σ n +0.1383S t -0.0820 In the formula, K Ⅰc is the type I fracture toughness index; K ⅡC is the type II fracture toughness index; σ n is the confining pressure on the crack surface, MPa; S t is the uniaxial tensile strength of rock, MPa.

8. The method according to claim 1, characterized in that: In step S5, the following reservoir compressibility calculation model is established according to the brittleness index, the shale horizontal differential stress coefficient and the shale fracture toughness index: In the formula, FI is the compressibility index; a and b are operation coefficients; B rit is the shale brittleness index; K h is the horizontal stress coefficient, dimensionless; K n is the shale fracture toughness index.

9. A storage medium, characterized in that: The storage medium stores program codes that can implement the method as claimed in any one of claims 1 to 8.

10. A shale reservoir compressibility evaluation system, characterized in that: The system executes the method according to any one of claims 1 to 8.

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