A method for rapidly estimating the rock mechanical parameters of laminated shale

Abstract

This invention discloses a method for rapidly estimating the mechanical parameters of laminated shale, relating to the field of petroleum exploration and development technology. Its key features include: selecting rocks with similar lithology and weak-plane structure distribution characteristics to the predicted target lithology and based on the required predicted lithological and microstructural features of the laminated shale; obtaining the compressive strength and elastic modulus of various types of uniaxially laminated and bedding shale through rock mechanics experiments; and deriving a method for calculating the compressive strength and elastic modulus of laminated shale based on McLamore's shale strength criterion under uniaxial conditions and the rock equivalent deformation assumption. The compressive strength and elastic modulus of laminated shale are then rapidly calculated based on the mechanical parameters of various types of uniaxially laminated and bedding shale obtained from mechanical experiments. This invention utilizes easily drillable rocks and achieves rapid prediction of the mechanical parameters of laminated shale through a small number of experiments, providing important data support for the exploration and development design of oil and gas reservoirs in this type of formation.

Description

Technical Field

[0001] This invention belongs to the field of petroleum exploration and development technology, specifically a method for rapidly estimating the rock mechanical parameters of layered shale. Background Technology

[0002] With the rapid development of the national economy, my country's dependence on foreign oil has been continuously increasing. Finding substitutes for conventional oil is crucial for improving people's lives, ensuring long-term social stability, and promoting national prosperity. my country is rich in shale oil resources, widely distributed in continental basins and frequently affected by orogenic and faulting activities, exhibiting characteristics of "multi-stage, multi-source, and multi-directional" formations. This results in significant vertical and planar lithological variations in reservoirs, large variations in interlayer pressure coefficients, and "sweet spots" exhibiting distinct thin interbedded characteristics. Typical shale oil reservoirs generally develop shale with various types of laminae, including silty, tuffaceous, or dolomitic shale, leading to complex and highly anisotropic rock mechanical properties, which restricts the safe and efficient development of shale oil.

[0003] Rock mechanical parameters are crucial for oil and gas reservoir exploration and development design. Current technologies typically involve drilling downhole cores, preparing standard rock samples, and obtaining formation rock mechanical parameters through rock mechanics experiments. However, laminated shale formations suffer from low core recovery rates and difficulties in sample preparation due to their well-developed lamellar structure, complex lithology, and strong heterogeneity. Therefore, it is necessary to develop a method for calculating the rock mechanical parameters of laminated shale. While some methods exist for estimating rock mechanical parameters, they do not fully consider the combined effects of various weak structural planes such as lamellarity, microfractures, and bedding on rock strain and mechanical parameters, and are not suitable for laminated shale. This patent fully considers the influence of various weak planes in laminated shale on rock mechanical parameters and establishes a rapid method for estimating the rock mechanical parameters of laminated shale. Summary of the Invention

[0004] The purpose of this invention is to rapidly predict the mechanical parameters of complex laminated shale (hereinafter referred to as complex laminated shale) by testing the rock mechanical properties of shale with single-type lamination and layered shale without lamination (hereinafter collectively referred to as layered shale).

[0005] The technical solution of the present invention is as follows:

[0006] 1. A method for rapidly estimating the rock mechanical parameters of layered shale, the method comprising:

[0007] Step 1: Assume that the target rock contains n types of lithology with laminae. Obtain the whole-rock mineral composition of the n types of laminae and shale matrix through X-ray diffraction experiments. Observe the distribution characteristics of laminae, bedding, and microcracks in the rock through polarized light microscopy. Based on the experimental results, select rocks with similar distribution characteristics of weak surface structure to the target lithology for mechanical experimental testing.

[0008] Step 2: Prepare cylindrical samples with different dip angles of lamination, each 50 mm long and 25 mm in diameter from the selected rocks. Through uniaxial mechanical experiments, measure the compressive strength and elastic modulus of n types of shale containing single-layer lamination and bedding shale.

[0009] Step 3: Based on McLamore's shale strength criterion under uniaxial conditions and the rock equivalent deformation assumption, the elastic modulus and compressive strength of complex laminated shale are calculated using the mechanical parameters of n types of single-layered shale and the mechanical parameters of bedding shale.

[0010] Furthermore, the specific steps of step 1 are as follows:

[0011] Step 1.1: Screening samples with similar lithology: Obtain the whole-rock mineral composition of n types of lamellar and shale matrix through X-ray diffraction experiments. Based on the test results of the whole-rock mineral composition, screen out n types of single-type lamellar shale and bedding shale with similar lithology to the predicted target, ensuring that the shale matrix mineral composition of single-type lamellar shale and the bedding shale mineral composition are similar to the predicted target shale matrix mineral composition; and ensuring that the lamellar mineral composition in single-type lamellar shale is similar to the lamellar mineral composition corresponding to the predicted target.

[0012] Since shale may contain trace mineral components with relatively large fluctuations in content, each mineral component in the screening sample should satisfy formula (1):

[0013] (1)

[0014] In the formula, To predict the content of a certain mineral in the target; To screen the content of a certain mineral in the target that is the same type as the mineral in the predicted target.

[0015] Step 1.2: Screening samples with similar structures: Cut the rock into thin sections parallel to the axis of the full-diameter core, observe and record the microscopic features of the rock using a polarizing microscope, and obtain digital images of the rock's microscopic features. Draw a set of parallel reference lines with a spacing of 1 mm in both the horizontal and vertical directions of the image. Count the number of intersections between the reference lines and the weak surfaces on the image, and then use formula (2) to calculate the density of the weak surfaces in the horizontal and vertical directions:

[0016] (2)

[0017] In the formula, The density is represented by the weak surface level; The density is represented vertically by weak surfaces; This represents the number of intersections between the weak surface structure and the horizontal reference line. The number of intersections between the weak surface structure and the vertical reference line; , These represent the number of horizontal and vertical reference lines, respectively. and These represent the lengths of the horizontal and vertical reference lines, respectively.

[0018] Based on step 1.1, according to formula (2), select targets that match the prediction target. and Single-layered shale and bedding shale with a relative error of no more than 10% were used for mechanical testing in step 2.

[0019] Furthermore, the specific steps of step 2 are as follows:

[0020] Step 2.1: Prepare the screened rocks into samples with a sampling angle of... , , , , , and A cylindrical sample with a length of 50mm ± 2mm and a diameter of 25mm ± 1mm. It is the angle between the lamination or bedding plane and the cylinder axis.

[0021] Step 2.2: Conduct uniaxial mechanical experiments using a uniaxial compressor, and calculate the compressive strength and elastic modulus of the experimental sample using formulas (3) and (4) respectively:

[0022] (3)

[0023] In the formula: The uniaxial compressive strength of the rock; The axial pressure during rock fracturing; The cross-sectional area of ​​the rock;

[0024] (4);

[0025] In the formula: The elastic modulus of the rock; The axial strain of the rock is 50% of its uniaxial compressive strength.

[0026] Step 2.3: Calculate the i-th type of shale containing single-layered laminae using the formula at a sampling angle of... The compressive strength and elastic modulus at that time are denoted as follows: and ; stratified shale at the sampling angle The compressive strength and elastic modulus at that time are denoted as follows: and .

[0027] Furthermore, the specific steps of step 3 are as follows:

[0028] Step 3.1, Calculation of compressive strength of complex layered shale: Layered shale and shale containing single-layered shale are rocks with weak surface structures. Their failure law under uniaxial conditions satisfies the anisotropic rock strength criterion proposed by McLamore, as shown in formula (5):

[0029] (5)

[0030] In the formula: The sampling angle is Rock compressive strength at that time; The angle corresponding to the minimum compressive strength; , , , For material strength parameters; , Let be a positive integer used to describe anisotropic types, when and When the value is 1 or 3, it indicates a two-dimensional planar anisotropy type; when it is 5 or 6, or higher, it indicates a linear anisotropy type.

[0031] According to experimental tests, the maximum uniaxial compressive strength of shale occurs at [a certain point]. or At that time, and The lowest value is found at an angle between 30° and 45°. Therefore, a more accurate uniaxial compressive strength model can be obtained based on formula (5), as shown in formula (6):

[0032] (6)

[0033] In the formula: As The minimum uniaxial compressive strength value during the process of increasing from 0° to 90°; The angle corresponding to the minimum compressive strength; , When , At that time, the uniaxial compressive strength of the rock; , The parameters are obtained by fitting the uniaxial compressive strength test under multiple conditions.

[0034] Based on the experiment in step 2, all parameters in formula (6) can be obtained, and then the results can be calculated. and The failure of any part of a shale containing bedding, microcracks, and multiple types of laminations will cause the failure of the entire rock. Therefore, the compressive strength of complex laminations containing multiple weak structural planes can be calculated by formula (7):

[0035] (7)

[0036] In the formula: The sampling angle is The compressive strength of complex layered shale at that time; For the nth type of shale containing single-layered laminae, at the sampling angle of The compressive strength at that time.

[0037] Step 3.2, Calculation of the elastic modulus of complex layered shale: According to the theory of equivalent deformation of rocks, the strain of complex layered shale during the compression elastic deformation stage of rocks is... It can be calculated using formula (8):

[0038] (8)

[0039] In the formula: The strain is that of the i-th type of shale containing a single layer of laminae; The strain of layered shale; denoted as strain of shale without weak surface structures; n represents the number of laminar types in the predicted target.

[0040] Based on the stress-strain relationship in the elastic strain stage of rock and the theory of equivalent deformation of rock, we can obtain formula (9):

[0041] (9)

[0042] In the formula: The sampling angle is The elastic modulus of complex layered shale; The elastic modulus of shale without weak surfaces.

[0043] Since it is almost impossible to find shale without the influence of bedding under natural conditions, considering the influence of weak surface structures such as bedding on the elastic deformation stage of rock compression deformation, it can be considered as... The elastic modulus of the layered shale at that time is the elastic modulus of the shale without weak surfaces. The other parameters in formula (9) are all measured in the mechanical experiment in step 2, so the elastic modulus of the complex layered shale at any sampling angle can be obtained.

[0044] In summary, due to the adoption of the above technical solution, the beneficial effects of the present invention are as follows:

[0045] Drilling cores from complex laminated shale is difficult, and obtaining its mechanical parameters is challenging. This invention, through mineral composition analysis and microstructural observation, identifies single-layered and bedding-like shales that can characterize the structural features of complex laminated shales with multiple types of lamination. The compressive strength and elastic modulus of these shales are obtained through mechanical experiments and rock mechanics parameter calculations. Furthermore, based on the rock strength criterion for anisotropic shale and the theory of equivalent rock deformation, a method for calculating the compressive strength and elastic modulus of complex laminated shale is established. Ultimately, this allows for the rapid prediction of the compressive strength and elastic modulus of complex laminated shale under different loading angles using a limited number of readily available rock mechanics parameters, providing crucial data support for geological engineering design in oil and gas reservoir exploration and development within this type of formation. Attached Figure Description

[0046] Figure 1 This is a flowchart of the present invention;

[0047] Figure 2 A schematic diagram illustrating the density of microscopic weak surfaces in complex layered shale;

[0048] Figure 3 Schematic diagram of cylindrical rock core sampling;

[0049] Figure 4 A schematic diagram of the equivalent deformation theory for complex layered shale;

[0050] Figure 5 A comparison chart of predicted and measured values ​​of compressive strength for complex layered shale;

[0051] Figure 6 A comparison chart of predicted and measured values ​​of elastic modulus for complex layered shale; Detailed Implementation

[0052] According to the invention process Figure 1 The specific implementation details of this technical solution are as follows:

[0053] Step 1: Shale containing two types of laminae, tuff and siltstone, is used as the prediction target. The mineral composition of tuff laminae, siltstone laminae, and shale matrix is ​​obtained by X-ray diffraction experiments. The distribution characteristics of laminae, bedding, and microcracks in the rocks are observed by polarized light microscopy. Based on the experimental results, rocks with similar lithology and weak surface structure distribution characteristics to the prediction target are selected for mechanical experimental testing.

[0054] Specifically, the details of step 1 are as follows:

[0055] Step 1.1: Extract the tuff lamellar, siltstone lamellar, and shale matrix components from the predicted target. The mineral and clay contents of each component are obtained through X-ray diffraction experiments, as shown in Tables 1 and 2.

[0056] Table 1. Whole-rock mineral composition of different media in complex layered shale.

[0057]

[0058] Table 2. Clay mineral composition of different media in complex layered shale.

[0059]

[0060] Sampling and X-ray diffraction analysis were conducted on non-complex laminated shale sections that were easy to drill through in the same geological strata as the predicted target. Layered shale and shale containing single-type laminates that satisfy formula (1) were then selected.

[0061] (1)

[0062] In the formula, To predict the content of a certain mineral in the target; To screen the content of a certain mineral in the target that is the same type as the mineral in the predicted target.

[0063] Step 1.2: Prepare a square thin section of the target rock with sides of 40 mm, parallel to the axis of the entire diameter core, ensuring one side of the square is parallel to the horizontal plane. Observe the thin section under a polarizing microscope to obtain a digital image of the rock's microscopic features. Further, draw a set of parallel reference lines with a spacing of 1 mm in both the horizontal and vertical directions of the image, such as... Figure 2 As shown. By counting the number of intersections between the reference line and the weak surface on the image, the density of the weak surface in the horizontal and vertical directions can be calculated using formula (2):

[0064] (2)

[0065] In the formula, The density is represented by the weak surface level; The density is represented vertically by weak surfaces; This represents the number of intersections between the weak surface structure and the horizontal reference line. The number of intersections between the weak surface structure and the vertical reference line; , These represent the number of horizontal and vertical reference lines, respectively. and These represent the lengths of the horizontal and vertical reference lines, respectively.

[0066] Calculations yielded results in complex layered shale. , , , The samples selected in step 1.1 are further filtered according to formula (2) to obtain samples that match the predicted target. exist to , exist to The sample was used for mechanical testing in step 2.

[0067] Step 2: Prepare cylindrical samples with different dip angles of lamination, each 50 mm long and 25 mm in diameter from the selected rocks. Through uniaxial mechanical experiments, measure the compressive strength and elastic modulus of the bedding shale, tuffaceous lamination shale, and silty lamination shale.

[0068] Specifically, the details of step 2 are as follows:

[0069] Step 2.1: Prepare the screened rocks into samples with a sampling angle of... , , , , , and A cylindrical sample with a length of 50mm ± 2mm and a diameter of 25mm ± 1mm. The angle between the lamination or bedding plane and the cylinder axis, such as... Figure 3 As shown.

[0070] Step 2.2: Conduct uniaxial mechanical experiments using a uniaxial compressor, and calculate the compressive strength and elastic modulus of the experimental sample using formulas (3) and (4) respectively:

[0071] (3)

[0072] In the formula: The uniaxial compressive strength of the rock; The axial pressure during rock fracturing; The cross-sectional area of ​​the rock;

[0073] (4);

[0074] In the formula: The elastic modulus of the rock; The axial strain of the rock is 50% of its uniaxial compressive strength.

[0075] Step 2.3: Calculate the single-layered shale using the formula at the sampling angle. The compressive strength and elastic modulus at that time are denoted as follows: and ; stratified shale at the sampling angle The compressive strength and elastic modulus at that time are denoted as follows: and Tables 3 and 4 show the compressive strength and elastic modulus of the layered shale, tuffaceous layered shale, and silty layered shale obtained from the tests, respectively.

[0076] Table 3 Statistical Table of Rock Compressive Strength

[0077]

[0078] Table 4 Statistical Table of Rock Elastic Modulus

[0079] .

[0080] Step 3: Based on McLamore's shale strength criterion and rock equivalent deformation assumption under uniaxial conditions, the elastic modulus and compressive strength of complex laminated shale are calculated using the rock mechanical parameters of three lithologies: bedding shale, tuffaceous laminated shale, and silty laminated shale.

[0081] Specifically, the details of step 3 are as follows:

[0082] Step 3.1, Calculation of compressive strength of complex layered shale: Layered shale and shale containing single-layered shale are rocks with weak surface structures. Their failure law under uniaxial conditions satisfies the anisotropic rock strength criterion proposed by McLamore, as shown in formula (5):

[0083] (5)

[0084] In the formula: The sampling angle is Rock compressive strength at that time; The angle corresponding to the minimum compressive strength; , , , For material strength parameters; , It is a positive integer used to describe anisotropic types.

[0085] According to experimental tests, the maximum uniaxial compressive strength of shale occurs at [a certain point]. or At that time, and The lowest value is found at an angle between 30° and 45°. Therefore, a more accurate uniaxial compressive strength model can be obtained based on formula (5), as shown in formula (6):

[0086] (6)

[0087] In the formula: As The minimum uniaxial compressive strength value during the process of increasing from 0° to 90°; The angle corresponding to the minimum compressive strength; , When , At that time, the uniaxial compressive strength of the rock; , The parameters are obtained by fitting the uniaxial compressive strength test under multiple conditions.

[0088] Based on the experiment in step 2, all parameters in formula (6) can be obtained, and then the results can be calculated. and The failure of any part of a shale containing bedding, microcracks, and multiple types of laminations will cause the failure of the entire rock. Therefore, the compressive strength of complex laminations containing multiple weak structural planes can be calculated by formula (7):

[0089] (7)

[0090] In the formula: The sampling angle is The compressive strength of complex layered shale at that time; For the nth type of shale containing single-layered laminae, at the sampling angle of The compressive strength at that time.

[0091] Step 3.2, Calculation of the elastic modulus of complex layered shale: According to the theory of equivalent deformation of rocks, the strain of complex layered shale during the compression elastic deformation stage of rocks is... It can be calculated using formula (8):

[0092] (8)

[0093] In the formula: The strain is that of the i-th type of shale containing a single layer of laminae; The strain of layered shale; denoted as strain of shale without weak surface structures; n represents the number of laminar types in the predicted target.

[0094] Integrating the stress-strain relationship in the elastic strain stage of rock and the theory of equivalent deformation of rock (see schematic diagram) Figure 4 As shown), we can obtain formula (9):

[0095] (9)

[0096] In the formula: The sampling angle is The elastic modulus of complex layered shale; The elastic modulus of shale without weak surfaces.

[0097] This method was applied to predict the mechanical parameters of six laminated shale formations. An analysis graph was plotted with the measured compressive strength as the ordinate and the predicted compressive strength as the abscissa, as shown below. Figure 5 and Figure 6 As shown in the figure, the small squares represent data points for complex layered shale. Comparing the rock mechanical parameters predicted by this method with those measured in actual experiments, it can be seen that the rock mechanical parameters rapidly predicted by this method are close to the measured values.

[0098] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for rapidly estimating lithologic shale rock mechanical parameters, characterized in that, The implementation steps are as follows: Step 1, assuming that the prediction target rock contains n types of lithologic laminae, the whole rock mineral composition of n types of laminae and shale matrix is obtained through X-ray diffraction experiment; the distribution characteristics of laminae, bedding and microcracks in the rock are observed by polarizing microscope, and the rock with consistent distribution characteristics of predicted lithology and weak plane structure is selected for mechanical experiment test according to the experimental results; Step 2, the selected rock is prepared into a cylindrical sample with different lamina inclination angles, length of 50mm and diameter of 25mm, the compressive strength and elastic modulus of n types of single lamina shale and bedded shale are measured respectively through uniaxial mechanical experiment; Step 3, according to the shale strength criterion of McLamore under uniaxial condition and the equivalent deformation assumption of rock, the elastic modulus and compressive strength of complex lamina shale are calculated based on the mechanical parameters of n types of single lamina shale and the mechanical parameters of bedded shale; specifically including the following steps: Step 3.1, the uniaxial compressive strength of bedded shale and single lamina shale is calculated according to the following anisotropic rock strength criterion: In the formula: As The minimum uniaxial compressive strength value during the process of increasing from 0° to 90°; The angle corresponding to the minimum compressive strength; , When , At that time, the uniaxial compressive strength of the rock; , The model parameters were obtained by fitting the uniaxial compressive strength test under multiple conditions. All the parameters in the above formula can be obtained according to the experiment in step 2, and then calculated and ; further, the following formula is used to calculate the compressive strength of the complex laminated shale: In the formula: is the compressive strength of the complex laminated shale when the sampling angle is is the compressive strength of the nth single-layer shale when the sampling angle is ​​ Step 3.2, according to the rock equivalent deformation theory and the measured rock mechanical parameters, the elastic modulus of the bedding shale at the time is the elastic modulus of the shale without weak surface; the elastic modulus of the complex laminated shale is calculated by the following formula: Step 3.2, according to the rock equivalent deformation theory and the measured rock mechanical parameters, the elastic modulus of the bedding shale at the time is the elastic modulus of the shale without weak surface; the elastic modulus of the complex laminated shale is calculated by the following formula: where: is the strain of complex laminated shale; n is the number of lamina types in the prediction target; is the strain of the ith single-lamina shale; is the strain of bedded shale; is the strain of shale without weak plane structure; is the elastic modulus of complex laminated shale when the sampling angle is is the elastic modulus of complex laminated shale when the sampling angle is is the elastic modulus of shale without weak plane.

2. The method for rapidly estimating the mechanical parameters of laminated shale rock according to claim 1, characterized in that, In the step 1, the following steps are included: Step 1.1, the whole rock mineral composition of n types of laminae and shale matrix is obtained through X-ray diffraction experiment, and n types of single lamina shale and bedded shale similar to the predicted target lithology are selected according to the test results of whole rock mineral composition, so as to ensure that the mineral composition of single lamina shale and bedded shale is similar to the mineral composition of shale matrix of the predicted target; ensure that the lamina mineral composition in single lamina shale is similar to the corresponding lamina mineral composition of the predicted target; each mineral composition in the selected sample should satisfy the following formula: In the formula, to predict the content of a certain mineral in the target; to screen the content of a certain mineral in the target which is the same type of mineral as in the prediction target; Step 1.2, the rock is cut into a thin section parallel to the full diameter core axis, the micro characteristics of the rock are observed and recorded by using polarizing microscope, and the digital image of the micro characteristics of the rock is obtained; a group of parallel reference lines with interval of 1mm are drawn in horizontal and vertical directions of the image; the number of intersection points of the reference lines and the weak plane is counted on the image, and the following formula is used to calculate the density of the weak plane in horizontal and vertical directions: wherein is the horizontal representation of the density of the weak face; is the vertical representation of the density of the weak face; is the number of intersections of the weak face structure with the horizontal reference lines; is the number of intersections of the weak face structure with the vertical reference lines; , denote the number of horizontal and vertical reference lines, respectively, and denote the length of the horizontal and vertical reference lines, respectively; Shales with monotonous lamination and bedded shales with relative errors not exceeding 10% were selected as the predicted targets according to steps 1.1 and 1.2 and Shales with monotonous lamination and bedded shales with relative errors not exceeding 10% were selected as the predicted targets according to steps 1.1 and 1.2 3. The method for rapidly estimating the mechanical parameters of laminated shale rock according to claim 1, characterized in that, In the step 2, the following steps are included: Step 2.1, the selected rocks are prepared into cylindrical samples with a sampling angle of , , , , , and 50 mm ± 2 mm in length and 25 mm ± 1 mm in diameter, is the angle between the bedding or stratification surface and the cylindrical axis; Step 2.2, the uniaxial mechanical experiment is carried out by using uniaxial compression machine, and the following formula is used to calculate the compressive strength and elastic modulus of the experimental sample respectively: In the formula: is the uniaxial compressive strength of the rock; is the axial pressure at which the rock fails; is the cross-sectional area of the rock; wherein: E is the elastic modulus of the rock; ε50 is the axial strain of the rock at 50% uniaxial compressive strength. Step 2.3, the compressive strength and elastic modulus of the i-th single-layer shale at the sampling angle of are calculated by formula as and respectively; the compressive strength and elastic modulus of the bedded shale at the sampling angle of are calculated by formula as and .

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