Stress relaxation simulation method and device of digital core, storage medium and processor

Abstract

The invention provides a stress relaxation simulation method and device of a digital core, a storage medium and a processor, and belongs to the technical field of oil and gas physical simulation. The method comprises the following steps: setting initial conditions and boundary conditions of a digital core, and establishing a uniform strain field at the initial strain moment of the digital core, so that a stress field and the strain field in the digital core change along with time, and the total strain of the digital core is kept unchanged; calculating a stress field of the digital rock core according to a rotary staggered grid finite difference algorithm; and solving a volume average for the stress field to obtain a stress relaxation curve of the digital rock core. Strain is set to be instantly established at the moment t = 0 in the boundary condition and the initial condition, that is, a strain function is a step function, and a time derivative of the step function is a pulse function of which the frequency spectrum is a full frequency band. Therefore, the obtained stress relaxation curve can be used for calculating the speed and attenuation of elastic waves in any frequency band.

Description

Technical Field

[0001] The present invention relates to the technical field of oil and gas physical simulation, and particularly to a method, device, storage medium and processor for simulating stress relaxation of digital cores. Background Art

[0002] When seismic waves propagate in rocks, velocity dispersion and energy attenuation occur. One of the reasons is that when the waves propagate in the rocks, the fluid pressures between adjacent parts are different, driving the fluid motion to form wave-induced flow. Numerical simulation is an important method for studying wave dispersion and attenuation. The numerical simulation method based on digital cores is called digital rock physics.

[0003] The elastic / viscoelastic simulation techniques in digital rock physics mainly include the following: ① static simulation; ② transmitted wave simulation; ③ dynamic damping method; ④ quasi-static stress-strain simulation; ⑤ dynamic stress-strain simulation. Among them, in methods ①, ② and ③, only the elastic and attenuation parameters at one frequency can be calculated in one simulation. Method ④ calculates the dispersion and attenuation in a wide frequency band, but the terms related to acceleration are ignored in the control equation, and only the dispersion and attenuation effects of elastic waves caused by pore fluid flow can be simply calculated. Method ⑤ calculates the dispersion and attenuation in a wide frequency band, and the control equation contains terms related to acceleration, and the elastic wave effects caused by wave-induced flow can be calculated. However, the existing simulation methods have not fully utilized this characteristic. Summary of the Invention

[0004] The purpose of the embodiments of the present invention is to provide a method for simulating stress relaxation of digital cores, by which the effective frequency band is continuously extended towards the high-frequency direction, so that the calculated dispersion and attenuation include both wave-induced flow effects and elastic scattering effects.

[0005] To achieve the above purpose, the embodiments of the present invention provide a method for simulating stress relaxation of digital cores, including:

[0006] Setting the initial conditions and boundary conditions of the digital core, and establishing a uniform strain field at the initial moment of strain of the digital core, so that the stress field and strain field inside the digital core change with time, and the total strain of the digital core remains unchanged;

[0007] Calculating the stress field of the digital core according to the rotated staggered grid finite difference algorithm; and

[0008] Taking the volume average of the stress field to obtain the stress relaxation curve of the digital core.

[0009] Optionally, the initial conditions are:

[0010] v x (x,z,t=0)=0 u x (x,z,t=0)=E0x

[0011] v z u(x,z,t=0)=0, z u(x,z,t=0)=0,

[0012] In the formula, v x and v z are respectively the velocity components of the mass points of the digital core in the X and Z directions. Among them, the X direction and the Z direction are respectively the positive X-axis direction and the positive Z-axis direction of the rectangular coordinate system where the digital core is located, and the X direction is the horizontal direction;

[0013] t is time; x and z are respectively the coordinates of the mass point on the X-axis and Z-axis of the rectangular coordinate system;

[0014] u x and u z are respectively the displacement components of the mass point in the X direction and the Z direction;

[0015] E0 is the coefficient that controls the positive strain level of the digital core in the x direction, and it is a constant with 0 < E0 ≤ 10 -6 .

[0016] Optionally, the left boundary of the boundary condition is:

[0017] The right boundary of the boundary condition is:

[0018] In the formula, E0 is the coefficient that controls the overall strain level of the digital core, and it is a constant with 0 < E0 ≤ 10 -6 .

[0019] L is the side length of the digital core.

[0020] Furthermore, when calculating the stress relaxation curve of the digital core according to the rotated staggered grid finite difference algorithm, the constitutive equations of the skeleton and pore fluid of the digital core are:

[0021]

[0022] The motion equation is:

[0023]

[0024] In the formula, λ and μ are Lame constants;

[0025] σ XX , σ ZZ , and σ ZX are respectively the normal stress of the mass point along the X direction, the normal stress along the Z direction, and the shear stress in the XOZ plane. Among them, the XOZ plane is the XOZ plane of the rectangular coordinate system where the digital core is located;

[0026] e xx 、e zz 、and e zx are respectively the normal strain of the particle along the X direction, the normal strain along the Z direction, and the shear strain in the XOZ plane;

[0027] η λ and η μ are respectively the fluid viscosity coefficients related to the volume deformation and the shear deformation, and ρ is the density of the digital core.

[0028] Optionally, the volume average of the stress field is obtained through the following formula:

[0029]

[0030] In the formula, i and j represent the indices of the model grid in the rotated staggered grid finite difference algorithm; <·> represents taking the volume average value; represents the normal stress of the (i, j) grid in the x direction at time t; τ xx (t) represents the average normal stress of the digital core in the x direction at time t; at t ≥ 0, τ xx (t) changes with time.

[0031] Furthermore, the stress relaxation simulation of the digital core also includes: performing noise reduction processing on the stress relaxation curve to obtain a noise-reduced stress relaxation curve.

[0032] Optionally, the noise reduction processing is performed by the exponential function fitting method or the logarithmic domain wavelet packet method.

[0033] Optionally, the noise reduction processing steps include:

[0034] In the linear coordinate system, an equally spaced time series is established, denoted as t = {Δt, 2Δt, …, NΔt};

[0035] The time series is represented in the logarithmic coordinate system, denoted as

[0036] Resample t l , so that t l is an equally spaced sequence in the logarithmic coordinate system, denoted as t lh ;

[0037] According to t lh resample the stress relaxation curve, denoted as S lh = S(t lh );

[0038] Use the wavelet packet denoising method to eliminate the high-frequency noise of S lh ;

[0039] The denoised Slh Transform it into the linear domain to obtain the denoised stress relaxation curve.

[0040] Furthermore, the stress relaxation simulation of the digital core also includes: calculating the dispersion curve and attenuation curve of the digital core according to the spectral ratio method and the stress relaxation curve.

[0041] Optionally, when calculating the dispersion curve and attenuation curve of the digital core, the dispersion and quality factor of the digital core are calculated through the complex elastic modulus:

[0042]

[0043] In the formula, FT[] represents the Fourier transform;

[0044] and respectively represent the stress rate and strain rate of the digital core in the X direction at time t, and is the zero-delay pulse function, where the X direction is the positive direction of the X-axis of the rectangular coordinate system where the digital core is located;

[0045] M(f) represents the complex elastic modulus at frequency f;

[0046] V P (f) represents the longitudinal wave velocity at frequency f;

[0047] Q P (f) represents the longitudinal wave quality factor at frequency f.

[0048] On the other hand, the present invention provides a stress relaxation simulation device for a digital core, including: a conditional loading module, a stress field calculation module, and a stress relaxation calculation module, where:

[0049] The conditional loading module sets the initial conditions and boundary conditions of the digital core, and establishes a uniform strain field at the initial moment of the strain of the digital core, so that the stress field and strain field inside the digital core change with time, and the overall strain of the digital core remains unchanged;

[0050] The stress field calculation module calculates the stress field of the digital core according to the rotated staggered grid finite difference algorithm; and

[0051] The stress relaxation calculation module takes the volume average of the stress field to obtain the stress relaxation curve of the digital core.

[0052] Optionally, the stress relaxation simulation device of the digital core further includes: a denoising module, which performs noise reduction processing on the stress relaxation curve to obtain the denoised stress relaxation curve.

[0053] Optionally, the apparatus for simulating stress relaxation of digital cores further includes: a dispersion and attenuation calculation module, which calculates the dispersion curve and attenuation curve of the digital core according to the spectral ratio method and the stress relaxation curve.

[0054] On the other hand, the present invention provides a machine-readable storage medium, on which instructions are stored, and the instructions are used to cause a machine to execute the method for simulating stress relaxation of digital cores in the present application.

[0055] On the other hand, the present invention provides a processor, which is characterized in that it is used to run a program, and when the program is run, it is used to execute the method for simulating stress relaxation of digital cores in the present application.

[0056] Through the above technical solution, it is first set in the boundary conditions and initial conditions that the strain is instantaneously established at t = 0, that is, the relationship between strain and time is a step function, the time derivative of the step function is an impulse function, and its frequency spectrum is all-frequency band. At the same time, it is also set in the boundary conditions and initial conditions that the total strain of the digital core is fixed and unchanged. Therefore, the stress relaxation curve obtained in the present application can be used to calculate the velocity and attenuation of elastic waves in any frequency band.

[0057] Other features and advantages of the embodiments of the present invention will be described in detail in the subsequent specific implementation part. Description of the Drawings

[0058] The drawings are used to provide a further understanding of the embodiments of the present invention, and constitute a part of the specification. Together with the following specific implementation, they are used to explain the embodiments of the present invention, but do not constitute a limitation to the embodiments of the present invention. In the drawings:

[0059] Figure 1 is a flowchart of an embodiment of the method for simulating stress relaxation of digital cores of the present invention;

[0060] Figure 2 is a schematic flowchart of another embodiment of the method for simulating stress relaxation of digital cores of the present invention;

[0061] Figure 3 is Figure 2 a schematic diagram of a rotated staggered grid in the embodiment;

[0062] Figure 4 is Figure 2 the sandstone digital core obtained in the embodiment and its numerical simulation dispersion and attenuation curves;

[0063] Figure 5 is a composition structure diagram of an embodiment of the apparatus for simulating stress relaxation of digital cores of the present invention; and

[0064] Figure 6It is the structural composition diagram of another embodiment of the stress relaxation simulation device for digital cores of the present invention. Detailed implementation manners

[0065] The following will describe in detail the specific implementation manners of the embodiments of the present invention with reference to the accompanying drawings. It should be understood that the specific implementation manners described herein are only used to illustrate and explain the embodiments of the present invention, and are not used to limit the embodiments of the present invention.

[0066] It should be noted that the digital core in Step 1 refers to a digital core model obtained according to digital core modeling technology. Digital core modeling is often used in core analysis fields such as sandstone, carbonate rock, and shale. The basic principle is based on two-dimensional scanning electron microscope images or three-dimensional CT scan images, using computer image processing technology, and through algorithms to complete the reconstruction of the digital core. The constructed digital core model is usually a regular cube.

[0067] The embodiments of the present invention provide a method for simulating stress relaxation of digital cores, which is used to calculate the stress relaxation curves of cores in different directions. In this embodiment, a two-dimensional coordinate system xoz is assumed, where o is the origin, the x-axis is along the horizontal direction, and the z-axis is along the vertical direction. The following will take the calculation of the dispersion and attenuation properties of elastic waves propagating along the x-axis direction as an example to illustrate the specific implementation steps of the present invention. The calculation method for the dispersion and attenuation properties of elastic waves propagating along the z-axis direction is the same. The following will describe Figure 1 the specific implementation manners of this embodiment.

[0068] As Figure 1 shown, this embodiment includes the following main steps:

[0069] Step 1: Set the initial conditions and boundary conditions of the digital core, and establish a uniform strain field at the initial moment of strain of the digital core, so that the stress field and strain field inside the digital core change with time, and the overall strain of the digital core remains unchanged;

[0070] Step 2: Calculate the stress field of the digital core according to the rotated staggered grid finite difference algorithm;

[0071] Step 3: Obtain the volume average of the stress field to obtain the stress relaxation curve of the digital core.

[0072] Regarding the boundary conditions, in Step 1, taking the stress relaxation curve of the mass points of the digital core propagating along a certain x-axis direction (i.e., the horizontal direction) as an example, the boundary conditions can be set as follows: at any moment of the simulation, the left and right boundaries of the digital core are fixed to keep the overall strain of the digital core unchanged in the propagation direction; the upper and lower boundaries satisfy the periodic boundary conditions, that is, the upper and lower boundaries of the digital core are connected to each other during the calculation process, and the wave field can propagate up and down. Similarly, if calculating the stress relaxation curve of the mass points of the digital core propagating along a certain z-axis direction (i.e., the vertical direction), the boundary conditions can be set as follows: at any moment of the simulation, the upper and lower boundaries of the digital core are fixed to keep the overall strain of the digital core unchanged in the propagation direction; the left and right boundaries satisfy the periodic boundary conditions, that is, the left and right boundaries of the digital core are connected to each other during the calculation process, and the wave field can propagate left and right.

[0073] Regarding the initial conditions, in Step 1, a uniform strain field is instantaneously established for the digital core at t = 0. Since the strain is instantaneously established, the relationship between strain and time is a step function, and the time derivative of the step function is an impulse function whose spectrum is full-band. Therefore, the method of the present invention can calculate the velocity and attenuation of the full band.

[0074] In Step 2, according to the rotated staggered grid finite difference algorithm, the stress field changing with time inside the digital core is calculated under the initial conditions and boundary conditions defined in Step 1.

[0075] In Step 3, the volume average of the stress field changing with time inside the digital core obtained in Step 2 is taken to obtain the stress relaxation curve of the digital core.

[0076] Compared with the prior art, the technical advantages of this embodiment are as follows:

[0077] The total strain of the digital core in the boundary conditions and initial conditions is fixed and unchanged, and the strain is instantaneously established at t = 0, that is, the relationship between strain and time is a step function, and the time derivative of the step function is an impulse function whose spectrum is full-band. Therefore, the stress relaxation curve obtained in this embodiment can calculate the velocity and attenuation of elastic waves in any frequency band.

[0078] In this embodiment, the obtained stress relaxation curve has a rapidly descending initial segment and a quite long oscillatory attenuation segment. The dispersion and attenuation directly calculated using the initial stress relaxation curve have strong high-frequency noise. To improve the above situation, on the basis of the above embodiment, the present invention proposes another preferred embodiment, including the denoising process of the initial stress relaxation curve, and at the same time, the process of calculating the initial stress relaxation curve is also improved to minimize the noise contained in the initial stress relaxation curve to the greatest extent.

[0079] In this embodiment, similar to the previous embodiment, a two-dimensional coordinate system xoz is assumed, with o as the origin, the x-axis along the horizontal direction, and the z-axis along the vertical direction. Taking the calculation of the dispersion and attenuation properties of elastic waves propagating along the x-axis as an example, the specific implementation steps of this embodiment are described. The calculation method for the dispersion and attenuation properties of elastic waves propagating along the z-axis is the same. The following is combined with Figures 2-4 to illustrate the specific implementation manner of this embodiment.

[0080] This embodiment is used to calculate the dispersion and attenuation curves of a sandstone digital core. As Figure 4 (a) shows, it is a slice of the sandstone digital core, where the white area represents the skeleton and the black area represents the pores. The bulk modulus, shear modulus, and density of the skeleton are 37.0 GPa, 44.0 GPa, and 2650.0 kg / m 3 respectively. The bulk modulus, density, and viscosity coefficient of the liquid are 2.3 GPa, 1000.0 kg / m 3 and 1.0 Pa·s respectively.

[0081] In the first stage, the initial stress relaxation curve is obtained first; in the second stage, the noise reduction processing of the initial stress relaxation curve is carried out to obtain the denoised stress relaxation curve; in the third stage, according to the denoised stress relaxation curve, the dispersion and attenuation curves of the digital core are obtained.

[0082] In the first stage, the initial stress relaxation curve is obtained through the following steps:

[0083] The first step is to set the initial conditions and boundary conditions

[0084] In this embodiment, the initial conditions of the digital core are set as

[0085] v x (x,z,t = 0) = 0 u x (x,z,t = 0) = E0x

[0086] v z (x,z,t = 0) = 0, u z (x,z,t = 0) = 0. (1)

[0087] In the formula, v x and v z are the velocity components of the particles of the digital core in the x and z directions; u x and u z are the displacement components of the particles in the x and z directions; t is the time; x and z are the coordinates of the particles on the X-axis and Z-axis of the rectangular coordinate system respectively; 0 < E0 ≤ 10 -6 , E0 is a coefficient that controls the overall strain level of the digital core, and during the stress relaxation simulation, E0 is a constant.

[0088] It should be noted that since this application takes the stress relaxation curve of a computing particle propagating in a certain x-axis direction (i.e., the horizontal direction) as an example for illustration, in the above initial conditions, E0x indicates that the coefficient E0 is the coefficient controlling the positive strain in the x direction and is a constant. In this embodiment, 0 < E0 ≤ 10 -6 .

[0089] In this embodiment, the boundary conditions of the left boundary are set as follows:

[0090] v x (0, z, t) = 0 u x (0, z, t) = 0

[0091] v z (0, z, t) = 0, u z (0, z, t) = 0. (2)

[0092] The boundary conditions of the right boundary are set as follows:

[0093] v x (L, z, t) = 0 u x (L, z, t) = E0L

[0094] v z (L, z, t) = 0, u z (L, z, t) = 0. (3)

[0095] In the formula, E0 is the coefficient controlling the overall strain level of the digital core and is a constant with 0 < E0 ≤ 10 -6 . L is the side length of the digital core. It should be noted that in this embodiment, the digital core is a cube model constructed by operations such as scanning and image processing of a sandstone.

[0096] Formulas (2)-(3) indicate that the left and right boundaries of the digital core are fixed at any moment during the simulation and do not move. In addition, the upper and lower boundaries of the digital core adopt periodic boundary conditions. With periodic boundary conditions, the upper and lower boundaries of the digital core are connected to each other during the calculation, and the wave field can propagate up and down. Therefore, the above initial and boundary conditions determine that the stress relaxation curve obtained in this embodiment can calculate the velocity and attenuation of elastic waves in any frequency band.

[0097] Second step, obtain the simulated stress field through the rotated staggered grid finite difference method

[0098] In this embodiment, the constitutive equations of the digital core's skeleton and pore fluid are as follows:

[0099]

[0100] In the formula, λ and μ are Lame constants; σ xx , σzz and σ zx are the normal stress in the X direction, the normal stress in the Z direction, and the shear stress in the XOZ plane of the particle, respectively; e xx , e zz and e zx are the normal strain in the X direction, the normal strain in the Z direction, and the shear strain in the XOZ plane of the particle, respectively; η λ and η μ are the fluid viscosity coefficients related to volume deformation and shear deformation, respectively. For fluids, the shear modulus μ = 0.

[0101] Within the ultrasonic frequencies commonly used in oil and gas exploration, for the solid skeleton, a viscosity coefficient equivalent to the value of the fluid viscosity coefficient can be set, and the impact on the velocity and attenuation values can be ignored, but it can significantly reduce the noise of the stress curve.

[0102] In this embodiment, the motion equation is constructed as:

[0103]

[0104]

[0105] where ρ is the density of the digital core.

[0106] A relatively mature finite difference technique - the rotated staggered grid finite difference technique is used to solve equations (4), (5), and (6) to obtain the changes of displacement, velocity, strain, and stress of the digital core over time. Among them, the schematic diagram of the rotated staggered grid is as Figure 3 shown.

[0107] The third step is to perform volume averaging on the stress field to calculate the stress relaxation time curve

[0108] At each time step of solving the stress and strain fields of the digital core by the finite difference method, calculate the average stress of the digital core

[0109]

[0110] where i, j represent the indices of the model grid in the rotated staggered grid finite difference algorithm; <·> represents taking the volume average; represents the normal stress in the x direction of the (i, j) grid at time t; τ xx (t) represents the average normal stress of the digital core in the x direction at time t; at t ≥ 0, τ xx (t) changes over time.

[0111] It should be noted that since the total strain of the digital core is set to be fixed in the boundary conditions and initial conditions, the value of the average stress of the digital core is a constant E0 (t≥0).

[0112] Thus, from the initial stress relaxation time curve obtained in the first stage, and because in this embodiment the constitutive equations of the fluid and the skeleton are expressed in a unified form, and the constitutive equation of the skeleton also includes a viscous term, it will not change the relationship between the velocity and attenuation and frequency within the frequency band of interest in oil and gas exploration, and also significantly reduces the noise in the obtained initial stress relaxation time curve, which is beneficial for further calculating the dispersion and attenuation curves.

[0113] The second stage, as Figure 5 shown, is to perform noise reduction processing on the initial stress relaxation curve obtained in the first stage, aiming to effectively extract the dispersion and attenuation caused by elastic scattering. Specifically, it can be to process the initial stress relaxation curve obtained in the first stage by exponential function fitting or logarithmic domain wavelet packet method. In this embodiment, the logarithmic domain wavelet packet method is adopted, and the specific steps of its denoising processing are as follows:

[0114] 1) In the linear coordinate system, establish an equally spaced time series, t = {Δt, 2Δt, …, NΔt};

[0115] 2) Represent this time series in the logarithmic coordinate system,

[0116] 3) Resample the above time series so that it is an equally spaced sequence in the logarithmic coordinate system, t lh ;

[0117] 4) According to t lh resample the stress relaxation curve, s lh = s(t lh );

[0118] 5) Use wavelet packet denoising technology to eliminate high-frequency noise;

[0119] 6) Transform the denoised stress relaxation curve to the linear domain to obtain the final denoised relaxation curve.

[0120] The third stage, as Figure 5 shown, is to process the denoised stress relaxation curve obtained in the second stage by the spectral ratio method to obtain the dispersion and attenuation curves of the digital core.

[0121] In this embodiment, when calculating the dispersion curve and attenuation curve of the digital core, the dispersion and quality factor of the digital core are calculated through the complex elastic modulus, and the complex elastic modulus of the digital core is the ratio of the Fourier transform of the stress rate and strain rate curves. The formulas used to calculate the longitudinal wave quality factor are:

[0122]

[0123] In the formula, FT[] represents the Fourier transform; and respectively represent the stress rate and strain rate of the digital core in the X direction at time t, and is the zero-delay pulse function, where the X direction is the positive direction of the X-axis of the rectangular coordinate system where the digital core is located; M(f) represents the complex elastic modulus at frequency f; V P (f) represents the longitudinal wave velocity at frequency f; Q P (f) represents the longitudinal wave quality factor at frequency f.

[0124] In this embodiment, the dispersion and attenuation curves of the longitudinal wave along the horizontal direction are as Figure 4 (b) shows that there are no fractures with low aspect ratios in the digital core, and the dispersion and attenuation curves mainly reflect the elastic scattering effect, and the wave-induced flow effect is weak.

[0125] Compared with the prior art, the technical advantages of this embodiment are as follows:

[0126] ① By setting the initial conditions and boundary conditions of the digital core, a uniform strain field is instantaneously established at t = 0, and as time goes by, the stress field and strain field inside the digital core change while the apparent change of the digital core remains unchanged. The stress relaxation curve of the digital core propagating along the specified direction can be obtained, and this stress relaxation curve can be used to further calculate the velocity and attenuation of elastic waves in any frequency band;

[0127] ② Express the constitutive equations of the fluid and the skeleton in a unified form. Not only is the form unified, but the constitutive equation of the skeleton also contains a viscous term, which does not change the relationship between the velocity and attenuation and frequency in the frequency band of interest for oil and gas exploration, and also significantly reduces the noise in the obtained stress relaxation curve, which is beneficial to calculating the dispersion and attenuation curves;

[0128] ③ The denoised stress relaxation curve contains richer physical connotations, and the dispersion and attenuation calculated using the denoised stress relaxation curve reduce high-frequency noise;

[0129] ④ The stress relaxation curve obtained through this embodiment can not only calculate the dispersion and attenuation of elastic waves caused by wave-induced flow motion, but also calculate the effect of elastic scattering on the dispersion and attenuation of elastic waves.

[0130] The embodiment of the present invention provides a stress relaxation simulation device for a digital core, as Figure 5 shown, including: a condition loading module, a stress field calculation module, and a stress relaxation calculation module, where:

[0131] A conditional loading module sets the initial conditions and boundary conditions of the digital core, and establishes a uniform strain field at the initial moment of strain of the digital core, so that the stress field and strain field inside the digital core change with time, and the total strain of the digital core remains unchanged.

[0132] A stress field calculation module calculates the stress field of the digital core according to the rotated staggered grid finite difference algorithm; and

[0133] A stress relaxation calculation module obtains the volume average of the stress field to obtain the stress relaxation curve of the digital core.

[0134] In some embodiments, as Figure 6 shown, the stress relaxation simulation device of the digital core further includes a denoising module and a dispersion and attenuation calculation module. Among them, the denoising module is used to perform noise reduction processing on the stress relaxation curve to obtain a noise-reduced stress relaxation curve; the dispersion and attenuation calculation module is used to calculate the dispersion curve and attenuation curve of the digital core according to the spectral ratio method and the stress relaxation curve.

[0135] In addition, there are other preferred embodiments or technical features of the stress relaxation simulation device of the digital core, which can refer to the embodiments of the above-mentioned stress relaxation simulation method of the digital core, and will not be elaborated here.

[0136] An embodiment of the present invention provides a stress relaxation simulation device for a digital core, including a processor and a memory. The above-mentioned conditional loading module, stress field calculation module, stress relaxation calculation module, denoising module, and dispersion and attenuation calculation module are all stored in the memory as program units, and the processor executes the above program units stored in the memory to implement corresponding functions.

[0137] The processor contains a kernel, and the kernel retrieves the corresponding program units from the memory. One or more kernels can be set, and the stress relaxation curve of the digital core is calculated by adjusting the kernel parameters, and the dispersion curve and attenuation curve of the digital core can be further calculated according to the obtained stress relaxation curve.

[0138] The memory may include non-permanent memory in a computer-readable medium, forms such as random access memory (RAM) and / or non-volatile memory, such as read-only memory (ROM) or flash memory (flash RAM), and the memory includes at least one storage chip.

[0139] An embodiment of the present invention provides a storage medium, on which a program is stored, and when the program is executed by a processor, it implements the stress relaxation simulation method of the digital core of the present application.

[0140] An embodiment of the present invention provides a processor for running a program, wherein when the program runs, it executes the stress relaxation simulation method of the digital core of the present application.

[0141] An embodiment of the present invention provides a device, which includes a processor, a memory, and a program stored on the memory and executable on the processor. When the processor executes the program, it implements the steps of the stress relaxation simulation method of the digital core of the present application. The device herein can be a server, a PC, a PAD, a mobile phone, etc.

[0142] The present application also provides a computer program product, which is adapted to execute a program initialized with the steps of the stress relaxation simulation method of the digital core of the present application when executed on a data processing device.

[0143] Those skilled in the art should understand that the embodiments of the present application can be provided as a method, a system, or a computer program product. Therefore, the present application can take the form of a complete hardware embodiment, a complete software embodiment, or an embodiment combining software and hardware aspects. Moreover, the present application can take the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0144] The present application is described with reference to the flowcharts and / or block diagrams of methods, devices (systems), and computer program products according to the embodiments of the present application. It should be understood that each flow and / or block in the flowchart and / or block diagram, as well as the combination of flows and / or blocks in the flowchart and / or block diagram, can be implemented by computer program instructions. These computer program instructions can be provided to the processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing devices to generate a machine, such that the instructions executed by the processor of the computer or other programmable data processing devices generate a device for implementing the functions specified in Figure 1 one process or multiple processes and / or blocks Figure 1 one block or multiple blocks.

[0145] These computer program instructions can also be stored in a computer-readable memory capable of guiding a computer or other programmable data processing device to work in a specific manner, such that the instructions stored in the computer-readable memory generate a manufactured article including an instruction device, and the instruction device implements the functions specified in Figure 1 one process or multiple processes and / or blocks Figure 1 one block or multiple blocks.

[0146] These computer program instructions can also be loaded onto a computer or other programmable data processing device, so that a series of operation steps are executed on the computer or other programmable device to generate a computer-implemented process. Thus, the instructions executed on the computer or other programmable device provide steps for implementing the functions specified in one process Figure 1 one process or multiple processes and / or blocks Figure 1 steps of the functions specified in one block or multiple blocks.

[0147] In a typical configuration, a computing device includes one or more processors (CPUs), an input / output interface, a network interface, and memory.

[0148] The memory may include non-permanent memory in the computer-readable medium, in the form of random access memory (RAM) and / or non-volatile memory, such as read-only memory (ROM) or flash memory (flash RAM). The memory is an example of a computer-readable medium.

[0149] Computer-readable media include permanent and non-permanent, removable and non-removable media, which can store information by any method or technology. The information can be computer-readable instructions, data structures, program modules, or other data. Examples of computer storage media include, but are not limited to, phase change memory (PRAM), static random access memory (SRAM), dynamic random access memory (DRAM), other types of random access memory (RAM), read-only memory (ROM), electrically erasable programmable read-only memory (EEPROM), flash memory or other memory technologies, compact disc read-only memory (CD-ROM), digital versatile disc (DVD) or other optical storage, magnetic cassette tapes, magnetic tape magnetic disk storage or other magnetic storage devices, or any other non-transmission media that can be used to store information accessible by a computing device. As defined herein, computer-readable media do not include transitory computer-readable media, such as modulated data signals and carrier waves.

[0150] It should also be noted that the term "comprising", "including" or any other variant thereof is intended to cover non-exclusive inclusion, so that a process, method, commodity or device comprising a series of elements not only includes those elements, but also includes other elements not expressly listed, or elements inherent to such process, method, commodity or device. Without further limitation, an element defined by the statement "comprising one..." does not exclude the presence of additional identical elements in the process, method, commodity or device comprising the element.

[0151] The above are only embodiments of the present application and are not intended to limit the present application. For those skilled in the art, various changes and modifications can be made to the present application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included within the scope of the claims of the present application.

Claims

1. A method for simulating stress relaxation of a digital core, comprising: Setting the initial conditions and boundary conditions of the digital core, and establishing a uniform strain field at the initial moment of strain of the digital core, so that the stress field and strain field inside the digital core change with time, and the total strain of the digital core remains unchanged; Calculating the stress field of the digital core according to the rotated staggered grid finite difference algorithm; And Performing volume averaging on the stress field to obtain the stress relaxation curve of the digital core.

2. The method according to claim 1, wherein The initial conditions are: v x u(x, z, t = 0) = 0 x E0x(x, z, t = 0) = E0x v z (x,z,t = 0)= 0, u z (x,z,t = 0)= 0, where v x and v z are the velocity components of the mass points of the digital core in the X and Z directions respectively, where the X direction and the Z direction are the positive directions of the X axis and the Z axis of the rectangular coordinate system where the digital core is located, and the X direction is the horizontal direction; t is time; x and z are the coordinates of the particle on the X-axis and Z-axis of the rectangular coordinate system respectively; u x and u z are the displacement components of the said particle in the said X direction and the said Z direction respectively; E0 is the coefficient for controlling the normal strain of the digital core in the x direction, and is a constant with 0 < E0 ≤ 10 -6 ​ 3. The method according to claim 2, wherein The left boundary of the said boundary conditions is: The right boundary of the said boundary condition is: wherein, E0 is a coefficient for controlling the overall strain level of the digital core, and is a constant satisfying 0 < E0 ≤ 10 -6 ; L is the side length of the digital core.

4. The method according to claim 3, characterized in that When calculating the stress relaxation curve of the digital core according to the rotated staggered grid finite difference algorithm, the constitutive equations of the skeleton and pore fluid of the digital core are: The motion equation is: In the formula, λ and μ are Lame constants; σ xx 、 σ zz 、 and σ zx are respectively the normal stress of the said particle along the X direction, the normal stress along the Z direction, and the shear stress in the XOZ plane, where the XOZ plane is the XOZ plane of the rectangular coordinate system where the digital core is located; e xx 、 e zz 、 and e zx are respectively the normal strain of the said particle along the said X direction, the normal strain along the said Z direction, and the shear strain in the XOZ plane; η λ and η μ are the fluid viscosity coefficients related to volume deformation and shear deformation respectively, and ρ is the density of the digital core.

5. The method according to claim 1, wherein Performing volume averaging on the stress field through the following formula: In the formula, i and j represent the indices of the model grid in the rotating staggered grid finite difference algorithm; <·> represents taking the volume average value; represents the normal stress in the x direction of the (i, j) grid at time t; τ xx (t) represents the average normal stress in the x direction of the digital core at time t; at time t≥0, τ xx (t) changes with time.

6. The method according to claim 1, further comprising: Performing noise reduction processing on the stress relaxation curve to obtain the noise-reduced stress relaxation curve.

7. The method according to claim 6, wherein the noise reduction processing is performed by an exponential function fitting method or a logarithmic domain wavelet packet method.

8. The method according to claim 6, wherein the noise reduction processing step comprises: In a linear coordinate system, establishing an equally spaced time series, denoted as t = {Δt, 2Δt,..., NΔt}; Represent the time series in a logarithmic coordinate system, denoted as Resample the t l such that the t l is an equally-spaced sequence in the logarithmic coordinate system, denoted as t lh ; According to the said t lh Resample the stress relaxation curve, denoted as S lh = S(t lh ); Use the wavelet packet denoising method to eliminate the high-frequency noise of the S lh ; Transform the denoised S lh to the linear domain to obtain the denoised stress relaxation curve.

9. The method according to claim 1 or 6, further comprising: Calculating the dispersion curve and attenuation curve of the digital core according to the spectral ratio method and the stress relaxation curve.

10. The method according to claim 9, wherein when calculating the dispersion curve and attenuation curve of the digital core, the dispersion and quality factor of the digital core are calculated through the complex elastic modulus: In the formula, FT[] represents the Fourier transform; and respectively represent the stress rate and strain rate of the digital core in the X direction at time t, and is the zero-delay pulse function, where The X direction is the positive direction of the X-axis of the rectangular coordinate system where the digital core is located; M(f) represents the complex elastic modulus at frequency f; V P (f) represents the longitudinal wave velocity at frequency f; Q P (f) represents the longitudinal wave quality factor at the frequency f.

11. A stress relaxation simulation device for digital cores, comprising: A condition loading module, a stress field calculation module, and a stress relaxation calculation module, wherein: The condition loading module sets the initial conditions and boundary conditions of the digital core, and establishes a uniform strain field at the initial moment of strain of the digital core, so that the stress field and strain field inside the digital core change with time, and the total strain of the digital core remains unchanged; The stress field calculation module calculates the stress field of the digital core according to the rotated staggered grid finite difference algorithm; and The stress relaxation calculation module performs volume averaging on the stress field to obtain the stress relaxation curve of the digital core.

12. The device according to claim 11, characterized in that It further comprises: A denoising module that performs noise reduction processing on the stress relaxation curve to obtain the noise-reduced stress relaxation curve.

13. The device according to claim 11, wherein It further comprises: A dispersion and attenuation calculation module that calculates the dispersion curve and attenuation curve of the digital core according to the spectral ratio method and the stress relaxation curve.

14. A machine-readable storage medium, on which instructions are stored, and the instructions are used to cause a machine to execute: the method for simulating stress relaxation of a digital core according to any one of claims 1-10.

15. A processor, characterized in that, For running a program, wherein when the program is run, it is used to execute: the method for simulating stress relaxation of a digital core according to any one of claims 1-10.

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