A method for predicting earth stress of a linear-elastic-viscous relaxed formation

Abstract

The present application provides a kind of linear elasticity-viscous relaxation formation's stress prediction method, comprising: S1, obtaining the generalized Hook's law of characterizing homogeneous isotropic elastic formation;S2, obtaining the formation stress logging calculation model of homogeneous isotropic linear elastic formation;S3, obtaining the horizontal stress difference calculation model of homogeneous isotropic linear elastic formation;S4, obtaining the horizontal stress difference prediction model of viscous relaxation formation;S5, obtain the basic model of horizontal principal stress difference of viscous relaxation formation;S6, obtain the horizontal maximum and horizontal minimum principal stress calculation model of viscous relaxation formation;S7, obtain the formation stress prediction model of linear elasticity-viscous relaxation formation;S8, model parameter acquisition method and rationality verification;S9, realize the formation stress profile prediction and formation stress longitudinal distribution characteristic analysis of linear elasticity-viscous relaxation formation;S10, obtain three-dimensional formation stress field prediction result, realize the formation stress spatial distribution characteristic analysis, solve the limitation of existing formation stress calculation model.

Description

Technical Field

[0001] This invention belongs to the field of oil and gas engineering geomechanics, and particularly relates to a method for predicting geostress in linear elastic-viscosity relaxed formations. Background Technology

[0002] In-situ stress is a key geomechanical parameter. Research on in-situ stress is crucial for structural analysis, hydrocarbon accumulation, wellbore stability, well network deployment, and fracturing stimulation throughout the entire oil and gas exploration, development, and production process. Especially as oil and gas exploration and development targets shift from shallow to deep and ultra-deep formations, and from conventional to unconventional and complex formations, the accurate prediction of in-situ stress presents even greater challenges and requirements.

[0003] However, many existing geostress calculation models have significant limitations. These models are primarily designed for homogeneous or transversely isotropic strata and typically assume that the strata undergo only linear elastic deformation. For complex, heterogeneous strata, external forces do not only induce simple linear elastic deformation; their plastic and inelastic deformation characteristics are equally significant. Existing models cannot accurately evaluate the stress characteristics of such strata, and more effective calculation models are needed for geostress assessment in complex strata. Summary of the Invention

[0004] The purpose of this invention is to address the limitations of existing geostress calculation models and the inability of existing technologies to handle complex and non-uniform geological formations.

[0005] To achieve the above objectives, the present invention provides a method for predicting in-situ stress in linear elastic-viscosity relaxed formations, comprising the following steps: S1. Based on the generalized Hooke's law expressed in terms of principal stresses under three-dimensional spatial stress conditions, derive the generalized Hooke's law characterizing homogeneous isotropic elastic strata; specifically: The generalized Hooke's law expressed in terms of principal stresses is: ; In the formula: , , These represent the total strain of the unit cell at its maximum horizontal position, minimum horizontal position, and vertical position, respectively. The normal strain generated in the direction of the maximum horizontal principal stress under the sole action of the maximum horizontal principal stress. The normal strain generated in the direction of the maximum horizontal principal stress under the sole action of the minimum horizontal principal stress. This refers to the normal strain generated in the direction of the maximum horizontal principal stress under the sole action of the vertical principal stress. The normal strain generated in the direction of the minimum horizontal principal stress under the sole action of the minimum horizontal principal stress. The normal strain generated in the direction of the minimum horizontal principal stress under the sole action of the maximum horizontal principal stress. This refers to the normal strain generated in the direction of the minimum horizontal principal stress under the sole action of the vertical principal stress. The normal strain generated in the direction of the vertical principal stress under the sole action of the vertical principal stress. The normal strain generated in the direction of the vertical principal stress under the action of the maximum horizontal principal stress alone. The normal strain generated in the direction of the vertical principal stress under the action of the minimum horizontal principal stress alone; Normal stress along the direction of maximum horizontal principal stress, Normal stress along the direction of minimum horizontal principal stress, It is the normal stress along the direction of the vertical principal stress; , , These are the maximum, minimum, and vertical elastic moduli of the unit cell, respectively. , , , , , All are Poisson's ratios. Because of Normal stress in the direction, Normal strain in the direction and The ratio of the normal strain in the direction; Assuming that the pure sandstone strata are homogeneous isotropic elastic bodies, the physical properties of the rock are the same along different directions. Therefore, the elastic parameters satisfy the following expression:

[0006] in , , These represent the Poisson's ratios for the horizontal maximum, horizontal minimum, and vertical values ​​of the unit cell, respectively. It is the elastic modulus and does not have directional information; It is Poisson's ratio and does not contain directional information; Substituting the expression for the elastic parameters into the generalized Hooke's law expressed in terms of principal stresses yields the generalized Hooke's law characterizing a homogeneous, isotropic, linearly elastic medium.

[0007] .

[0008] S2. The Hooke's law, expressed in terms of strain, in step S1 is transformed into a stress calculation model expressed in terms of the horizontal maximum principal stress and the horizontal minimum principal stress components, thus obtaining the geostress logging calculation model for homogeneous isotropic linear elastic formations. ; in For vertical principal stress, For Biots coefficients, This refers to the formation pore pressure.

[0009] S3. Using the two horizontal principal stress components from step S2, a horizontal stress difference calculation model for homogeneous isotropic linear elastic strata is further obtained, namely: ; S4. Based on the known differential stress model of viscous relaxed formations, a horizontal stress difference prediction model for viscous relaxed formations is obtained, specifically: The creep compliance function in linear viscoelastic theory Using the principle of linear superposition, a calculation model for the horizontal stress difference in viscous relaxed formations is derived: ; in Differential stress caused by loading at a constant strain rate The deformation coefficient is related to the elastic modulus, t is the effective duration of deformation, and n is the power-law constitutive parameter. The strain rate is constant.

[0010] Based on the fact that E is approximately equal to 1 / B, the horizontal stress difference calculation model for viscous relaxed formations can be transformed into the following form: ; S5. Compare the form similarity between the horizontal stress difference calculation model of the viscous relaxed strata in step S4 and the horizontal stress difference calculation model of the linear elastic strata in step S3, and derive the basic model of the horizontal principal stress difference of the viscous relaxed mudstone strata in reverse. ; S6. Based on the construction action terms in step S5, considering the overlying formation pressure, formation pore pressure and rock mechanical parameters, derive the calculation model for the maximum and minimum horizontal principal stresses of the viscous relaxed strata. ; S7. Taking into account the linear elastic characteristics of hard rock formations and the viscous relaxation characteristics of soft rock formations, and combining the geostress logging calculation model of homogeneous isotropic linear elastic formations in step S2, the calculation model of the maximum and minimum horizontal principal stresses of viscous relaxed formations in step S6, and the three basic assumptions, a geostress prediction model for linear elastic-viscous relaxed formations is obtained. ; in This refers to the mud content.

[0011] S8. Parameter acquisition method and model rationality verification of the geostress prediction model for linear elastic-viscous relaxation strata; S9. Combining well logging data from drilling, we can predict the geostress profile of linear elastic-viscosity relaxed formations and analyze the longitudinal distribution characteristics of geostress. S10. By combining seismic data, geostress well logging prediction results, three-dimensional rock mechanics parameter field, and three-dimensional formation pore pressure field, the prediction results of the three-dimensional geostress field are obtained, and the spatial distribution characteristics of geostress are analyzed.

[0012] Beneficial effects: This invention focuses on the influence of lithology (especially alternating hard and soft strata) on in-situ stress, and reflects the heterogeneity and elastic-plastic deformation characteristics of formations. When calculating in-situ stress, it fully considers the rock mechanical properties, pore pressure, tectonic activity, clay content, and the differential deformation characteristics of hard and soft strata. It derives a linear elastic-viscosity relaxation in-situ stress prediction model suitable for alternating hard and soft strata. Furthermore, by combining well logging and seismic data, it achieves in-situ stress prediction, accurate characterization of stress differences, and analysis of the vertical, lateral, and spatial distribution characteristics of in-situ stress in alternating hard and soft strata. The in-situ stress prediction results obtained based on this invention will provide supporting evidence for engineering sweet spot selection, wellbore trajectory optimization, wellbore stability evaluation, fracturing stimulation construction, and efficient oil and gas exploration and development. Attached Figure Description

[0013] Figure 1 This is a flowchart of the geostress prediction method for linear elastic-viscous relaxation formations provided by the present invention. Figure 2 This is a comparison chart of the geostress calculation results for a sandstone section of a horizontal well provided by this invention; Figure 3 This is a comparison chart of the predicted and measured values ​​of ground stress in a certain well provided by the present invention; Figure 4 This is a comparison chart of predicted and measured values ​​of formation fracture pressure in a certain well provided by the present invention; Figure 5 This invention provides the geostress prediction results for a certain well based on a linear elastic-viscous relaxation model. Figure 6 This is a well stress logging prediction profile provided by the present invention; Figure 7 This is the prediction result of the maximum principal stress at the horizontal level provided by the present invention; Figure 8 This is the prediction result of the lowest horizontal principal stress provided by the present invention; Figure 9 This is the vertical principal stress prediction result provided by the present invention; Figure 10 This is a comparison chart of predicted values ​​obtained from different geostress calculation models provided by this invention. Detailed Implementation

[0014] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0015] The application principle of the present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0016] Example 1: like Figure 1 As shown, a method for predicting geostress in linear elastic-viscosity relaxed formations is as follows: S1. Based on the generalized Hooke's law expressed in terms of principal stresses under classical three-dimensional spatial stress states, a generalized Hooke's law characterizing homogeneous isotropic elastic strata is derived. The generalized Hooke's law expressed in terms of principal stresses is as follows: (1); In the formula: , , These represent the total strain of the unit cell at its maximum horizontal position, minimum horizontal position, and vertical position, respectively. The normal strain generated in the direction of the maximum horizontal principal stress under the sole action of the maximum horizontal principal stress. The normal strain generated in the direction of the maximum horizontal principal stress under the sole action of the minimum horizontal principal stress. This refers to the normal strain generated in the direction of the maximum horizontal principal stress under the sole action of the vertical principal stress. The normal strain generated in the direction of the minimum horizontal principal stress under the sole action of the minimum horizontal principal stress. The normal strain generated in the direction of the minimum horizontal principal stress under the sole action of the maximum horizontal principal stress. This refers to the normal strain generated in the direction of the minimum horizontal principal stress under the sole action of the vertical principal stress. The normal strain generated in the direction of the vertical principal stress under the sole action of the vertical principal stress. The normal strain generated in the direction of the vertical principal stress under the action of the maximum horizontal principal stress alone. The normal strain generated in the direction of the vertical principal stress under the action of the minimum horizontal principal stress alone; Normal stress along the direction of maximum horizontal principal stress, Normal stress along the direction of minimum horizontal principal stress, It is the normal stress along the direction of the vertical principal stress; , , These are the maximum, minimum, and vertical elastic moduli of the unit cell, respectively. , , , , , All are Poisson's ratios. This can be understood as due to Normal stress in the direction, Normal strain in the direction and The ratio of the normal strain in the direction.

[0017] Assuming that the pure sandstone strata are homogeneous isotropic elastic bodies, the physical properties of the rock are the same along different directions. Therefore, the elastic parameters satisfy the following expression: (2); in , , These are the maximum, minimum, and vertical elastic moduli of the unit cell, respectively. , , These represent the Poisson's ratios for the horizontal maximum, horizontal minimum, and vertical values ​​of the unit cell, respectively. It is the elastic modulus and does not have directional information; It is Poisson's ratio and does not contain directional information.

[0018] Substituting equation (2) into equation (1) of the generalized Hooke's law expressed in terms of principal stress, we can obtain the generalized Hooke's law characterizing a homogeneous isotropic linear elastic medium.

[0019] (3); S2. Based on the generalized Hooke's law of homogeneous isotropic linear elastic medium as shown in equation (3), the Hooke's law expressed in terms of strain is transformed into a stress calculation model expressed in terms of the horizontal maximum principal stress and the horizontal minimum principal stress components. This model is the geostress logging calculation model for homogeneous isotropic linear elastic formations.

[0020] (4); in For vertical principal stress, For Biots coefficients, This refers to the formation pore pressure.

[0021] S3. Based on the two horizontal principal stress components shown in equation (4), a horizontal stress difference calculation model for homogeneous isotropic linear elastic strata is further calculated, namely: (5); (6); (7); (8); S4. Based on the known differential stress model induced by constant strain rate tectonic loading in viscous relaxed strata, a horizontal stress difference prediction model for viscous relaxed mudstone strata is obtained, namely: By applying the principle of linear superposition to the creep compliance function in the linear viscoelastic theory shown in equation (9), the horizontal stress difference calculation model of the viscous relaxed strata shown in equation (10) is obtained: (9); (10); in Differential stress caused by loading at a constant strain rate The deformation coefficient is related to the elastic modulus, t is the effective duration of deformation, and n is the power-law constitutive parameter. The strain rate is constant.

[0022] The relationship between the deformation coefficient and the elastic modulus is modeled as follows: ;

[0023] Based on the relationship model between the deformation coefficient and the elastic modulus, the horizontal stress difference calculation model of viscous relaxed formations can be transformed into the horizontal stress difference calculation model form of viscous relaxed formations as shown in Equation (11): (11); S5. Comparing the horizontal stress difference calculation model (11) of the viscous loose strata with the horizontal stress difference calculation model (8) of the linear elastic strata, and by comparing their formal similarities, and drawing on the derivation mode from the geostress prediction model of the linear elastic strata to the horizontal principal stress difference model, we can perform reverse derivation to obtain the basic model of the horizontal principal stress difference of the viscous loose mudstone strata, namely: (12); (13); S6. Taking the two parameters on the right side of equation (13) as tectonic terms in the calculation model of the maximum and minimum horizontal principal stresses of viscous loose strata, and further considering the overlying strata pressure, formation pressure, and rock mechanics parameters, the derived calculation model of the maximum and minimum horizontal principal stresses of viscous loose strata can be expressed as in equation (14), i.e.: (14); S7. Derivation of the stress prediction model for linear elastic-viscosity relaxed strata: Taking into account the linear elastic characteristics of hard rock strata and the viscosity relaxation characteristics of soft rock strata, based on the stress calculation model for linear elastic hard rock strata as shown in Equation (4) and the stress calculation model for viscosity relaxed soft rock strata as shown in Equation (14), and based on the following assumptions I, II, and III, the stress prediction model for linear elastic-viscosity relaxed strata as shown in Equation (15) is derived. I. The strata consist of homogeneous, isotropic, linearly elastic sandstone strata and viscoelastic mudstone strata superimposed on each other; II. The composition of geostress comprehensively considers the self-weight of the strata rocks, the pore pressure of the strata, the mechanical properties of the rocks, and tectonic activity; III. It is believed that the stress characteristics of strata are affected by lithology. Under stress, sandstone undergoes linear elastic deformation, while mudstone undergoes viscous relaxation. The mud content is introduced to characterize different lithologies.

[0024] (15); in The constant structural strain rate; t is the effective duration of deformation; n is the power-law constitutive parameter; This refers to the mud content.

[0025] S8. Parameter acquisition method and model rationality verification of the geostress prediction model for linear elastic-viscous relaxation strata.

[0026] S81. Model Parameter Acquisition: Reasonable acquisition of various parameter values ​​in the model is crucial for the effective application of the model. All parameters in the model can be acquired in the following ways.

[0027] I. Overlying strata pressure

[0028] The overlying formation pressure caused by the weight of the rock can be obtained by integrating density logging data, and the specific calculation formula is shown in equation (16): (16); In the formula: The overlying formation pressure is given in MPa; H represents the depth of the calculation point in meters. The logging density values ​​vary with depth, in g / cm³. 3 g is the acceleration due to gravity, kg·m / s² 2 .

[0029] II. Elastic modulus E The elastic modulus can be calculated based on the longitudinal wave time difference and transverse wave time difference obtained from cross-dipole array acoustic logging, combined with the density logging curve in conventional logging. The specific calculation formula is shown in equation (17): (17); In the formula: The bulk density of the formation is expressed in g / cm³. 3 ; For the shear wave time difference of the strata, ; For the P-wave time difference of the strata, ; The unit conversion factor is set to 10. 9 .

[0030] III. Poisson's ratio

[0031] Poisson's ratio can also be calculated based on the P-wave and S-wave time difference curves obtained from cross-dipole array acoustic logging. The specific calculation formula is shown in equation (18): (18); IV. Clay content

[0032] (19); (20); In the formula: SH is the relative value of natural gamma; GR is the natural gamma logging value of the target layer, API; API (Advanced Phosphorus Logging) values ​​for pure lithological formations; API value for natural gamma ray logging in pure mudstone formation; The value represents the clay content, a decimal; GCUR is an empirical coefficient related to stratigraphic age, with 3.7 for new strata and 2 for old strata.

[0033] V. Formation pressure

[0034] Formation pressure can be obtained based on the traditional Eaton method, equivalent depth method, and effective stress method. For details, please refer to the existing literature Liu Xiangjun and Liang Lixi. Oil and Gas Engineering Logging Theory and Application [M]. Science Press, 2015.

[0035] VI. Structural strain coefficient , ,

[0036] Maximum horizontal structural strain coefficient and the minimum structural strain coefficient of the horizontal It can be calculated back from fracturing data. Constant tectonic strain rate. The constant deformation rate of the plate where the study area is located can be directly taken, which is usually a regional empirical value. For details, please refer to the existing literature Wang Qi, Zhang Peizhen, et al. Current crustal movement and tectonic deformation in mainland China [J]. Science in China Series D, 2001, 31(7):8.

[0037] VII. Biots coefficient

[0038] The Biot coefficient has no clear physical meaning and no precise method for determination. It can usually be determined experimentally, and its calculation formula is shown in equation (21). Its value is between 0 and 1, but there are many experimental methods and the measured values ​​are very scattered. In addition, it can also be determined empirically, generally taking a value between 0.5 and 0.8, and then fitting the result.

[0039] (twenty one); In the formula: is the compression coefficient of the skeleton volume. is the compressibility coefficient of the rock's apparent volume.

[0040] VIII. Effective deformation duration t: This value can be taken as the deformation time experienced by the target layer from deposition to the present.

[0041] IX. Power-law constitutive parameter n: n represents the creep parameter, which can be approximately taken as 666.7 / E.

[0042] S82. Model Rationality Verification: The accuracy of well logging prediction results for linear elastic-viscosity relaxation formations was verified through the following three methods: I. The rationality of a model can be verified by its regression simplification effect. The geostress prediction model of linear elastic-viscosity relaxed strata proposed in this invention, as shown in equation (15), is applied to pure sandstone strata ( When =0), it can be simplified to the geostress calculation model of homogeneous isotropic linear elastic sandstone strata as shown in equation (4); when applied to pure mudstone strata ( When =1), it can be simplified to the geostress calculation model of viscous relaxed mudstone formations as shown in equation (14). Figure 2 The results of geostress calculation for a horizontal well in a sandstone section (with low clay content) show that the calculation results obtained by the formula derived in this invention are in good agreement with the results obtained by the linear elastic sandstone formation geostress calculation model. When the well is in a pure sandstone formation, the calculation results of the two models will be completely consistent.

[0043] II. In addition to verifying the model's rationality through regression simplification performance, its rationality was also demonstrated by comparing predicted values ​​with measured in-situ stress values. Figure 3 The comparison between the measured and predicted values ​​shows that the predicted values ​​are in good agreement with the measured values, proving that the linear elastic-viscosity relaxation stress prediction model has good applicability.

[0044] III. In addition to comparing measured and predicted values, formation collapse pressure and formation fracturing pressure were further calculated based on the in-situ stress prediction results. The accuracy of the model was also verified by comparing the consistency between the predicted and inversely calculated values ​​of formation collapse and fracturing pressures and the fracturing data. Figure 4 The results show that the geostress values ​​obtained based on the linear elastic-viscosity relaxation geostress prediction model and the predicted formation fracture pressure values ​​calculated further are in good agreement with the formation fracture pressure values ​​calculated from the hydraulic fracturing data, which further illustrates the rationality of the geostress calculation model proposed in this invention.

[0045] S9. Based on this model, combined with well logging data from drilled wells, rock mechanics parameters, and formation pore pressure prediction results, the following can be obtained: Figure 5 and Figure 6 The geostress logging prediction results shown demonstrate the prediction of geostress profiles and the analysis of the vertical distribution characteristics of geostress in linear elastic-viscosity relaxation formations. From... Figure 5 The results show that the sandstone and mudstone sections exhibit significant differences in geostress. The mudstone section, influenced by variations in mud content, plastic deformation, and viscous relaxation, shows large stress fluctuations and lower stress values ​​compared to the adjacent sandstone section. The sandstone section primarily undergoes elastic deformation with no obvious stress relaxation, exhibiting smaller stress fluctuations and significantly higher stress values ​​than the adjacent mudstone section. Due to stress relaxation characteristics, the maximum and minimum horizontal principal stresses in the mudstone strata gradually converge, resulting in a smaller horizontal stress difference compared to the sandstone section. Figure 6 The single-well in-situ stress profile obtained from the linear elastic-viscosity relaxation formation stress prediction model shows that, influenced by lithology, the vertical distribution of in-situ stress exhibits significant lithology-controlled characteristics. Specifically, in sandstone strata, stress fluctuations are small, stress values ​​are high, and horizontal stress differences are large. Conversely, in mudstone strata, stress fluctuations are large, stress values ​​are low, and horizontal stress differences are low. Furthermore, the vertical distribution characteristics of in-situ stress show good consistency with the vertical distribution characteristics of rock mechanical parameters. In summary, this demonstrates that the linear elastic-viscosity relaxation formation stress prediction model proposed in this invention reasonably characterizes the stress differences between sandstone and mudstone strata.

[0046] S10. Based on the model proposed in this invention, combined with seismic data, geostress well logging prediction results, three-dimensional rock mechanics parameter field, and three-dimensional formation pore pressure field, the following can be obtained: Figures 7-9 The three-dimensional geostress field prediction results shown enable the analysis of the spatial distribution characteristics of geostress. From... Figure 7 and Figure 8The three-dimensional geostress field prediction results obtained from the linear elastic-viscosity relaxation formation geostress prediction model show that, in addition to exhibiting an overall characteristic of increasing with depth, the maximum and minimum horizontal principal stresses also possess heterogeneous and nonlinear variation characteristics in both the vertical and horizontal directions, indicating that the stress differences among different lithologies have been effectively characterized. Figure 9 Compared to the spatial distribution characteristics of vertical principal stress, the lithological control of the maximum and minimum horizontal principal stresses is more pronounced. Since the vertical principal stress is mainly influenced by the density of the overlying strata, it exhibits a uniformly varying spatial distribution with no significant lithological control. In summary, this demonstrates that the maximum and minimum horizontal principal stresses obtained from the linear elastic-viscosity relaxation formation stress calculation model can effectively characterize the stress in sandstone and mudstone strata and achieve accurate prediction.

[0047] like Figure 10 The image shows a comparison of the geostress profiles obtained from the linear elastic-viscosity relaxation formation stress prediction model proposed in this invention and the currently widely used combined spring model. From... Figure 10 The results show that, because the combined spring model assumes that the formation only undergoes linear elastic deformation, there is no significant difference in the predicted in-situ stress between the mudstone and sandstone sections. However, the linear elastic-viscosity relaxation formation in-situ stress prediction model proposed in this invention assumes that the mudstone formation undergoes plastic deformation and viscosity relaxation, while the sandstone formation undergoes linear elastic deformation. Therefore, the predicted in-situ stress exhibits significant lithological control characteristics, reasonably reflecting the stress differences between different formations.

[0048] 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 predicting in-situ stress in linear elastic-viscosity relaxed formations, characterized in that, Includes the following steps: S1. Based on the generalized Hooke's law expressed in terms of principal stresses under three-dimensional spatial stress conditions, derive the generalized Hooke's law characterizing homogeneous isotropic elastic strata; specifically: The generalized Hooke's law expressed in terms of principal stresses is: ； In the formula: , , These represent the total strain of the unit cell at its maximum horizontal position, minimum horizontal position, and vertical position, respectively. The normal strain generated in the direction of the maximum horizontal principal stress under the sole action of the maximum horizontal principal stress. The normal strain generated in the direction of the maximum horizontal principal stress under the sole action of the minimum horizontal principal stress. This refers to the normal strain generated in the direction of the maximum horizontal principal stress under the sole action of the vertical principal stress. The normal strain generated in the direction of the minimum horizontal principal stress under the sole action of the minimum horizontal principal stress. The normal strain generated in the direction of the minimum horizontal principal stress under the action of the maximum horizontal principal stress alone; This refers to the normal strain generated in the direction of the minimum horizontal principal stress under the sole action of the vertical principal stress. This refers to the normal strain generated in the direction of the vertical principal stress under the sole action of the vertical principal stress. The normal strain generated in the direction of the vertical principal stress under the action of the maximum horizontal principal stress alone; The normal strain generated in the direction of the vertical principal stress under the action of the minimum horizontal principal stress alone; It is the normal stress along the direction of the maximum horizontal principal stress; It is the normal stress along the direction of the minimum principal stress in the horizontal direction; It is the normal stress along the direction of the vertical principal stress; , , These are the maximum, minimum, and vertical elastic moduli of the unit cell, respectively. , , , , , All are Poisson's ratios. Because of Normal stress in the direction, Normal strain in the direction and The ratio of the normal strain in the direction; Assuming that the pure sandstone strata are homogeneous isotropic elastic bodies, the physical properties of the rock are the same along different directions. Therefore, the elastic parameters satisfy the following expression: ; in , , These represent the Poisson's ratios for the horizontal maximum, horizontal minimum, and vertical values ​​of the unit cell, respectively. It is the elastic modulus and does not have directional information; It is Poisson's ratio and does not contain directional information; Substituting the expression for the elastic parameters into the generalized Hooke's law expressed in terms of principal stresses, we obtain the generalized Hooke's law characterizing a homogeneous, isotropic, linear elastic medium. ； S2. The generalized Hooke's law characterizing the homogeneous isotropic linear elastic medium in step S1 is derived into a stress calculation model represented by the horizontal maximum principal stress and the horizontal minimum principal stress components, thus obtaining the geostress logging calculation model for the homogeneous isotropic linear elastic formation. ； in For vertical principal stress, For Biots coefficients, Formation pore pressure; S3. Using the two horizontal principal stress components from step S2, a horizontal stress difference calculation model for homogeneous isotropic linear elastic strata is further obtained, namely: ; S4. Based on the known differential stress model of viscous relaxed formations, a horizontal stress difference prediction model for viscous relaxed formations is obtained, specifically: The creep compliance function in linear viscoelastic theory Using the principle of linear superposition, a calculation model for the horizontal stress difference in viscous relaxed formations is derived: ; in Differential stress caused by loading at a constant strain rate The deformation coefficient is related to the elastic modulus, t is the effective duration of deformation, and n is the power-law constitutive parameter. The strain rate is constant. Based on the relationship model between deformation coefficient and elastic modulus The horizontal stress difference calculation model for viscous relaxed formations is transformed into the following form: ; S5. Compare the form similarity between the horizontal stress difference calculation model of the viscous relaxed strata in step S4 and the horizontal stress difference calculation model of the linear elastic strata in step S3, and derive the basic model of the horizontal principal stress difference of the viscous relaxed strata in reverse. ； S6. Based on the construction action terms in step S5, considering the overlying formation pressure, formation pore pressure and rock mechanical parameters, derive the calculation model for the maximum and minimum horizontal principal stresses of the viscous relaxed strata. ； S7. Taking into account the linear elastic characteristics of hard rock formations and the viscous relaxation characteristics of soft rock formations, and combining the geostress logging calculation model for homogeneous isotropic linear elastic formations from step S2, the calculation models for the maximum and minimum horizontal principal stresses of viscous relaxed formations from step S6, and the three basic assumptions, the geostress prediction model for linear elastic-viscous relaxed formations is obtained: ； in The content of clay; The three assumptions are as follows: I. The strata consist of homogeneous, isotropic, linearly elastic sandstone strata and viscoelastic mudstone strata superimposed on each other; II. The composition of geostress comprehensively considers the self-weight of the strata rocks, the pore pressure of the strata, the mechanical properties of the rocks, and tectonic activity; III. The stress characteristics of strata are affected by lithology. Under stress, sandstone undergoes linear elastic deformation, while mudstone undergoes viscous relaxation. The mud content is introduced to characterize different lithologies. S8. Parameter acquisition method and model rationality verification of the geostress prediction model for linear elastic-viscous relaxation strata; S9. Combining well logging data from drilling, we can predict the geostress profile of linear elastic-viscosity relaxed formations and analyze the longitudinal distribution characteristics of geostress. S10. By combining seismic data, geostress well logging prediction results, three-dimensional rock mechanics parameter field, and three-dimensional formation pore pressure field, the prediction results of the three-dimensional geostress field are obtained, and the spatial distribution characteristics of geostress are analyzed.

2. The method for predicting geostress in linear elastic-viscosity relaxed formations according to claim 1, characterized in that, The method for obtaining the parameters in step S8 is as follows: I. Overlying strata pressure The overlying formation pressure caused by the weight of the rock is obtained by integrating density logging data: ; In the formula: The overlying formation pressure is given in MPa; H represents the depth of the calculation point in meters. The logging density values ​​vary with depth, in g / cm³. 3 g is the acceleration due to gravity, kg·m / s² 2 ; II. Elastic modulus E The elastic modulus is calculated based on the P-wave and S-wave transit times obtained from cross-dipole array acoustic logging, combined with density logging curves from conventional logging. ; In the formula: The bulk density of the formation is expressed in g / cm³. 3 ; For the shear wave time difference of the strata, ; For the P-wave time difference of the strata, ; The unit conversion factor is set to 10. 9 ; III. Poisson's ratio Poisson's ratio is calculated based on the P-wave and S-wave transit time curves obtained from cross-dipole array acoustic logging. ； IV. Clay content ； ； In the formula: SH is the relative value of natural gamma; GR is the natural gamma logging value of the target layer, API; API (Advanced Phosphorus Logging) values ​​for pure lithological formations; API value for natural gamma ray logging in pure mudstone formation; The value represents the clay content, a decimal; GCUR is an empirical coefficient related to stratigraphic age, with 3.7 for new strata and 2 for old strata. V. Formation pressure Formation pressure is obtained based on the traditional Eaton method, equivalent depth method, and effective stress method; VI. Structural strain coefficient , , Maximum horizontal structural strain coefficient and the minimum structural strain coefficient of the horizontal Obtained from back-calculation of fracturing data; constant structural strain rate. The constant deformation rate of the plate where the study area is located is directly taken; VII. Biots coefficient Biot coefficient The experimental method or empirical method is used; the calculation for the experimental method is as follows: ,That The value is between 0 and 1. is the compression coefficient of the skeleton volume. The compressibility coefficient of the rock's apparent volume is determined empirically. Values ​​were selected between 0.5 and 0.8, and then a fit was performed. VIII. Effective deformation duration t: This value is the deformation time experienced by the target layer from deposition to the present. IX. Power-law constitutive parameter n: n represents the creep parameter, which is 666.7 / E.

3. The method for predicting geostress in linear elastic-viscosity relaxed formations according to claim 1, characterized in that, Step S8, the model rationality verification, specifically involves: I. Regression simplification was used for verification. When the geostress prediction model of linear elastic-viscosity relaxed strata was applied to pure sandstone strata, the geostress calculation model of homogeneous isotropic linear elastic sandstone strata was regressed and simplified. When applied to pure mudstone strata, the geostress calculation model of viscosity relaxed mudstone strata was regressed and simplified. The verification results were obtained by comparing the calculation results of the two models. II. By verifying the rationality of the predicted values ​​and the measured values ​​of in-situ stress, if the predicted values ​​and the measured values ​​are in good agreement, then the linear elastic-viscosity relaxation in-situ stress prediction model has good applicability; III. Based on the geostress prediction results, the formation collapse pressure and formation fracturing pressure were further calculated. The accuracy of the model was verified by comparing the consistency between the predicted values ​​of formation collapse pressure and fracturing pressure and the back-calculated values ​​from the fracturing data.

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