Ground stress prediction method for linear elasticity-viscidity relaxation stratum

Abstract

The invention provides a crustal stress prediction method for a linear elasticity-viscosity relaxation stratum. The crustal stress prediction method comprises the following steps: S1, obtaining a generalized Hooke's law representing a homogeneous isotropic elastic stratum; s2, obtaining a ground stress logging calculation model of the homogeneous isotropic linear elastic stratum; s3, obtaining a horizontal stress difference calculation model of the homogeneous isotropic linear elastic stratum; s4, obtaining a horizontal stress difference prediction model of the viscous relaxation stratum; s5, obtaining a basic model of the horizontal principal stress difference of the viscous relaxation stratum; s6, obtaining a horizontal maximum principal stress calculation model and a horizontal minimum principal stress calculation model of the viscous relaxation stratum; s7, obtaining a crustal stress prediction model of the linear elasticity-viscidity relaxation stratum; s8, performing model parameter acquisition method and reasonability verification; s9, realizing crustal stress profile prediction and crustal stress longitudinal distribution characteristic analysis of the linear elasticity-viscidity relaxation stratum; and S10, a three-dimensional crustal stress field prediction result is obtained, crustal stress space distribution feature analysis is realized, and the limitation of an existing crustal stress calculation model is solved.

Description

Technical Field

[0001] The invention belongs to the field of oil and gas engineering geomechanics, and in particular relates to a method for predicting ground stress of a linear elastic-viscous relaxation formation. Background Art

[0002] Geostress is a key geomechanical parameter. The study of geostress is crucial for structural analysis, oil and gas accumulation, wellbore stability, well pattern deployment, and fracturing transformation in the entire oil and gas exploration, development, and production process. In particular, as the objects of oil and gas exploration and development gradually shift from shallow formations to deep and ultra-deep formations, and from conventional formations to unconventional and complex formations, the accurate prediction of geostress has higher requirements and difficulties.

[0003] However, a large number of existing geostress calculation models have strong limitations. These geostress calculation models are mainly proposed for homogeneous isotropic formations or transversely isotropic formation conditions, and usually assume that the formation only undergoes linear elastic deformation. For complex and heterogeneous formations, under the action of external forces, not only simple linear elastic deformation occurs, but also its plastic and inelastic deformation characteristics cannot be ignored. Existing models cannot accurately evaluate the stress characteristics of such formations. For the evaluation of geostress in complex formations, a more effective calculation model is needed. Summary of the invention

[0004] The purpose of the present invention is to solve the limitations of the existing geostress calculation model and the problem that the existing technology cannot handle complex and non-uniform strata.

[0005] To achieve the above object, the present invention provides a method for predicting ground stress in a linear elastic-viscous relaxation formation, comprising the following steps:

[0006] S1. Based on the generalized Hooke's law expressed by principal stress under three-dimensional stress state, the generalized Hooke's law characterizing homogeneous isotropic elastic formation is derived; specifically:

[0007] The generalized Hooke's law expressed as principal stress is:

[0008]

[0009] Where: ε H , ε h , ε v are the maximum horizontal strain, minimum horizontal strain and total vertical strain of the unit body respectively; ε' H is the normal strain in the direction of the horizontal maximum principal stress under the action of the horizontal maximum principal stress alone, ε” H is the normal strain in the direction of the maximum horizontal principal stress under the action of the minimum horizontal principal stress alone, ε”' Hε' is the normal strain generated in the direction of the maximum horizontal principal stress under the action of the vertical principal stress alone; h is the normal strain in the direction of the horizontal minimum principal stress under the action of the horizontal minimum principal stress alone, ε” h is the normal strain in the direction of the horizontal minimum principal stress under the action of the horizontal maximum principal stress alone, ε”' h ε' is the normal strain generated in the direction of the horizontal minimum principal stress under the action of the vertical principal stress alone; v is the normal strain in the direction of the vertical principal stress under the action of the vertical principal stress alone, ε” v is the normal strain in the direction of the vertical principal stress under the action of the horizontal maximum principal stress alone, ε”' v is the normal strain generated in the direction of the vertical principal stress under the action of the horizontal minimum principal stress alone; σ H is the normal stress along the direction of the maximum horizontal principal stress, σ h is the normal stress along the horizontal minimum principal stress direction, σ v is the normal stress along the direction of the vertical principal stress; E H 、E h 、E v are the maximum horizontal, minimum horizontal and vertical elastic moduli of the unit cell respectively; v Hh 、v Hv 、v hH 、v hv 、v vH 、v vh are Poisson's ratio, V ij is the ratio of the normal strain in the j direction to the normal strain in the i direction due to the normal stress in the i direction;

[0010] Assuming that the pure sandstone formation is a homogeneous isotropic elastic body, the physical properties of the rock in different directions are the same, so the elastic parameters satisfy the following expression:

[0011]

[0012] where v H 、v h 、v v are the maximum horizontal, minimum horizontal and vertical elastic moduli of the unit cell respectively; E is the elastic modulus, which does not have directional information; v is the Poisson's ratio, which does not have directional information;

[0013] Substituting the elastic parameter expressions into the generalized Hooke's law expressed in terms of principal stresses, we obtain the generalized Hooke's law that characterizes homogeneous isotropic linear elastic media.

[0014]

[0015] S2, converting the Hooke's law expressed in strain in step S1 into a stress calculation model expressed in horizontal maximum principal stress and horizontal minimum principal stress components, that is, obtaining a geostress logging calculation model for a homogeneous isotropic linear elastic formation;

[0016]

[0017] where σ v is the vertical principal stress, α is the Biots coefficient, P P is the formation pore pressure.

[0018] S3, further calculating the horizontal stress difference calculation model of the homogeneous isotropic linear elastic formation through the two horizontal principal stress components in step S2, that is:

[0019]

[0020] S4. Based on the known differential stress model of the viscous relaxed formation, a horizontal stress difference prediction model of the viscous relaxed formation is obtained, which is specifically:

[0021] The creep compliance function J(t) in the linear viscoelastic theory is converted into Bt n , using the linear superposition principle, the horizontal stress difference calculation model of viscous relaxed formation is calculated:

[0022]

[0023] Where J(t) is the differential stress caused by constant strain rate structural loading, B is the deformation coefficient related to the elastic modulus, t is the effective duration of deformation, and n is the power law constitutive parameter. is a constant structural strain rate.

[0024] Based on the fact that E is approximately equal to 1 / B, the horizontal stress difference calculation model of viscous relaxed formation can be transformed into the horizontal stress difference calculation model form of viscous relaxed formation:

[0025]

[0026] S5, comparing the horizontal stress difference calculation model of the viscous relaxed formation in step S4 with the horizontal stress difference calculation model of the linear elastic formation in step S3 for similarity in form, and reversely deriving a basic model of the horizontal principal stress difference of the viscous relaxed mudstone formation;

[0027]

[0028] S6. Constructing action terms according to step S5, taking into account overlying formation pressure, formation pore pressure and rock mechanics parameters, deriving a calculation model for horizontal maximum and horizontal minimum principal stresses of a viscous relaxed formation;

[0029]

[0030] S7, comprehensively considering the linear elastic characteristics of the hard rock formation and the viscous relaxation characteristics of the soft rock formation, combining the in-situ stress logging calculation model of the homogeneous isotropic linear elastic formation in step S2, the horizontal maximum and horizontal minimum principal stress calculation model of the viscous relaxation formation in step S6 and the three-point basic assumption, to obtain a in-situ stress prediction model for the linear elastic-viscous relaxation formation;

[0031]

[0032] Where V sh The mud content.

[0033] S8. Parameter acquisition method of the geostress prediction model for linear elastic-viscous relaxation formations and model rationality verification;

[0034] S9. Combined with the logging data of drilling, the prediction of the geostress profile of the linear elastic-viscous relaxation formation and the analysis of the vertical distribution characteristics of the geostress are realized;

[0035] S10. Combining seismic data, geostress logging prediction results, three-dimensional rock mechanics parameter field, and three-dimensional formation pore pressure field, obtain three-dimensional geostress field prediction results and realize spatial distribution characteristic analysis of geostress.

[0036] Beneficial effects:

[0037] The present invention focuses on the influence of lithology (especially soft and hard alternating strata) on geostress, and reflects the heterogeneity and elastic-plastic deformation characteristics of strata. When calculating geostress, it fully considers the mechanical properties of strata rock, pore pressure of strata, tectonic action, mud content, and differential deformation characteristics of soft and hard strata, and derives a linear elastic-viscous relaxation strata geostress prediction model suitable for soft and hard alternating strata. It further combines logging data and seismic data to achieve geostress prediction of soft and hard alternating strata, accurate characterization of stress differences, and analysis of longitudinal, lateral and spatial distribution characteristics of geostress of soft and hard alternating strata. The geostress prediction results obtained based on the invention will provide support for engineering sweet spot optimization, wellbore trajectory optimization, wellbore stability evaluation, fracturing transformation construction, and efficient oil and gas exploration and development. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 It is a flow chart of the geostress logging prediction method of linear elastic-viscous relaxation formation provided by the present invention;

[0039] Figure 2 It is a comparison diagram of the calculation results of the in-situ stress of a sandstone section of a horizontal well provided by the present invention;

[0040] Figure 3It is a comparison diagram of the predicted value and the measured value of the in-situ stress of a certain well provided by the present invention;

[0041] Figure 4 It is a comparison diagram of the predicted value and the measured value of the formation breakdown pressure of a certain well provided by the present invention;

[0042] Figure 5 It is the ground stress prediction result of a certain well based on the linear elastic-viscous relaxation model provided by the present invention;

[0043] Figure 6 It is a well-stress logging prediction profile of a certain well provided by the present invention;

[0044] Figure 7 is the horizontal maximum principal stress prediction result provided by the present invention;

[0045] Figure 8 It is the prediction result of the horizontal lowest principal stress provided by the present invention;

[0046] Fig. 9 It is the vertical principal stress prediction result provided by the present invention;

[0047] Fig.10 It is a comparison chart of predicted values ​​obtained by different geostress calculation models provided by the present invention. DETAILED DESCRIPTION

[0048] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0049] The application principle of the present invention is further described below in conjunction with the accompanying drawings and specific embodiments.

[0050] Embodiment 1:

[0051] like Figure 1 As shown in FIG. 1 , a method for predicting ground stress in a linear elastic-viscous relaxation formation is described as follows:

[0052] S1. Based on the generalized Hooke's law expressed by principal stress under the classic three-dimensional stress state, the generalized Hooke's law characterizing homogeneous isotropic elastic formations is derived. The generalized Hooke's law expressed by principal stress is:

[0053]

[0054] Where: ε H , ε h , ε v are the maximum horizontal strain, minimum horizontal strain and total vertical strain of the unit body respectively; ε' His the normal strain in the direction of the horizontal maximum principal stress under the action of the horizontal maximum principal stress alone, ε” H is the normal strain in the direction of the maximum horizontal principal stress under the action of the minimum horizontal principal stress alone, ε”' H ε' is the normal strain generated in the direction of the maximum horizontal principal stress under the action of the vertical principal stress alone; h is the normal strain in the direction of the horizontal minimum principal stress under the action of the horizontal minimum principal stress alone, ε” h is the normal strain in the direction of the horizontal minimum principal stress under the action of the horizontal maximum principal stress alone, ε”' h ε' is the normal strain generated in the direction of the horizontal minimum principal stress under the action of the vertical principal stress alone; v is the normal strain in the direction of the vertical principal stress under the action of the vertical principal stress alone, ε” v is the normal strain in the direction of the vertical principal stress under the action of the horizontal maximum principal stress alone, ε”' v is the normal strain generated in the direction of the vertical principal stress under the action of the horizontal minimum principal stress alone; σ H is the normal stress along the direction of the maximum horizontal principal stress, σ h is the normal stress along the horizontal minimum principal stress direction, σ v is the normal stress along the direction of the vertical principal stress; E H 、E h 、E v are the maximum horizontal, minimum horizontal and vertical elastic moduli of the unit cell respectively; v Hh 、v Hv 、v hH 、v hv 、v vH 、v vh are Poisson's ratio, V ij They can be understood as the ratio of the normal strain in the j direction to the normal strain in the i direction due to the normal stress in the i direction.

[0055] Assuming that the pure sandstone formation is a homogeneous isotropic elastic body, the physical properties of the rock in different directions are the same. Therefore, the elastic parameters satisfy the following expression:

[0056]

[0057] Where E H 、E h 、E v are the maximum horizontal, minimum horizontal and vertical elastic moduli of the unit cell respectively; v H 、v h 、v vThey are the maximum horizontal, minimum horizontal and vertical elastic moduli of the unit cell respectively; E is the elastic modulus and has no directional information; v is the Poisson's ratio and has no directional information.

[0058] Substituting equation (2) into the generalized Hooke's law equation (1) expressed in terms of principal stress, we can obtain the generalized Hooke's law that characterizes homogeneous isotropic linear elastic media.

[0059]

[0060] S2. Based on the generalized Hooke's law of homogeneous isotropic linear elastic medium as shown in formula (3), the Hooke's law expressed in strain is converted into a stress calculation model expressed in horizontal maximum principal stress and horizontal minimum principal stress components. This model is the in-situ stress logging calculation model for homogeneous isotropic linear elastic formations.

[0061]

[0062] where σ v is the vertical principal stress, α is the Biots coefficient, P P is the formation pore pressure.

[0063] S3. Based on the two horizontal principal stress components shown in formula (4), the horizontal stress difference calculation model of the homogeneous isotropic linear elastic formation is further calculated, that is:

[0064]

[0065] S4. Based on the known differential stress model caused by constant strain rate structural loading in viscous relaxed formations, a horizontal stress difference prediction model for viscous relaxed mudstone formations is obtained, namely:

[0066] The creep compliance function in the linear viscoelastic theory shown in equation (9) is applied with the linear superposition principle to calculate the horizontal stress difference calculation model of the viscous relaxation formation as shown in equation (10):

[0067] J(t)=Bt n (9)

[0068]

[0069] Where J(t) is the differential stress caused by constant strain rate structural loading, B is the deformation coefficient related to the elastic modulus, t is the effective duration of deformation, and n is the power law constitutive parameter. is a constant structural strain rate.

[0070] The relationship model between the deformation coefficient related to the elastic modulus and the elastic modulus is:

[0071]

[0072] Based on the relationship model between the deformation coefficient and the elastic modulus, the horizontal stress difference calculation model of the viscous relaxation formation can be transformed into the horizontal stress difference calculation model of the viscous relaxation formation as shown in formula (11):

[0073]

[0074] S5. Compare the horizontal stress difference calculation model formula (11) of viscous relaxed formation with the horizontal stress difference calculation model formula (8) of linear elastic formation. By comparing the similarity of the two forms, the derivation mode from the in-situ stress prediction model of linear elastic formation to the horizontal principal stress difference model is used for reverse deduction, and the basic model of horizontal principal stress difference of viscous relaxed mudstone formation is obtained, namely:

[0075]

[0076] S6. The two parameters on the right side of formula (13) are regarded as the tectonic action terms in the calculation model of the horizontal maximum principal stress and the horizontal minimum principal stress of the viscous relaxed formation. Further considering the overlying formation pressure, formation pressure and rock mechanics parameters, the calculation model of the horizontal maximum and horizontal minimum principal stress of the viscous relaxed formation can be derived as formula (14), that is:

[0077]

[0078] S7. Derivation result of linear elastic-viscous relaxation formation ground stress prediction model: Taking into account the linear elastic characteristics of hard rock formations and the viscous relaxation characteristics of soft rock formations, based on the linear elastic hard rock formation ground stress calculation model such as formula (4) and the viscous relaxation soft rock formation ground stress calculation model such as formula (14), based on the following assumptions I, II, III, the linear elastic-viscous relaxation formation ground stress prediction model shown in formula (15) is derived;

[0079] Ⅰ. The strata are composed of homogeneous, isotropic, linear elastic sandstone strata and viscoelastic mudstone strata superimposed;

[0080] II. The composition of ground stress comprehensively considers the deadweight of formation rock, formation pore pressure, rock mechanical properties and tectonic action;

[0081] III. It is believed that the stress characteristics of the formation are affected by the lithology. Under the action of stress, sandstone undergoes linear elastic deformation, while mudstone undergoes viscous relaxation. The mud content is introduced to characterize different lithologies.

[0082]

[0083] in is the constant structural strain rate; t is the effective duration of deformation; n is the power law constitutive parameter; V sh The mud content.

[0084] S8. Parameter acquisition method of the geostress prediction model for linear elastic-viscous relaxation formations and verification of the rationality of the model.

[0085] S81. Model parameter acquisition: Reasonable acquisition of various parameter values ​​in the model is crucial to the effective application of the model. All parameters in the model can be obtained in the following ways.

[0086] Ⅰ. Overlying formation pressure σ V

[0087] The overlying formation pressure caused by the deadweight of the rock can be obtained by integrating the density logging data. The specific calculation formula is as follows:

[0088]

[0089] Where: V is the overlying formation pressure, MPa; H is the depth of the calculation point, in m; ρ(h) is the logging density value that varies with depth, g / cm 3 ; g is the acceleration due to gravity, kg·m / s 2 .

[0090] Ⅱ. Elastic modulus E

[0091] The elastic modulus can be calculated based on the longitudinal wave time difference and transverse wave time difference obtained by the cross-dipole array acoustic logging, combined with the density logging curve in conventional logging. The specific calculation formula is as follows:

[0092]

[0093] Where: b is the bulk density of the formation, g / cm 3 ; Δt s is the shear wave time difference of the formation, μs / m; Δt c is the longitudinal wave time difference of the formation, μs / m; α is the unit conversion coefficient, which is 10 9 .

[0094] III. Poisson's ratio v

[0095] Poisson's ratio can also be calculated based on the P-wave time difference and S-wave time difference curves obtained by cross-dipole array acoustic logging. The specific calculation formula is as follows:

[0096]

[0097] Ⅳ. Mud content V sh

[0098]

[0099] Where: SH is the relative value of natural gamma; GR is the natural gamma logging value of the target layer, API; GR min is the natural gamma logging value of pure lithologic formation, API; GR max is the natural gamma logging value of pure mudstone formation, API; V sh is the mud content, a decimal; GCUR is an empirical coefficient related to the age of the strata, 3.7 for new strata and 2 for old strata.

[0100] V. Formation pressure P P

[0101] The 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, Liang Lixi. Oil and Gas Engineering Well Logging Theory and Application [M]. Science Press, 2015.

[0102] VI. Structural strain coefficient ε H , ε h ,

[0103] Horizontal maximum structural strain coefficient ε H and the horizontal minimum structural strain coefficient ε h It can be calculated based on the fracturing data. Constant structural 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.

[0104] VII. Biots coefficient α

[0105] The Biot coefficient has no clear physical meaning and no precise determination method. It can usually be determined by experimental methods. Its calculation formula is shown in formula (21). Its value is between 0 and 1, but there are many experimental methods and the measured values ​​are also scattered. In addition, it can also be determined by empirical methods, generally taking a value between 0.5 and 0.8, and then fitting.

[0106]

[0107] Where: C s is the compression coefficient of the skeleton volume, C b is the compression coefficient of the rock's apparent volume.

[0108] VIII. Effective duration of deformation t: This value can be taken as the deformation time experienced by the target layer from deposition to the present.

[0109] Ⅸ. Power-law constitutive parameter n: n represents the creep parameter, which can be roughly taken as 666.7 / E.

[0110] S82. Model rationality verification: The accuracy of the linear elastic-viscous relaxation formation in-situ stress logging prediction results was verified by the following three methods:

[0111] I. The rationality of a model can be verified by its regression simplification effect. The linear elastic-viscous relaxation formation in-situ stress prediction model proposed in the present invention as shown in formula (15) is applied to pure sandstone formation (V sh =0), it can be regressed and simplified into the in-situ stress calculation model of homogeneous isotropic linear elastic sandstone formation as shown in formula (4); when applied to pure mudstone formation (V sh =1), it can be regressed and simplified into the calculation model of the in-situ stress of the viscous relaxed mudstone formation as shown in formula (14). Figure 2 The calculation results of the geostress of a horizontal well in a sandstone section (with low mud content) of a horizontal well show that the calculation results obtained by the formula derived by the present invention are very consistent with the results obtained by the geostress calculation model of the linear elastic sandstone formation. When the well is a pure sandstone formation, the calculation results of the two models will be completely consistent.

[0112] Ⅱ. In addition to verifying the rationality of the model through the regression simplification performance of the model, the rationality of the model is also demonstrated through the predicted values ​​and the measured values ​​of the ground stress. Figure 3 The comparison between the measured values ​​and the predicted values ​​shows that the predicted values ​​are in good agreement with the measured values, which proves that the linear elastic-viscous relaxation stress prediction model has good applicability.

[0113] III. In addition to comparing the measured values ​​with the predicted values, the formation collapse pressure and formation fracture pressure are further calculated based on the ground stress prediction results. By comparing the consistency between the predicted values ​​of formation collapse pressure and fracture pressure and the back-calculated values ​​of fracturing data, the accuracy of the model can also be verified. Figure 4 It can be seen from the results that the in-situ stress value obtained based on the linear elastic-viscous relaxation in-situ stress prediction model and the formation fracture pressure prediction value further calculated are in good agreement with the formation fracture pressure value back-calculated from the fracturing data, which further illustrates the rationality of the in-situ stress calculation model proposed in the present invention.

[0114] S9. Based on this model, combined with the logging data of the drilled wells and the rock mechanics parameters and formation pore pressure logging prediction results, the following can be obtained: Figure 5 and Figure 6 The geostress logging prediction results shown in the figure can realize the prediction of geostress profile and analyze the vertical distribution characteristics of geostress in linear elastic-viscous relaxation formations. Figure 5The results shown show that the sandstone and mudstone sections show obvious differences in ground stress. The mudstone section is affected by the change in mud content, plastic deformation and viscous relaxation characteristics, and the ground stress fluctuation amplitude is large and the ground stress value is lower than that of the adjacent sandstone section. The sandstone section mainly undergoes elastic deformation, and there is no obvious stress relaxation phenomenon. The ground stress fluctuation amplitude is small and the ground stress value is significantly higher than that of the adjacent mudstone section. Due to the stress relaxation characteristics of the mudstone formation, the horizontal maximum principal stress and the horizontal minimum principal stress gradually approach each other, and the horizontal stress difference is smaller than the horizontal stress difference of the sandstone section. Figure 6 It can be seen from the single-well geostress profile obtained based on the linear elastic-viscous relaxation formation geostress prediction model that, affected by the lithology, the longitudinal distribution characteristics of the geostress show significant lithology control characteristics, that is, the geostress fluctuation is small, the stress value is high, and the horizontal stress difference is high in the sandstone layer. In the mudstone layer, the geostress fluctuation is large, the stress value is low, and the horizontal stress difference is low. And the longitudinal distribution characteristics of the geostress are in good consistency with the longitudinal distribution characteristics of the rock mechanics parameters. In summary, it is explained that the linear elastic-viscous relaxation formation geostress prediction model proposed in the present invention reasonably characterizes the stress difference between sandstone formations and mudstone formations.

[0115] S10. Based on the model proposed by the present invention, combined with seismic data, geostress logging prediction results, three-dimensional rock mechanics parameter field, and three-dimensional formation pore pressure field, the following can be obtained: Figure 7-Figure 9 The three-dimensional geostress field prediction results shown in the figure can realize the spatial distribution characteristics analysis of geostress. Figure 7 and Figure 8 The three-dimensional geostress field prediction results based on the linear elastic-viscous relaxation formation geostress prediction model show that the horizontal maximum and minimum principal stresses not only show the overall characteristics of increasing with increasing depth, but also have the characteristics of vertical and horizontal heterogeneity and nonlinear variation, indicating that the stress difference characteristics of different lithologies have been effectively characterized. Fig. 9 Compared with the spatial distribution characteristics of the vertical principal stress shown in the figure, the horizontal maximum and minimum principal stresses are more significantly controlled by lithology. Since the vertical principal stress is mainly affected by the density of the overlying rock formation, it presents a uniform gradient in space and has no significant lithology control characteristics. In summary, it is shown that the horizontal maximum principal stress and horizontal minimum principal stress obtained based on the linear elastic-viscous relaxation formation in-situ stress calculation model can effectively characterize the in-situ stress of sandstone and mudstone formations and achieve accurate prediction.

[0116] like Fig.10 The figure shows a comparison of the in-situ stress profiles obtained based on the linear elastic-viscous relaxation in-situ stress prediction model proposed by the present invention and the currently widely used combined spring model. Fig.10It can be seen from the results that since the combined spring model assumes that the formation only undergoes linear elastic deformation, the predicted ground stress values ​​have no significant stress difference between the mudstone layer and the sandstone layer. The linear elastic-viscous relaxation ground stress prediction model proposed in the present invention assumes that the mudstone formation undergoes plastic deformation and viscous relaxation, while the sandstone formation undergoes linear elastic deformation. Therefore, the predicted ground stress value will show significant lithology control characteristics, which reasonably reflects the stress difference characteristics of different formations.

[0117] 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 in the protection scope of the present invention.

Claims

1. A method for predicting ground stress in a linear elastic-viscous relaxation formation, characterized in that: The following steps are involved: S1. Based on the generalized Hooke's law expressed by principal stress under three-dimensional stress state, the generalized Hooke's law characterizing homogeneous isotropic elastic formation is derived; specifically: The generalized Hooke's law expressed as principal stress is: Where: ε H , ε h , ε v are the maximum horizontal strain, minimum horizontal strain and total vertical strain of the unit body respectively; ε' H is the normal strain in the direction of the horizontal maximum principal stress under the action of the horizontal maximum principal stress alone, ε' H ' H is the normal strain in the direction of the horizontal maximum principal stress under the action of the horizontal minimum principal stress alone, ε' H ” is the normal strain generated in the direction of the maximum horizontal principal stress under the action of the vertical principal stress alone; ε' h is the normal strain in the direction of the horizontal minimum principal stress under the action of the horizontal minimum principal stress alone, ε' h ' is the normal strain generated in the direction of the horizontal minimum principal stress under the action of the horizontal maximum principal stress alone, ε' h ” is the normal strain generated in the direction of the horizontal minimum principal stress under the action of the vertical principal stress alone; ε' v is the normal strain in the direction of the vertical principal stress under the action of the vertical principal stress alone, ε' v ' is the normal strain generated in the direction of the vertical principal stress under the action of the horizontal maximum principal stress alone, ε' v ” is the normal strain generated in the direction of vertical principal stress under the action of horizontal minimum principal stress alone; σ H is the normal stress along the direction of the maximum horizontal principal stress, σ h is the normal stress along the horizontal minimum principal stress direction, σ v is the normal stress along the direction of the vertical principal stress; E H 、E h 、E v are the maximum horizontal, minimum horizontal and vertical elastic moduli of the unit cell respectively; v Hh 、v Hv 、v hH 、v hv 、v vH 、v vh are Poisson's ratio, V ij is the ratio of the normal strain in the j direction to the normal strain in the i direction due to the normal stress in the i direction; Assuming that the pure sandstone formation is a homogeneous isotropic elastic body, the physical properties of the rock in different directions are the same, so the elastic parameters satisfy the following expression: where v H 、v h 、v v are the maximum horizontal, minimum horizontal and vertical elastic moduli of the unit cell respectively; E is the elastic modulus, which does not have directional information; v is the Poisson's ratio, which does not have directional information; Substituting the elastic parameter expression into the generalized Hooke's law expressed in terms of principal stress, we obtain the generalized Hooke's law that characterizes homogeneous isotropic linear elastic media. S2, converting the generalized Hooke's law characterizing the homogeneous isotropic linear elastic medium in step S1 into a stress calculation model represented by horizontal maximum principal stress and horizontal minimum principal stress components, that is, obtaining a geostress logging calculation model for the homogeneous isotropic linear elastic formation; where σ v is the vertical principal stress, α is the Biots coefficient, P P is the formation pore pressure; S3, further calculating the horizontal stress difference calculation model of the homogeneous isotropic linear elastic formation through the two horizontal principal stress components in step S2, that is: S4. Based on the known differential stress model of the viscous relaxed formation, a horizontal stress difference prediction model of the viscous relaxed formation is obtained, which is specifically: The creep compliance function J(t) in the linear viscoelastic theory is converted into Bt n , using the linear superposition principle, the horizontal stress difference calculation model of viscous relaxed formation is calculated: Where J(t) is the differential stress caused by constant strain rate structural loading, B is the deformation coefficient related to the elastic modulus, t is the effective duration of deformation, and n is the power law constitutive parameter. is a constant structural strain rate; Based on the relationship model between deformation coefficient and elastic modulus The horizontal stress difference calculation model of viscous relaxed formation is converted into the horizontal stress difference calculation model form of viscous relaxed formation: S5, comparing the horizontal stress difference calculation model of the viscous relaxed formation in step S4 with the horizontal stress difference calculation model of the linear elastic formation in step S3 for similarity in form, and reversely deriving a basic model of the horizontal principal stress difference of the viscous relaxed mudstone formation; S6. Constructing action terms according to step S5, taking into account overlying formation pressure, formation pore pressure and rock mechanics parameters, deriving a calculation model for horizontal maximum and horizontal minimum principal stresses of a viscous relaxed formation; S7, comprehensively considering the linear elastic characteristics of the hard rock formation and the viscous relaxation characteristics of the soft rock formation, combining the in-situ stress logging calculation model of the homogeneous isotropic linear elastic formation in step S2, the horizontal maximum and horizontal minimum principal stress calculation model of the viscous relaxation formation in step S6 and the three-point basic assumption, to obtain a in-situ stress prediction model for the linear elastic-viscous relaxation formation; Where V sh is the mud content; S8. Parameter acquisition method of the geostress prediction model for linear elastic-viscous relaxation formations and model rationality verification; S9. Combined with the logging data of drilling, the prediction of the geostress profile of the linear elastic-viscous relaxation formation and the analysis of the vertical distribution characteristics of the geostress are realized; S10. Combining seismic data, geostress logging prediction results, three-dimensional rock mechanics parameter field, and three-dimensional formation pore pressure field, obtain three-dimensional geostress field prediction results and realize spatial distribution characteristic analysis of geostress.

2. The method for predicting ground stress of a linear elastic-viscous relaxation formation according to claim 1, characterized in that: The three assumptions of step S7 are as follows: Ⅰ. The strata are composed of homogeneous, isotropic, linear elastic sandstone strata and viscoelastic mudstone strata superimposed; II. The composition of ground stress comprehensively considers the deadweight of formation rock, formation pore pressure, rock mechanical properties and tectonic action; III. The stress characteristics of the stratum are affected by the lithology. Under the action of stress, sandstone undergoes linear elastic deformation, while mudstone undergoes viscous relaxation. The mud content is introduced to characterize different lithologies.

3. The method for predicting ground stress of a linear elastic-viscous relaxation formation according to claim 1, characterized in that: The method for obtaining the parameters in step S8 is: Ⅰ. Overlying formation pressure σ V The overlying formation pressure caused by the rock's own weight is obtained by integrating the density logging data: Where: V is the overlying formation pressure, MPa; H is the depth of the calculation point, in m; ρ(h) is the logging density value that varies with depth, g / cm 3 ; g is the acceleration due to gravity, kg·m / s 2 ; Ⅱ. Elastic modulus E The elastic modulus is calculated based on the P-wave time difference and S-wave time difference obtained by cross-dipole array acoustic logging, combined with the density logging curve in conventional logging: Where: b is the bulk density of the formation, g / cm 3 ; Δt s is the shear wave time difference of the formation, μs / m; Δt c is the longitudinal wave time difference of the formation, μs / m; α is the unit conversion coefficient, which is 10 9 ; III. Poisson's ratio v The Poisson's ratio is calculated based on the P-wave and S-wave time difference curves obtained from the cross-dipole array acoustic logging: Ⅳ. Mud content V sh Where: SH is the relative value of natural gamma; GR is the natural gamma logging value of the target layer, API; GR min is the natural gamma logging value of pure lithologic formation, API; GR max is the natural gamma logging value of pure mudstone formation, API; V sh is the mud content, decimal; GCUR is an empirical coefficient related to the age of the strata, 3.7 for new strata and 2 for old strata; V. Formation pressure P P The formation pressure is obtained based on the traditional Eaton method, equivalent depth method and effective stress method; VI. Structural strain coefficient ε H , ε h , Horizontal maximum structural strain coefficient ε H and the horizontal minimum structural strain coefficient ε h Based on the back calculation of fracturing data; constant structural strain rate Directly take the constant deformation rate of the plate where the study area is located; VII. Biots coefficient α The Biot coefficient α is calculated using experimental or empirical methods; the experimental method is calculated as: Its α value is between 0 and 1, C s is the compression coefficient of the skeleton volume, C b is the compression coefficient of the rock's apparent volume; it is determined by empirical methods, α is taken between 0.5 and 0.8, and then fitting is performed; VIII. Effective duration of deformation t: This value is the deformation time experienced by the target layer from its deposition to the present; Ⅸ. Power-law constitutive parameter n: n represents the creep parameter, which is taken as 666.7 / E.

4. The method for predicting ground stress of a linear elastic-viscous relaxation formation according to claim 1, characterized in that: Step S8: Model rationality verification is specifically as follows: Ⅰ. Verification by regression simplification: When the geostress prediction model of the linear elastic-viscous relaxation formation is applied to pure sandstone formations, the geostress calculation model of the homogeneous isotropic linear elastic sandstone formation is regressed and simplified; when it is applied to pure mudstone formations, the geostress calculation model of the viscous relaxation mudstone formation is regressed and simplified. By comparing the calculation results of the two models, the verification results are obtained; Ⅱ. The rationality is demonstrated by comparing the predicted values ​​with the measured values ​​of the in-situ stress. If the predicted values ​​are consistent with the measured values, the linear elastic-viscous relaxation in-situ stress prediction model has good applicability; III. Based on the prediction results of geostress, the formation collapse pressure and formation fracture pressure were further calculated. The accuracy of the model was verified by comparing the consistency between the predicted values ​​of formation collapse pressure and fracture pressure and the back-calculated values ​​of fracturing data.

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