CN113343336A - Numerical simulation method for well wall collapse progressive damage process - Google Patents

Numerical simulation method for well wall collapse progressive damage process Download PDF

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CN113343336A
CN113343336A CN202110604345.8A CN202110604345A CN113343336A CN 113343336 A CN113343336 A CN 113343336A CN 202110604345 A CN202110604345 A CN 202110604345A CN 113343336 A CN113343336 A CN 113343336A
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well
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CN113343336B (en
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马天寿
邱艺
刘阳
付建红
白杨
任海涛
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Southwest Petroleum University
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Abstract

The invention discloses a numerical simulation method for a well wall collapse progressive destruction process, which comprises the following steps: determining rock basic physical property parameters, rock mechanical parameters, stratum crustal stress, stratum pore pressure and wellbore pressure of a target well stratum according to indoor experiments and logging information; establishing a flow-solid coupling finite element mathematical model of the well circumferential stress distribution; calculating the stress distribution around the well; calculating a damage variable F; determining a damaged area, and updating the Young modulus, the porosity and the permeability of the damaged area; repeating the steps to iterate until the damage variable F is less than or equal to 0; drawing a borehole wall instability area graph and determining the width phi of the instability areabAnd depth rb. The invention overcomes the defect that the conventional finite element model updates the geometric model shape to update the inner boundary, and the invention updates the formation parameters of the damaged area, such as Young modulus, porosity and permeabilityThe penetration rate makes the units in the damage range disappear in an equivalent way and cannot bear stress loading, thereby achieving the purpose of updating the inner boundary.

Description

Numerical simulation method for well wall collapse progressive damage process
Technical Field
The invention relates to a numerical simulation method for a well wall collapse progressive destruction process, and belongs to the field of petroleum drilling engineering, rock mechanics and engineering.
Background
Borehole wall instability is the expansion of an initially circular borehole due to stress concentrations and continuous borehole failure. Borehole wall instability is a gradual damage process, and if the damage degree is low, the process is called borehole wall collapse; after excessive well wall damage occurs, the well wall collapses. The well wall collapse and the well wall collapse are important reference indexes for predicting the ground stress, and experiments and theories prove that the direction of the well wall collapse of the vertical well is consistent with the direction of the minimum horizontal ground stress. The advantage of using borehole wall breakout to predict the state of geostress is that they are widely present in the borehole, and that their location, size, and breakout orientation can be detected using logging techniques. In deep wells, hydraulic fracturing is difficult to achieve in-situ stress measurement, and wellbore breakout is the only method for obtaining formation stress state information.
In order to simulate the progressive damage and instability of the well wall, a large number of models have been proposed by scholars at home and abroad, such as experiments: observing the collapse or damaged shape of the well through a true triaxial experiment, wherein the result is generally influenced by a boundary effect due to the small size of an experimental device, and a large amount of manpower and material resources are needed for cost; resolving: the elastic theory, the elastoplasticity theory and the hole elastoplasticity theory are applied to calculate the stress distribution around the well, the damage of stratum rocks is predicted by combining the criterion, and the analytical solution has the advantages of few input parameters and convenience and easiness in calculation, but is only suitable for predicting the beginning of well wall collapse and a simple geometric model; discrete element model: the limitation of the study of inter-particle damage by discretizing the rock into individual particles is the precise description of the contact behavior between the particles due to the excessive computer power required. Deep learning model: the relation between the ground stress and the stable well wall collapse shape is subjected to highly linear/nonlinear fitting by using an AI algorithm, the defect is that a large amount of data sets are needed for training the AI algorithm, and the crack initiation, expansion and collapse formation of a collapse or collapse area cannot be predicted; therefore, the finite element is a method which is convenient and easy to realize, and a finite element model comprises the following steps: the stress distribution of any borehole shape can be calculated, the failure zone is determined according to the criterion, and the geometric inner boundary is updated. But methods for updating geometric boundaries are often difficult to implement.
Therefore, the invention provides a numerical simulation method for a well wall collapse progressive damage process. The mechanical parameters of the damaged area are changed by adopting a continuous damage theory, the area is considered to be equivalently excavated, and four stages of progressive damage development of the well wall are simulated: crack initiation, propagation, breakout formation and stabilization.
Disclosure of Invention
The invention mainly overcomes the defects in the prior art and provides a numerical simulation method for the gradual collapse and damage process of the well wall; the method adopts a continuous damage theory, changes the mechanical parameters of a damaged area, and regards the mechanical parameters as equivalent excavation of the area, thereby simulating four stages of gradual damage development of the well wall, namely crack initiation, expansion, collapse formation and stability.
The technical scheme provided by the invention for solving the technical problems is as follows: a numerical simulation method for a well wall collapse progressive damage process comprises the following steps:
s10, determining rock basic physical parameters, rock mechanical parameters, stratum ground stress, stratum pore pressure and shaft pressure of the target well stratum according to indoor experiments and logging information;
s20, establishing a flow-solid coupling finite element mathematical model of the well circumferential stress distribution;
step S30, calculating the well circumferential stress distribution according to the flow-solid coupling finite element mathematical model of the well circumferential stress distribution;
step S40, calculating a damage variable F according to the stress distribution around the well;
step S50, determining a damaged area according to the damage variable F, and updating the Young modulus, the porosity and the permeability of the damaged area;
step S60, repeating the steps S30 to S50 to iterate until the damage variable F is less than or equal to 0; at the moment, no new damage area appears, the well bore finally tends to be stable, and the well is drawnWall instability region map, determining instability region width phibAnd depth rb。
The further technical scheme is that the basic physical parameters of the rock in the step S10 comprise porosity, permeability and Biot coefficient; the rock mechanical parameters comprise Young modulus, Poisson ratio, internal friction angle and cohesion; the formation ground stress comprises maximum horizontal stress and minimum horizontal ground stress.
The further technical solution is that the specific establishment process of step S20 is:
step S21, establishing a control equation:
Figure BDA0003093851230000031
Figure BDA0003093851230000032
in the formula: g and λ are Lame constants; k is porous media permeability; μ is the viscosity of the fluid; u and p are the displacement and pore pressure of the porous medium, respectively; the subscript t represents the derivative of time; phi is the porosity of the porous medium; kf、KmBulk modulus for fluid and rock, respectively; i isT＝[1,1,1,0,0,0](ii) a D is an elastic stiffness matrix; α is the Biot coefficient;
s22, approximating a control equation by using a Galerkin finite element method to obtain a finite element solution format of the control equation;
Figure BDA0003093851230000033
wherein the content of the first and second substances,
M＝∫VBTDBdV
Figure BDA0003093851230000034
Figure BDA0003093851230000035
Figure BDA0003093851230000036
Figure BDA0003093851230000037
B＝LNu
Figure BDA0003093851230000041
Figure BDA0003093851230000042
in the formula: m, H, S, C are respectively elastic stiffness, flow capacity, and coupling matrix; b is a strain matrix related to strain and displacement; the superscript T is a matrix transposition; n is a radical ofuAnd NpRespectively a displacement shape function and a pressure shape function; l differential operator; rhosAnd ρwDensity of rock and fluid respectively; g is the acceleration of gravity;
Figure BDA0003093851230000043
and
Figure BDA0003093851230000044
respectively corresponding to a force boundary condition and a flow boundary condition in the boundary gamma; q. q.swIs the pressure flow acting on the boundary; u and p are vectors of unknown variables u and p, respectively; u. oftAnd ptThe time derivatives of the variables u and p, respectively; f. ofu、fpThe vector of node loads and the vector of the fluid sink are respectively.
The further technical scheme is that the specific process of the step S30 is as follows: flow-solid coupling finite element mathematical model according to well circumferential stress distributionEstablishing a finite element model for simulating the process of the gradual collapse and damage of the well wall, assigning material parameters, applying boundary conditions and dividing finite element grids to the finite element model for simulating the process of the gradual collapse and damage of the well wall, and finally calculating the distribution of the stress around the well to obtain the maximum principal stress sigma1Intermediate principal stress σ2And minimum principal stress σ3。
The further technical scheme is that the method for establishing the finite element model for simulating the borehole wall collapse progressive failure process according to the borehole stress distribution flow-solid coupling finite element mathematical model comprises the following steps: according to plane strain, axisymmetric conditions and a well circumferential stress distribution flow-solid coupling finite element mathematical model, a finite element model for simulating a well wall collapse progressive failure process is established, the radius of a well shaft is R, and the model is a geometric model of 50R multiplied by 50R so as to eliminate the influence of a boundary effect and reduce the calculated amount.
The further technical scheme is that the finite element model assignment material parameters for simulating the well wall collapse progressive damage process comprise: solving the domain assignment material parameter of the finite element model simulated in the process of the gradual collapse and damage of the well wall, and setting the rock basic physical property parameter and the rock mechanical parameter of the solving domain.
The further technical scheme is that the boundary conditions include: inner boundary applied drilling fluid pressure PmRespectively applying maximum horizontal ground stress sigma to the outer boundaryHAnd minimum horizontal ground stress σhApplying pore pressure P over the entire areap。
A further technical scheme is that in the step 30, since the direction of the progressive well wall damage is always parallel to the direction of the minimum horizontal ground stress, in order to improve the calculation accuracy, the grid is gradually thickened far away from the well shaft, so that the number of units to be calculated is reduced.
The further technical solution is that the calculation formula in step S40 is as follows:
Figure BDA0003093851230000051
F1＝-σ3-fto
Figure BDA0003093851230000052
wherein the content of the first and second substances,
Figure BDA0003093851230000053
I1＝σ1+σ2+σ3
Figure BDA0003093851230000054
Figure BDA0003093851230000055
in the formula I1Is a first invariant of stress; j. the design is a square2A second invariant for stress deflection tension;
Figure BDA0003093851230000056
is the internal friction angle of the rock; c is cohesion; f. oftoIs the tensile strength of the rock; alpha is alpha0And k0Is a material constant, related to rock cohesion and internal friction angle; f1As a function of the state of maximum tensile stress; f2Is Drucker-Prager state function; f is a damage variable; sigma1Is the maximum principal stress; sigma2Is the intermediate principal stress; sigma3Is the minimum principal stress.
The further technical solution is that the calculation formula in step S50 is:
E＝(1-F)E0
Figure BDA0003093851230000057
Figure BDA0003093851230000058
in the formula: E. e0The Young's modulus before and after rock damage respectively; phi, phi0Porosity before and after rock damage respectively; K. k0Permeability before and after rock damage respectively; f is the lesion variable.
The invention has the following beneficial effects: the invention overcomes the defect that the inner boundary is updated by updating the geometric model shape of the conventional finite element model, and the invention enables the units in the damage range to be equivalent and disappear by updating the Young modulus, the porosity and the permeability of the formation parameters of the damage region, and can not bear stress loading, thereby achieving the purpose of updating the inner boundary.
Drawings
FIG. 1 is a flow chart of an embodiment;
FIG. 2 is a diagram of a quarter geometric model;
FIG. 3 is a graph of the results of applying boundary conditions;
FIG. 4 is a finite element mesh partition;
FIG. 5 is a diagram of the jumping-down area of each iteration;
fig. 6 is a graph of the shape values of breakouts for each iteration.
Detailed Description
The present invention will be further described with reference to the following examples and the accompanying drawings.
As shown in fig. 1, the numerical simulation method for the progressive collapse process of the borehole wall of the present invention includes the following steps:
s10, determining basic parameters such as rock basic physical property parameters, rock mechanical parameters, stratum ground stress, stratum pore pressure, shaft pressure and the like of the simulated target well stratum in the process of well wall collapse progressive destruction according to indoor experiments and logging information;
wherein the rock fundamental parameters include porosity, permeability, and Biot coefficient; the rock mechanical parameters comprise Young modulus, Poisson ratio, internal friction angle and cohesion; the formation ground stress comprises maximum horizontal stress and minimum horizontal ground stress;
s20, establishing a flow-solid coupling finite element mathematical model of the well circumferential stress distribution;
the specific process is as follows:
step S21, establishing a control equation:
Figure BDA0003093851230000071
Figure BDA0003093851230000072
wherein G and λ are Lame constants; k is porous media permeability; μ is the viscosity of the fluid; u and p are the displacement and pore pressure of the porous medium, respectively; the subscript t represents the derivative of time; phi is the porosity of the porous medium; kf、KmBulk modulus for fluid and rock, respectively; i isT＝[1,1,1,0,0,0](ii) a D is an elastic stiffness matrix; α is the Biot coefficient;
step S21, approximating the control equation by using the galaogen finite element method, and obtaining a finite element solution format of the control equations (1) and (2):
Figure BDA0003093851230000073
wherein the content of the first and second substances,
M＝∫VBTDBdV-------------------------------------(4)
Figure BDA0003093851230000074
Figure BDA0003093851230000075
Figure BDA0003093851230000076
wherein, M, H, S, C are respectivelyIs the elastic stiffness, flow capacity and coupling matrix; b is a strain matrix related to strain and displacement; the superscript T is a matrix transposition; n is a radical ofuAnd NpRespectively a displacement shape function and a pressure shape function; u and p are vectors of unknown variables u and p, respectively; u. oftAnd ptThe time derivatives of the variables u and p, respectively; f. ofu、fpRespectively a vector of node load and a vector of a fluid source sink;
Figure BDA0003093851230000077
B＝LNu-----------------------------------------(9)
Figure BDA0003093851230000081
Figure BDA0003093851230000082
in the formula, L is a differential operator; rhosAnd ρwDensity of rock and fluid respectively; g is the acceleration of gravity;
Figure BDA0003093851230000083
and
Figure BDA0003093851230000084
respectively corresponding to a force boundary condition and a flow boundary condition in the boundary gamma; q. q.swIs the pressure flow acting on the boundary;
step S30, establishing a finite element model for simulating the borehole wall collapse progressive failure process according to the well circumferential stress distribution flow-solid coupling finite element mathematical model, assigning material parameters to the finite element model, applying boundary conditions, dividing finite element grids, and calculating the well circumferential stress distribution;
step S31, establishing a finite element model for simulating the well wall collapse progressive failure process according to plane strain and axial symmetry conditions and a well circumferential stress distribution flow-solid coupling finite element mathematical model, wherein the radius of a well shaft is R, and the whole finite element model is a geometric model of 50R multiplied by 50R so as to eliminate the influence of a boundary effect and reduce the calculated amount;
s32, solving the finite element model to obtain the assigned material parameters of the solution domain, and setting the basic physical parameters and rock mechanical parameters of the solution domain, including the basic parameters of porosity, permeability, Biot coefficient, Young modulus, Poisson ratio, internal friction angle, cohesion and the like;
step S33, applying boundary conditions to the finite element model, wherein the inner boundary (wellbore) applies drilling fluid pressure PmRespectively applying maximum horizontal ground stress sigma to the outer boundaryHAnd minimum horizontal ground stress σhApplying pore pressure P over the entire areap；
Step S34, dividing finite element meshes for the finite element model, wherein the direction of gradual well wall damage is always parallel to the direction of minimum horizontal ground stress, and in order to improve the calculation precision, particularly, the direction of minimum horizontal ground stress is subjected to mesh encryption, and the meshes are gradually thickened to reduce the number of units to be calculated and are far away from a shaft;
step S35, finally, solving the grid model of the finite element model, and calculating the stress distribution around the well, thereby obtaining the maximum principal stress sigma1Intermediate principal stress σ2And minimum principal stress σ3；
Step S40, calculating a damage variable F according to the stress obtained in the step S30;
step S41, determining a stress state function when the stratum rock is damaged: a maximum tensile stress state function and a Drucker-Prager stress state function.
F1＝-σ3-fto-----------------------------------(12)
Figure BDA0003093851230000091
Wherein the content of the first and second substances,
Figure BDA0003093851230000092
I1＝σ1+σ2+σ3------------------------------------(15)
Figure BDA0003093851230000093
Figure BDA0003093851230000094
in the formula I1Is a first invariant of stress; j. the design is a square2A second invariant for stress deflection tension;
Figure BDA0003093851230000095
is the internal friction angle of the rock; c is cohesion; f. oftoIs the tensile strength of the rock; alpha is alpha0And k0Is a material constant, related to rock cohesion and internal friction angle; f1As a function of the state of maximum tensile stress; f2Is a Drucker-Prager state function.
Step S42, according to the maximum tensile stress state function F1And Drucker-Prager status function F2Calculating a damage variable F of the stratum rock;
Figure BDA0003093851230000096
wherein F is a damage variable; when F is 0, corresponding to a non-damaged unit state, and F is 1, corresponding to a complete damaged state, namely the rock is fractured or damaged; f1More than or equal to 0 represents that the stratum rock is subjected to tensile failure damage; f2The condition that the shearing damage of the stratum rock is generated is more than or equal to 0; dF1>0 and dF2>0 is respectively a continuous loading state after two kinds of damage, and can cause the increase of damage variables; when dF1<0 and dF2<0 represents the unloading state, no new damage is generated, and the damage variable keeps the value of the last loading step (time step);
step S50, determining a damage area according to the damage variable F, and updating the Young modulus, the porosity and the permeability of formation parameters of the damage area to enable units in the damage area to lose the bearing stress capacity;
step S51, according to the isotropic elastic damage theory, the young 'S modulus of the unit will gradually degrade along with the damage process, and the young' S modulus after damage can be expressed as:
E＝(1-F)E0------------------------------------(19)
in the formula: E. e0The Young's modulus before and after rock damage respectively; since the theory of elastic damage is used here, when F is 1, the rock is damaged destructively, and in order to eliminate the error problem caused by numerical calculation, it is assumed that the young modulus of the unit cell body with F being 1 is one ten thousandth of the initial young modulus (0.0001E)0) Instead, the Young's modulus of the unit cell with updated F ═ 1 was 0.0001E0；
S52, the rock damaged by stress concentration is carried away from the original position by the circulation of the drilling fluid, and the damaged area is equivalently completely supported by the drilling fluid, namely the damaged area is regarded as a new inner boundary; therefore, it is necessary to incorporate porosity and permeability as a function of rock element damage, with physical field parameters as shown in equations (20) and (21):
Figure BDA0003093851230000101
Figure BDA0003093851230000102
for a damage area, the permeability should be infinite, the infinite variable cannot be input for numerical simulation, and the progressive damage result is not obviously influenced when the permeability is greater than 4 darcy. Therefore, during subsequent numerical simulation, when borehole wall damage occurs, the permeability is at least 4 darcy;
step S60, repeating the steps S30 to S50 to iterate until the damage variable F is less than or equal to 0, at this time, no new damage area appears, and the well holeFinally, the stability is approached, the numerical simulation of the gradual collapse and damage process of the well wall is realized, the instability area diagram of the well wall is drawn, and the width phi of the instability area is determinedbAnd depth rb。
Examples
And S10, determining basic parameters such as rock basic physical property parameters, rock mechanical parameters, stratum ground stress, stratum pore pressure, shaft pressure and the like of the simulated target well stratum in the process of the gradual collapse and damage of the well wall according to indoor experiments and logging information. The parameters are shown in table 1:
TABLE 1 basic parameters
Figure BDA0003093851230000111
Step S20, establishing a mathematical model of fluid-solid coupling;
s30, establishing a finite element model for simulating the borehole wall collapse progressive failure process according to the borehole wall stress distribution flow-solid coupling finite element mathematical model in the step S20, assigning material parameters to the finite element model, applying boundary conditions, dividing finite element grids, and calculating the borehole stress distribution;
step S40, calculating a damage variable F according to the stress obtained in the step S30;
step S50, determining a damaged area according to the damage variable F in the step S40, and updating the Young modulus, the porosity and the permeability of formation parameters of the damaged area to enable units in the damaged area to lose the capacity of bearing stress;
step S60, repeating the steps S30 to S50 to iterate until the damage variable F is less than or equal to 0, no new damage area appears at the moment, the borehole finally tends to be stable, a borehole wall instability area graph is drawn, and the width phi of the instability area is determinedbAnd depth rb。
FIGS. 5 and 6 show the well wall collapse zone and the corresponding instability zone width Φ for each iteration stepbAnd depth rb. It can be seen that: the width phi of the destabilizing region increases with the number of iteration stepsbAfter the first iteration, the depth r of the destabilizing zone remains constantbFollowing an iterative movementIncreasing and continuously increasing, and reaching a stable state after 6 steps of iteration, which shows that the gradual damage process of the collapse of the well wall is mainly at the depth r of the instability areabThe development and the evolution are gradually carried out, and the situation is consistent with the actual situation and the knowledge in the field.
Although the present invention has been described with reference to the above embodiments, it should be understood that the present invention is not limited to the above embodiments, and those skilled in the art can make various changes and modifications without departing from the scope of the present invention.

Claims (10)

1. A numerical simulation method for a well wall collapse progressive damage process is characterized by comprising the following steps:
s10, determining rock basic physical parameters, rock mechanical parameters, stratum ground stress, stratum pore pressure and shaft pressure of the target well stratum according to indoor experiments and logging information;
s20, establishing a flow-solid coupling finite element mathematical model of the well circumferential stress distribution;
step S30, calculating the well circumferential stress distribution according to the flow-solid coupling finite element mathematical model of the well circumferential stress distribution;
step S40, calculating a damage variable F according to the stress distribution around the well;
step S50, determining a damaged area according to the damage variable F, and updating the Young modulus, the porosity and the permeability of the damaged area;
step S60, repeating the steps S30 to S50 to iterate until the damage variable F is less than or equal to 0; at the moment, no new damage area appears, the borehole finally tends to be stable, a borehole wall instability area graph is drawn, and the width phi of the instability area is determinedbAnd depth rb。
2. The numerical simulation method for the process of progressive collapse of borehole wall according to claim 1, wherein said rock basic property parameters in step S10 include porosity, permeability and Biot coefficient; the rock mechanical parameters comprise Young modulus, Poisson ratio, internal friction angle and cohesion; the formation ground stress comprises maximum horizontal stress and minimum horizontal ground stress.
3. The numerical simulation method for the process of progressive collapse of borehole wall according to claim 1, wherein the specific establishment process of step S20 is as follows:
step S21, establishing a control equation:
Figure FDA0003093851220000011
Figure FDA0003093851220000012
in the formula: g and λ are Lame constants; k is porous media permeability; μ is the viscosity of the fluid; u and p are the displacement and pore pressure of the porous medium, respectively; the subscript t represents the derivative of time; phi is the porosity of the porous medium; kf、KmBulk modulus for fluid and rock, respectively; i isT＝[1,1,1,0,0,0](ii) a D is an elastic stiffness matrix; α is the Biot coefficient;
s22, approximating a control equation by using a Galerkin finite element method to obtain a finite element solution format of the control equation;
Figure FDA0003093851220000021
wherein the content of the first and second substances,
M＝∫VBTDBdV
Figure FDA0003093851220000022
Figure FDA0003093851220000023
Figure FDA0003093851220000024
Figure FDA0003093851220000025
B＝LNu
Figure FDA0003093851220000026
Figure FDA0003093851220000027
in the formula: m, H, S, C are respectively elastic stiffness, flow capacity, and coupling matrix; b is a strain matrix related to strain and displacement; the superscript T is a matrix transposition; n is a radical ofuAnd NpRespectively a displacement shape function and a pressure shape function; l differential operator; rhosAnd ρwDensity of rock and fluid respectively; g is the acceleration of gravity;
Figure FDA0003093851220000028
and
Figure FDA0003093851220000029
respectively corresponding to a force boundary condition and a flow boundary condition in the boundary gamma; q. q.swIs the pressure flow acting on the boundary; u and p are vectors of unknown variables u and p, respectively; u. oftAnd ptThe time derivatives of the variables u and p, respectively; f. ofu、fpThe vector of node loads and the vector of the fluid sink are respectively.
4. The numerical simulation method for the process of progressive collapse of borehole wall according to claim 3, wherein the specific process of step S30 is as follows: establishing a finite element model for simulating the borehole wall collapse progressive failure process according to the well circumferential stress distribution flow-solid coupling finite element mathematical model, assigning material parameters, applying boundary conditions and dividing finite element grids to the finite element model for simulating the borehole wall collapse progressive failure process, and finally calculating the well circumferential stress distribution to obtain the maximum principal stress sigma1Intermediate principal stress σ2And minimum principal stress σ3。
5. The numerical simulation method for the gradual collapse damage process of the well wall according to claim 4, wherein the step of establishing the finite element model for the gradual collapse damage process simulation of the well wall according to the well circumferential stress distribution flow-solid coupling finite element mathematical model comprises the following steps: according to plane strain, axisymmetric conditions and a well circumferential stress distribution flow-solid coupling finite element mathematical model, a finite element model for simulating a well wall collapse progressive failure process is established, the radius of a well shaft is R, and the finite element model is a geometric model of 50R multiplied by 50R so as to eliminate the influence of a boundary effect and reduce the calculated amount.
6. The numerical simulation method for the process of progressive collapse of borehole wall according to claim 4, wherein the assigning the material parameters to the finite element model comprises: and solving the domain assignment material parameters of the finite element model, and setting the rock basic physical property parameters and the rock mechanical parameters of the solution domain.
7. The numerical simulation method for the process of progressive collapse and damage of the borehole wall according to claim 4, wherein the boundary conditions comprise: inner boundary applied drilling fluid pressure PmRespectively applying maximum horizontal ground stress sigma to the outer boundaryHAnd minimum horizontal ground stress σhApplying pore pressure P over the entire areap。
8. The numerical simulation method for the process of progressive borehole wall collapse according to claim 4, wherein in the step 30, since the progressive borehole wall collapse direction is always parallel to the direction of the minimum horizontal ground stress, in order to improve the calculation accuracy, the grid is gradually thickened away from the borehole so as to reduce the number of cells to be calculated.
9. The numerical simulation method for the process of progressive collapse of borehole wall according to claim 4, wherein the calculation formula in step S40 is as follows:
Figure FDA0003093851220000041
F1＝-σ3-fto
Figure FDA0003093851220000042
wherein the content of the first and second substances,
Figure FDA0003093851220000043
I1＝σ1+σ2+σ3
Figure FDA0003093851220000044
Figure FDA0003093851220000045
in the formula I1Is a first invariant of stress; j. the design is a square2A second invariant for stress deflection tension;
Figure FDA0003093851220000048
is the internal friction angle of the rock; c is cohesion; f. oftoIs the tensile strength of the rock; alpha is alpha0And k0Is a material constant, related to rock cohesion and internal friction angle; f1As a function of the state of maximum tensile stress; f2Is Drucker-Prager state function; f is a damage variable; sigma1Is the maximum principal stress; sigma2Is the intermediate principal stress; sigma3Is the minimum principal stress.
10. The numerical simulation method for the process of progressive collapse of borehole wall according to claim 9, wherein the calculation formula in step S50 is:
E＝(1-F)E0
Figure FDA0003093851220000046
Figure FDA0003093851220000047
in the formula: E. e0The Young's modulus before and after rock damage respectively; phi, phi0Porosity before and after rock damage respectively; K. k0Permeability before and after rock damage respectively; f is the lesion variable.
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