Method for predicting service life of ultra-inclination cased hole casing

Abstract

The application discloses a method for predicting the service life cycle of a casing under the condition of ultra-deviation, and belongs to the technical field of oil and gas well engineering. The method specifically comprises the following steps: step S1, establishing a casing stress model under the condition of ultra-deviation; step S2, verifying the casing stress model under the condition of ultra-deviation through multi-physical field coupling; step S3, developing a simulation evaluation of casing dynamic corrosion; step S4, calculating the residual internal pressure strength, extrusion strength and tensile strength of the casing at the position of ultra-deviation; and step S5, establishing a casing service life cycle prediction model under the condition of ultra-deviation. The application not only provides a clear theoretical basis for wellbore repair decision and emergency response, but also provides a new method for predicting the service life of the casing in the non-safe service life, and lays a theoretical foundation for establishing the casing service life cycle under the condition of ultra-deviation.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of oil and gas well engineering, and particularly relates to a method for predicting the service life of a casing used for well cementing under an over-deviated condition. BACKGROUND

[0002] Due to the influence of complex geological conditions such as high temperature, high pressure and high ground stress, deep exploration is most difficult. Although there are certain design requirements for the deviation of the drilling well during deep exploration drilling, the increase of the deviation beyond the design requirements is difficult to avoid. Firstly, the difference in deep geological structure leads to different stratum inclinations or the alternation of hard and soft strata, which makes the drilling tendency of the drill bit deviate, resulting in the increase of the deviation. Secondly, improper selection of the drilling assembly and incomplete optimization of drilling parameters lead to the bending of the drill string in the well, the formation of a contact fulcrum between the drill string and the well wall, the generation of an elastic moment, the inclination of the drill bit, the deviation of the drill bit and the increase of the deviation.

[0003] With the increase of the drilling depth, the deviation continuously increases, which brings many problems to the deep exploration operation. For example, the casing cementing of a ten-thousand-meter scientific ultra-deep well faces the problem of casing cementing. The casing cementing under the condition of over-deviation not only has the risk of encountering resistance during casing lowering, but also faces the risk of casing damage and failure due to the high temperature and high pressure deep environment (the casing in the over-deviated well section frequently fails early, and the casing damage rate in some blocks is as high as 35%).

[0004] An invention patent with the publication number CN120124225A discloses a method and system for predicting the residual life of a downhole casing throughout its life cycle, which comprises obtaining the working pressure, internal pressure safety factor and extrusion safety factor of the casing to be measured; obtaining the internal pressure safety service time of the casing to be measured based on the working pressure, internal pressure safety factor and the first model constructed in advance of the casing to be measured; obtaining the extrusion safety service time of the casing to be measured based on the working pressure, extrusion safety factor and the second model constructed in advance of the casing to be measured; and obtaining the residual life of the casing to be measured according to the internal pressure safety service time and the extrusion safety service time.

[0005] The existing technology cannot accurately evaluate the residual strength and service life of the casing under the condition of over-deviation, which leads to frequent well repair operations or direct wellbore scrapping. SUMMARY

[0006] The purpose of the present application is to provide a method for predicting the service life of a casing used for well cementing under an over-deviated condition, which calculates the residual internal pressure resistance, extrusion resistance and tensile strength of the casing under the over-deviated condition, and establishes a model for predicting the service life of the casing under the over-deviated condition.

[0007] To solve the above technical problems, the technical solution adopted by the present application is as follows:

[0008] The method for predicting the service life cycle of the casing under the ultra-inclination condition comprises the following steps:

[0009] Step S1: establishing a casing stress model under the ultra-inclination condition;

[0010] Step S2: verifying the casing stress model under the ultra-inclination condition through multi-physical field coupling;

[0011] Step S3: developing a simulation evaluation of casing dynamic corrosion;

[0012] Step S4: calculating the residual internal pressure strength, extrusion strength and tensile strength of the casing at the ultra-inclination position;

[0013] Step S5: establishing a casing service life cycle prediction model under the ultra-inclination condition.

[0014] In one embodiment of the present application, in step S1, the stress function is set in the form of , wherein A is the real part and B is the imaginary part.

[0015] The stress function is as follows:

[0016]

[0017] In the formula, is the casing radial stress, is the casing circumferential stress, and is the abstract form of the stress potential function on the plane, m is a variable related to the wellbore size, is the casing outer wall shear force, R is the radial distance, is the casing outer wall normal pressure, D is the casing outer diameter, is the internal pressure strength, d is the casing inner diameter, v is the tangential displacement, is the Poisson's ratio, is the cement sheath elastic modulus.

[0018] In one embodiment of the present application, in step S2, the judgment standard for the stress value at the casing outer wall passing the multi-physical field test is:

[0019] A: the stress value at the casing outer wall based on step S1 can construct a stress field conforming to the actual situation;

[0020] B: the multi-field coupling with the stress field as the core can be successfully realized, and the parameter changes of different physical fields reflecting the same physical property are consistent;

[0021] C: there is no data distortion phenomenon in the multi-field coupling process;

[0022] The above criteria must be met simultaneously to ensure that the stress value at the outer wall of the casing passes the multi-physical field test.

[0023] In one embodiment of the present disclosure, in step S3, the regression equation of the dynamic change of the corrosion rate with time is fitted by using multiple sets of single-point corrosion rate data, and the form of the regression equation is as follows:

[0024]

[0025] In the formula:

[0026] is the dynamic value of the corrosion rate under any temperature and pressure environment;

[0027] n is the number of sets of single-point corrosion rate data participating in fitting;

[0028] is any time point;

[0029] is a set of regression coefficients, which is calculated by statistical data interpolation.

[0030] In one embodiment of the present disclosure, in step S3, under the condition of excessive well deviation, the casing wall thickness calculation formula considering the dynamic corrosion of the casing is obtained by using a downhole multi-physical field coupling model.

[0031]

[0032] In the formula: is the casing wall thickness under dynamic corrosion; is the dynamic corrosion amount.

[0033] In one embodiment of the present disclosure, in step S4, the upper limit values of the residual internal pressure strength, the residual collapse strength and the residual tensile strength of the casing are respectively taken as three threshold values, and the internal pressure safe service life, the collapse safe service life and the tensile safe service life of the casing are respectively calculated.

[0034] In one embodiment of the present disclosure, the internal pressure safe service life calculation formula is as follows:

[0035]

[0036] In the formula: is the minimum wall thickness corresponding to the upper limit of the collapse strength.

[0037] In one embodiment of the present disclosure, the tensile safe service life calculation formula is as follows:

[0038]

[0039] In the formula: The minimum wall thickness corresponding to the upper limit of the tensile strength.

[0040] In one embodiment of the present disclosure, the tensile safety service life calculation formula is:

[0041]

[0042] In the formula: The minimum wall thickness corresponding to the upper limit of the tensile strength;

[0043] The tensile strength.

[0044] In one embodiment of the present disclosure, in step S5, the upper and lower limits of the service life of the ultra-deviated well cementing casing are:

[0045]

[0046] In the formula: The casing failure risk occurrence probability;

[0047] The lower limit of the service life of the ultra-deviated well cementing casing;

[0048] The casing full life test service life cycle of the simulated drilling;

[0049] The minimum value of the safe service life;

[0050] The casing failure risk occurrence probability;

[0051] The upper limit of the service life of the ultra-deviated well cementing casing;

[0052] The maximum value of the safe service life;

[0053] The minimum value of the safe service life; As the predicted service life cycle of the ultra-deviated well cementing casing, a service life cycle prediction model of the casing under the ultra-deviated condition is established.

[0054] Compared with the prior art, the present application has the following beneficial effects:

[0055] The application establishes a casing stress model under the condition of ultra-inclination, and the stress value at the outer wall of the casing solved is verified by multi-physical field to ensure that it can be used as a basic parameter for predicting the service life of the ultra-inclination cementing casing; at the same time, based on the stress data at the outer wall of the casing, the casing corrosion simulation research is carried out; combined with the casing wall thickness change under dynamic corrosion, the residual internal pressure strength, residual collapse strength and residual tensile strength of the casing are calculated according to the API standard, and finally the casing service life prediction model under the condition of ultra-inclination is established.

[0056] The application closely surrounds the ultra-inclination working condition, efficiently utilizes multi-field coupling and other numerical simulation methods, carries out precise mechanical analysis and corrosion simulation research, comprehensively considers the casing failure type, failure consequence severity and occurrence probability, combines with the risk matrix method, scientifically divides the wellbore safety risk level of casing failure risk, not only provides clear theoretical basis for wellbore repair decision and emergency response, but also provides a new method for predicting the service life of the casing in the non-safety service life, and lays a theoretical foundation for establishing the casing service life cycle under the condition of ultra-inclination. BRIEF DESCRIPTION OF DRAWINGS

[0057] In order to more clearly illustrate the technical solutions of the embodiments of the application, the following will briefly introduce the drawings needed to be used in the embodiments. It should be understood that the following drawings only show some embodiments of the application, and therefore should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can also be obtained without creative labor.

[0058] Figure 1 It is a stress plane projection diagram of the casing, cement sheath and formation combination body described in the application.

[0059] Figure 2 It is an elliptical stress state of the ultra-inclination cementing casing described in the application.

[0060] Figure 3 It is a multi-field coupling grid model and ultra-inclination segment three-dimensional space trajectory schematic diagram of the application.

[0061] Figure 4 It is a whole flow chart of the application. DETAILED DESCRIPTION

[0062] In the following, only some exemplary embodiments are simply described. As those skilled in the art can recognize, the described embodiments can be modified in various different ways without departing from the spirit or scope of the embodiments of the application. Therefore, the drawings and the description are considered to be exemplary in nature rather than limiting. The embodiments of the application are described in detail below with reference to the drawings.

[0063] Reference Figure 4This embodiment discloses a method for predicting the service life cycle of inclined cementing casing in over-wells, which specifically includes the following steps:

[0064] Step S1: Establish a casing stress model under over-well deviation conditions;

[0065] Step S2: Verify the casing stress model under over-slant well conditions through multiphysics coupling;

[0066] Step S3: Conduct dynamic corrosion simulation assessment of the casing;

[0067] Step S4: Calculate the remaining internal pressure resistance, extrusion resistance, and tensile strength of the casing at the over-well deviation;

[0068] Step S5: Establish a casing service life cycle prediction model under over-inclination conditions.

[0069] In one embodiment of the present invention, in step S1, with The stress function is expressed as follows: A is the real part and B is the imaginary part.

[0070] The stress function is as follows:

[0071]

[0072] In the formula: For the radial stress of the casing, For the circumferential stress of the casing, and for An abstract form of the stress potential function on a plane, where m is a variable related to the wellbore size. R represents the shear force on the outer wall of the casing, and R represents the radial distance. The positive pressure on the outer wall of the casing is D, where D is the outer diameter of the casing. The internal pressure resistance is given by d, where d is the inner diameter of the casing and v is the tangential displacement. Poisson's ratio, This refers to the elastic modulus of the cement ring.

[0073] In one embodiment of the present invention, in step S2, the criterion for determining whether the stress value at the outer wall of the sleeve passes the multiphysics field test is:

[0074] A: Based on the stress values ​​at the outer wall of the casing obtained in step S1, a stress field that conforms to reality can be constructed.

[0075] B: Multi-field coupling with stress field as the core can be successfully achieved, and the parameter changes of different physical fields reflecting the same physical property are consistent;

[0076] C: No data loss or distortion occurs during multi-field coupling;

[0077] The above criteria must be met simultaneously to ensure that the stress value at the outer wall of the casing passes the multi-physical field test.

[0078] In one embodiment of the present disclosure, in step S3, the regression equation of the dynamic change of the corrosion rate with time is fitted by using multiple sets of single-point corrosion rate data, and the form of the regression equation is as follows:

[0079]

[0080] In the formula:

[0081] is the dynamic value of the corrosion rate under any temperature and pressure environment;

[0082] n is the number of sets of single-point corrosion rate data participating in fitting;

[0083] is any time point;

[0084] is a set of regression coefficients, which is calculated by statistical data interpolation.

[0085] In one embodiment of the present disclosure, in step S3, under the condition of excessive well deviation, the casing wall thickness calculation formula considering the dynamic corrosion of the casing is obtained by using the downhole multi-physical field coupling model.

[0086]

[0087] In the formula: is the casing wall thickness under dynamic corrosion; is the dynamic corrosion amount.

[0088] In one embodiment of the present disclosure, in step S4, the upper limit values of the residual internal pressure strength, the residual collapse strength and the residual tensile strength of the casing are respectively taken as three threshold values, and the internal pressure safety service life, the collapse safety service life and the tensile safety service life of the casing are respectively calculated.

[0089] In one embodiment of the present disclosure, the internal pressure safety service life calculation formula is as follows:

[0090]

[0091] In the formula: is the minimum wall thickness corresponding to the upper limit of the internal pressure strength.

[0092] In one embodiment of the present disclosure, the collapse safety service life calculation formula is as follows:

[0093]

[0094] In the formula: The minimum wall thickness corresponding to the upper limit of tensile strength.

[0095] In one embodiment of the present disclosure, the tensile safety service life calculation formula is:

[0096]

[0097] In the formula: The minimum wall thickness corresponding to the upper limit of tensile strength;

[0098] Tensile strength of the casing.

[0099] In one embodiment of the present disclosure, in step S5, the upper and lower limits of the service life of the ultra-deviation cementing casing are:

[0100]

[0101] In the formula: The probability of occurrence of casing failure risk;

[0102] The lower limit of the service life of the ultra-deviation cementing casing;

[0103] The casing full life test service life period under simulated drilling;

[0104] The minimum value of the safe service life;

[0105] The probability of occurrence of casing failure risk;

[0106] The upper limit of the service life of the ultra-deviation cementing casing;

[0107] The maximum value of the safe service life;

[0108] The minimum value of the safe service life; As the predicted service life of the ultra-deviation cementing casing, a casing service life prediction model under the condition of ultra-deviation is established.

[0109] In order to facilitate further understanding of the present application by those skilled in the art, the present application is further described in detail below.

[0110] A method for predicting the service life of an ultra-deviation cementing casing, a casing stress model under the condition of ultra-deviation is established, and the stress value at the outer wall of the casing obtained by solving is verified by multi-physical field coupling to ensure that it can be used as a basic parameter for predicting the service life of the ultra-deviation cementing casing;

[0111] At the same time, based on the stress data at the outer wall of the casing, a casing corrosion simulation study is carried out;

[0112] Combined with the casing wall thickness change under dynamic corrosion, the residual internal pressure strength, residual collapse strength and residual tensile strength of the casing are calculated according to the API standard, and finally the casing service life prediction model under the condition of excessive well deviation is established.

[0113] Specifically, the following steps are included:

[0114] Step S1: establishing a casing stress model under the condition of excessive well deviation;

[0115] Step S2: verifying the casing stress model under the condition of excessive well deviation through multi-physical field coupling;

[0116] Step S3: developing a dynamic corrosion simulation evaluation of the casing;

[0117] Step S4: calculating the residual internal pressure strength, collapse strength and tensile strength of the casing at the position of excessive well deviation;

[0118] Step S5: establishing a casing service life prediction model under the condition of excessive well deviation based on risk management and control.

[0119] In specific implementation, in order to ensure that the casing service life prediction is in line with the actual situation, the casing under the condition of excessive well deviation should also be coupled with the multi-physical field under the formation condition for verification to ensure the accuracy and effectiveness of the related data.

[0120] Further, the cementation condition and fluid erosion condition of each contact interface of the casing-cement sheath-formation should be considered, and combined with the stress evolution degree of the casing, a multi-physical field model and its coupling model under the condition of excessive well deviation are constructed.

[0121] When performing multi-physical field coupling verification under the condition of excessive well deviation, a casing stress model under the condition of excessive well deviation should be established first to form the stress field required for coupling.

[0122] In actual drilling and completion operations, the proportion of casing cementing in the straight well section is higher, so most of the casing cementing considers the service life of the straight well section. When the casing in the straight well section fails, it will directly affect the integrity of the wellbore.

[0123] In specific embodiments, the research on excessive well deviation is aimed at the situation that the well deviation in the straight well section exceeds the standard, and the well deviation angle in the straight well section is generally required to be less than 60°. If a large displacement well or a deviated well is encountered, an extension analysis can be performed based on the research of the present embodiment.

[0124] Specifically, referring to Figure 4 , the following steps are included:

[0125] Step S1: establishing a casing stress model under the condition of excessive well deviation;

[0126] When well deviation exceeds design requirements, the casing in deep formations exhibits nonlinear stress characteristics, such as the coupling effect of bending stress and contact stress. Currently, there is a lack of analytical research on nonlinear stress, and existing casing models under excessive well deviation conditions often rely more on statics, neglecting or downplaying the influence of multiple physical fields under formation conditions.

[0127] It should be noted that when the well deviation in the vertical well section exceeds the standard, the stress balance state around the well will be different from that in the normal vertical well section; the wellbore edge along the direction of maximum horizontal stress tends to be compressed, while the wellbore edge along the direction of minimum horizontal stress tends to be elongated, ultimately forming an elliptical non-uniform stress field at the cementing casing of the deviated section.

[0128] Based on the theory of elasticity and combined with the complex variable function method, a planar mechanical model of the "casing-cement sheath-formation" within an elliptical wellbore is established under the combined action of non-uniform stress and uniform internal pressure, as follows: Figure 1 As shown, the semi-analytical solution is derived, and the influence of the geometric and elastic parameters of the cement sheath on the casing stress is investigated, laying a theoretical foundation for establishing a multi-physics coupling model under over-inclination conditions.

[0129] Furthermore, because the deformation of the casing and cement sheath along the wellbore axis is restricted, the casing, cement sheath, and formation assembly model is simplified to an elliptical plane strain model.

[0130] According to wellbore stability theory, the maximum horizontal stress is... The direction should coincide with the direction of the minor axis of the ellipse, and the entire assembly will be subjected to the maximum horizontal stress. Minimum horizontal stress Internal pressure of the casing The three work together.

[0131] Based on the principle of superposition of stress and deformation in elastic mechanics, the entire assembly is divided into three parts: "the infinitely large stratum with the central elliptical hole, the cement ring, and the casing".

[0132] Because the well wall is elliptical, there is geometric asymmetry in the contact surfaces between the three parts of the assembly. Therefore, in addition to the normal pressure, shear force also acts on the well wall and the outer wall of the casing.

[0133] Let the normal pressure acting on the outer wall of the casing be... Shear force is ,because It is related to the well inclination angle The relevant polar coordinate functions can be approximated by the following first-order approximation:

[0134]

[0135] In the formula:

[0136] Shear force of casing outer wall, unit: MPa

[0137] Deviation angle of well, unit: °

[0138] Respectively, the resultant stress of casing outer wall surface, the cement shearing elastic interface of ring

[0139] Maximum horizontal stress Minimum horizontal stress Casing internal pressure Determined by corresponding radius synthesis.

[0140] The processing cement ring at the boundary of the elliptical resultant stress solution, that is, the solution of the radial stress, tangential stress synthesized by casing outer wall normal pressure Shear force .

[0141] As Figure 2 Shown, with complex function “Roukovski conformal transformation”, the ellipse well on the complex plane is transformed into the auxiliary plane Z on the ellipse well is transformed into the auxiliary plane On the unit circle , the casing outer wall circle is transformed into the concentric outer circle of the unit circle.

[0142] The established casing concentric outer circle description formula is:

[0143]

[0144] The transformation formula is:

[0145]

[0146] In the formula:

[0147] The distance of the long semi-axis of the ellipse, unit: m

[0148] The distance of the short semi-axis of the ellipse, unit: m

[0149] The imaginary unit

[0150] D is the outer diameter of the casing, unit: m

[0151] The The center distance of the circle at any point on the plane:

[0152] Z is: the physical quantity on the Z plane

[0153] is: Physical quantity on the plane;

[0154] R is: the radius of the sleeve concentric circle;

[0155] e is: natural base, about equal to 2.71828;

[0156] is: Coordinate on the plane.

[0157] It should be noted that the above transformation is only valid when the sleeve size does not exceed .

[0158] The process of analytically solving the radial stress and tangential stress of the sleeve outer wall by using the complex function method is:

[0159] ① Assume that there is a stress potential function abstract form on the complex plane Z;

[0160] ② Substitute the stress potential function abstract form into the stress boundary condition to solve;

[0161] ③ After obtaining the specific form of the stress potential function, substitute it into the simplified super-inclination stress equation set to calculate the stress function;

[0162] ④ Separate the real part and the imaginary part of the obtained stress function, and solve the resultant stress and displacement at the sleeve outer wall.

[0163] The stress boundary condition based on the complex function method is:

[0164]

[0165] In the formula:

[0166] is: the first-order derivative of function Z;

[0167] is: the first-order derivative of function G;

[0168] : integral sign;

[0169] is: integral variable;

[0170] The simplified super-inclination stress equation set is essentially the relationship expression of the stress potential function and the radial stress, tangential stress, radial displacement, and tangential displacement at the measured position, which is in the form of:

[0171]

[0172] In the formula:

[0173] is the radial stress, unit: MPa;

[0174] is the tangential stress, unit: MPa;

[0175] is the radial displacement, unit m;

[0176] is the tangential displacement, unit m;

[0177] is the Poisson's ratio;

[0178] is the cement sheath elastic modulus, unit MPa.

[0179] According to the performance form of real part + imaginary part , the stress function is:

[0180]

[0181] In the formula: d is the inner diameter of the casing, m.

[0182] Each separation of the real part and the imaginary part of the radial stress and the tangential stress can be solved to obtain the stress and displacement at the outer wall of the casing, but because we are more concerned about the service life of the casing, here we only need to extract the real part of the two respectively:

[0183] and ;

[0184] to calculate the numerical value of the radial stress and the tangential stress.

[0185] The stress value at the outer wall of the casing solved out still needs to be verified by multiple physical fields, so as to be used as a basic parameter for predicting the service life of the casing in the ultra-inclined well.

[0186] Step S2: The casing stress model under the condition of ultra-inclination is verified by multiple physical field coupling;

[0187] To predict the service life of the casing, the temperature and pressure changes of the ultra-inclined well section need to be considered, and the influences of "formation fluid seepage", "phase change of formation fluid" and the like also need to be taken into account. In order to accurately evaluate the temperature and pressure changes of the ultra-inclined well section and their influences on the cementing casing, a multiple physical field coupling model including temperature field, stress field, seepage field and chemical field needs to be established.

[0188] Under the condition of ultra-inclination, a stress field model of the cementing casing is constructed based on the stress value at the outer wall of the casing.

[0189] Temperature change causes the casing material to expand or contract, resulting in thermal stress of a certain size, and the stress field will affect the heat conduction path through material deformation.

[0190] Under the condition of ultra-inclination, the temperature field model of the cementing casing is constructed, which is coupled with the stress field. The change of formation pore pressure caused by formation fluid seepage changes the distribution of effective stress around the casing, and the stress field will react on the seepage process through the change of porosity and permeability.

[0191] Under the condition of ultra-inclination, the seepage field model of the cementing casing is constructed, which is coupled with the stress field.

[0192] In different regions, due to different geothermal gradients, fluid will be driven by natural convection, and at the same time, fluid flow will take away heat, forming a heat convection-heat conduction coupling. The phase change of formation fluid (such as hydrate decomposition, CO2 dissolution, mineral precipitation, etc.) changes the fluid density and viscosity, affecting the seepage characteristics, and the chemical field will affect the mechanical properties of the rock around the well through ion diffusion and physical and chemical reactions.

[0193] Under the condition of ultra-inclination, the chemical field model of the cementing casing is constructed, which is coupled with the seepage field, temperature field and stress field.

[0194] Based on the linear elastic constitutive relation, considering the effects of thermal strain and pore pressure, the stress field control equation is in the form of:

[0195]

[0196] In the formula:

[0197] is the stress of a certain point of the stress field, unit: MPa;

[0198] is the elastic tensor, which is composed of the radial stress and tangential stress around the well;

[0199] is the elastic tensor coefficient;

[0200] is the dimensionless thermal strain value, the thermal strain data of a certain rock is constant, which can be researched in literature;

[0201] is the temperature change, unit: K;

[0202] is the thermal expansion coefficient, unit: MPa / K.

[0203] Based on the Fourier heat conduction law, considering the heat source term (such as geothermal gradient, fluid friction heat), the temperature field control equation is in the form of:

[0204]

[0205] wherein:

[0206] is the formation fluid density, g / cm3;

[0207] is the formation fluid specific heat capacity, J / (kg·K);

[0208] is the formation rock thermal conductivity, W / (m·K);

[0209] is the heat source term, J;

[0210] is the gradient operator;

[0211] is the temperature gradient;

[0212] is the rate of change of temperature with time.

[0213] Based on Darcy's law, considering non-isothermal effect and fluid phase transition, the control equation of the involved seepage field is in the form of:

[0214]

[0215] wherein:

[0216] is the flow rate at a point in the seepage field, m3 / s;

[0217] is the formation rock permeability, mD;

[0218] is the formation fluid viscosity, mPa·s;

[0219] is the gravitational acceleration, m / s2;

[0220] is the fluid thermal expansion coefficient, 1 / K;

[0221] is the pressure gradient.

[0222] Based on the mass conservation equation, considering molecular diffusion and convection, the control equation of the involved chemical field is in the form of:

[0223]

[0224] wherein:

[0225] For the molecular concentration value;

[0226] For the diffusion coefficient;

[0227] For the reaction term;

[0228] For the partial derivative of the molecular concentration value with respect to time;

[0229] Under the condition of super-inclination, the multi-physical field coupling of stress field, temperature field, seepage field and chemical field is simply referred to as multi-field coupling. The multi-field coupling can be selected using software such as COMSOL or ansys, and the specific steps are as follows:

[0230] (I) Geometric modeling and meshing

[0231] 1. Geometric construction: import the three-dimensional model of the super-inclination well section, as shown in the specific embodiment of FIG. 1, including casing, cement sheath, formation and perforated section; define different material domains (such as casing steel, cement, formation). Figure 3

[0232] 2. Meshing: encrypt the mesh (such as using free tetrahedral mesh, maximum unit size ≤0.1 m) of the casing-cement sheath interface and other key areas of the super-inclination section; the fluid domain adopts sweep meshing to ensure calculation accuracy.

[0233] (II) Material property definition

[0234] For example, for shale, the casing of the super-inclination section is set to have an elastic modulus of 210 GPa, a Poisson's ratio of 0.3, a thermal expansion coefficient of 1.2×10⁻ 5 K⁻¹, a thermal conductivity of 50 W / (m·K), and a specific heat capacity of 460 J / (kg·K).

[0235] For example, for shale, the cement sheath of the super-inclination section is set to have an elastic modulus of 10 GPa, a Poisson's ratio of 0.25, a permeability of 0.01 mD, a thermal conductivity of 1.5 W / (m·K), and a specific heat capacity of 800 J / (kg·K).

[0236] For example, for shale, the formation of the super-inclination section is set to have a permeability of 0.01 mD, a porosity of 20%, a Young's modulus of 10 GPa, a geothermal gradient of 0.03 K / m, and a fluid viscosity of 1 mPa·s (at 100°C).

[0237] (III) Physical field setting

[0238] 1. Stress field boundary conditions: wellhead fixed, well bottom axial displacement free, side boundary normal displacement constrained; initial conditions: geostress field (vertical stress gradient 22 MPa / km, horizontal stress ratio 0.8).

[0239] ​2. Temperature field boundary conditions: bottom-hole temperature 120℃, wellhead temperature 30℃, adiabatic at the formation side boundary; heat source term: considering fluid friction heat generation (calculated by Darcy-Weisbach formula).

[0240] 3. Seepage field boundary conditions: wellhead pressure 10 MPa, constant pressure (40 MPa) at the formation side boundary; considering phase change: setting a nonlinear relationship of fluid density with temperature / pressure (such as near the critical point of CO2).

[0241] 4. Chemical field boundary conditions: CO2 concentration of 1 mol / L injected at the wellhead, no flux at the formation side boundary; reaction term: considering carbonate dissolution caused by CO2 dissolution (calculated by chemical equilibrium constant).

[0242] (Four) Multi-physical field coupling implementation

[0243] (1) Temperature and stress coupling: enable "thermal expansion" multi-physical field coupling in the "solid mechanics" interface, and enable the "deformation heat" option in the "heat conduction" interface.

[0244] (2) Seepage and stress coupling:

[0245] Enable "Biot theory" in the "porous elasticity" interface, define the pore elastic parameters (Biot coefficient: 0.8), and enable the "effective stress" option in the "Darcy's law" interface.

[0246] (3) Temperature, stress, seepage and chemical coupling:

[0247] First, seepage and chemical coupling, define concentration-dependent fluid density in the "thin substance transfer" interface, and enable the "concentration-dependent viscosity" option in the "Darcy's law" interface, and then match the temperature field and stress field variation trend.

[0248] (Five) Numerical solution and post-processing

[0249] Steady-state analysis: linear system is solved by direct solver, relative tolerance 1×10⁻ 6 , maximum iteration number 100.

[0250] Transient analysis: time step is dynamically adjusted (initial step 1 day, maximum step 30 days), second-order accuracy.

[0251] The standard for judging whether the stress value at the outer wall of the casing passes the multi-physical field test is:

[0252] A: The stress value at the outer wall of the casing based on step S1 can construct a stress field that conforms to reality;

[0253] B: The multi-field coupling can be successfully realized with stress field as the core, and different physical fields are consistent in reflecting the parameter change of the same physical property;

[0254] C: There is no data distortion in the multi-field coupling process.

[0255] The above criteria must be met simultaneously to ensure that the stress value at the outer wall of the casing passes the multi-physical field test.

[0256] Step S3: Conduct dynamic corrosion simulation evaluation of the casing;

[0257] Although the stress on the super-inclination section is uneven, it is easy to cause deformation of the casing. However, only when the stress around the well increases to the yield strength of the casing material, the casing body will be deformed and broken. However, corrosion can directly cause corrosion and breakage of the casing material below the yield strength of the casing material. When uniform corrosion occurs, the casing material corrosion surface will be uniformly damaged, reducing the strength of the pipe material and reducing the safe service life of the casing.

[0258] Based on the stress data at the outer wall of the casing, the corrosion simulation of the casing is carried out.

[0259] The stress corrosion test of the casing material under simulated working conditions is carried out by using a high temperature and high pressure kettle. The corrosion rate of different experimental time is calculated through the corrosion weight loss experiment. Specifically, the casing material is clamped by triaxial compression to simulate the stress environment at the outer wall of the casing. Then the whole device is placed in a larger volume high temperature and high pressure kettle to obtain the corrosion rate under different corrosion media (such as CO2, H2S).

[0260] The uniform corrosion rate is represented by the material loss (or thickness) of a certain point or a certain area on the commonly used material per unit time.

[0261] The single-point corrosion rate measurement equation established by simulating the temperature and pressure changes of the downhole casing is:

[0262]

[0263] In the formula:

[0264] is the corrosion rate, mm / h;

[0265] is the pressure change, MPa;

[0266] t is the test time, h;

[0267] M is the mass of the sample before the test, g;

[0268] is the mass of the sample after the test, g;

[0269] A is the total surface area of the sample, cm2;

[0270] Density of the casing material, g / m3;

[0271] P is: initial pressure, MPa;

[0272] T is: initial temperature, ℃.

[0273] The regression equation form of the corrosion rate dynamic change with time is fitted by multiple sets of single-point corrosion rate data:

[0274]

[0275] In the formula:

[0276] The corrosion rate dynamic value (instantaneous value) under any temperature and pressure environment, mm / h;

[0277] n is the number of single-point corrosion rate data sets involved in fitting; is the time point, h; is a set of regression coefficients, which is calculated by statistical data interpolation.

[0278] The corrosion rate dynamic value under any temperature and pressure environment is input into the numerical simulation software, and the dynamic change amount of the casing wall thickness with time can be calculated. Under the condition of well deviation, the casing wall thickness calculation formula considering the dynamic corrosion of the casing is:

[0279] ;

[0280] In the formula: is the casing wall thickness under dynamic corrosion, mm.

[0281] Step S4: Calculate the residual internal pressure strength, collapse strength, and tensile strength of the casing at the well deviation;

[0282] The casing failure types are:

[0283] ① Mechanical damage: casing rupture, deformation, perforation (such as crushing);

[0284] ② Corrosion failure: uniform corrosion, local corrosion (such as pitting, stress corrosion cracking);

[0285] ③ Sealing failure: cement sheath debonding, channeling, leading to interlayer fluid channeling;

[0286] ④ Fatigue failure: alternating load leading to casing fatigue fracture.

[0287] According to the summary of the casing failure forms, it can be found that the mechanical performance parameters of the casing at the well deviation angle are easy to be changed, specifically, the three of the internal pressure resistance, the collapse resistance and the tensile strength of the casing after corrosion will change, and the residual internal pressure resistance, the residual collapse resistance and the residual tensile strength of the casing need to be calculated.

[0288] According to the API standard, the residual internal pressure resistance, the residual collapse resistance and the residual tensile strength of the casing are calculated.

[0289] The residual internal pressure resistance calculation formula is:

[0290]

[0291] In the formula,

[0292] is the internal pressure resistance, MPa;

[0293] is the yield strength of the casing material, MPa.

[0294] The collapse resistance calculation formula is:

[0295]

[0296] In the formula,

[0297] is the collapse resistance, MPa.

[0298] The residual tensile strength calculation formula is:

[0299]

[0300] In the formula, is the tensile strength, MPa; is the minimum yield strength of the casing material, MPa.

[0301] Step S5: establishing a casing service life prediction model under the well deviation angle condition;

[0302] Taking the upper limit values of the residual internal pressure resistance, the residual collapse resistance and the residual tensile strength of the casing as three threshold values respectively, the internal pressure safety service life, the collapse safety service life and the tensile safety service life of the casing are calculated respectively.

[0303] The internal pressure safety service life calculation formula is:

[0304] ;

[0305] In the formula, is the minimum wall thickness corresponding to the upper limit of the internal pressure resistance, mm.

[0306] The anti-extrusion safe service life calculation formula is:

[0307]

[0308] In the formula: is the minimum wall thickness corresponding to the upper limit of the anti-extrusion strength, mm.

[0309] The anti-tension safe service life calculation formula is:

[0310]

[0311] In the formula: is the minimum wall thickness corresponding to the upper limit of the anti-tension strength, mm.

[0312] It should be noted that the casing is not damaged and failed immediately after exceeding the safe service life, but there is a different probability of damage and failure. The service life of the casing exceeding the safe service life is analyzed and predicted by using the risk control method.

[0313] The casing failure risk level is determined by the severity of the failure consequences SS and the failure frequency PP. According to the wellbore function, environmental impact and economic loss, the failure consequences and failure frequency are divided into the following 5 levels according to Tables 1 and 2:

[0314] Table 1: Inductive statistics of casing failure consequences:

[0315]

[0316] Table 2: Inductive statistics of casing failure frequency:

[0317]

[0318] Further technical solutions are that in combination with the SS and PP levels, the risk matrix method is used to divide the casing failure risk level into three levels of high, medium and low, and the casing failure risk level calculation is shown in Table 3.

[0319] Table 3: Casing failure risk level calculation table:

[0320]

[0321] Using the empirical statistical method, the average value of the casing under the ground test condition in the whole life cycle is:

[0322]

[0323] In the formula:

[0324] is the simulation drilling casing full life test service life period;

[0325] Total number of wells in the structure;

[0326] Total number of existing intact wells in the structure;

[0327] Average number of casing damage wells per year in the previous three years for prediction;

[0328] Equivalent stress of ground stress on casing in the structure, converted from radial stress and tangential stress at the outer wall of the casing;

[0329] Corrosion perforation influence coefficient, 1.05-2.0.

[0330] External extrusion force that casing can withstand uniform load;

[0331] Corrosion influence coefficient, 1.1-2.0.

[0332] Under the condition of excessive well inclination, the casing needs to deduct the safe service life based on the full life cycle under the surface test condition, and superimpose the non-safe service life, which can constitute the upper and lower limits of the cementing casing service life cycle range.

[0333] Further, the minimum and maximum values of the safe service life are calculated by weighting the anti-internal pressure safe service life, the anti-extrusion safe service life and the anti-tension safe service life.

[0334] The minimum value formula of the safe service life established is:

[0335]

[0336] The maximum value formula of the safe service life established is:

[0337]

[0338] In the formula: The minimum and maximum values of the anti-internal pressure safe service life are respectively:

[0339] The minimum and maximum values of the anti-extrusion safe service life are respectively:

[0340] The minimum and maximum values of the anti-tension safe service life are respectively:

[0341] The lower limit of the service life of the cementing casing under excessive inclination is:

[0342]

[0343] The upper limit of the service life of the casing for the super-inclination well is:

[0344]

[0345] In the formula: is the casing failure risk occurrence probability;

[0346] : the lower limit of the service life of the casing for the super-inclination well;

[0347] : the casing full life test service life period of the simulated drilling;

[0348] : the minimum value of the safe service life;

[0349] : the casing failure risk occurrence probability;

[0350] : the upper limit of the service life of the casing for the super-inclination well;

[0351] : the maximum value of the safe service life;

[0352] In the implementation, the predicted service life of the casing for the super-inclination well is taken as the service life of the casing for the super-inclination well. The service life prediction model of the casing for the super-inclination well is established.

[0353] The present application closely surrounds the super-inclination working condition, efficiently utilizes multi-field coupling and other numerical simulation methods, carries out precise mechanical analysis and corrosion simulation research, comprehensively considers the casing failure type, failure consequence severity and occurrence probability, combines the risk matrix method, scientifically divides the wellbore safety risk grade of the casing failure risk, not only provides a clear theoretical basis for the wellbore repair decision and emergency response, but also provides a new method for predicting the service life of the casing in the non-safe service life, and lays a theoretical foundation for establishing the service life of the casing under the super-inclination condition.

[0354] Although the preferred embodiments of the present application have been described, those skilled in the art can make further changes and modifications to the embodiments once they know the basic inventive concept. Therefore, the appended claims are intended to be interpreted as including all changes and modifications falling within the scope of the present application.

[0355] The above only describes the preferred embodiments of the present application and is not intended to limit the present application. It should be pointed out that any modification, equivalent replacement and improvement made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A method for predicting the service life cycle of inclined cementing casing in over-wells, characterized in that, Specifically, the steps include the following: Step S1: Establish a casing stress model under over-well deviation conditions; Step S2: Verify the casing stress model under over-slant well conditions through multiphysics coupling; Step S3: Conduct dynamic corrosion simulation assessment of the casing; Step S4: Calculate the remaining internal pressure resistance, extrusion resistance, and tensile strength of the casing at the over-well deviation; Step S5: Establish a casing service life prediction model under over-deviation conditions; In step S1, with The stress function is expressed as follows: A is the real part, and B is the imaginary part. The stress function is as follows: In the formula: For the radial stress of the casing, For the circumferential stress of the casing, and for Abstract form of the stress potential function on a plane. For variables related to wellbore size, Shear force on the outer wall of the casing. Radial distance, The positive pressure on the outer wall of the casing. The outer diameter of the casing. For internal pressure resistance, The inner diameter of the casing. For tangential displacement, Poisson's ratio, The elastic modulus of the cement ring; In step S2, the criterion for determining whether the stress value at the outer wall of the casing passes the multiphysics test is: A: Based on the stress values ​​at the outer wall of the casing obtained in step S1, a stress field that conforms to reality can be constructed. B: Multi-field coupling with stress field as the core can be successfully achieved, and the parameter changes of different physical fields reflecting the same physical property are consistent; C: No data loss or distortion occurs during multi-field coupling; The above standards must be met simultaneously to ensure that the stress value at the outer wall of the casing passes the multi-physics field test; In step S3, the regression equation for the dynamic change of corrosion rate over time is fitted using multiple sets of single-point corrosion rate data as follows: In the formula: The dynamic value of corrosion rate under any temperature and pressure environment; The number of single-point corrosion rate data sets participating in the fitting; For any point in time; These are a set of regression coefficients, calculated by interpolating statistical data; In step S3, under the condition of over-deviation, the casing wall thickness calculation formula considering dynamic corrosion of the casing is as follows, using the downhole multiphysics coupling model: In the formula: The casing wall thickness under dynamic corrosion; This represents the dynamic corrosion rate.

2. The method for predicting the service life cycle of inclined cementing casing in over-wells according to claim 1, characterized in that: In step S4, the remaining internal pressure resistance, remaining extrusion resistance, and remaining tensile strength of the casing are used as three threshold values ​​to calculate the safe service life of the casing against internal pressure, extrusion, and tensile strength, respectively.

3. The method for predicting the service life cycle of inclined cementing casing in over-wells according to claim 1, characterized in that: In step S5, the service life cycle of the cementing casing under the condition of over-well deviation is predicted, and then a prediction model of the service life cycle of the casing under the condition of over-well deviation is established.

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