Well wall stability analysis method for deep-water shallow-layer long horizontal well

Abstract

The invention relates to the field of oil and gas reservoir drilling, and provides a borehole wall stability analysis method for a deepwater shallow long horizontal well, which comprises the following steps: carrying out mechanical parameter test on a weakly cemented rock sample to obtain basic mechanical characteristic parameters; performing time-varying strength modeling on the weak cementation constitutive structure to obtain a damage evolution model; performing three-dimensional finite element modeling on the stratum around the borehole to obtain a numerical calculation model; establishing a control equation set, and discretely solving the control equation set through the numerical calculation model to obtain a field variable solving result; and well wall stability analysis is carried out based on the field variable solving result, and an analysis result is obtained. According to the method, the well wall stability prediction precision of the deep-water shallow-layer long horizontal well is improved, and the safety of well drilling is improved.

Description

Technical Field

[0001] This invention relates to the field of oil and gas reservoir drilling technology, and in particular to a wellbore stability analysis method for long horizontal wells in deep water shallow formations. Background Technology

[0002] Deepwater shallow gas reservoirs are characterized by high porosity, weak cementation, and low effective stress, making them highly susceptible to wellbore necking, spalling, and even large-scale collapse when disturbed during drilling. Existing wellbore stability studies mainly focus on deepwater deep reservoirs or onshore oil fields, based on elastoplastic assumptions or the Mohr-Coulomb and Drucker-Prager strength criteria, and assess the collapse pressure window using analytical methods or simplified two-dimensional finite element analysis.

[0003] However, existing technologies have significant shortcomings in the application of long horizontal wells in shallow, weakly cemented formations in deep water: Firstly, the evolution of the mechanical properties of weakly cemented formations is not fully characterized, and the traditional linear elastic-ideal plasticity model assumes c, The constant neglect of the impact of cementation damage evolution on the deterioration of strength parameters leads to a large error in the predicted collapse pressure. Secondly, there is a lack of specialized horizontal wellbore stability models in deep-water shallow environments. Existing studies mostly focus on vertical wells or short deviated wells, without considering the coupling effect of stress and trajectory in long horizontal wells, as well as the special combination of high hydrostatic pressure on the seabed and low ground stress at shallow depths. In addition, hydrate phase change coupling analysis is still in the proof-of-concept stage and lacks a multi-field coupling simulation framework applicable to shallow long horizontal wells, making it impossible to accurately predict the impact of hydrate decomposition caused by drilling disturbance on wellbore stability. Summary of the Invention

[0004] This invention provides a wellbore stability analysis method for deep-water shallow long horizontal wells to overcome the shortcomings of existing technologies.

[0005] This invention provides a wellbore stability analysis method for long horizontal wells in deep water shallow formations, comprising:

[0006] S1: Mechanical parameters of weakly cemented rock samples are tested to obtain basic mechanical characteristic parameters;

[0007] S2: Time-varying intensity modeling is performed on the weak cementation constitutive model to obtain a damage evolution model;

[0008] S3: Perform three-dimensional finite element modeling of the formation surrounding the wellbore to obtain a numerical calculation model;

[0009] S4: Establish a set of control equations, and solve the set of control equations discretizedly using the numerical calculation model to obtain the solution results of the field variables;

[0010] S5: Based on the solution results of the field variables, perform wellbore stability analysis to obtain the analysis results.

[0011] According to the present invention, a wellbore stability analysis method for deep-water shallow long horizontal wells is provided. The basic mechanical characteristic parameters in step S1 include: initial elastic modulus, Poisson's ratio, cohesion, friction angle, and cemented damage evolution parameters, wherein the cemented damage evolution parameters include damage rate coefficient and damage index.

[0012] According to the wellbore stability analysis method for long horizontal wells in deep water shallow formations provided by the present invention, step S1 further includes:

[0013] S11: Identify the in-situ stress state using well logging data and acoustic imaging data to obtain in-situ stress field parameters;

[0014] S12: Triaxial rate creep combined test was conducted on weakly cemented rock samples and hydrate-covered sediments to obtain basic mechanical characteristic parameters.

[0015] According to the wellbore stability analysis method for long horizontal wells in shallow deep water provided by the present invention, step S2 further includes:

[0016] S21: Introduce cementation damage variables into the modified Cambridge model, establish the yield surface, and obtain the yield surface equation;

[0017] S22: Based on the yield surface equation, establish a damage evolution model.

[0018] According to the wellbore stability analysis method for deep-water shallow long horizontal wells provided by the present invention, the expression of the yield surface equation in step S21 is as follows:

[0019]

[0020] Where F is the yield function, q is the deviatoric stress invariant, and p ′ For effective average stress, D c Let M be the cementation damage variable, and p be the critical stress ratio parameter. c Pre-consolidation pressure;

[0021] The expression for the damage evolution model in step S22 is:

[0022] c(t)=c0(1-D c );

[0023]

[0024] Where c(t) is the cohesive force varying with time, and c0 is the initial cohesive force. The friction angle varies with time. Let be the initial friction angle, and m be the friction angle damage coefficient. Let A be the rate of change of the damage variable over time, and let n be the damage rate coefficient and n be the damage exponent. This is the equivalent plastic strain.

[0025] According to the wellbore stability analysis method for deep-water shallow long horizontal wells provided by the present invention, in step S3, when performing three-dimensional finite element modeling of the formation around the wellbore, the wellbore is densified by using hybrid up elements.

[0026] According to the wellbore stability analysis method for long horizontal wells in deep water shallow formations provided by the present invention, step S3 further includes:

[0027] When the physical field triggers the hydrate phase transition decomposition condition, the coupling equation used to balance the effects of hydrate decomposition is invoked;

[0028] The expression for the coupling equation is:

[0029]

[0030] χ hyd(t) =1-βS h,dis (t);

[0031] Where ρ is density, C eff The effective specific heat capacity is given by T, temperature, t, λ, thermal conductivity, Q (heat source term), and L. d For the latent heat of hydrate decomposition, ρ h S is the density of the hydrate. h v represents the hydrate saturation. f Let α be the fluid seepage velocity. d χ is the hydrate decomposition coefficient. hyd(t) S is a function of the relative content of hydrates, β is the influence coefficient of hydrate decomposition, and S h,dis (t) represents the saturation of the decomposed hydrates.

[0032] According to the wellbore stability analysis method for long horizontal wells in deep water shallow formations provided by the present invention, the expression of the governing equations in step S4 is as follows:

[0033]

[0034]

[0035] Where σ is stress, ρ b Let g be the rock bulk density, g be the gravitational acceleration, and ε be the total strain tensor. e For elastic strain, For plastic strain that is related to cementation damage variable and time, v f Let ρ be the fluid seepage velocity, k be the permeability tensor, μ be the fluid dynamic viscosity, p be the pore pressure, and ρ be the pore velocity. f Let be the fluid density, z represent the vertical direction, φ be the porosity, t be the time, and Q be the fluid density.f For fluid source and sink terms.

[0036] According to the present invention, a wellbore stability analysis method for long horizontal wells in deep water shallow formations is provided, wherein step S5 specifically includes:

[0037] Output safety parameters for wellbore stability; these safety parameters include: plastic zone radius, wellbore enlargement rate, and safe mud density window.

[0038] A dual-threshold determination is performed on the wellbore stability state to obtain a determination result that includes both safe and unsafe conditions.

[0039] According to the present invention, a wellbore stability analysis method for long horizontal wells in deep water shallow formations is provided, wherein the determination expression for determining the stable state using a dual-threshold method is as follows:

[0040]

[0041] in, For equivalent plastic strain, ε cr τ is the critical plastic strain threshold, F is the strength failure exponent, and τ max The maximum shear stress is given by c(t), and the time-varying cohesion is given by σ. n For normal stress, The friction angle varies with time.

[0042] This invention provides a wellbore stability analysis method for deep-water shallow horizontal wells. Firstly, by introducing cementation damage variables and establishing a time-varying strength criterion based on a modified Cambridge model, it can dynamically track the real-time decay process of cohesion and friction angle in weakly cemented formations during drilling disturbances. Compared to the traditional linear elastic-ideal plasticity model, this significantly improves the accuracy of wellbore strength prediction, reduces collapse pressure prediction errors, and effectively avoids the systematic overestimation of wellbore bearing capacity by traditional methods. Secondly, this invention provides a reliable physical basis for the damage evolution model by obtaining key parameters such as the damage rate coefficient and damage index through triaxial rate creep joint tests on weakly cemented rock samples and hydrate-coated sediments. This allows the model to accurately reflect the essential characteristics of deep-water shallow weakly cemented formations: "low cementation and rapid cementation damage evolution." Secondly, this invention employs hybrid up-element meshing to refine the wellbore area at 1mm-10cm levels, which not only improves numerical computation efficiency but also accurately captures the abrupt changes in the stress-strain field near the wellbore. Combined with 3D finite element modeling technology, it fully considers the influence of the long horizontal well trajectory and the angle between the minimum principal stress on the wellbore stress distribution, thus improving the accuracy of plastic zone radius prediction compared to traditional 2D models. Furthermore, in this invention, when the physical field triggers the hydrate phase transition decomposition condition, by invoking the THMC multi-field coupling equation containing the latent heat of hydrate decomposition, decomposition coefficient, and relative content function, it can accurately simulate the destructive effect of mud circulation cooling and depressurization on the steady-state equilibrium of hydrates during drilling. This enables a quantitative assessment of the pore pressure increase and strength decay caused by hydrate decomposition, effectively avoiding the risk of underestimating collapse pressure that might result from ignoring the phase transition effect in typical operating conditions. Subsequently, this invention establishes a complete set of governing equations including equilibrium equations, strain decomposition, Darcy flow, and mass conservation. By coupling and solving key field variables such as stress tensor divergence, pore pressure gradient, and fluid flow velocity, it can comprehensively describe the complex physical processes of fluid-structure interaction in deep-water shallow reservoirs, providing a solid theoretical foundation for wellbore stability analysis. Finally, a dual-threshold criterion of equivalent plastic strain and strength failure index is adopted, which has higher reliability and adaptability compared to a single instability criterion. It can simultaneously consider both deformation failure and strength failure modes of the material. By outputting key safety parameters such as plastic zone radius, wellbore enlargement rate, and safe mud density window in real time, it provides a quantitative risk assessment basis for field drilling operations, enabling early warning information to be issued in advance. This buys valuable time for the well site to adjust mud density and pump pressure parameters in a timely manner, significantly reducing the probability of accidents such as wellbore necking, blockage, and large-scale collapse during drilling of long horizontal wells in deep-water shallow gas reservoirs, and improving the safety and economic benefits of drilling operations. Attached Figure Description

[0043] To more clearly illustrate the technical solutions in this invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0044] Figure 1 This is a schematic flowchart of a wellbore stability analysis method for deep-water shallow long horizontal wells provided in an embodiment of the present invention;

[0045] Figure 2 This is a schematic diagram comparing the calculation effects of the safe mud density window provided in the embodiments of the present invention. Detailed Implementation

[0046] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, embodiments of this invention, and should not be construed as limiting the invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention. In the description of this invention, it should be understood that the terminology used is for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0047] The embodiments of the present invention are described below with reference to the figures.

[0048] like Figure 1 As shown, this invention provides a wellbore stability analysis method for long horizontal wells in deep water shallow formations, comprising:

[0049] S1: Mechanical parameters of weakly cemented rock samples were tested to obtain basic mechanical characteristic parameters.

[0050] The basic mechanical characteristic parameters in step S1 include: initial elastic modulus, Poisson's ratio, cohesion, friction angle, and cementation damage evolution parameters, wherein the cementation damage evolution parameters include damage rate coefficient and damage index.

[0051] Furthermore, the initial elastic modulus represents the ratio of stress to strain in the rock during the elastic deformation stage, Poisson's ratio reflects the ratio of lateral deformation to longitudinal deformation in the rock, cohesion and friction angle are Mohr-Coulomb strength parameters describing the shear strength of the rock, and cementation damage evolution parameters include damage rate coefficient and damage index. The damage rate coefficient controls the rate of damage accumulation, and the damage index determines the nonlinear characteristics of damage evolution.

[0052] Step S1 further includes:

[0053] S11: Identify the in-situ stress state using well logging data and acoustic imaging data to obtain in-situ stress field parameters.

[0054] In step S11, geological engineering data is first collected, specifically, data from well logging, acoustic imaging, seabed geological surveys, and coring are collected. Subsequently, the in-situ principal stress σ is determined based on the above data. H σ h σ v With initial pore pressure P p0 , where σ H For the maximum horizontal principal stress, σ h For the minimum horizontal principal stress, σ v The vertical stress is calculated first from the density logging curve using well logging data, followed by the calculation of the vertical stress. For acoustic imaging data, the direction and magnitude of the horizontal principal stress are determined by identifying the elliptic deformation characteristics of the wellbore. The major axis of the ellipse corresponds to the direction of the maximum horizontal principal stress, and the minor axis corresponds to the direction of the minimum horizontal principal stress. Furthermore, the formation elastic modulus distribution is calculated using acoustic logging data. Combined with drilling fluid density and wellbore enlargement data, the magnitude of the horizontal principal stress is inversely calculated using Kirsch analytical methods.

[0055] S12: Triaxial rate creep combined test was conducted on weakly cemented rock samples and hydrate-covered sediments to obtain basic mechanical characteristic parameters.

[0056] Furthermore, the triaxial rate creep combined test is a composite test method that combines the characteristics of triaxial compression test and creep test. Before the experiment, the preparation of weakly cemented rock samples requires strict control of water content and density. The hydrate-coated sediments are prepared by artificial synthesis. Methane gas is mixed with saturated water sediments under low temperature and high pressure conditions to form hydrates. During the experiment, this invention comprehensively analyzes the creep data under different confining pressures, uses damage evolution equations to describe the cementation damage evolution process, and uses nonlinear least squares method to fit the experimental data to obtain the damage rate coefficient and damage index.

[0057] S2: Time-varying intensity modeling is performed on the weak cementation constitutive model to obtain a damage evolution model.

[0058] Furthermore, the weak cement constitutive model is a mathematical model describing the stress-strain relationship of weak cemented strata under external forces. In step S2 of this invention, the modeling method considering the decay of strength parameters over time is used to obtain a damage evolution model for quantitatively describing the evolution of cemented damage over time and the accumulation of plastic deformation.

[0059] Step S2 further includes:

[0060] S21: Introduce cementation damage variables into the modified Cambridge model, establish the yield surface, and obtain the yield surface equation.

[0061] The expression for the yield surface equation in step S21 is as follows:

[0062]

[0063] Where F is the yield function, q is the deviatoric stress invariant, and p ′ For effective average stress, D c Let M be the cementation damage variable, and p be the critical stress ratio parameter. c This is the pre-consolidation pressure.

[0064] This invention introduces a cementation damage variable into the modified Cambridge model, which characterizes the degree of attenuation of cementation strength. Specifically, this invention first substitutes the cementation damage evolution parameters obtained in step S1 into the damage variable calculation formula to obtain the cementation damage variable. The critical stress ratio parameter is determined through triaxial test data, and the preconsolidation pressure represents the maximum effective stress that the soil has historically endured, determined through the e-logp' curve of the compression test. After substituting the parameters into the yield surface equation, the equation can be obtained and numerical calculations can be performed. That is, when the equation = 0, the stress state is located on the yield surface; when the equation < 0, it is in an elastic state; and when the equation > 0, plastic deformation occurs.

[0065] S22: Based on the yield surface equation, establish a damage evolution model.

[0066] The expression for the damage evolution model in step S22 is as follows:

[0067] c(t)=c0(1-D c );

[0068]

[0069] Where c(t) is the cohesive force varying with time, and c0 is the initial cohesive force. The friction angle varies with time. Let be the initial friction angle, and m be the friction angle damage coefficient. Let A be the rate of change of the damage variable over time, and let n be the damage rate coefficient and n be the damage exponent. This is the equivalent plastic strain.

[0070] Furthermore, the aforementioned damage evolution model describes the variation of cohesion and friction angle with cementation damage variables and time. In step S22, a cohesion attenuation function is first established. The initial cohesion obtained in step S1 is multiplied by the damage variable to calculate the current cohesion. The friction angle attenuation function adopts a linear attenuation model, where is the friction angle damage coefficient, determined through regression analysis of experimental data. The time-varying rate of change of the damage variable is calculated using a numerical differentiation method. The damage rate coefficient and damage exponent are substituted into the damage evolution equation for numerical integration, and the fourth-order Runge-Kutta method is used to solve the system of ordinary differential equations. The damage variables at different times are obtained through numerical solutions, and the corresponding cohesion and friction angle values ​​are then calculated. Finally, a damage evolution database as shown in the above expression is obtained, storing the correspondence between time, equivalent plastic strain, damage variable, cohesion, and friction angle. The strength parameter values ​​at any time are obtained through interpolation algorithms.

[0071] In addition, the present invention also provides a nonlinear elastic-plastic analytical verification of the wellbore, that is, the circumferential stress of the wellbore is derived based on the nonlinear elastic-plastic closed solution, and the critical mud density that decays with time is obtained.

[0072] S3: Perform three-dimensional finite element modeling of the formation surrounding the wellbore to obtain a numerical calculation model.

[0073] In step S3, when performing three-dimensional finite element modeling of the formation around the wellbore, the wellbore wall is densified using hybrid up elements.

[0074] Furthermore, in step S3 of this invention, a three-dimensional finite element model is constructed, in which the wellbore radius r is set. w The horizontal well trajectory has an angle α with the minimum principal stress. The well wall is finely densified in 1mm–10cm layers. A hybrid up-element method is used to improve numerical efficiency. The model boundary is the top seafloor free surface, i.e., σ. n =0, apply σ to the side boundary H σ h Bottom boundary σ v .

[0075] Step S3 further includes:

[0076] When the physical field triggers the hydrate phase transition decomposition condition, the coupling equation used to balance the effects of hydrate decomposition is invoked;

[0077] The expression for the coupling equation is:

[0078]

[0079] χ hyd(t) =1-βS h,dis (t);

[0080] Where ρ is density, Ceff The effective specific heat capacity is given by T, temperature, t, λ, thermal conductivity, Q (heat source term), and L. d For the latent heat of hydrate decomposition, ρ h S is the density of the hydrate. h v represents the hydrate saturation. f Let α be the fluid seepage velocity. d χ is the hydrate decomposition coefficient. hyd(t) S is a function of the relative content of hydrates, β is the influence coefficient of hydrate decomposition, and S h,dis (t) represents the saturation of the decomposed hydrates.

[0081] Furthermore, the physical field of this invention adopts Biot fluid-structure interaction and combines it with real-time evolution of temperature and pressure fields. When necessary (when the hydrate phase transition decomposition condition is triggered), the energy conservation equation and the hydrate phase transition module are invoked. This module is used to activate the above-mentioned coupling equation when wellbore disturbance causes the hydrate-bearing layer to satisfy ΔP / ΔT crossing the hydrate steady-state curve. The three expressions in the coupling equation are energy conservation, mass conservation, and hydrate saturation evolution, respectively. The coupling equation is based on the subsequent control equations and additionally superimposed with the influence of hydrate phase transition, that is, to increase the computational complexity when necessary (when the phase transition condition is triggered).

[0082] S4: Establish a set of control equations, and solve the set of control equations discretly using the numerical calculation model to obtain the solution results of the field variables.

[0083] The expression for the governing equations in step S4 is as follows:

[0084]

[0085] Where σ is stress, ρ b Let g be the rock bulk density, g be the gravitational acceleration, and ε be the total strain tensor. e For elastic strain, For plastic strain that is related to cementation damage variable and time, v f Let ρ be the fluid seepage velocity, k be the permeability tensor, μ be the fluid dynamic viscosity, p be the pore pressure, and ρ be the pore velocity. f Let be the fluid density, z represent the vertical direction, φ be the porosity, t be the time, and Q be the fluid density. f For fluid source and sink terms.

[0086] In step S4, within the grid established in step S3, the present invention constructs fluid-structure interaction control equations. If necessary, the aforementioned temperature and hydrate phase change control equations of THMC are superimposed to form a multi-field coupled model of the co-evolution of solid mechanics, seepage, and thermo-chemical processes. Boundary conditions include seafloor hydrostatic pressure, lateral and bottom in-situ stresses, and time-varying mud pressure inside the wellbore. The specific solver adopts an implicit-explicit time integration strategy to ensure that the numerical stability is maintained during the weak cementation deformation stage.

[0087] S5: Based on the solution results of the field variables, perform wellbore stability analysis to obtain the analysis results.

[0088] Specifically, step S5 includes:

[0089] Output safety parameters for wellbore stability; these safety parameters include: plastic zone radius, wellbore enlargement rate, and safe mud density window.

[0090] A dual-threshold determination is performed on the wellbore stability state to obtain a determination result that includes both safe and unsafe conditions.

[0091] According to the present invention, a wellbore stability analysis method for long horizontal wells in deep water shallow formations is provided, wherein the determination expression for determining the stable state using a dual-threshold method is as follows:

[0092]

[0093] in, For equivalent plastic strain, ε cr τ is the critical plastic strain threshold, F is the strength failure exponent, and τ max The maximum shear stress is given by c(t), and the time-varying cohesion is given by σ. n For normal stress, The friction angle varies with time, and "or" indicates that an unstable state is determined when either of the expressions on the left or right occurs.

[0094] Furthermore, in step S5, based on the simulation process in S4, the present invention continuously tracks indicators such as plastic strain, shear stress, and cementation damage variables. When the combination of indicators reaches the set instability threshold, it is determined that a local collapse has occurred at the corresponding location. The model outputs the plastic zone expansion radius, well diameter enlargement rate, and time-related safe mud density window in real time to guide drilling operations.

[0095] In addition, based on model convergence, this invention also performs global sensitivity analysis on geological parameters (such as permeability and cementation degradation) and engineering parameters (such as mud density and drilling speed). By comparing the impact of different parameter disturbances on wellbore stability, key control factors are identified, and mud parameters or drilling processes are adjusted accordingly to reduce the risk of wellbore instability.

[0096] Simultaneously, the model is integrated with the well site monitoring system, allowing real-time measurements of mud pressure, drilling speed, and bottom hole temperature to be directly updated into the numerical model. The system periodically transmits the latest calculated safe density window and risk level to the site, enabling timely adjustments to pump pressure or density.

[0097] Through the above description of the embodiments, those skilled in the art can clearly understand that each embodiment can be implemented by means of software plus necessary general-purpose hardware platforms, and of course, it can also be implemented by hardware. Based on this understanding, the above technical solutions, in essence or the part that contributes to the prior art, can be embodied in the form of a software product. This computer software product can be stored in a computer-readable storage medium, such as ROM / RAM, magnetic disk, optical disk, etc., and includes several instructions to cause a computer device (which may be a personal computer, server, or network device, etc.) to execute the methods described in the various embodiments or some parts of the embodiments.

[0098] The following detailed description of a wellbore stability analysis method for deep-water shallow-depth long horizontal wells, based on a specific embodiment, illustrates the present invention.

[0099] This example focuses on a planned well in a certain sea area. The operating water depth is 1520m. The wellbore design, with a horizontal section vertical depth of 240–305m, penetrates an ultra-shallow gas layer. Well logging and seabed geological sampling show that the reservoir is medium- to fine-grained, weakly cemented siltstone with an average porosity of 38% and a permeability of 55mD. Overlying this is a 15m thick layer of interbedded clay hydrates at a vertical depth of 195–210m. The measured hydrostatic pressure is 15.1 MPa, and the pore pressure gradient is 0.97 g / cm³. 3 In-situ principal stress is inverted through drain testing and imaging logging: σ v ≈6.2MPa, σ H ≈5.4MPa (northeast 85° direction), σ h ≈4.1MPa.

[0100] After acquiring the data, laboratory tests were conducted on weakly cemented rock samples and hydrated mudstone. Specifically, siltstone cores at a vertical depth of 265m were taken and subjected to conventional and rate-controlled triaxial loading (confining pressure 4MPa) and 24h creep-recovery tests. The results showed an elastic modulus E0≈1.25GPa, Poisson's ratio ν0≈0.23, cohesion c0≈0.51MPa, and an internal friction angle of 0.51MPa. The damage evolution coefficient A≈2.5×10-3h-1 and the exponent n≈0.83. The uniaxial strength of the hydrate-bearing mudstone is about 2.2MPa under the conditions of 4℃ and 12MPa, providing mechanical and thermodynamic parameters for phase transformation coupling.

[0101] Next, a two-way calibration of well logging and experimentation and verification of the geostress model were conducted. The experimental elastic modulus was compared with the back-calculated results of longitudinal and transverse wave velocities from well logging to correct the longitudinal distribution of lithology. Furthermore, the effective stress coefficient and Biot coefficient were inverted using the corrected density and porosity curves. Combined with marine acoustic refraction data, a three-dimensional initial geostress and pore pressure field covering a 200m×200m×150m area around the wellbore was established.

[0102] The fourth step involves constructing a three-dimensional finite element geometric mesh and wellbore trajectory. Based on the aforementioned stress field, a cylindrical model with a radius of 50m and a height of 120m is generated, with a wellbore radius of 0.108m. The horizontal section follows the σ... h The well layout is oriented with a well wall density of 0.001–0.01m layers. The outer far-field mesh is gradually thickened, and the entire model has approximately 280,000 tetrahedral elements, which can simultaneously ensure both accuracy and efficiency.

[0103] After establishing the simulation field, a multiphysics coupling model and solution settings were implemented. A seepage-solid coupling control equation was established within the mesh, with a Biot coefficient of 0.95. Based on the difference between the mud circulation temperature of 4℃ and the original wellbore temperature of 8.5℃, a temperature-pressure threshold was set to trigger a hydrate phase transition. The mud pressure inside the wellbore was input using a real-time recorded displacement-density curve, while the seawater-formation static pressure was maintained at the outer boundary. The solver used the generalized-α method, with an initial time step of 10s and an upper limit of 120s.

[0104] After simulation, wellbore instability was assessed and simulation results were output. During the 8-hour simulation period, the maximum radius of the plastic zone was 0.46 m, and the wellbore enlargement rate was 11.2%. The first local collapse was assessed at 4.2 hours into the simulation. If hydrate decomposition is ignored, the lower limit of the safe mud density window is underestimated by 0.07–0.08 g / cm³. The model recommends maintaining 1.26–1.31 g / cm³ in the field. 3 The mud density is controlled to a certain level, and the circulating temperature difference is limited to no more than 3°C to ensure that the degree of strength degradation is controllable.

[0105] In addition, this invention also includes parameter sensitivity analysis and process optimization, as well as real-time data transmission risk warning. Seven parameters, including permeability, cementation attenuation index n, mud density, and drilling speed, were selected for global sensitivity assessment within a reasonable range of ±10%. The results show that n and the mud density gradient are the two most important controlling factors.

[0106] The model interfaces with the protocol used in the field, automatically reading information such as mud pool density, pump pressure, and bottom hole temperature every 30 seconds and updating the prediction results. When the comprehensive risk index reaches a threshold, the system automatically pushes a suggestion to increase mud density, which helps to avoid wellbore spalling and collapse.

[0107] like Figure 2The figure shows the effect of the method of the present invention in improving the reliability of the calculation of the safe mud density window. The vertical axis represents the collapse pressure density and the horizontal axis represents the depth. As can be seen from the figure, the traditional method is based on static parameters or simplified models, which leads to a wider predicted safe window range or deviation from the actual needs. The present invention, by dynamically coupling multiple factors such as weak cementation damage evolution and hydrate phase transformation, calculates results that are closer to actual drilling conditions and the safe window range is more accurate, verifying the reliability of the present invention under complex conditions of deep water and shallow layers (weak cementation + hydrate).

[0108] This invention considers a time-varying strength constitutive model for weakly cemented damage in deep-water shallow formation environments. By introducing cemented damage variables, the cohesion, friction angle, and elastic modulus decay in real time with drilling disturbances, overcoming the systematic overestimation of wellbore strength by traditional constants and assumption models. Furthermore, by using triaxial creep data of weakly cemented sandstone for inversion, the average error in predicting collapse pressure decreases, improving reliability.

[0109] This invention characterizes the unique geostress and mechanical parameters of deep-water shallow environments by simultaneously applying the top seafloor free surface, hydrostatic pressure, and shallow low principal stress boundaries in a three-dimensional finite element method, while also considering stress evolution along the horizontal well path. Compared to traditional two-dimensional models that only consider lateral pressure boundaries, the predicted plastic zone radius is increased, which better matches the increased well diameter data measured in the South China Sea shallow layers, thus avoiding the risk of overestimating the safety window range.

[0110] When temperature and pressure disturbances trigger hydrate decomposition, this invention considers the hydrate decomposition heat, pore pressure increase, and strength decay. In practice, for a horizontal well with a hydrate bottom boundary 3m from the wellbore, coupled calculations show that the lower limit of the safe mud density window needs to be increased by 0.08 g / cm³. 3 If the phase transition is ignored, the collapse pressure may be underestimated, which verifies the necessity of this module for risk identification.

[0111] This invention utilizes nonlinear elastoplastic analytical solutions to provide initial boundaries and convergence criteria for finite element methods, and improves computational efficiency through local adaptive meshing and the generalized -α method. Under the same accuracy, the overall computation time is reduced from 3.8h to 1.3h, and the numerical convergence rate in the rapid expansion stage of the plastic zone is increased to 100%, ensuring the stability of long-term drilling simulation.

[0112] This invention also employs a dual threshold judgment of equivalent plastic strain and strength failure index, and outputs the calculation results to the well site through an interface. It can issue risk warnings 4–6 minutes earlier than offline analysis, providing a quantitative basis for real-time control of mud density and pump pressure.

[0113] In general, this invention aims to propose a method for selectively coupled wellbore stability analysis of weakly cemented hydrates in long horizontal wells in deep-water shallow reservoirs. By establishing time-varying weakly cemented constitutive and strength criteria, coupling the deep-water-shallow-reservoir stress field, selectively integrating the THMC phase transition model of hydrates, and combining efficient three-dimensional finite element method with analytical solutions, the method dynamically simulates the four-dimensional evolution law of wellbore stress-strain-failure, accurately calculates and updates the safe mud density window in real time, reduces the risk of wellbore collapse and diameter reduction, and achieves safe and efficient drilling of long horizontal wells in deep-water shallow gas reservoirs.

[0114] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A wellbore stability analysis method for long horizontal wells in shallow deep water, characterized in that, include: S1: Mechanical parameters of weakly cemented rock samples are tested to obtain basic mechanical characteristic parameters; S2: Time-varying intensity modeling is performed on the weak cementation constitutive model to obtain a damage evolution model; S3: Perform three-dimensional finite element modeling of the formation surrounding the wellbore to obtain a numerical calculation model; S4: Establish a set of control equations, and solve the set of control equations discretizedly using the numerical calculation model to obtain the solution results of the field variables; S5: Based on the solution results of the field variables, perform wellbore stability analysis to obtain the analysis results.

2. The wellbore stability analysis method for deep-water shallow long horizontal wells according to claim 1, characterized in that, The basic mechanical characteristic parameters in step S1 include: initial elastic modulus, Poisson's ratio, cohesion, friction angle, and cementation damage evolution parameters, including damage rate coefficient and damage index.

3. The wellbore stability analysis method for deep-water shallow long horizontal wells according to claim 1, characterized in that, Step S1 further includes: S11: Identify the in-situ stress state using well logging data and acoustic imaging data to obtain in-situ stress field parameters; S12: Triaxial rate creep combined test was conducted on weakly cemented rock samples and hydrate-covered sediments to obtain basic mechanical characteristic parameters.

4. The wellbore stability analysis method for deep-water shallow long horizontal wells according to claim 1, characterized in that, Step S2 further includes: S21: Introduce cementation damage variables into the modified Cambridge model, establish the yield surface, and obtain the yield surface equation; S22: Based on the yield surface equation, establish a damage evolution model.

5. The wellbore stability analysis method for deep-water shallow long horizontal wells according to claim 4, characterized in that, The expression for the yield surface equation in step S21 is: Where F is the yield function, q is the deviatoric stress invariant, and p ′ For effective average stress, D c Let M be the cementation damage variable, and p be the critical stress ratio parameter. c Pre-consolidation pressure; The expression for the damage evolution model in step S22 is: c(t)=c0(1-D c ); Where c(t) is the cohesive force varying with time, and c0 is the initial cohesive force. The friction angle varies with time. Let be the initial friction angle, and m be the friction angle damage coefficient. Let A be the rate of change of the damage variable over time, and let n be the damage rate coefficient and n be the damage exponent. This is the equivalent plastic strain.

6. The wellbore stability analysis method for deep-water shallow long horizontal wells according to claim 1, characterized in that, In step S3, when performing three-dimensional finite element modeling of the formation around the wellbore, the wellbore wall is densified using hybrid elements.

7. The wellbore stability analysis method for deep-water shallow long horizontal wells according to claim 1, characterized in that, Step S3 also includes: When the physical field triggers the hydrate phase transition decomposition condition, the coupling equation used to balance the effects of hydrate decomposition is invoked; The expression for the coupling equation is: x hyd(t) =1-βS h,dis (t); Where ρ is density, C eff The effective specific heat capacity is given by T, temperature, t, λ, thermal conductivity, Q (heat source term), and L. d For the latent heat of hydrate decomposition, ρ h S is the density of the hydrate. h v represents the hydrate saturation. f Let α be the fluid seepage velocity. d χ is the hydrate decomposition coefficient. hyd(t) S is a function of the relative content of hydrates, β is the influence coefficient of hydrate decomposition, and S h,dis (t) represents the saturation of the decomposed hydrates.

8. The wellbore stability analysis method for deep-water shallow long horizontal wells according to claim 1, characterized in that, The expression for the governing equations in step S4 is: Where σ is stress, ρ b Let g be the rock bulk density, g be the gravitational acceleration, and ε be the total strain tensor. e For elastic strain, For plastic strain that is related to cementation damage variable and time, v f Let ρ be the fluid seepage velocity, k be the permeability tensor, μ be the fluid dynamic viscosity, p be the pore pressure, and ρ be the pore velocity. f Let ρ be the fluid density, z represent the vertical direction, φ be the porosity, y be time, and Q be... f For fluid source and sink terms.

9. The wellbore stability analysis method for deep-water shallow long horizontal wells according to claim 1, characterized in that, Step S5 specifically includes: Output safety parameters for wellbore stability; these safety parameters include: plastic zone radius, wellbore enlargement rate, and safe mud density window. A dual-threshold determination is performed on the wellbore stability state to obtain a determination result that includes both safe and unsafe conditions.

10. The wellbore stability analysis method for deep-water shallow long horizontal wells according to claim 9, characterized in that, The decision expression used for determining the steady state using a dual threshold method is as follows: in, For equivalent plastic strain, ε cr τ is the critical plastic strain threshold, F is the strength failure exponent, and τ max The maximum shear stress is given by c(t), and the time-varying cohesion is given by σ. n For normal stress, The friction angle varies with time.

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