Method and device for determining lower limit of drilling fluid density and storage medium

Abstract

This application discloses a method, apparatus, equipment, and storage medium for determining the lower limit of drilling fluid density. The method includes obtaining deviatoric stresses and long-term strength data of hydrate-bearing samples under different stress levels and hydrate saturation conditions through creep experiments; determining creep characteristics based on the long-term strength data and deviatoric stresses of the hydrate-bearing samples, and establishing a creep constitutive model based on the parameters and creep characteristics of the creep experiments; determining stress balance equations, geometric equations, and boundary conditions based on the stress characteristics of the surrounding rock of the well to be logged; determining the wellbore reduction calculation equation based on the creep constitutive model, stress balance equations, geometric equations, and boundary conditions; and determining the lower limit of drilling fluid density in hydrate formations under preset wellbore reduction conditions based on the wellbore reduction calculation equation and combined with logging data. This application can achieve the determination of the lower limit of drilling fluid density, thereby enabling safe drilling in hydrate formations.

Description

Technical Field

[0001] The embodiments of the present invention relate to the technical field of oil and gas well drilling, and particularly to a method, apparatus, equipment, and storage medium for determining the lower limit of drilling fluid density. Background Technology

[0002] With the increasing depletion of traditional oil and gas resources, natural gas hydrates are considered one of the most promising clean energy sources for exploitation. Natural gas hydrates are cage-like crystalline compounds formed from methane gas and water under low temperature and high pressure. They are widely distributed in shallow deep-water formations and permafrost zones, possessing characteristics such as abundant reserves, high energy density, and no pollution after combustion. Currently, the United States, Japan, China, and other countries have conducted pilot production operations of hydrates, confirming the feasibility of deep-water hydrate exploitation. However, due to their weak cementation and easy deformation, wellbore instability and reservoir sand production are highly likely to occur during drilling and production, severely restricting the safe and efficient exploitation of hydrates. Therefore, research on wellbore stability during drilling in hydrate formations has significant engineering implications.

[0003] Numerous studies have shown that hydrate formations exhibit strong creep characteristics. During drilling, stress concentration around the wellbore can lead to wellbore shrinkage and stuck pipe, severely impacting drilling safety in hydrate formations. However, current research on the creep characteristics of hydrate formations mainly focuses on experimental analysis and creep model construction, lacking methods for controlling wellbore stability under the influence of creep characteristics. Summary of the Invention

[0004] The purpose of this invention is to provide at least one method, apparatus, equipment, and storage medium for determining the lower limit of drilling fluid density, which can at least solve the technical problem of the lack of a method for controlling wellbore stability under the influence of creep characteristics in the prior art.

[0005] To address the aforementioned technical problems, at least one embodiment of this application provides a method for determining the lower limit of drilling fluid density, comprising:

[0006] Creep tests were conducted on hydrate-containing samples to obtain the deviatoric stress of the samples under different stress levels and the long-term strength data of the samples under different hydrate saturation conditions.

[0007] Creep characteristics were determined based on the long-term strength data and deviatoric stress of the hydrate-containing sample, and a creep constitutive model was established based on the parameters of the creep experiment and the creep characteristics.

[0008] Determine the stress balance equation, geometric equation, and boundary conditions based on the stress characteristics of the rock surrounding the well to be logged;

[0009] The wellbore reduction calculation equation is determined based on the creep constitutive model, stress balance equation, geometric equation, and boundary conditions.

[0010] Based on the wellbore reduction calculation equation and combined with well logging data, the lower limit of drilling fluid density in hydrate formations under preset wellbore reduction conditions is determined.

[0011] At least one embodiment of this application also provides a drilling fluid density lower limit determination device, comprising:

[0012] The acquisition module is used to acquire the deviatoric stress of the hydrate-containing sample under different stress levels and the long-term strength data of the hydrate-containing sample under different hydrate saturation conditions through creep experiments of the hydrate-containing sample.

[0013] The model building module is used to determine creep characteristics based on the long-term strength data and deviatoric stress of the hydrate-containing sample, and to establish a creep constitutive model based on the parameters of the creep experiment and the creep characteristics.

[0014] The first determining module is used to determine the stress balance equation, geometric equation and boundary conditions based on the stress characteristics of the rock surrounding the well to be tested.

[0015] The second determining module is used to determine the wellbore reduction calculation equation based on the creep constitutive model, stress balance equation, geometric equation and boundary conditions;

[0016] The calculation module is used to determine the lower limit of drilling fluid density in hydrate formations under preset wellbore reduction conditions based on the wellbore reduction calculation equation and in combination with logging data.

[0017] At least one embodiment of this application also provides an electronic device, including: at least one processor; and a memory communicatively connected to the at least one processor; wherein the memory stores instructions executable by the at least one processor, the instructions being executed by the at least one processor to enable the at least one processor to perform the above-described drilling fluid density lower limit determination method.

[0018] At least one embodiment of this application also provides a computer-readable storage medium storing a computer program that, when executed by a processor, implements the above-described method for determining the lower limit of drilling fluid density.

[0019] The embodiments of this application provide a method, apparatus, electronic device, and computer-readable storage medium for determining the lower limit of drilling fluid density. By establishing wellbore shrinkage models of hydrate formations under two stress states—decaying creep and accelerated creep—the recommended short start-up and stop times for specific drilling fluid densities are determined. Furthermore, a method for determining the lower limit of drilling fluid density under specific drilling conditions is proposed, thereby achieving safe drilling in hydrate formations, reducing the risk of stuck pipe, and overcoming the shortcomings of current methods for determining drilling fluid density under wellbore shrinkage conditions in hydrate formations.

[0020] In some optional embodiments, the step of determining the wellbore reduction calculation equation based on the creep constitutive model, stress balance equation, geometric equation, and boundary conditions includes:

[0021] The formation shrinkage rate is determined by combining the aforementioned creep constitutive model, stress balance equation, geometric equation, and the aforementioned boundary conditions.

[0022] The equation for calculating wellbore reduction is determined based on the formation's shrinkage rate.

[0023] In some optional embodiments, the step of determining the lower limit of drilling fluid density in hydrate formations under preset wellbore reduction conditions based on the wellbore reduction calculation equation and in conjunction with logging data includes:

[0024] Acquire logging data, which includes the maximum allowable wellbore diameter reduction and the preset drilling time;

[0025] Based on the wellbore reduction calculation equation, the minimum drilling fluid column pressure required for the preset drilling time to reach the maximum allowable wellbore reduction is determined.

[0026] The lower limit of drilling fluid density is determined based on the required minimum drilling fluid column pressure.

[0027] In some alternative embodiments,

[0028] The steps of determining creep characteristics based on the long-term strength data and deviatoric stress of the hydrate-containing sample, and establishing a creep constitutive model based on the parameters of the creep experiment and the creep characteristics, include:

[0029] When the deviatoric stress of the hydrate-containing sample is lower than the long-term strength data, the creep characteristic is determined to be decaying creep, and a decaying creep constitutive model is established based on the parameters of the creep experiment and the creep characteristic.

[0030] When the deviatoric stress of the hydrate-containing sample is higher than the long-term strength data, the creep characteristic is determined to be accelerated creep, and an accelerated creep constitutive model is established based on the parameters of the creep experiment and the creep characteristic.

[0031] In some optional embodiments, the expression for the decay creep constitutive model is:

[0032]

[0033] The expression for the accelerated creep constitutive model is:

[0034]

[0035] Where ε is the creep strain; σ is the shear stress in the creep experiment; t is the creep time; E1 is the instantaneous elastic modulus; E2 is the elastic modulus of the viscoelastic element; η1 is the viscosity coefficient of the viscoelastic element; k is a fitting parameter less than 1; η2 is the viscosity coefficient of the viscous element; η3 is the viscosity coefficient of the viscous element; σ s The long-term strength of the hydrate-containing sample.

[0036] In some optional embodiments, the step of determining the wellbore reduction calculation equation based on the creep constitutive model, stress balance equation, geometric equation, and boundary conditions includes:

[0037] Based on the aforementioned decay creep constitutive model, stress balance equation, geometric equation, and boundary conditions, the equation for calculating wellbore reduction when the wellbore stress state is in decay creep is determined; and...

[0038] Based on the aforementioned accelerated creep constitutive model, stress balance equation, geometric equation, and boundary conditions, the equation for calculating wellbore reduction when the wellbore stress is under accelerated creep is determined.

[0039] The expression for the wellbore reduction calculation equation when the wellbore stress state is in decay creep is as follows:

[0040]

[0041] When the wellbore stress state is under accelerated creep, the expression for the wellbore reduction calculation equation is:

[0042]

[0043] Among them, u a σ is the wellbore shrinkage; t0 is the drilling time; a is the wellbore radius; σ h For effective horizontal ground stress; p i Density of drilling fluid; μ c E2(S) is Poisson's ratio; h ), η1(S h ), η2(S h ), k(S h ), η3(S h ) represents the creep parameter.

[0044] In some optional embodiments, the step of obtaining the deviatoric stress of the hydrate-containing sample under different stress levels and the long-term strength data of the hydrate-containing sample under different hydrate saturation conditions through creep experiments includes:

[0045] By conducting triaxial mechanical compression experiments on hydrate-containing samples under different hydrate saturation conditions, triaxial mechanical compression experimental data of hydrate-containing samples under different hydrate saturation conditions were obtained.

[0046] Based on the triaxial mechanical compression test data, the deviatoric stress of the hydrate-containing sample in the creep test under different stress levels was determined;

[0047] Triaxial creep tests were conducted on the hydrate-containing samples under different hydrate saturation conditions to obtain long-term strength data of the hydrate-containing samples. Attached Figure Description

[0048] One or more embodiments are illustrated by way of example with reference to the accompanying drawings, and these illustrative descriptions do not constitute a limitation on the embodiments.

[0049] Figure 1 This is a flowchart of a method for determining the lower limit of drilling fluid density provided in one embodiment of this application;

[0050] Figure 2 This is a flowchart of a method for determining the lower limit of drilling fluid density provided in another embodiment of this application;

[0051] Figure 3 This is another embodiment of the present application, showing the creep rate of hydrate formations over time under different loading stresses (hydrate saturation is 0%).

[0052] Figure 4 This is another embodiment of the present application providing safe drilling fluid density (wellbore reduction ratio of 15%) under different constraint times.

[0053] Figure 5 Another embodiment of this application provides the density variation of safe drilling fluid for hydrate formations with depth based on well logging results (wellbore diameter reduction rate of 15%).

[0054] Figure 6 This is a schematic diagram of a drilling fluid density lower limit determination device provided in another embodiment of this application;

[0055] Figure 7 This is a schematic diagram of the structure of an electronic device provided in another embodiment of this application.

[0056] In the accompanying drawings, the same parts are referred to by the same reference numerals, and the drawings are not drawn to scale. Detailed Implementation

[0057] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the various embodiments of this application will be described in detail below with reference to the accompanying drawings. However, those skilled in the art will understand that many technical details have been provided in the various embodiments of this application to help readers better understand this application. However, the technical solutions claimed in this application can be implemented even without these technical details and various changes and modifications based on the following embodiments. The division of the various embodiments below is for the convenience of description and should not constitute any limitation on the specific implementation of this application. The various embodiments can be combined with and referenced by each other without contradiction.

[0058] It should be noted that the terms "first," "second," etc., in the specification, claims, and accompanying drawings of this disclosure are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate so that the embodiments of this disclosure described herein can be implemented in orders other than those illustrated or described herein. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion; for example, a process, method, system, product, or apparatus that comprises a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0059] It should be noted that the steps shown in the flowchart in the accompanying drawings can be executed in a computer system such as a set of computer-executable instructions, and although a logical order is shown in the flowchart, in some cases the steps shown or described may be executed in a different order than that shown here.

[0060] This invention proposes a method for determining the lower limit of drilling fluid density. The implementation details of the drilling fluid density lower limit determination method in this embodiment are described below. The following content is only for the convenience of understanding and is not necessary for implementing this solution.

[0061] Example 1:

[0062] The specific process of the drilling fluid density lower limit determination method in this embodiment can be as follows: Figure 1 As shown, it includes:

[0063] Step 110: Through creep tests on hydrate-containing samples, obtain the deviatoric stress of the hydrate-containing samples under different stress levels under different hydrate saturation conditions, as well as the long-term strength data of the hydrate-containing samples under different hydrate saturation conditions.

[0064] Specifically, creep testing is an experimental method for studying the deformation behavior of materials under long-term constant stress or strain conditions. In creep tests on hydrate-containing specimens, the specimens are typically placed under triaxial stress, and different axial and confining pressures (i.e., back pressures) are applied to simulate the stress environment in actual engineering. During creep, the deformation of the specimen, including axial and radial strain, can be measured and recorded. The deviatoric stress of the specimen is then calculated, typically expressed as σ1-σ3, where σ1 is the axial stress and σ3 is the confining pressure. The deviatoric stress is an important parameter describing the stress state of the specimen. In creep tests, the confining pressure is usually a fixed value, while the deviatoric stress is changed by adjusting the axial force.

[0065] In this embodiment, the compressive strength of the hydrate-containing sample at different hydrate saturation levels is first obtained. A series of stress levels are then established based on the compressive strength; these stress levels can be percentages of the compressive strength. A creep test is performed on the hydrate-containing sample using a creep apparatus, and the deviatoric stress at different stress levels is determined by applying axial pressure and micro-pressure to the sample.

[0066] In some cases, hydrate-containing samples with different hydrate saturations were prepared using marine soil, and compressive strength tests were conducted under standard conditions to obtain the compressive strength value at each hydrate saturation level.

[0067] Long-term strength data refers to the maximum stress a material can withstand without failure under sustained loading over a long period of time. In creep tests, by continuously observing the deformation of the specimen, the strength state of the specimen at different time points can be determined. When the specimen fails at a certain time point, the corresponding stress value is the long-term strength at that time.

[0068] In some cases, triaxial creep tests are conducted on hydrate-containing specimens under different hydrate saturation conditions, and the long-term strength expression of the hydrate-containing specimens is obtained using the isochronous curve method. Specifically, based on the creep test data, isochronous curves are plotted at different stress levels. These isochronous curves represent the stress values ​​corresponding to the specimen reaching the same strain at different time points. The corresponding failure points are found on the isochronous curves; these points represent the stress values ​​at which the specimen fails at different time points. Long-term strength curves are plotted using these failure points and their corresponding times, and the long-term strength expression is obtained by fitting these curves using mathematical methods.

[0069] Step 120: Determine the creep characteristics based on the long-term strength data and deviatoric stress of the hydrate-containing sample, and establish a creep constitutive model based on the parameters of the creep experiment and the creep characteristics.

[0070] Specifically, hydrate formations exhibit strong creep characteristics. During drilling, stress concentration around the wellbore can lead to wellbore shrinkage and stuck pipe, severely impacting drilling safety in hydrate formations. The creep characteristics of hydrate formations include accelerated creep and decaying creep. Accelerated creep refers to the acceleration of creep after it begins due to certain conditions; while decaying creep refers to the phenomenon where creep deformation is more pronounced initially under a small constant load, but the creep rate gradually decreases over time, eventually reaching a stable value. The corresponding creep constitutive models for accelerated creep and decaying creep are not the same. In this embodiment, the creep characteristics are determined based on the long-term strength data and deviatoric stress of the hydrate-bearing sample. That is, when the creep characteristics of the hydrate-bearing sample conform to accelerated creep, an accelerated creep constitutive model is established based on the conformed accelerated creep characteristics; when the creep characteristics of the hydrate-bearing sample conform to decaying creep, a decaying creep constitutive model is established based on the conformed decaying creep characteristics.

[0071] Step 130: Determine the stress balance equation, geometric equation, and boundary conditions based on the stress characteristics of the rock surrounding the well to be tested.

[0072] Specifically, for any stratum micro-element, it satisfies the stress balance equation and the geometric equation. The stress balance equation describes the equilibrium condition that the internal stress distribution of the stratum micro-element must satisfy when subjected to external forces. The geometric equation describes the relationship between the internal strain and displacement of the stratum micro-element when subjected to external forces. The boundary conditions describe the stress and displacement state of the rock at the boundaries.

[0073] Furthermore, for any infinitesimal element of the formation, the stress balance equation and the geometric equation can be expressed as follows:

[0074]

[0075]

[0076] Assuming the wellbore size is 'a', the radial stress at the wellbore location is equal to the drilling fluid column pressure, and the radial stress at infinity is equal to the horizontal ground stress σ. h :

[0077] σ r =P m (r=a)

[0078] σ r =σ h (r→∞)

[0079] Therefore, the stress distribution of the surrounding rock of the wellbore at the initial moment can be obtained as follows:

[0080] σ r =P m -φ(P m -P p )

[0081] σ θ =2σ h -P m +K1(P m -P p )

[0082] σ z =σ v +K1(P m -P p )

[0083]

[0084] In the formula, K1 is the seepage effect coefficient; φ is the porosity; P p δ represents the formation pore pressure; δ is the permeability coefficient, which is taken as 1 here; α is the Biot coefficient, which is taken as 1 here.

[0085] In some cases, the generalized shear stress (i.e., deviatoric stress) of the wellbore surrounding rock is calculated based on the stress distribution. The expression for the generalized shear stress of the wellbore surrounding rock is:

[0086]

[0087] The peak strength of the rock under these conditions was calculated according to the Mohr-Coulomb criterion, and its long-term strength was further calculated.

[0088] The expression for long-term strength is:

[0089] σ 1f =σ3·tan 2 (45°+φ / 2)+2c(S h )·tan(45°+φ / 2)

[0090] Step 140: Determine the wellbore reduction calculation equation based on the creep constitutive model, stress balance equation, geometric equation, and boundary conditions.

[0091] Specifically, based on the accelerated creep constitutive model, stress balance equations, geometric equations, and boundary conditions, the wellbore shrinkage calculation equations are determined when the wellbore stress is under accelerated creep; and based on the decaying creep constitutive model, stress balance equations, geometric equations, and boundary conditions, the wellbore shrinkage calculation equations are determined when the wellbore stress is under decaying creep. That is, wellbore shrinkage models for hydrate formations are established based on both decaying and accelerated creep stress states, and wellbore shrinkage calculation equations are constructed to ensure the accuracy of subsequent drilling fluid density lower limit calculations.

[0092] Step 150: Based on the wellbore reduction calculation equation and combined with logging data, determine the lower limit of drilling fluid density in hydrate formations under preset wellbore reduction conditions.

[0093] Specifically, logging data includes drilling time, safe wellbore shrinkage, and hydrate saturation. Based on the wellbore shrinkage calculation equation and logging data, the lower limit of drilling fluid density in hydrate formations under preset wellbore shrinkage conditions is determined. That is, the wellbore shrinkage rate and shrinkage amount are determined under different hydrate saturation conditions, drilling fluid densities, and drilling times. A safe lower limit of drilling fluid density is determined based on specific drilling fluid density, drilling time, and safe wellbore shrinkage as constraints.

[0094] In this embodiment, the influence of hydrate formation creep on wellbore shrinkage is comprehensively considered. Based on the creep characteristics of hydrate formation creep, wellbore shrinkage models for hydrate formations under two stress states, namely decay creep and accelerated creep, are established. These models constitute the wellbore shrinkage calculation equations. Recommended short start-up and stop times are determined for specific drilling fluid densities. Furthermore, a method for determining the lower limit of drilling fluid density under specific drilling conditions is proposed. This enables safe drilling in hydrate formations, reduces the risk of stuck pipe, and overcomes the shortcomings of current methods for determining drilling fluid density under wellbore shrinkage conditions in hydrate formations.

[0095] In some embodiments,

[0096] The steps of determining creep characteristics based on the long-term strength data and deviatoric stress of the hydrate-containing sample, and establishing a creep constitutive model based on the parameters of the creep experiment and the creep characteristics, include:

[0097] When the deviatoric stress of the hydrate-containing sample is lower than the long-term strength data, the creep characteristic is determined to be decaying creep, and a decaying creep constitutive model is established based on the parameters of the creep experiment and the creep characteristic.

[0098] When the deviatoric stress of the hydrate-containing sample is higher than the long-term strength data, the creep characteristic is determined to be accelerated creep, and an accelerated creep constitutive model is established based on the parameters of the creep experiment and the creep characteristic.

[0099] In some embodiments, the expression for the decay creep constitutive model is:

[0100]

[0101] The expression for the accelerated creep constitutive model is:

[0102]

[0103] Where ε is the creep strain; σ is the shear stress in the creep experiment, σ = σ1 - σ3 under triaxial loading stress, MPa; t is the creep time; E1 is the instantaneous elastic modulus, MPa; E2 is the elastic modulus of the viscoelastic element, MPa; η1 is the viscosity coefficient of the viscoelastic element, MPa·h; k is a fitting parameter less than 1, dimensionless; η2 is the viscosity coefficient of the viscous element, MPa·h k η3 is the viscosity coefficient of the viscous element, in MPa·h 2 ;σ s The value is the long-term strength of the hydrate-containing sample, in MPa.

[0104] Specifically, the relationship between long-term strength data and deviatoric stress is determined. When the deviatoric stress of the hydrate-containing sample is lower than the long-term strength, its creep characteristics conform to decay creep, and a decay creep constitutive model is established based on decay creep. When the deviatoric stress of the hydrate-containing sample is higher than the long-term strength, its creep characteristics conform to accelerated creep, and an accelerated creep constitutive model is established based on accelerated creep.

[0105] Based on the results of creep experiments, the creep constitutive equation is constructed as follows:

[0106]

[0107]

[0108] In the formula, ε is the creep strain; σ is the shear stress in the creep experiment, σ = σ1 - σ3 under triaxial loading stress, MPa; t is the creep time; E1 is the instantaneous elastic modulus, MPa; E2 is the elastic modulus of the viscoelastic element, MPa; η1 is the viscosity coefficient of the viscoelastic element, MPa·h; k is a fitting parameter less than 1, dimensionless; η2 is the viscosity coefficient of the viscous element, MPa·h k η3 is the viscosity coefficient of the viscous element, in MPa·h 2 ;σ s The value is the long-term strength of the hydrate-containing sample, in MPa.

[0109] In some embodiments, the step of determining the wellbore reduction calculation equation based on the creep constitutive model, stress balance equation, geometric equation, and boundary conditions includes:

[0110] The formation shrinkage rate is determined by combining the aforementioned creep constitutive model, equilibrium equations, geometric equations, and the aforementioned boundary conditions.

[0111] The wellbore reduction calculation equation is determined based on the formation's shrinkage rate.

[0112] In this embodiment, by combining the stress balance equation, geometric equation, creep constitutive model and simultaneous boundary conditions, the formation shrinkage rate can be obtained. Based on the formation shrinkage rate, the wellbore shrinkage rate under a specific drilling fluid density and drilling time is further determined; thus, the calculation equation for the wellbore shrinkage amount under any drilling fluid density and drilling time is determined.

[0113] In some embodiments, the step of determining the wellbore reduction calculation equation based on the creep constitutive model, stress balance equation, geometric equation, and boundary conditions includes:

[0114] Based on the aforementioned decay creep constitutive model, stress balance equation, geometric equation, and boundary conditions, the equation for calculating wellbore reduction when the wellbore stress state is in decay creep is determined; and...

[0115] Based on the aforementioned accelerated creep constitutive model, stress balance equation, geometric equation, and boundary conditions, the equation for calculating wellbore reduction when the wellbore stress is under accelerated creep is determined.

[0116] The expression for the wellbore reduction calculation equation when the wellbore stress state is in decay creep is:

[0117]

[0118] When the wellbore stress state is under accelerated creep, the expression for the wellbore reduction calculation equation is:

[0119]

[0120] Among them, u a σ is the wellbore shrinkage; t0 is the drilling time; a is the wellbore radius; σ h For effective horizontal ground stress; p i Density of drilling fluid; μ c E2(S) is Poisson's ratio; h ), η1(S h ), η2(S h ), k(S h ), η3(S h ) represents the creep parameter.

[0121] Specifically, a wellbore shrinkage model for hydrate formations under decaying creep is established based on the decaying creep state, i.e., the wellbore shrinkage calculation equation when the wellbore stress state is in decaying creep. A wellbore shrinkage model for hydrate formations under accelerated creep is established based on the accelerated creep state, i.e., the wellbore shrinkage calculation equation when the wellbore stress state is in accelerated creep.

[0122] The following example illustrates the situation when the wellbore stress state is in a state of decaying creep.

[0123] Geometric model ε r With ε θ Differentiating by time yields the creep rate equation:

[0124]

[0125] Will Taking the derivative with respect to time, we get:

[0126]

[0127] To simplify the calculation, we assume that: (1) hydrostatic pressure does not produce creep; (2) the generalized creep rate and the stress deviator have the same principal direction, i.e.:

[0128]

[0129] In the formula,

[0130] According to the theory of plasticity, we can obtain:

[0131]

[0132] In the formula:

[0133]

[0134]

[0135] Furthermore, the creep rate equation for hydrate formations under two-dimensional conditions can be obtained as follows:

[0136]

[0137] Under polar coordinate conditions, the creep rate equation for hydrate formations is:

[0138]

[0139]

[0140]

[0141] In the formula:

[0142]

[0143] Based on the plane strain assumption: ε z =0、 We can obtain:

[0144]

[0145] Furthermore, the creep rate equation for hydrate formations in polar coordinates can be obtained as follows:

[0146]

[0147]

[0148] Combining the equilibrium equations, geometric equations, and constitutive equations, we can obtain:

[0149]

[0150]

[0151] By combining the boundary conditions, we can obtain:

[0152]

[0153] From geometric equations The shrinkage rate of the formation can be obtained as follows:

[0154]

[0155] When r = a, it can represent the drilling fluid density and the downhole shrinkage rate at a specific drilling time:

[0156]

[0157] Integrating the above equation over time yields the wellbore shrinkage at any drilling fluid density and drilling time t0:

[0158]

[0159] Similarly, when the wellbore stress is under accelerated creep, the calculation equation for wellbore diameter reduction is determined based on the accelerated creep constitutive model, stress balance equation, geometric equation, and boundary conditions.

[0160] When the wellbore stress state is under accelerated creep, the expression for the wellbore reduction calculation equation is:

[0161]

[0162] Among them, u aσ is the wellbore shrinkage; t0 is the drilling time; a is the wellbore radius; σ h For effective horizontal ground stress; p i Density of drilling fluid; μ c E2(S) is Poisson's ratio; h ), η1(S h ), η2(S h ), k(S h ), η3(S h ) represents the creep parameter.

[0163] In some embodiments, the step of determining the lower limit of drilling fluid density in hydrate formations under preset wellbore reduction conditions based on the wellbore reduction calculation equation and in conjunction with logging data includes:

[0164] Acquire logging data, which includes the maximum allowable wellbore diameter reduction and the preset drilling time;

[0165] Based on the wellbore reduction calculation equation, the minimum drilling fluid column pressure required for the preset drilling time to reach the maximum allowable wellbore reduction is determined.

[0166] The lower limit of drilling fluid density is determined based on the required minimum drilling fluid column pressure.

[0167] Specifically, based on the accelerated creep constitutive model, stress balance equation, geometric equation, and boundary conditions, the equation for calculating wellbore reduction when the wellbore stress is in accelerated creep is determined, and the minimum drilling fluid column pressure required to reach the maximum allowable wellbore reduction within a preset drilling time is determined; the lower limit of drilling fluid density is determined based on the required minimum drilling fluid column pressure. Similarly, based on the decaying creep constitutive model, stress balance equation, geometric equation, and boundary conditions, the equation for calculating wellbore reduction when the wellbore stress is in decaying creep is determined, and the minimum drilling fluid column pressure required to reach the maximum allowable wellbore reduction within a preset drilling time is determined; the lower limit of drilling fluid density is determined based on the required minimum drilling fluid column pressure.

[0168] Furthermore, let's take the example of wellbore stress in a state of decaying creep as an example for illustration.

[0169] Assuming the maximum allowable wellbore reduction during this drilling time is u0, the minimum required drilling fluid column pressure is:

[0170]

[0171] Furthermore, the lower limit of drilling fluid density can be obtained as follows:

[0172]

[0173] In the formula, ρ mThe lower limit of drilling fluid density, g / cm³ 3 ;σ h Effective horizontal ground stress, MPa; g is gravitational acceleration, m / s². 2 H represents the formation depth, in meters; μ c E2(S) is Poisson's ratio; h ), η1(S h ), η2(S h ), k(S h ), η3(S h ) represents the creep parameter; a represents the wellbore radius (m); u0 represents the wellbore shrinkage (m); and t0 represents the wellbore drilling time (h).

[0174] Similarly, when the wellbore stress is under accelerated creep, the creep equation is:

[0175]

[0176] The constitutive equation in the form of creep rate is:

[0177]

[0178] Furthermore, the equation for calculating the lower limit of drilling fluid density under this condition can be obtained as follows:

[0179]

[0180] In the formula, ρ m The lower limit of drilling fluid density, g / cm³ 3 ;σ h Effective horizontal ground stress, MPa; g is gravitational acceleration, m / s². 2 H represents the formation depth, in meters; μ c E2(S) is Poisson's ratio; h ), η1(S h ), η2(S h ), k(S h ), η3(S h ) represents the creep parameter; a represents the wellbore radius (m); u0 represents the wellbore shrinkage (m); and t0 represents the wellbore drilling time (h).

[0181] In some examples, σ h The calculation method is as follows:

[0182] In the formula, ρ(H) is the formation density; p p α represents the formation pressure; α is the Biot coefficient, which is set to 1.

[0183] In some embodiments, the step of obtaining the deviatoric stress of the hydrate-containing sample under different stress levels and the long-term strength data of the hydrate-containing sample under different hydrate saturation conditions through creep testing of the hydrate-containing sample includes:

[0184] By conducting triaxial mechanical compression experiments on hydrate-containing samples under different hydrate saturation conditions, triaxial mechanical compression experimental data of hydrate-containing samples under different hydrate saturation conditions were obtained.

[0185] Based on the triaxial mechanical compression test data, the deviatoric stress of the hydrate-containing sample in the creep test under different stress levels was determined;

[0186] Triaxial creep tests were conducted on the hydrate-containing samples under different hydrate saturation conditions to obtain long-term strength data of the hydrate-containing samples.

[0187] Specifically, hydrate-containing samples were prepared using marine soil, and triaxial mechanical compression tests were conducted under different confining pressures and hydrate saturation conditions. Based on the experimental results, the compressive strength, Poisson's ratio, cohesion, and internal friction angle of the hydrate samples at different saturation levels were obtained.

[0188] k is obtained based on compressive strength at different stress levels. d (0.2, 0.4, 0.6, 0.8) Deviatoric stress σ in creep tests.

[0189] Triaxial creep tests were conducted on hydrate-containing samples under different hydrate saturation conditions. The long-term strength expression of the hydrate-containing samples was obtained by the isochronous curve method. When the deviatoric stress of the sample is lower than the long-term strength σ, s When the creep characteristics conform to decay creep, the deviatoric stress of the specimen is higher than the long-term strength σ. s At that time, its creep characteristics conform to accelerated creep.

[0190] Among them, the correspondence between long-term intensity and peak intensity under different hydrate saturation was obtained: σ s =f(S) h ,σ p ).

[0191] Based on the differences in parameters under different hydrate saturation levels, a parametric equation related to hydrate saturation is further constructed: E1=E1(S h E2 = E2(S) h ), η1=η1(S h ), k = k(S) h ), η2=η2(S h ), η3=η3(S h ).

[0192] The formation hydrate saturation S was obtained from the well logging results. h =S h (H), formation density distribution ρ=ρ(H).

[0193] Preferably, both the triaxial mechanical tests and creep tests of hydrates are performed using a low-temperature triaxial testing apparatus.

[0194] In this embodiment, the shrinkage rate and diameter reduction of the wellbore are calculated under different hydrate saturation conditions, drilling fluid density, and drilling time. Based on specific drilling fluid density, drilling time, and safe wellbore shrinkage as constraints, a lower limit of safe drilling fluid density can be determined. At the same time, a chart of the lower limit of safe drilling fluid density in hydrate formations can be established, thereby achieving safe drilling in hydrate formations and reducing the risk of stuck pipe.

[0195] Example 2:

[0196] Another embodiment of this application relates to a method for determining the lower limit of drilling fluid density considering the influence of hydrate formation creep on wellbore diameter reduction. The specific process can be as follows: Figure 2 As shown, it includes the following steps:

[0197] Step (1): Prepare hydrate-containing samples using marine soil and conduct triaxial mechanical compression tests under different confining pressures and hydrate saturation conditions. Based on the experimental results, obtain the compressive strength, Poisson's ratio, cohesion, and internal friction angle of the hydrate samples under different saturation conditions.

[0198] Step (2): Obtain k under different stress levels based on compressive strength. d (0.2, 0.4, 0.6, 0.8) Deviatoric stress σ in creep tests.

[0199] Step (3): Conduct triaxial creep tests on hydrate-containing samples under different hydrate saturation conditions, and obtain the long-term strength expression of the hydrate-containing samples by the isochronous curve method. When the deviatoric stress of the sample is lower than the long-term strength σ s When the creep characteristics conform to decay creep, the deviatoric stress of the specimen is higher than the long-term strength σ. s At that time, its creep characteristics conform to accelerated creep.

[0200] Step (4): Based on the creep experiment results, construct the creep constitutive equation as shown in the following equation:

[0201]

[0202]

[0203] In the formula, σ is the shear stress in the creep experiment, and under triaxial loading stress, σ = σ1 - σ3, MPa; t is the creep time; E1 is the instantaneous elastic modulus, MPa; E2 is the elastic modulus of the viscoelastic element, MPa; η1 is the viscosity coefficient of the viscoelastic element, MPa·h; k is a fitting parameter less than 1, dimensionless; η2 is the viscosity coefficient of the viscous element, MPa·h k η3 is the viscosity coefficient of the viscous element, in MPa·h 2 ;σ s The value is the long-term strength of the hydrate-containing sample, in MPa.

[0204] Step (5): Obtain the correspondence between long-term strength and peak strength under different hydrate saturation levels: σ s =f(S) h ,σ p ).

[0205] Step (6): Based on the differences in parameters under different hydrate saturation levels, further construct the parametric equation related to hydrate saturation: E1=E1(S h E2 = E2(S) h ), η1=η1(S h ), k = k(S) h ), η2=η2(S h ), η3=η3(S h ).

[0206] Step (7): Obtain the formation hydrate saturation S based on the well logging results. h =S h (H), formation density distribution ρ=ρ(H).

[0207] Step (8): The following formula can be used to determine the calculation equation for the lower limit of drilling fluid density under a certain wellbore shrinkage in hydrate formations. When the wellbore stress state is in decay creep, the calculation equation is:

[0208]

[0209] When the wellbore stress state is under accelerated creep, the calculation equation is:

[0210]

[0211] In the formula, ρ m The lower limit of drilling fluid density, g / cm³ 3 ;σ h Effective horizontal ground stress, MPa; g is gravitational acceleration, m / s². 2 H represents the formation depth, in meters; μ c E2(S) is Poisson's ratio; h ), η1(S h), η2(S h ), k(S h ), η3(S h ) represents the creep parameter; a represents the wellbore radius (m); u0 represents the wellbore shrinkage (m); and t0 represents the wellbore drilling time (h).

[0212] Preferably, both the triaxial mechanical tests and creep tests of hydrates are performed using a low-temperature triaxial testing apparatus.

[0213] In the above steps, σ h The calculation method is as follows:

[0214]

[0215] In the formula, ρ(H) is the formation density; p p α represents the formation pressure; α is the Biot coefficient, which is set to 1.

[0216] The formula for determining the drilling fluid density in the above steps is derived as follows:

[0217] For any infinitesimal element in the formation, which satisfies the stress equilibrium equation and the geometric equation, it can be expressed as follows:

[0218]

[0219]

[0220] Assuming the wellbore size is 'a', the radial stress at the wellbore location is equal to the drilling fluid column pressure, and the radial stress at infinity is equal to the horizontal ground stress σ. h :

[0221] σ r =P m (r=a)

[0222] σ r =σ h (r→∞)

[0223] Therefore, the stress distribution of the surrounding rock of the wellbore at the initial moment can be obtained as follows:

[0224] σ r =P m -φ(P m -P p )

[0225] σ θ =2σ h -P m +K1(P m -P p )

[0226] σ z =σv +K1(P m -P p )

[0227]

[0228] In the formula, K1 is the seepage effect coefficient; φ is the porosity; P p δ represents the formation pore pressure; δ is the permeability coefficient, which is taken as 1 here; α is the Biot coefficient, which is taken as 1 here.

[0229] Calculate the generalized shear stress of the surrounding rock of the wellbore based on its stress distribution:

[0230]

[0231] The peak strength of the rock under this condition was calculated using the Mohr-Coulomb criterion, and its long-term length was further calculated:

[0232] σ 1f =σ3·tan 2 (45°+φ / 2)+2c(S h )·tan(45°+φ / 2)

[0233] Determine whether the magnitude of the generalized shear stress is greater than the long-term strength. If it is less than the long-term strength, it indicates that the surrounding rock of the wellbore is in a state of decaying creep; if it is greater than the long-term strength, it indicates that the surrounding rock of the wellbore is in a state of accelerated creep.

[0234] Geometric model ε r With ε θ Differentiating by time yields the creep rate equation:

[0235]

[0236] Will Taking the derivative with respect to time, we get:

[0237]

[0238] To simplify the calculation, we assume that: (1) hydrostatic pressure does not produce creep; (2) the generalized creep rate and the stress deviator have the same principal direction, i.e.:

[0239]

[0240] In the formula,

[0241] According to the theory of plasticity, we can obtain:

[0242]

[0243] In the formula:

[0244]

[0245]

[0246] Furthermore, the creep rate equation for hydrate formations under two-dimensional conditions can be obtained as follows:

[0247]

[0248] Under polar coordinate conditions, the creep rate equation for hydrate formations is:

[0249]

[0250] In the formula:

[0251]

[0252] Based on the plane strain assumption: ε z =0、 We can obtain:

[0253]

[0254] Furthermore, the creep rate equation for hydrate formations in polar coordinates can be obtained as follows:

[0255]

[0256]

[0257] Combining the equilibrium equations, geometric equations, and constitutive equations, we can obtain:

[0258]

[0259]

[0260] By combining the boundary conditions, we can obtain:

[0261]

[0262]

[0263]

[0264] From geometric equations The shrinkage rate of the formation can be obtained as follows:

[0265]

[0266] When r = a, it can represent the drilling fluid density and the downhole shrinkage rate at a specific drilling time:

[0267]

[0268] Integrating the above equation over time yields the wellbore shrinkage at any drilling fluid density and drilling time t0:

[0269]

[0270] Assuming the maximum allowable wellbore reduction during this drilling time is u0, the minimum required drilling fluid column pressure is:

[0271]

[0272] Furthermore, the lower limit of drilling fluid density can be obtained as follows:

[0273]

[0274] Similarly, if the generalized shear stress is greater than the long-term strength, it indicates that the surrounding rock of the wellbore is in an accelerated creep state, and its creep equation is:

[0275]

[0276] The constitutive equation in the form of creep rate is:

[0277]

[0278] Furthermore, the equation for calculating the lower limit of drilling fluid density under this condition can be obtained as follows:

[0279]

[0280] In this embodiment, the wellbore shrinkage rate and diameter reduction are calculated under different hydrate saturation conditions, drilling fluid densities, and drilling times. Based on specific drilling fluid densities, drilling times, and safe wellbore shrinkage as constraints, a lower limit for safe drilling fluid density can be determined, and a chart of the lower limit for safe drilling fluid density in hydrate formations can be established. By establishing wellbore shrinkage models for hydrate formations under both decaying creep and accelerated creep stress states, recommended short start-up and start-up times are determined for specific drilling fluid densities. Furthermore, a method for determining the lower limit of drilling fluid density under specific drilling conditions is proposed, thereby achieving safe drilling in hydrate formations, reducing the risk of stuck pipe, and overcoming the shortcomings of current methods for determining drilling fluid density under wellbore diameter reduction conditions in hydrate formations.

[0281] Example 3:

[0282] Another embodiment of this application relates to a method for determining the lower limit of drilling fluid density considering the influence of hydrate formation creep on wellbore diameter reduction. The specific process is as follows:

[0283] Step 11: Take a certain amount of marine soil and prepare hydrate-containing sediment samples with a diameter of 25 mm and a height of 50 mm by the air saturation method. Conduct triaxial mechanical compression tests under hydrate saturation conditions. Based on the experimental results, obtain the compressive strength, Poisson's ratio, cohesion, and internal friction angle of the hydrate samples under different saturation conditions.

[0284] Step 12: Obtain k under different stress levels based on compressive strength. d (0.2, 0.4, 0.6, 0.8) Deviatoric stress σ in creep tests.

[0285] Step 13: Conduct triaxial creep tests on hydrate-containing samples under different hydrate saturation conditions, and obtain the long-term strength expression σ of the hydrate-containing samples using the isochronous curve method. s =(0.0059·ln(S) h +0.1)+0.5889)·σ p When the deviatoric stress of the specimen is lower than the long-term strength σ s When the creep characteristics conform to decay creep, the deviatoric stress of the specimen is higher than the long-term strength σ. s At this time, its creep characteristics conform to accelerated creep, where σ p Indicates peak intensity.

[0286] Step 14: Construct the creep constitutive equation based on the creep experiment results, as shown in the following equation:

[0287]

[0288] In the formula, σ is the shear stress in the creep experiment, and under triaxial loading stress, σ = σ1 - σ3, MPa; t is the creep time; E1 is the instantaneous elastic modulus, MPa; E2 is the elastic modulus of the viscoelastic element, MPa; η1 is the viscosity coefficient of the viscoelastic element, MPa·h; k is a fitting parameter less than 1, dimensionless; η2 is the viscosity coefficient of the viscous element, MPa·h k η3 is the viscosity coefficient of the viscous element, in MPa·h 2 ;σ s The value is the long-term strength of the hydrate-containing sample, in MPa.

[0289] Among them, such as Figure 3 As shown, Figure 3 The graph shows the creep rate of hydrate formations over time under different loading stresses, where the hydrate saturation is 0%.

[0290] Step 15: Further construct the parameters related to hydrate saturation as shown below:

[0291] Table 1 Calculation parameters for hydrates

[0292]

[0293] / MPa / MPa·h <![CDATA[ / MPa·h k ]]> <![CDATA[ / MPa·h 2 ]]> 0 1532 622.14 2134.22 0.28 29892.48 0.2 1720 725.136 1383.40 0.28 26757.57 0.4 2832 920.35 1931.20 0.33 21379.00 0.6 4218 1039.15 1862.2 0.34 18067.67

[0294] Step 16: The water depth in the Shenhu area of ​​the South China Sea is 1000-1700m; in this example, 1300m is used. The hydrate burial depth is between 195m and 250m, and the hydrate saturation is between 0% and 40%. The formation hydrate saturation S is obtained based on the well logging results. h =S h (H), formation density distribution ρ=ρ(H), and calculate formation stress distribution.

[0295] Step 17: Based on the stress distribution of the surrounding rock in the wellbore, determine whether the drilling fluid density is greater than 0.98 g / cm³ under this stress state. 3 At this point, the generalized shear stress is less than the long-term strength, and the surrounding rock of the wellbore is in a state of decaying creep. The calculation equation for the lower limit of drilling fluid density under a certain wellbore shrinkage is as follows:

[0296]

[0297] In the formula, ρ m The lower limit of drilling fluid density, g / cm³ 3 ;σ h Effective horizontal ground stress, MPa; g is gravitational acceleration, m / s². 2 H represents the formation depth, in meters; μ c E2(S) is Poisson's ratio; h ), η1(S h ), η2(S h ), k(S h ), η3(S h ) represents the creep parameter; a represents the wellbore radius (m); u0 represents the wellbore shrinkage (m); and t0 represents the wellbore drilling time (h).

[0298] Step 18: To prevent stuck pipe caused by excessive wellbore reduction, a reduction of 15% of the wellbore size is taken as the critical value. This yields the lower limit of drilling fluid density considering creep effects. According to the calculation results, for a certain hydrate layer in the Shenhu sea area, when the hydrate saturation is 0.2, to maintain the wellbore enlargement requirement within 20 hours, the drilling fluid density should be controlled to be no less than 1.06 g / cm³. 3 .

[0299] Step 19: As Figure 4 As shown, by combining the logging results, the distribution of the lower limit of safe drilling fluid density under different drilling time conditions can be further obtained.

[0300] Furthermore, by combining well logging data, the lower limit of drilling fluid density in hydrate formations under different wellbore diameter reduction conditions was obtained, and drilling density curves were plotted, such as... Figure 5 As shown.

[0301] Example 4:

[0302] Another embodiment of this application relates to a drilling fluid density lower limit determination device. The implementation details of this embodiment's drilling fluid density lower limit determination device are described below. The following details are provided for ease of understanding and are not essential for implementing this solution. A schematic diagram of this embodiment's drilling fluid density lower limit determination device can be seen as follows: Figure 6 As shown, it includes an acquisition module 801, a model construction module 802, a first determination module 803, a second determination module 804, and a calculation module 805.

[0303] The acquisition module 801 is used to acquire, through creep experiments of hydrate-containing samples, the deviatoric stress of the hydrate-containing samples under different stress levels under different hydrate saturation conditions, and the long-term strength data of the hydrate-containing samples under different hydrate saturation conditions.

[0304] The model building module 802 is used to determine the creep characteristics based on the long-term strength data and deviatoric stress of the hydrate-containing sample, and to establish a creep constitutive model based on the parameters of the creep experiment and the creep characteristics.

[0305] The first determining module 803 is used to determine the stress balance equation, geometric equation and boundary conditions based on the stress characteristics of the rock surrounding the well to be tested.

[0306] The second determining module 804 is used to determine the wellbore reduction calculation equation based on the creep constitutive model, stress balance equation, geometric equation and boundary conditions.

[0307] The calculation module 805 is used to determine the lower limit of drilling fluid density in hydrate formations under preset wellbore reduction conditions based on the wellbore reduction calculation equation and in combination with logging data.

[0308] In some optional embodiments, the second determining module 804 is further configured to combine the creep constitutive model, stress balance equation, geometric equation, and the boundary conditions to determine the formation shrinkage rate;

[0309] The equation for calculating wellbore reduction is determined based on the formation's shrinkage rate.

[0310] In some optional embodiments, the calculation module 805 is further configured to acquire logging data, which includes the maximum allowable wellbore reduction and a preset drilling time.

[0311] Based on the wellbore reduction calculation equation, the minimum drilling fluid column pressure required for the preset drilling time to reach the maximum allowable wellbore reduction is determined.

[0312] The lower limit of drilling fluid density is determined based on the required minimum drilling fluid column pressure.

[0313] In some optional embodiments, the model building module 802 is further configured to determine that the creep characteristic is decaying creep when the deviatoric stress of the hydrate-containing sample is lower than the long-term strength data, and to establish a decaying creep constitutive model based on the parameters of the creep experiment and the creep characteristic.

[0314] When the deviatoric stress of the hydrate-containing sample is higher than the long-term strength data, the creep characteristic is determined to be accelerated creep, and an accelerated creep constitutive model is established based on the parameters of the creep experiment and the creep characteristic.

[0315] In some optional embodiments, the second determining module 804 is further configured to determine, based on the decaying creep constitutive model, stress balance equation, geometric equation, and boundary conditions, the wellbore diameter reduction calculation equation when the wellbore stress state is in decaying creep; and,

[0316] Based on the aforementioned accelerated creep constitutive model, stress balance equation, geometric equation, and boundary conditions, the equation for calculating wellbore reduction when the wellbore stress is under accelerated creep is determined.

[0317] The expression for the wellbore reduction calculation equation when the wellbore stress state is in decay creep is as follows:

[0318]

[0319] When the wellbore stress state is under accelerated creep, the expression for the wellbore reduction calculation equation is:

[0320]

[0321] Among them, u a σ is the wellbore shrinkage; t0 is the drilling time; a is the wellbore radius; σ h For effective horizontal ground stress; p i Density of drilling fluid; μ c E2(S) is Poisson's ratio; h ), η1(S h ), η2(S h ), k(S h ), η3(S h ) represents the creep parameter.

[0322] In some optional embodiments, the acquisition module 801 is further configured to acquire triaxial mechanical compression test data of hydrate-containing samples under different hydrate saturation conditions by conducting triaxial mechanical compression tests on hydrate-containing samples under different hydrate saturation conditions.

[0323] Based on the triaxial mechanical compression test data, the deviatoric stress of the hydrate-containing sample in the creep test under different stress levels was determined;

[0324] Triaxial creep tests were conducted on the hydrate-containing samples under different hydrate saturation conditions to obtain long-term strength data of the hydrate-containing samples.

[0325] It is worth mentioning that all modules involved in this embodiment are logical modules. In practical applications, a logical unit can be a physical unit, a part of a physical unit, or a combination of multiple physical units. Furthermore, to highlight the innovative aspects of this application, this embodiment does not introduce units that are not closely related to solving the technical problems proposed in this application; however, this does not mean that other units are absent in this embodiment.

[0326] In the embodiments provided in this application, it should be understood that the disclosed apparatus and methods can also be implemented in other ways. The apparatus embodiments described above are merely illustrative. For example, the flowcharts and block diagrams in the accompanying drawings illustrate the architecture, functionality, and operation of possible implementations of apparatus, methods, and computer program products according to various embodiments of this disclosure. In this regard, each block in a flowchart or block diagram may represent a module, segment, or portion of code containing one or more executable instructions for implementing a specified logical function. It should also be noted that in some alternative implementations, the functions marked in the blocks may occur in a different order than those marked in the drawings. For example, two consecutive blocks may actually be executed substantially in parallel, and they may sometimes be executed in reverse order, depending on the functions involved. It should also be noted that each block in a block diagram and / or flowchart, and combinations of blocks in block diagrams and / or flowcharts, can be implemented using a dedicated hardware-based system that performs the specified function or action, or using a combination of dedicated hardware and computer instructions.

[0327] It should be noted that, in this disclosure, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitation, an element limited by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes that element.

[0328] Example 5:

[0329] Another embodiment of this application relates to an electronic device, such as... Figure 7As shown, it includes: at least one processor; and a memory communicatively connected to the at least one processor; wherein the memory stores instructions executable by the at least one processor, the instructions being executed by the at least one processor to enable the at least one processor to implement the above method steps.

[0330] The memory and processor are connected via a bus, which can include any number of interconnecting buses and bridges, connecting various circuits of one or more processors and memories. The bus can also connect various other circuits, such as peripheral devices, voltage regulators, and power management circuits, which are well known in the art and will not be described further herein. The bus interface provides an interface between the bus and the transceiver. The transceiver can be a single element or multiple elements, such as multiple receivers and transmitters, providing a unit for communicating with various other devices over a transmission medium. Data processed by the processor is transmitted over the wireless medium via an antenna, which further receives data and transmits it to the processor.

[0331] The processor manages the bus and general processing, and also provides various functions, including timing, peripheral interfaces, voltage regulation, power management, and other control functions. Memory is used to store data used by the processor during operation.

[0332] The processor may include, but is not limited to, one or more processors or microprocessors. Each processor may be implemented as an Application Specific Integrated Circuit (ASIC), Digital Signal Processor (DSP), Digital Signal Processing Device (DSPD), Programmable Logic Device (PLD), Field Programmable Gate Array (FPGA), controller, microcontroller, microprocessor, or other electronic component, for executing the methods in the above embodiments.

[0333] Example 6:

[0334] Another embodiment of this application relates to a computer-readable storage medium storing a computer program. When executed by a processor, the computer program implements the above-described method steps.

[0335] That is, those skilled in the art will understand that all or part of the steps in the methods of the above embodiments can be implemented by a program instructing related hardware. This program is stored in a storage medium and includes several instructions to cause a device (which may be a microcontroller, chip, etc.) or processor to execute all or part of the steps of the methods described in the various embodiments of this application. The aforementioned storage medium includes various media capable of storing program code, such as USB flash drives, portable hard drives, read-only memory (ROM), random access memory (RAM), magnetic disks, or optical disks.

[0336] Computer-readable storage media may also store at least one computer-executable program / instruction, such as computer-readable instructions. Computer-readable storage media include, but are not limited to, volatile memory and / or non-volatile memory. Volatile memory may include, for example, random access memory (RAM) and / or cache memory. Computer-readable storage media may include, for example, read-only memory (ROM), hard disk, flash memory, etc. For example, a non-transitory computer-readable storage medium may be connected to a computing device such as a computer, and then, when the computing device executes the computer-readable instructions stored on the computer-readable storage medium, the various methods described above can be performed.

[0337] In addition, the computer device may include (but is not limited to) a data bus, an input / output (I / O) bus, a display, and input / output devices (e.g., keyboard, mouse, speakers, etc.).

[0338] The processor can communicate with external devices via the I / O bus through wired or wireless networks.

[0339] Those skilled in the art will understand that the above embodiments are specific embodiments for implementing this application, and in practical applications, various changes can be made to them in form and detail without departing from the spirit and scope of this application.

Claims

1. A method for determining the lower limit of drilling fluid density, characterized in that, include: Creep tests were conducted on hydrate-containing samples to obtain the deviatoric stress of the samples under different stress levels and the long-term strength data of the samples under different hydrate saturation conditions. Creep characteristics were determined based on the long-term strength data and deviatoric stress of the hydrate-containing sample, and a creep constitutive model was established based on the parameters of the creep experiment and the creep characteristics. Determine the stress balance equation, geometric equation, and boundary conditions based on the stress characteristics of the rock surrounding the well to be logged; The wellbore reduction calculation equation is determined based on the creep constitutive model, stress balance equation, geometric equation, and boundary conditions. Based on the wellbore reduction calculation equation and combined with well logging data, the lower limit of drilling fluid density in hydrate formations under preset wellbore reduction conditions is determined.

2. The method for determining the lower limit of drilling fluid density according to claim 1, characterized in that, The steps for determining the wellbore reduction calculation equation based on the creep constitutive model, stress balance equation, geometric equation, and boundary conditions include: The formation shrinkage rate is determined by combining the aforementioned creep constitutive model, stress balance equation, geometric equation, and the aforementioned boundary conditions. The equation for calculating wellbore reduction is determined based on the formation's shrinkage rate.

3. The method for determining the lower limit of drilling fluid density according to claim 1, characterized in that, The step of determining the lower limit of drilling fluid density in hydrate formations under preset wellbore reduction conditions based on the wellbore reduction calculation equation and in conjunction with logging data includes: Acquire logging data, which includes the maximum allowable wellbore diameter reduction and the preset drilling time; Based on the wellbore reduction calculation equation, the minimum drilling fluid column pressure required for the preset drilling time to reach the maximum allowable wellbore reduction is determined. The lower limit of drilling fluid density is determined based on the required minimum drilling fluid column pressure.

4. The method for determining the lower limit of drilling fluid density according to claim 1, characterized in that, The steps of determining creep characteristics based on the long-term strength data and deviatoric stress of the hydrate-containing sample, and establishing a creep constitutive model based on the parameters of the creep experiment and the creep characteristics, include: When the deviatoric stress of the hydrate-containing sample is lower than the long-term strength data, the creep characteristic is determined to be decaying creep, and a decaying creep constitutive model is established based on the parameters of the creep experiment and the creep characteristic. When the deviatoric stress of the hydrate-containing sample is higher than the long-term strength data, the creep characteristic is determined to be accelerated creep, and an accelerated creep constitutive model is established based on the parameters of the creep experiment and the creep characteristic.

5. The method for determining the lower limit of drilling fluid density according to claim 4, characterized in that, The expression for the decay creep constitutive model is: The expression for the accelerated creep constitutive model is: Where ε is the creep strain; σ is the shear stress in the creep experiment; t is the creep time; E1 is the instantaneous elastic modulus; E2 is the elastic modulus of the viscoelastic element; η1 is the viscosity coefficient of the viscoelastic element; k is a fitting parameter less than 1; η2 is the viscosity coefficient of the viscous element; η3 is the viscosity coefficient of the viscous element; σ s The long-term strength of the hydrate-containing sample.

6. The method for determining the lower limit of drilling fluid density according to claim 4, characterized in that, The steps for determining the wellbore reduction calculation equation based on the creep constitutive model, stress balance equation, geometric equation, and boundary conditions include: Based on the aforementioned decay creep constitutive model, stress balance equation, geometric equation, and boundary conditions, the equation for calculating wellbore reduction when the wellbore stress state is in decay creep is determined; and... Based on the aforementioned accelerated creep constitutive model, stress balance equation, geometric equation, and boundary conditions, the equation for calculating wellbore reduction when the wellbore stress is under accelerated creep is determined. The expression for the wellbore reduction calculation equation when the wellbore stress state is in decay creep is as follows: When the wellbore stress state is under accelerated creep, the expression for the wellbore reduction calculation equation is: Among them, u a σ is the wellbore shrinkage; t0 is the drilling time; a is the wellbore radius; σ h For effective horizontal ground stress; p i Density of drilling fluid; μ c E2(S) is Poisson's ratio; h ), η1(S h ), η2(S h ), k(S h ), η3(S h ) represents the creep parameter.

7. The method for determining the lower limit of drilling fluid density according to any one of claims 1-6, characterized in that, The steps of obtaining the deviatoric stress of the hydrate-containing sample under different stress levels and the long-term strength data of the hydrate-containing sample under different hydrate saturation conditions through creep experiments include: By conducting triaxial mechanical compression experiments on hydrate-containing samples under different hydrate saturation conditions, triaxial mechanical compression experimental data of hydrate-containing samples under different hydrate saturation conditions were obtained. Based on the triaxial mechanical compression test data, the deviatoric stress of the hydrate-containing sample in the creep test under different stress levels was determined; Triaxial creep tests were conducted on the hydrate-containing samples under different hydrate saturation conditions to obtain long-term strength data of the hydrate-containing samples.

8. A device for determining the lower limit of drilling fluid density, characterized in that, include: The acquisition module is used to acquire the deviatoric stress of the hydrate-containing sample under different stress levels and the long-term strength data of the hydrate-containing sample under different hydrate saturation conditions through creep experiments of the hydrate-containing sample. The model building module is used to determine creep characteristics based on the long-term strength data and deviatoric stress of the hydrate-containing sample, and to establish a creep constitutive model based on the parameters of the creep experiment and the creep characteristics. The first determining module is used to determine the stress balance equation, geometric equation and boundary conditions based on the stress characteristics of the rock surrounding the well to be tested. The second determining module is used to determine the wellbore reduction calculation equation based on the creep constitutive model, stress balance equation, geometric equation and boundary conditions; The calculation module is used to determine the lower limit of drilling fluid density in hydrate formations under preset wellbore reduction conditions based on the wellbore reduction calculation equation and in combination with logging data.

9. An electronic device, characterized in that, include: At least one processor; as well as, A memory communicatively connected to the at least one processor; wherein, The memory stores instructions that can be executed by the at least one processor to enable the at least one processor to perform the drilling fluid density lower limit determination method as described in any one of claims 1 to 7.

10. A computer-readable storage medium storing a computer program, characterized in that, When the computer program is executed by the processor, it implements the method for determining the lower limit of drilling fluid density as described in any one of claims 1 to 7.

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