Method for predicting and evaluating steady state or decomposition of stratum hydrate under mass and heat transfer of oil well cement

Abstract

The invention belongs to the field of oil and gas well cementation, and relates to a method for predicting and evaluating formation hydrate steady state or decomposition under oil well cement mass and heat transfer. The method comprises the steps that firstly, a cement sheath-casing-displacing fluid model in the actual hydrate layer well cementation process is built and simulated; then a cement sheath hydration heat source-well wall hydrate decomposition coupling temperature field model and a hydrate layer-filtrate molecular dynamics model are established and combined for simulation, and a regulation and control interval chart for maintaining hydrate stability in the corresponding hydrate layer well cementation process based on certain cement slurry system mass and heat transfer conditions is obtained. According to the method, thermodynamic simulation and molecular simulation results are combined, the stability condition of the stratum hydrate in the well cementation process of the target stratum through certain cement paste can be reasonably predicted, guidance is provided for the hydration heat condition of a cement paste system, and important guarantee is provided for the stability of the stratum hydrate in actual well cementation and the well cementation quality.

Description

Technical Field

[0001] The invention belongs to the technical field of hydrate layer cementing, and in particular relates to a method for predicting and evaluating the steady state or decomposition of formation hydrate under mass and heat transfer of oil well cement. Background Art

[0002] During the process of deepwater oil and gas development and well construction, it is inevitable to drill through shallow hydrate layers, which requires cementing of the drilled shallow hydrate layers. However, oil well cement generates a large amount of hydration heat during the solidification process, which leads to the decomposition of hydrates in the cementing layer, posing a huge safety hazard to the cement sheath that has not yet developed sufficient strength, thus causing cementing quality problems and even safety accidents. For deepwater shallow formation cementing, many scholars at home and abroad have developed cement slurry systems such as particle graded cement slurry and low-density fast-setting cement slurry and designed supporting cementing completion technologies, which have effectively solved the problem of slow cement hydration and low early strength under low temperatures in deep water. In order to solve the problem of cement hydration heat causing hydrate decomposition in deepwater shallow layers, some scholars have begun to study how to reduce the hydration heat of cement. There are two main methods: (1) reducing the hydration heat energy of cement itself, such as low-heat cement, slag fly ash and other low-heat admixture cement systems; (2) adding admixtures that absorb cement hydration heat energy, such as paraffin, inorganic salt hydrates, synthetic polymer materials, etc. These studies have reduced the hydration exothermic temperature of cement to a certain extent, laying the foundation for cementing deep-water shallow hydrate layers. However, the question of whether the thermodynamic properties of cement slurry systems can achieve safe sealing of deep-water shallow hydrate layers still requires in-depth and systematic research. Therefore, simply developing low-density, early-strength and low-hydration heat cement systems is difficult to fundamentally guarantee the stability and safety of deep-water shallow hydrate layers.

[0003] Patent CN114417663A discloses a method for designing safe and long-term cementing performance of marine natural gas hydrates, which involves a wellbore temperature field model, using cement temperature rise as the only heat source, combined with a traditional heat transfer model, to calculate the temperature heat transfer change from the wellbore to the formation during cement hydration. This prediction model takes into account the effect of cement hydration heat release on the formation temperature field, but does not consider the complex situation of the formation containing phase change heat storage materials such as hydrates, and does not involve the effect of cement slurry filtrate on formation heat transfer after infiltration into the formation.

[0004] Patent CN116090271A provides a method for predicting the temperature of offshore natural gas hydrate drilling. It uses the acquired wellbore structural parameters and thermodynamic parameters to establish a temperature model to predict the wellbore temperature distribution during the drilling cycle of a horizontal well in a offshore natural gas hydrate reservoir. The prediction model takes into account the influence of the wellbore fluid on the formation temperature field during the drilling process, but does not consider the influence of the heterogeneous heat release of cement slurry hydration on the hydrate decomposition of the formation during the subsequent cementing process.

[0005] Therefore, it is necessary to study the relationship between the radial heat distribution law of the cement sheath and the cement hydration time, establish a heat source model of the cement sheath in the cement hydration process, and study and clarify the critical conditions for the stability of the hydrate layer, so as to propose the cement hydration exothermic conditions and control methods to maintain the stability of the hydrate, and lay the foundation for safe cementing of shallow hydrate layers. Summary of the invention

[0006] In view of the above problems existing in the prior art, the present invention provides a prediction and evaluation method for the steady state or decomposition of formation hydrates under the mass and heat transfer of oil well cement. Based on the collected experimental data, a coupled temperature field model of cement ring hydration heat source and well wall hydrate decomposition is established by simulation. Numerical simulation and molecular simulation are combined to study the evolution law of hydrate phase in the near-well zone under the influence of cement slurry filtrate caused by cement hydration temperature rise, and determine the critical value of cement hydration heat release to maintain hydrate stability.

[0007] The method for predicting and evaluating the steady state or decomposition of formation hydrate under mass and heat transfer of oil well cement described in the present invention is based on thermodynamic simulation and molecular simulation, and comprises the following steps: (1) According to the actual hydrate occurrence state and cementing process in deep and shallow marine layers, the hydrate formation related parameters, casing related parameters and displacement fluid parameters are obtained and calculated, and a cement sheath-casing-displacement fluid model is established to simulate the actual hydrate layer cementing process; (2) Based on the characteristics of the hydrate formation, the cement slurry system to be used for cementing is pre-selected, and the conventional properties, hydration heat release parameters and heat transfer parameters of the cement slurry system under different temperature and pressure conditions are collected and calculated using experimental methods or cement hydration kinetic models; (3) Based on the cement hydration kinetics model, and according to the conventional properties, hydration exothermic parameters, and heat transfer parameters of the cement slurry system collected and calculated in step (2), a cement ring hydration heat source-wellbore hydrate decomposition coupled temperature field model is established; (4) Based on the coupled temperature field model of cement ring hydration heat source and wellbore hydrate decomposition, the phase change of the second interface hydrate layer during the cement ring hydration process under the use conditions of the preselected cement slurry system is simulated, and the influence of the cement ring heat transfer on the stability of the second interface hydrate layer is obtained; (5) According to the cement slurry system used, collect the filtrate of the cement slurry and establish a molecular dynamics model of the hydrate layer-filtrate; (6) The coupled temperature field model of cement ring hydration heat source and wellbore hydrate decomposition was combined with the molecular dynamics model of hydrate layer and filtrate to simulate and obtain the control range map for maintaining hydrate stability during cementing of the corresponding hydrate layer under the mass and heat transfer conditions of a certain cement slurry system.

[0008] The present invention combines the results of thermodynamic simulation with molecular simulation, which can reasonably predict the stability of the hydrate formation in the target formation during cementing using a certain cement slurry, and provide guidance on the hydration heat of the cement slurry system, thus providing important guarantees for the stability of the hydrate formation and the cementing quality in actual cementing.

[0009] Furthermore, the hydrate formation related parameters are mineral composition, particle size composition, porosity, hydrate occurrence type, hydrate saturation, pore water content, formation temperature, specific heat capacity, density, thermal conductivity and thermal convection coefficient, and the casing related parameters are density, specific heat capacity, inner and outer diameters, and thermal conductivity.

[0010] Furthermore, the hydrate saturation, formation temperature, specific heat capacity, thermal conductivity and thermal convection coefficient change in real time with the heat release of oil well cement hydration.

[0011] Furthermore, conventional properties and heat transfer parameters of oil well cement slurry are hydration heat release, cement density, specific heat capacity, thermal conductivity, and cement annulus radius.

[0012] Furthermore, the hydration heat release is correlated with the hydration time, which is obtained through experiments or cement hydration kinetics models and is the only heat-increasing item during the cementing waiting period.

[0013] Furthermore, the cement sheath hydration heat source-wellbore hydrate decomposition coupled temperature field model is divided into three parts: (1) cement sheath heat conduction model per unit time period; (2) cement sheath-casing heat conduction model; (3) cement sheath-formation temperature field model.

[0014] Furthermore, the expression and display software of the cement sheath hydration heat source-well wall hydrate decomposition coupled temperature field model includes but is not limited to comsol, ANSYS, Fluent, SolidWorks Simulation and other thermodynamic simulation software.

[0015] The formula of the cement sheath heat conduction model per unit time period is: Formula (1); In the formula, T c is the cement ring temperature, K; Q ct For t Hydration heat of cement sheath at the moment, J·g -1 ; c c is the specific heat capacity of cement sheath, J·kg -1 ·K -1 ; ρ c is the cement sheath density, kg·m-3 ; r c is the outer diameter of the casing, m; K cp is the heat transfer coefficient from cement sheath to casing, W·m -2 ·K -1 ; T p is the temperature of the casing, K; r f is the radius of the two interfaces, m; K cf is the heat transfer coefficient from cement sheath to formation, W·m -2 ·K -1 ; T f is the formation temperature, K.

[0016] Furthermore, the cement sheath-casing heat conduction model formula is: Formula (2); In the formula, T p is the temperature of the casing, K; T p1 is the boundary temperature of the casing at an interface, K; T p2 is the temperature of the boundary between casing and displacement fluid, K; c p is the specific heat capacity of the casing, J·kg -1 ·K -1 ; ρ p is the casing density, kg·m -3 ; r c is the outer diameter of the casing, m; K cp is the heat transfer coefficient from cement sheath to casing, W·m -2 ·K -1 ; T c1 is the boundary temperature of cement sheath at an interface, K; r p is the inner diameter of the casing, m; K pd is the heat transfer coefficient from casing to displacement fluid, W·m -2 ·K -1 ; T d is the temperature of the displacement fluid, K.

[0017] Furthermore, the cement sheath-formation temperature field model formula is: Formula (3); In the formula, T c is the cement ring temperature, K; T f is the formation temperature, K; K f is the effective heat transfer coefficient of the formation, W·m -2 ·K -1 ; x is the distance from the stratum to the second interface, m; h f is the effective thermal convection coefficient of the formation fluid, W·m -2 ·K -1 ; Q p is the heat absorbed by the hydrate phase change in the formation, J·g -1 ; c c is the specific heat capacity of cement sheath, J·kg -1 ·K -1 ; ρ c is the cement sheath density, kg·m -3 .

[0018] The function of the above three model formulas is to construct the heat transfer relationship between the cement sheath itself, the cement sheath to the casing, and the cement sheath to the formation. During the application process, the three model formulas need to be input into the software to establish a heat transfer model between each heat transfer unit cell, and then the relevant formation, casing, and cement sheath heat transfer parameters are input to form the heat transfer relationship between each unit cell. Finally, the corresponding curve values ​​of cement hydration time and hydration heat release are input into the software for operation, which can simulate the heat transfer of cement to the formation and casing at each moment during the hydration process. Through the construction and interpretation of the heat transfer relationship by the three model formulas, the temperature changes of any position of the casing, cement sheath, and formation during the cement hydration process can be obtained (such as Fig. 9 ).

[0019] Furthermore, the molecular dynamics model of the hydrate layer includes structural types such as type I hydrate, type II hydrate and type H hydrate, and the hydrate molecular structure adopts the CVFF standpoint and TIP4P model.

[0020] Furthermore, the filtrate is the filtrate that the cement slurry may penetrate into the hydrate formation, and the solute molecules in the filtrate include but are not limited to: Ca 2+ 、Na + , Cl - , -COOH, -SO3 and -CO-NH2, etc.

[0021] Furthermore, the molecular dynamics model analysis means of the hydrate layer-filtrate is radial distribution function, root mean square displacement, mean square displacement and diffusion coefficient, etc. According to the hydration temperature of the cement sheath-formation interface, the present invention combines the decomposition of the hydrate model under the influence of cement filtrate molecules with the analysis means to determine the critical value of cement hydration heat release for maintaining hydrate stability.

[0022] Furthermore, the process of obtaining the control interval map is as follows: (6-1) Based on the critical value of phase transition of hydrate molecules without filtrate influence at cement hydration temperature, the diffusion coefficient of hydrate without filtrate influence is determined by molecular dynamics model analysis; (6-2) Substituting the diffusion coefficient of hydrate without the influence of filtrate into the molecular simulation results under the condition of cement slurry filtrate, the phase transition critical temperature of hydrate under the influence of cement slurry filtrate is determined; (6-3) The critical temperature of the phase change of hydrate under the influence of cement slurry filtrate is matched with the hydration heat released by the cement slurry system, and a control range chart for maintaining hydrate stability during cementing of the corresponding hydrate layer under the mass and heat transfer conditions of the cement slurry system is obtained.

[0023] Compared with the prior art, the present invention has the following advantages: Innovatively combining the mass transfer and heat transfer of cement ring, the influence of cement ring hydration heat release on hydrate formations was studied and analyzed. Through the collection of relevant physical and chemical parameters of the actual formation and the applied cement slurry system, the mass transfer and heat transfer of a certain cement slurry system on formations with different hydrate saturations were studied in a targeted and specific manner, and an evaluation method for the influence of cement slurry hydration on the stability of hydrate layers was established based on the hydrate phase change state as the standard.

[0024] By combining two completely different macroscopic and microscopic simulation methods, thermodynamic simulation and molecular dynamics simulation, a coupled temperature field model of cement hydration heat source and wellbore hydrate decomposition was constructed to reveal the effects of cement slurry filtrate and exothermic parameters on the phase evolution of hydrates in the near-wellbore zone during the cement hydration exothermic process. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is the equivalent hydration exothermic curve of A# low-density cement at different temperatures.

[0026] Figure 2 Schematic diagram of the coupled temperature field model of cement sheath hydration heat source and wellbore hydrate decomposition.

[0027] Figure 3 This is the temperature field of the A# low-density cement hydration process model.

[0028] Figure 4This is the curve of temperature variation with radius at the peak of the second interface temperature during the hydration process of A# low-density cement.

[0029] Figure 5 This is the initial configuration of the molecular dynamics model of the hydrate layer-dodecylamide solution.

[0030] Figure 6 This is the relationship curve between temperature and diffusion coefficient.

[0031] Figure 7 is the critical temperature of phase transition of hydrate under different pressures under the influence of amide groups.

[0032] Figure 8 It is the relationship between the initial temperature of hydrate formation and the critical value of cement hydration exothermicity.

[0033] Fig. 9 This is the curve of the change of the temperature of the two interfaces with time during the hydration process of low-density cement. DETAILED DESCRIPTION

[0034] In order to make the present invention easier to understand, the present invention will be described in detail below in conjunction with embodiments. It should be noted that the embodiments are only illustrative and do not limit the application scope of the present invention.

[0035] (1) According to the actual hydrate occurrence state and cementing process in deep and shallow marine layers, the hydrate formation related parameters, casing related parameters and displacement fluid parameters are obtained and calculated, and a cement sheath-casing-displacement fluid model is established to simulate the actual hydrate layer cementing process.

[0036] Take the shallow hydrate block in Shenhu area of ​​South China Sea as an example. The casing and displacement fluid parameters of a well with a water depth of 1500 m in the South China Sea were selected as reference, and the simulation was carried out with the initial environment temperature of 4-18℃ and the initial formation pressure of 10 MPa. The casing related parameters include: inner diameter 404 mm, outer diameter 468 mm, density 7.8 g·cm -3 , the specific heat capacity is 0.460 J·g -1 ·K -1 , thermal conductivity is 43.0 W·m -1 ·K -1 ; The displacement fluid parameters in the casing include: density 1.1 g·cm -3 , specific heat capacity is 3.180 J·g -1 ·K -1 , thermal conductivity is 1.0 W·m -1 ·K -1 By analyzing the mineral composition, particle size and hydrate saturation of the hydrate formation in this area, a simulated cement sheath-casing-displacement fluid model was constructed. The material parameters used in the cement sheath-casing-displacement fluid model are shown in Table 1: Table 1 Material parameters of the simulated cement sheath-casing-displacement fluid model .

[0037] (2) Based on the characteristics of the hydrate formation, the cement slurry system used for cementing is pre-selected, and the conventional properties, hydration exothermic parameters, and heat transfer parameters of the cement slurry system under different temperature and pressure conditions are collected and calculated using experimental methods or cement hydration kinetic models.

[0038] Cement slurry system selection is targeted at deep-water shallow hydrate formations in the ocean. The low-density cement slurry system currently used in deep-water cementing is based on G-grade oil well cement. The cement slurry formula and properties are shown in Table 2-3 and Figure 1 As shown: Table 2 Cement slurry formula ; Table 3 Properties of cement used in hydrate layer cementing experiment .

[0039] (3) Based on the cement hydration kinetics model, and according to the conventional properties, hydration exothermic parameters, and heat transfer parameters of the cement slurry system collected and calculated in step (2), a coupled temperature field model of cement ring hydration heat source and wellbore hydrate decomposition is established.

[0040] According to the heat transfer process of cement hydration heat source, combined with the heat transfer model relationship between cement sheath and itself, casing and formation in formulas (1), (2) and (3), the following equations are established using Comsol Multiphysics software: Figure 2 The coupled temperature field model of cement sheath hydration heat source and wellbore hydrate decomposition is shown.

[0041] The relevant parameters of the hydrate formation and casing are input, and the model is designed according to the cementing structure of the deep-water formation in the ocean, including displacement fluid, casing, cement sheath and hydrate formation, among which structure A is the displacement fluid in the casing, structure B is the casing, structure C is the cement sheath, structure D is the hydrate formation, and structure E is the infinite element domain of the hydrate formation. According to the coupled temperature field model of cement sheath hydration heat source and wellbore hydrate decomposition, the casing and cement sheath are solid in the combined model, the displacement fluid in the casing is fluid, the hydrate formation is a porous medium formation, the porosity is 52.25%, and the phase change latent heat of hydrate is 736.49 J·g -1 Before the hydrate phase change and decomposition, the pores contained water and natural gas hydrate, and the hydrate saturation was 36.90%.

[0042] (4) Based on the coupled temperature field model of cement ring hydration heat source and wellbore hydrate decomposition, the phase change of the second interface hydrate layer during the cement ring hydration process under the use conditions of the preselected cement slurry system is simulated, and the influence of the cement ring heat transfer on the stability of the second interface hydrate layer is obtained.

[0043] According to the temperature field control and heat transfer equation, the porous media heat transfer module is selected for numerical simulation. The equation form is transient, and the thermodynamic process of hydrate phase change in the porous medium formation pores is coupled with the heat conduction process of cement ring hydration heat release, and the influence of the hydrate layer stability during cement hydration in the target block is studied. The conditions set in the numerical simulation of the coupled temperature field of cement ring hydration heat source-wellbore hydrate decomposition are as follows: (1) The displacement fluid in the casing, casing, cement sheath, and hydrate formation are all homogeneous models; (2) The displacement fluid, casing, cement sheath and hydrate formation in the casing are in close contact, the temperature field is continuous, and the cementing quality is good; (3) Thermodynamic parameters of hydrate formations change with temperature during the hydrate phase change and decomposition process; (4) The cement sheath is the only heat source, and the heat released per unit volume is the same during the hydration and setting process; (5) The initial temperature of the wellbore and the hydrate formation is the same (which can be defined as the temperature at the end of the cementing process); (6) The hydrate formation is infinite in radial direction.

[0044] The simulation is carried out using the initial environment temperature of 4-18°C and the initial hydrate formation pressure of 10 MPa as an example.

[0045] During the numerical simulation, the cement sheath is the only heat source, and the heat release rate is based on the equivalent hydration heat release fitted in the cement hydration kinetic model according to the different simulated temperature conditions. The heat transfer of the cement sheath, casing, and hydrate formation conforms to the coupled temperature model of cement sheath hydration heat source-hydrate decomposition. The thermodynamic parameters of the hydrate formation change with the change of temperature. When the hydrate is not decomposed, the formation is in a heat transfer state of hydrate heat conduction between the formation skeleton and pores, and pore water heat convection; when the hydrate is in a phase change decomposition state, the hydrate phase change heat absorption part also acts on the formation heat transfer process; after the hydrate is decomposed, the formation is in a heat transfer state of heat conduction between the formation skeleton and pore water and methane heat convection.

[0046] During the simulation, the initial hydration degree of cement was set to 0, cement began to hydrate at time 0 of the model operation, and the initial temperatures were set to 4°C, 6°C, 8°C, 10°C, 12°C, 14°C, 16°C, and 18°C, respectively. The relevant parameters of the hydrate formation and cement slurry were put into the model for operation, and the temperature changes of the casing-cement sheath-hydrate formation when the cement slurry was hydrated and set at different temperatures were obtained, as shown in Figure 2. Figure 3 shown.

[0047] The time point of the highest temperature of the two interfaces is determined through the simulation results. The simulation time is set to this time point, and the temperature variation curve with radius starting from the center of the model circle is drawn to determine the temperature variation curve of the displacement fluid, casing, cement sheath, and hydrate formation in the casing, such as Figure 4 The simulation results show that when the initial temperature of the hydrated formation is 4-18°C and the initial pressure is 10 MPa, the peak temperature rise of cement hydration is between 279.1-292.0 K (5.95-18.95°C), and the critical temperature of the hydrate phase change is 291.5 K.

[0048] (5) According to the cement slurry system adopted, the filtrate of the cement slurry is collected, and a molecular dynamics model of the hydrate layer-filtrate is established using molecular simulation software.

[0049] After preparing cement paste according to API standards, ICP-MS and ion spectroscopy were used to test the main ions and functional groups in the cement paste filtrate. Then, IR (infrared spectroscopy) was used to obtain the main functional groups in the cement admixture, and molecular simulation software was used to establish a molecular dynamics model of the hydrate layer-filtrate. This case test took the amide (-CO-NH2), the main functional group in the cement paste filtrate, as an example, and established a molecular dynamics model of the hydrate layer-amide solution (such as Figure 5 as shown).

[0050] (6) The coupled temperature field model of cement ring hydration heat source and wellbore hydrate decomposition was combined with the molecular dynamics model of hydrate layer and filtrate to simulate and obtain the control range map for maintaining hydrate stability during cementing of the corresponding hydrate layer under the mass and heat transfer conditions of a certain cement slurry system.

[0051] Specifically: (6-1) Based on the critical temperature of phase change of hydrate molecules without the influence of filtrate at the cement hydration temperature, the diffusion coefficient of hydrate without the influence of filtrate is determined by molecular dynamics model analysis.

[0052] (6-1-1) Combined with the peak temperature rise range of cement hydration at the interface at 4-18°C obtained by thermodynamic simulation, the hydrate layer-pure water molecular dynamics model and the hydrate layer-amide solution molecular dynamics model of hydrate, water molecules, and dodecylamide molecules were established using MS software (e.g. Figure 5), the supercell parameters were exported as data files, and then the lammps software was used to run the molecular model to simulate the decomposition of the hydrate model in pure water system and dodecylamide solution system at 277-293 K. The relationship curves between temperature and diffusion coefficient under the influence of amide functional groups and without the influence of filtrate were obtained. Figure 6 ).

[0053] (6-1-2) According to the phase transition critical temperature, the corresponding diffusion coefficient without the influence of filtrate is determined. Taking the initial formation pressure of 10 MPa as an example, without the influence of filtrate, the phase transition critical temperature of hydrate is 291.5 K, and the corresponding diffusion coefficient of hydrate without the influence of filtrate is 0.86 m·s -1 , so it is set that when the diffusion coefficient is greater than 0.86 m·s -1 The hydrate begins to decompose.

[0054] (6-2) Substituting the diffusion coefficient of hydrate without the influence of filtrate into the molecular simulation results under the condition of cement slurry filtrate, the phase transition critical temperature of hydrate under the influence of cement slurry filtrate is determined; The diffusion coefficient is 0.86 m·s -1 Substituting the temperature into the relationship curve between the temperature and the diffusion coefficient under the influence of the amide group, the critical decomposition temperature of the phase change of the hydrate under the influence of the amide group is 291.2 K. That is, compared with the formation hydrate without the influence of the filtrate, the critical temperature of the phase change of the formation hydrate under the influence of the amide group is reduced by 0.3 K.

[0055] The critical temperature of phase transition of hydrates under different pressures under the influence of amide groups was calculated by simulation using this method, such as Figure 7 shown.

[0056] pass Figure 7 It can be obtained that at 10-15 MPa, when there is no influence of filtrate, the critical temperature range of phase change of formation hydrate is 291.5-295.0 K. Under the influence of amide groups, the critical temperature range of phase change is 291.2-294.8 K. This indicates that under the influence of filtrate, at a certain pressure, the phase change temperature point of hydrate decreases.

[0057] (6-3) The critical temperature of the phase change of hydrate under the influence of cement slurry filtrate is matched with the hydration heat released by the cement slurry system, and a control range chart for maintaining hydrate stability during cementing of the corresponding hydrate layer under the mass and heat transfer conditions of the cement slurry system is obtained.

[0058] Comsol is used to simulate the effect of cement hydration heat release on the stability of hydrates in the formation at different initial temperatures and pressures. The simulation is set at a certain pressure. When the hydrate temperature in the formation reaches Figure 7The corresponding critical temperature of phase change under the influence of the amide group is the phase change decomposition of hydrate in the formation pores. The relationship between the critical value of cement hydration exothermicity to maintain hydrate stability at different initial formation temperatures is plotted as follows: Figure 8 shown.

[0059] Through examples, it can be obtained that by simulating the decomposition of hydrate formations under cement hydration heat release at different temperatures and pressures, and taking the influence of cement slurry filtrate into account, a relationship chart of the critical value of hydration heat release under the conditions of cement mass transfer and heat transfer to maintain hydrate stability at different initial formation temperatures can be established, and the relationship between the initial formation temperature and pressure and the critical value of cement hydration heat release can be clarified, providing guidance for the regulation of hydration heat of on-site oil well cement.

Claims

1. A method for predicting and evaluating the steady state or decomposition of formation hydrates under mass and heat transfer of oil well cement, characterized in that: Based on thermodynamic simulation and molecular dynamics simulation, the following steps are included: (1) According to the actual hydrate occurrence state and cementing process in deep and shallow marine layers, the hydrate formation related parameters, casing related parameters and displacement fluid parameters are obtained and calculated, and a cement sheath-casing-displacement fluid model is established to simulate the actual hydrate layer cementing process; (2) Based on the characteristics of the hydrate formation, the cement slurry system to be used for cementing is pre-selected, and the conventional properties, hydration heat release parameters and heat transfer parameters of the cement slurry system under different temperature and pressure conditions are collected and calculated using experimental methods or cement hydration kinetic models; (3) Based on the cement hydration kinetics model, and according to the conventional properties, hydration exothermic parameters, and heat transfer parameters of the cement slurry system collected and calculated in step (2), a cement ring hydration heat source-wellbore hydrate decomposition coupled temperature field model is established; (4) Based on the coupled temperature field model of cement ring hydration heat source and wellbore hydrate decomposition, the phase change of the second interface hydrate layer during the cement ring hydration process under the use conditions of the preselected cement slurry system is simulated, and the influence of the cement ring heat transfer on the stability of the second interface hydrate layer is obtained; (5) According to the cement slurry system used, collect the filtrate of the cement slurry and use molecular simulation software to establish a molecular dynamics model of the hydrate layer-filtrate; (6) The coupled temperature field model of cement ring hydration heat source and wellbore hydrate decomposition was combined with the molecular dynamics model of hydrate layer and filtrate to simulate and obtain the control range map for maintaining hydrate stability during cementing of the corresponding hydrate layer under the mass and heat transfer conditions of a certain cement slurry system.

2. The method for predicting and evaluating the steady state or decomposition of formation hydrate under mass and heat transfer of oil well cement according to claim 1, characterized in that: In step (1), the hydrate formation related parameters are mineral composition, particle size composition, porosity, hydrate occurrence type, hydrate saturation, pore water content, formation temperature, specific heat capacity, density, thermal conductivity and thermal convection coefficient; the casing related parameters are density, specific heat capacity, inner and outer diameters, and thermal conductivity.

3. The method for predicting and evaluating the steady state or decomposition of formation hydrate under mass and heat transfer of oil well cement according to claim 1, characterized in that: In step (2), the heat transfer parameters of the cement slurry system are cement density, specific heat capacity, thermal conductivity, and cement ring radius.

4. The method for predicting and evaluating the steady state or decomposition of formation hydrate under mass and heat transfer of oil well cement according to claim 1, characterized in that: In step (3), the cement sheath hydration heat source-wellbore hydrate decomposition coupled temperature field model includes a cement sheath heat conduction model per unit time period, a cement sheath-casing heat conduction model and a cement sheath-formation temperature field model.

5. The method for predicting and evaluating the steady state or decomposition of formation hydrate under mass and heat transfer of oil well cement according to claim 4, characterized in that: The formula of the cement sheath heat conduction model per unit time period is: ； In the formula, T c is the cement ring temperature, K; Q ct For t Hydration heat of cement sheath at the moment, J·g -1 ; c c is the specific heat capacity of cement sheath, J·kg -1 ·K -1 ; ρ c is the cement sheath density, kg·m -3 ; r c is the outer diameter of the casing, m; K cp is the heat transfer coefficient from cement sheath to casing, W·m -2 ·K -1 ; T p is the temperature of the casing, K; r f is the radius of the two interfaces, m; K cf is the heat transfer coefficient from cement sheath to formation, W·m -2 ·K -1 ; T f is the formation temperature, K.

6. The method for predicting and evaluating the steady state or decomposition of formation hydrate under mass and heat transfer of oil well cement according to claim 4, characterized in that: The cement sheath-casing heat conduction model formula is: ； In the formula, T p is the temperature of the casing, K; T p1 is the boundary temperature of the casing at an interface, K; T p2 is the temperature of the boundary between casing and displacement fluid, K; c p is the specific heat capacity of the casing, J·kg -1 ·K -1 ; ρ p is the casing density, kg·m -3 ; r c is the outer diameter of the casing, m; K cp is the heat transfer coefficient from cement sheath to casing, W·m -2 ·K -1 ; T c1 is the boundary temperature of cement sheath at an interface, K; r p is the inner diameter of the casing, m; K pd is the heat transfer coefficient from casing to displacement fluid, W·m -2 ·K -1 ; T d is the temperature of the displacement fluid, K.

7. The method for predicting and evaluating the steady state or decomposition of formation hydrate under mass and heat transfer of oil well cement according to claim 4, characterized in that: The cement sheath-formation temperature field model formula is: ； In the formula, T c is the cement ring temperature, K; T f is the formation temperature, K; K f is the effective heat transfer coefficient of the formation, W·m -2 ·K -1 ; x is the distance from the stratum to the second interface, m; h f is the effective thermal convection coefficient of the formation fluid, W·m -2 ·K -1 ; Q p is the heat absorbed by the hydrate phase change in the formation, J·g -1 ; c c is the specific heat capacity of cement sheath, J·kg -1 ·K -1 ; ρ c is the cement sheath density, kg·m -3 .

8. The method for predicting and evaluating the steady state or decomposition of formation hydrate under mass and heat transfer of oil well cement according to claim 1, characterized in that: The process of obtaining the control interval map is as follows: (6-1) Based on the critical value of phase transition of hydrate molecules without filtrate influence at cement hydration temperature, the diffusion coefficient of hydrate without filtrate influence is determined by molecular model analysis; (6-2) Substitute the diffusion coefficient of hydrate without the influence of filtrate into the molecular simulation results under the condition of cement slurry filtrate to determine the critical temperature of hydrate phase transition under the influence of cement slurry filtrate; (6-3) The critical temperature of hydrate phase change under the influence of cement slurry filtrate is matched with the hydration heat released by the cement slurry system, and a control range chart for maintaining hydrate stability during cementing of the corresponding hydrate layer under the mass and heat transfer conditions of the cement slurry system is obtained.

Images