Prediction and evaluation method for steady state or decomposition of formation hydrates under mass and heat transfer of oil well cement

Abstract

The present invention belongs to the field of oil and gas well cementing, and relates to a prediction and evaluation method for the steady state or decomposition of formation hydrates under heat and mass transfer of oil well cement. First, the invention establishes a cement sheath-casing-displacement fluid model during the actual hydrate layer cementing process; then, a coupled temperature field model of the heat source of cement sheath hydration and the decomposition of wellbore hydrates and a molecular dynamics model of the hydrate layer-filtrate are established, and they are combined for simulation to obtain a control interval chart for maintaining the stability of hydrates during the cementing process of the corresponding hydrate layer under the heat and mass transfer conditions of a certain cement slurry system. By combining the results of thermodynamic simulation and molecular simulation, the present invention can reasonably predict the stability of formation hydrates during the cementing process of the target formation using a certain cement slurry, provide guidance for the heat of hydration of the cement slurry system, and provide an important guarantee for the stability of formation hydrates and the cementing quality in actual cementing.

Description

Technical Field

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

[0002] During the well construction process of deepwater oil and gas development, it is inevitable to drill through the shallow hydrate layer. This requires cementing the drilled shallow hydrate layer. However, a large amount of hydration heat is generated during the solidification process of oil well cement, resulting in the decomposition of hydrates in the cementing section, 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 cementing in deepwater shallow formations, many domestic and foreign scholars have developed cement slurry systems such as particle size graded cement slurry and low-density quick-setting cement slurry, and designed supporting cementing and completion technologies, which have better solved the problems of slow cement hydration and low early strength at low temperatures in deepwater. To solve the problem of hydrate decomposition in deepwater shallow layers caused by cement hydration heat release, some scholars have started to study reducing the hydration heat release of cement. There are mainly two methods: (1) reducing the hydration heat release energy of cement itself, such as low-heat cement, slag fly ash and other low-heat admixture cement systems; (2) adding admixtures that absorb the heat energy of cement hydration, such as paraffin, inorganic salt hydrates, synthetic polymer materials, etc. These studies have reduced the hydration heat release temperature of cement to a certain extent and laid a foundation for cementing deepwater shallow hydrate layers. However, at present, in-depth and systematic research is still needed on whether the thermodynamic properties of the cement slurry system can achieve the safe sealing of deepwater shallow hydrate layers. Therefore, simply developing low-density, early-strength and low-hydration heat cement systems is difficult to fundamentally ensure the stability and safety of deepwater shallow hydrate layers.

[0003] In the method for designing the safe and long-term cementing performance of marine natural gas hydrates disclosed in Patent CN114417663A, a wellbore temperature field model is involved. Using the cement temperature rise as the only heat source and combining with the traditional heat transfer model, the temperature heat transfer change from the wellbore to the formation during cement hydration is calculated. This prediction model considers the influence 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 substances such as hydrates, and does not involve the influence of cement slurry filtrate infiltrating into the formation on formation heat transfer.

[0004] Patent CN116090271A provides a method for predicting the drilling temperature of natural gas hydrates in the sea area. By using the obtained wellbore structure parameters and thermodynamic parameters, a temperature model is established to predict the wellbore temperature distribution during the drilling circulation of horizontal wells in natural gas hydrate reservoirs in the sea area. This prediction model considers the influence of the fluid in the wellbore on the formation temperature field during the drilling process, but does not consider the influence of non-uniform heat release during cement slurry hydration on the decomposition of hydrates in the formation during the subsequent cementing process.

[0005] Therefore, it is necessary to study the relationship between the heat distribution law of the cement sheath along the radial direction and the cement hydration time, establish a heat source model of the cement sheath during the cement hydration process, and study and clarify the critical conditions for the stability of the hydrate layer, so as to propose the heat release conditions and control methods of cement hydration to maintain the stability of hydrates, and lay a foundation for the 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 transfer and heat transfer of oil well cement. Based on the collected experimental data, a coupled temperature field model of the cement sheath hydration heat source - wellbore hydrate decomposition is established by means of simulation. Combining numerical simulation and molecular simulation, the phase state evolution law of hydrates in the near-wellbore zone under the influence of the temperature rise during cement hydration under the influence of cement slurry filtrate is studied, and the critical value of the heat release amount of cement hydration to maintain the stability of hydrates is determined.

[0007] The prediction and evaluation method for the steady state or decomposition of formation hydrates under the mass transfer and heat transfer of oil well cement according to the present invention, based on thermodynamic simulation and molecular simulation, includes the following steps:

[0008] (1) According to the actual occurrence state of hydrates and the cementing process in the deep - water shallow layer of the ocean, obtain and calculate the relevant parameters of the hydrate formation, the relevant parameters of the casing, and the parameters of the displacement fluid, and establish a cement sheath - casing - displacement fluid model during the cementing process of the actual hydrate layer;

[0009] (2) Aiming at the characteristics of the hydrate formation, pre - select the cement slurry system used for cementing, and collect and calculate the conventional properties, hydration heat release parameters, and heat transfer parameters of the cement slurry system under different temperature and pressure conditions by means of experiments or cement hydration kinetics models;

[0010] (3) According to the cement hydration kinetics model, based on the conventional properties, hydration heat release parameters, and heat transfer parameters of the cement slurry system collected and calculated in step (2), establish a coupled temperature field model of the cement sheath hydration heat source - wellbore hydrate decomposition;

[0011] (4) Based on the coupled temperature field model of the cement sheath hydration heat source - wellbore hydrate decomposition, simulate the phase state changes of the hydrate layer at the second interface during the cement sheath hydration process under the use conditions of the pre - selected cement slurry system, and obtain the influence of the heat transfer of the cement sheath on the stability of the hydrate layer at the second interface;

[0012] (5) According to the adopted cement slurry system, collect the filtrate situation of the cement slurry, and establish a molecular dynamics model of the hydrate layer - filtrate;

[0013] (6) Combine the coupled temperature field model of the hydration heat source of the cement sheath - hydrate decomposition on the wellbore wall with the molecular dynamics model of the hydrate layer - filtrate for simulation to obtain a control interval chart for maintaining the stability of hydrates during the cementing process of the corresponding hydrate layer under the mass transfer and heat transfer conditions of a certain cement slurry system.

[0014] The present invention combines the results of thermodynamic simulation and molecular simulation, which can reasonably predict the stability of the hydrate formation during the cementing process of the target formation using a certain cement slurry, provide guidance for the hydration heat situation of the cement slurry system, and provide an important guarantee for the stability of the hydrate formation and the cementing quality in actual cementing.

[0015] Furthermore, the relevant parameters of the hydrate formation are mineral composition, particle size composition, porosity, hydrate occurrence type, hydrate saturation, pore moisture content, formation temperature, specific heat capacity, density, thermal conductivity, and heat convection coefficient, and the relevant parameters of the casing are density, specific heat capacity, inner and outer diameters, and thermal conductivity.

[0016] Furthermore, the hydrate saturation, formation temperature, specific heat capacity, thermal conductivity, and heat convection coefficient change in real time with the exothermic hydration of the oil well cement.

[0017] Furthermore, the conventional properties and heat transfer parameters of the oil well cement slurry are the amount of heat released during hydration, cement density, specific heat capacity, thermal conductivity, and cement sheath radius.

[0018] Furthermore, the relationship between the amount of heat released during hydration and the hydration time is an associated relationship, which is obtained through experiments or a cement hydration kinetics model and is the only heat increase term during the waiting period for cement setting.

[0019] Furthermore, the coupled temperature field model of the hydration heat source of the cement sheath - hydrate decomposition on the wellbore wall is divided into three parts: (1) the heat conduction model of the cement sheath in a unit time period; (2) the heat conduction model of the cement sheath - casing; (3) the temperature field model of the cement sheath - formation.

[0020] Furthermore, the software for expressing and displaying the coupled temperature field model of the hydration heat source of the cement sheath - hydrate decomposition on the wellbore wall includes but is not limited to thermodynamic simulation software such as Comsol, ANSYS, Fluent, SolidWorks Simulation, etc.

[0021] The formula for the heat conduction model of the cement sheath in a unit time period is:

[0022] Formula (1);

[0023] In the formula, T c is the temperature of the cement sheath, K; Q ct is att Hydration heat of the cement sheath at a certain moment, J·g -1 ; c c Specific heat capacity of the cement sheath, J·kg -1 ·K -1 ; ρ c Density of the cement sheath, kg·m -3 ; r c Outer diameter of the casing, m; K cp Heat transfer coefficient from the cement sheath to the casing, W·m -2 ·K -1 ; T p Temperature of the casing, K; r f Radius of the second interface, m; K cf Heat transfer coefficient from the cement sheath to the formation, W·m -2 ·K -1 ; T f Formation temperature, K.

[0024] Furthermore, the formula of the cement sheath-casing heat conduction model is:

[0025] Formula (2);

[0026] Wherein, T p Temperature of the casing, K; T p1 Boundary temperature of the casing at the first interface, K; T p2 Temperature of the boundary between the casing and the displacement fluid, K; c p Specific heat capacity of the casing, J·kg -1 ·K -1 ; ρ p Density of the casing, kg·m -3 ; r c Outer diameter of the casing, m; K cp Heat transfer coefficient from the cement sheath to the casing, W·m -2 ·K -1 ; T c1 Boundary temperature of the cement sheath at the first interface, K; r p Inner diameter of the casing, m; Kpd is the heat transfer coefficient from the casing to the displacement fluid, W·m -2 ·K -1 ; T d is the temperature of the displacement fluid, K.

[0027] Furthermore, the formula of the cement sheath - formation temperature field model is:

[0028] Formula (3);

[0029] In the formula, T c is the cement sheath 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 formation to the second interface, m; h f is the effective heat convection coefficient of the formation fluid, W·m -2 ·K -1 ; Q p is the heat absorption during hydrate phase change in the formation, J·g -1 ; c c is the specific heat capacity of the cement sheath, J·kg -1 ·K -1 ; ρ c is the density of the cement sheath, kg·m -3 .

[0030] The functions of the above three model formulas are to constitute the heat transfer relationships of the cement sheath itself, from the cement sheath to the casing, and from the cement sheath to the formation. During the application process, the three model formulas need to be input into the software to establish the heat transfer model between each heat transfer cell, and then the relevant heat transfer parameters of the formation, casing, and cement sheath are input to form the heat transfer relationships between each cell. Finally, the corresponding curve values of the cement hydration time and the heat of hydration are input into the software to run, and the heat transfer conditions of the cement to the formation and the casing at each moment during the hydration process can be simulated. Through the construction and interpretation of the heat transfer relationships by the three model formulas, the temperature change conditions at any position of the casing, cement sheath, and formation during the cement hydration process can be obtained (such as Figure 9 ).

[0031] Furthermore, the molecular dynamics model of the hydrate layer is structural types such as type I hydrate, type II hydrate, and type H hydrate, and the hydrate molecular structure adopts the CVFF force field and the TIP4P model.

[0032] Further, the filtrate is the filtrate that cement slurry may infiltrate into the hydrate formation. The solute molecules in the filtrate include, but are not limited to, Ca 2+ , Na + , Cl - , -COOH, -SO3, and -CO-NH2, etc.

[0033] Further, the molecular dynamics model analysis methods for the hydrate layer - filtrate are radial distribution function, root mean square displacement, mean square displacement, diffusion coefficient, etc. According to the hydration temperature of the cement sheath - formation interface of the present invention, the decomposition of the hydrate model under the influence of cement filtrate molecules is combined with the above analysis methods to determine the critical value of the heat of cement hydration release for maintaining the stability of the hydrate.

[0034] Further, the process of obtaining the regulation interval chart is as follows:

[0035] (6 - 1) According to the phase change critical value of the hydrate molecules without the influence of filtrate at the cement hydration temperature, based on the molecular dynamics model analysis method, determine the diffusion coefficient of the hydrate without the influence of filtrate;

[0036] (6 - 2) Substitute the diffusion coefficient of the hydrate without the influence of filtrate into the molecular simulation results under the condition of containing cement slurry filtrate to determine the phase change critical temperature value of the hydrate under the influence of cement slurry filtrate;

[0037] (6 - 3) Correlate the phase change critical temperature value of the hydrate under the influence of cement slurry filtrate with the heat of hydration release of the cement slurry system to obtain the regulation interval chart for maintaining the stability of the hydrate during the cementing process of the corresponding hydrate layer under the mass transfer and heat transfer conditions of this kind of cement slurry system.

[0038] Compared with the prior art, the present invention has the following advantages:

[0039] Innovatively combine the two aspects of mass transfer and heat transfer of the cement sheath, and study and analyze the influence of the heat release of cement sheath hydration on the hydrate formation. Through the collection of relevant physical and chemical parameters of the actual formation and the applied cement slurry system, specifically study the mass transfer and heat transfer conditions of a certain cement slurry system for different hydrate saturation formations, and establish an evaluation method for the influence of cement slurry hydration on the stability of the hydrate layer based on the phase change state of the hydrate as the standard.

[0040] Adopt a combination of two completely different macroscopic and microscopic simulations of thermodynamic simulation and molecular dynamics simulation to construct a coupled temperature field model of cement hydration heat source - wellbore hydrate decomposition, and reveal the evolution law of the phase state of hydrates in the near - wellbore zone under the influence of cement slurry filtrate and heat release parameters during the cement hydration heat release process. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 It is the equivalent hydration heat release curve of A# low-density cement at different temperatures.

[0042] Figure 2 It is a schematic diagram of the coupled temperature field model of the hydration heat source in the cement sheath and the decomposition of hydrates on the wellbore wall.

[0043] Figure 3 It is the temperature field of the A# low-density cement hydration process model.

[0044] Figure 4 It is the curve of the temperature change with the radius at the peak temperature of the two interfaces during the hydration process of A# low-density cement.

[0045] Figure 5 It is the initial configuration of the molecular dynamics model of the hydrate layer - dodecylamide solution.

[0046] Figure 6 It is the relationship curve between temperature and diffusion coefficient.

[0047] Figure 7 It is the phase change critical temperature of hydrates under different pressures affected by the amide group.

[0048] Figure 8 It is the relationship between the initial temperature of the hydrate formation and the critical value of cement hydration heat release.

[0049] Figure 9 It is the curve of the temperature change with time at the two interfaces during the hydration process of low-density cement. Specific implementation manner

[0050] 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 not limited to the application scope of the present invention.

[0051] (1) According to the actual occurrence state of hydrates and the cementing process in the shallow layer of deep ocean water, relevant parameters of the hydrate formation, relevant parameters of the casing, and parameters of the displacement fluid are obtained and calculated, and a cement sheath - casing - displacement fluid model for simulating the actual cementing process of the hydrate layer is established.

[0052] Taking the shallow hydrate block in the Shenhu Sea Area of the 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 area are selected as a reference, and the simulation is carried out with the initial environmental temperature of 4 - 18 °C and the initial formation pressure of 10 MPa. The relevant parameters of the casing include: inner diameter 404 mm, outer diameter 468 mm, density 7.8 g·cm -3 , specific heat capacity of 0.460 J·g -1 ·K -1 , thermal conductivity of 43.0 W·m -1 ·K -1; The displacement fluid parameters inside the casing include: density 1.1 g·cm -3 , specific heat capacity 3.180 J·g -1 ·K -1 , thermal conductivity 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 simulation model of the cement sheath - casing - displacement fluid is constructed. The material parameters applied in the cement sheath - casing - displacement fluid model are shown in Table 1:

[0053] Table 1 Material parameters of the simulation model of the cement sheath - casing - displacement fluid

[0054] .

[0055] (2) For the characteristics of the hydrate formation, pre - select the cement slurry system used for cementing. Use experimental means or a cement hydration kinetics model to collect and calculate the conventional properties, hydration heat release parameters, and heat transfer parameters of the cement slurry system under different temperature and pressure conditions.

[0056] The selected cement slurry system is a low - density cement slurry system currently used in deep - water offshore cementing, with G - grade oil well cement as the base material for the shallow hydrate formation in deep - water offshore. The cement slurry formula and performance are shown in Tables 2 - 3 and Figure 1 as follows:

[0057] Table 2 Cement slurry formula

[0058] ;

[0059] Table 3 Performance of the cement used in the hydrate layer cementing experiment

[0060] .

[0061] (3) According to the cement hydration kinetics model, based on the conventional properties, hydration heat release parameters, and heat transfer parameters of the cement slurry system collected and calculated in step (2), establish a coupled temperature field model of the cement sheath hydration heat source - wellbore hydrate decomposition.

[0062] According to the heat transfer process of the cement hydration heat source, combined with the heat transfer model relationships between the cement sheath and itself, the casing, and the formation in formulas (1), (2), and (3), use the comsol Multiphysics software to establish a coupled temperature field model of the cement sheath hydration heat source - wellbore hydrate decomposition as Figure 2 shown.

[0063] Input the relevant parameters of the hydrate formation and casing. The model is designed according to the primary cementing structure of the deep - water marine formation, including the displacement fluid, casing, cement sheath, and hydrate formation. Among them, Structure A is the displacement fluid inside 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 the heat source of cement hydration and hydrate decomposition on the wellbore wall, in the combined model, the casing and cement sheath are set as solids, the displacement fluid inside the casing is set as a fluid, the hydrate formation is set as a porous - medium formation with a porosity of 52.25%, and the latent heat of phase change of hydrate is 736.49 J·g -1 , and before the phase - change decomposition of hydrate, the pores are filled with water and gas hydrate, and the hydrate saturation is 36.90%.

[0064] Based on the coupled temperature - field model of the heat source of cement hydration and hydrate decomposition on the wellbore wall, simulate the phase - state change of the hydrate layer at the second interface during the cement - sheath hydration process under the use conditions of the pre - selected cement slurry system, and obtain the influence of the heat transfer of the cement sheath on the stability of the hydrate layer at the second interface.

[0065] According to the temperature - field control and heat - transfer equation, select the porous - medium heat - transfer module for numerical simulation. The equation form is transient. Couple the thermodynamic process of the phase - state change of hydrate in the pores of the porous - medium formation with the heat - conduction process of the heat release during the setting and hardening of the cement sheath, and study the influence of the stability of the hydrate layer during the cement hydration process in the target block. The set conditions during the numerical simulation of the coupled temperature - field of the heat source of cement hydration and hydrate decomposition on the wellbore wall are as follows:

[0066] (1) The displacement fluid inside the casing, the casing, the cement sheath, and the hydrate formation are all homogeneous models;

[0067] (2) The displacement fluid inside the casing, the casing, the cement sheath, and the hydrate formation are in close contact, the temperature field is continuous, and the cementing quality is good;

[0068] (3) The thermodynamic parameters of the hydrate formation change with temperature during the phase - change decomposition of hydrate;

[0069] (4) The cement sheath is the only heat - source term, and the heat release per unit volume is the same during the setting and hardening process;

[0070] (5) The initial temperatures of the wellbore and the hydrate formation are the same (which can be defined as the temperature at the end of the cement - injection process);

[0071] (6) The hydrate formation is infinitely large in the radial direction.

[0072] Take the ambient initial temperature of 4 - 18°C and the initial pressure of the hydrate formation of 10 MPa as an example for simulation.

[0073] During the numerical simulation process, the cement sheath serves as the sole heat source. The heat release rate is based on the equivalent heat of hydration fitted in the cement hydration kinetics model according to 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 temperature. When the hydrate is not decomposed, the formation is in a heat transfer state of heat conduction between the formation skeleton and pore hydrates and heat convection of pore water; when the hydrate is in the phase change decomposition state, the heat absorption part of the hydrate phase change also acts on the formation heat transfer process; when the hydrate is decomposed, the formation is in a heat transfer state of heat conduction of the formation skeleton and heat convection of pore water and methane.

[0074] During the simulation process, the initial degree of cement hydration is set to 0. The cement starts to hydrate at the 0th moment of model operation. The initial temperatures are 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 are put into the model for operation, and the temperature changes of the casing - cement sheath - hydrate formation during the setting time of the cement slurry at different temperatures are obtained, as Figure 3 shown.

[0075] Determine the time point of the highest temperature at the second interface through the simulation results. Set the simulation time to this time point, and draw the curve of temperature change with radius starting from the center of the model, so as to determine the temperature change curves of the displacement fluid in the casing, casing, cement sheath, and hydrate formation, as Figure 4 shown. It can be seen from the simulation results that when the initial temperature of the hydrate formation is 4 - 18°C and the initial pressure is 10 MPa, the peak range of the temperature rise of cement hydration is between 279.1 - 292.0 K (5.95 - 18.95°C), and the phase change critical temperature value of the hydrate is 291.5 K.

[0076] (5) According to the cement slurry system adopted, collect the filtrate situation of the cement slurry, and establish a molecular dynamics model of the hydrate layer - filtrate using molecular simulation software.

[0077] After configuring the cement slurry according to the API standard, use ICP - MS and ion spectroscopy to test the main existing ions and functional groups in the cement slurry filtrate, then use IR (infrared spectroscopy) to test and obtain the main functional groups in the cement additives, and use molecular simulation software to establish a molecular dynamics model of the hydrate layer - filtrate. In this case test, taking the main existing functional group in the cement slurry filtrate as amide (-CO - NH2) as an example, a molecular dynamics model of the hydrate layer - amide solution is established (as Figure 5 shown).

[0078] (6) Combine the coupled temperature field model of the hydration heat source of the cement sheath - hydrate decomposition in the wellbore with the molecular dynamics model of the hydrate layer - filtrate to perform simulations, and obtain the control interval chart for maintaining hydrate stability during the cementing process of the corresponding hydrate layer under the mass transfer and heat transfer conditions of a certain cement slurry system.

[0079] Specifically:

[0080] (6-1) According to the phase change critical temperature value of hydrate molecules without the influence of filtrate at the cement hydration temperature, determine the diffusion coefficient of hydrates without the influence of filtrate based on the analysis method of the molecular dynamics model.

[0081] (6-1-1) Combine the temperature rise peak range caused by cement hydration at the second interface when the hydrate is at 4 - 18 °C obtained from thermodynamic simulations. After establishing the hydrate layer - pure water molecular dynamics model and hydrate layer - amide solution molecular dynamics model of hydrates, water molecules, and dodecylamide molecules using MS software (such as Figure 5 ), output the supercell parameters as a data file, and then use lammps software to perform the operation simulation of the molecular model to analyze the decomposition of the hydrate model under pure water system and dodecylamide solution system at 277 - 293 K temperature, and obtain the relationship curves of temperature and diffusion coefficient under the influence of no filtrate and amide functional groups ( Figure 6 ).

[0082] (6-1-2) Determine the value of the diffusion coefficient corresponding to the phase change critical temperature under the influence of no filtrate. Taking the initial formation pressure of 10 MPa as an example, under the influence of no filtrate, the phase change critical temperature value of the hydrate is 291.5 K at this time, and the diffusion coefficient of the hydrate under the influence of no filtrate is 0.86 m·s -1 , so it is set that the hydrate starts to decompose when the diffusion coefficient is greater than 0.86 m·s -1 .

[0083] (6-2) Substitute the diffusion coefficient of the hydrate without the influence of filtrate into the molecular simulation results under the condition of cement slurry filtrate to determine the phase change critical temperature value of the hydrate under the influence of cement slurry filtrate;

[0084] Substitute the diffusion coefficient of 0.86 m·s -1 into the relationship curve of temperature and diffusion coefficient under the influence of amide groups to obtain the phase change critical decomposition temperature of the hydrate under the influence of amide groups at this time as 291.2 K, that is, compared with the formation hydrate without the influence of filtrate, the phase change critical temperature of the formation hydrate under the influence of amide groups is reduced by 0.3 K.

[0085] Use this method to simulate and calculate the phase change critical temperature of hydrates at different pressures under the influence of amide groups, such as Figure 7as shown

[0086] It can be obtained that Figure 7 when there is no filtrate influence, the phase change critical temperature range of formation hydrate is 291.5 - 295.0 K at 10 - 15 MPa. Under the influence of amide groups, the phase change critical temperature range 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.

[0087] (6 - 3) Corresponding the phase change critical temperature value of hydrate under the influence of cement slurry filtrate with the heat of hydration release of the cement slurry system, the control interval chart for maintaining the stability of hydrate during the cementing process of the hydrate layer under the mass transfer and heat transfer conditions of this kind of cement slurry system is obtained.

[0088] By using Comsol to simulate the influence of cement hydration heat release on the stability of hydrates in the formation under different initial temperatures and pressures. Set the simulation to be carried out at a certain pressure. When the temperature of hydrates in the formation reaches Figure 7 the corresponding phase change critical temperature value under the influence of amide groups in [reference] is the phase change decomposition of hydrates in the formation pores. The relationship chart of the critical value of cement hydration heat release for maintaining the stability of hydrates at different initial formation temperatures is drawn as Figure 8 as shown

[0089] It can be obtained from examples that by simulating the decomposition of hydrate formations under different temperatures and pressures under the heat release of cement hydration and taking into account the influence of cement slurry filtrate, a relationship chart of the critical value of heat release during cement mass transfer and heat transfer for maintaining the stability of hydrates at different initial formation temperatures can be established, clarifying the relationship between the initial formation temperature, pressure and the critical value of cement hydration heat release, providing guidance for the adjustment of on-site oil well cement in terms of hydration heat.

Claims

1. A prediction and evaluation method for the steady state or decomposition of formation hydrates under mass transfer and heat transfer of oil well cement, characterized in that, Based on thermodynamic simulation and molecular dynamics simulation, it includes the following steps: (1) According to the actual occurrence state of hydrate and the cementing process in the shallow layer of deep ocean water, obtain and calculate the relevant parameters of the hydrate formation, the relevant parameters of the casing, and the displacement fluid parameters, and establish a cement sheath-casing-displacement fluid model for simulating the cementing process in the actual hydrate layer; (2) Aiming at the characteristics of the hydrate formation, preselect the cement slurry system used for cementing, and use experimental means or cement hydration kinetics models to collect and calculate the conventional properties, hydration heat release parameters, and heat transfer parameters of the cement slurry system under different temperature and pressure conditions; (3) According to the cement hydration kinetics model, based on the conventional properties, hydration heat release parameters, and heat transfer parameters of the cement slurry system collected and calculated in step (2), establish a coupled temperature field model of the heat source of cement sheath hydration and the decomposition of hydrate on the wellbore wall; (4) Based on the coupled temperature field model of the heat source of cement sheath hydration and the decomposition of hydrate on the wellbore wall, simulate the phase state change of the hydrate layer at the second interface during the hydration process of the cement sheath under the use conditions of the preselected cement slurry system, and obtain the influence of the heat transfer of the cement sheath on the stability of the hydrate layer at the second interface; (5) According to the cement slurry system adopted, collect the filtrate condition of the cement slurry, and use molecular simulation software to establish a molecular dynamics model of the hydrate layer-filtrate; (6) Combine the coupled temperature field model of the heat source of cement sheath hydration and the decomposition of hydrate on the wellbore wall with the molecular dynamics model of the hydrate layer-filtrate for simulation, and obtain a control interval chart for maintaining the stability of the hydrate during the cementing process of the corresponding hydrate layer under the mass transfer and heat transfer conditions of a certain cement slurry system; The process of obtaining the said control interval chart is as follows: (6-1) According to the phase change critical value of the hydrate molecule without the influence of filtrate at the cement hydration temperature, determine the diffusion coefficient of the hydrate without the influence of filtrate based on the molecular model analysis method; (6-2) Substitute the diffusion coefficient of the hydrate without the influence of filtrate into the molecular simulation results under the condition of cement slurry filtrate, and determine the critical temperature value of hydrate phase change under the influence of cement slurry filtrate; (6-3) Correlate the critical temperature value of hydrate phase change under the influence of cement slurry filtrate with the hydration heat release of the cement slurry system, and obtain a control interval chart for maintaining the stability of the hydrate during the cementing process of the corresponding hydrate layer under the mass transfer and heat transfer conditions of this cement slurry system.

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

3. The prediction and evaluation method for the steady state or decomposition of hydrates in the formation water under mass transfer 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 sheath radius.

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

5. The prediction and evaluation method for the steady state or decomposition of hydrates in the formation water under mass and heat transfer of oil well cement according to claim 4, wherein The formula of the heat conduction model of the cement sheath per unit time is: ； In the formula, T c is the temperature of the cement sheath, in K; Q ct is the hydration heat of the cement sheath at t moment, in J·g -1 ; c c is the specific heat capacity of the cement sheath, in J·kg -1 ·K -1 ; ρ c is the density of the cement sheath, in kg·m -3 ; r c is the outer diameter of the casing, in m; K cp is the heat transfer coefficient from the cement sheath to the casing, in W·m -2 ·K -1 ; T p is the temperature of the casing, in K; r f is the radius of the second interface, in m; K cf is the heat transfer coefficient from the cement sheath to the formation, in W·m -2 ·K -1 ; T f is the formation temperature, in K.

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

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

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