A method for assessing the risk of submarine landslides during the exploitation of hydrates

Abstract

The present invention discloses a method for evaluating the risk of submarine landslides during the process of hydrate exploitation. First, a thermal-fluid-mechanical-phase change coupling model is established based on a mechanical model and its coupling relationship with a thermal-fluid-phase change coupling model. Subsequently, a multi-physical field coupling numerical simulation platform for hydrate exploitation at the site scale is built according to the established thermal-fluid-mechanical-phase change coupling model. By using the multi-physical field coupling numerical simulation platform for hydrate exploitation at the site scale, a reservoir model of natural gas hydrate in the target area is established based on on-site geological exploration data. The formation settlement behavior is quantitatively analyzed according to the calculation results, and the stress monitoring area is selected. At the same time, the risk of submarine landslides during the exploitation process can be quantitatively evaluated by combining the stress evolution curve in the monitoring area with the Mohr-Coulomb failure curve.

Description

Technical Field

[0001] The present invention belongs to the field of natural gas hydrate resource development engineering, and particularly relates to a method for evaluating the risk of submarine landslides during the process of hydrate exploitation. Background Art

[0002] With the development of industrialization, the demand for energy has increased sharply, resulting in the shortage of traditional fossil energy and a series of environmental pollution problems. The development and utilization of alternative energy sources are imperative. Natural gas hydrate is a cage-like crystalline compound formed by water molecules and light hydrocarbon gas molecules (mainly methane gas) under high pressure and low temperature conditions, and is mainly distributed in terrestrial permafrost and deep-sea sediments in nature.

[0003] With the continuous growth of the global demand for clean energy, the advantages of huge reserves, cleanliness and pollution-free make natural gas hydrate increasingly attractive as a potential future energy source. In the prior art, most are to simulate and analyze the reservoir settlement during the decomposition process of natural gas hydrate through experimental devices at the laboratory scale. However, there are few monitoring and analysis technologies for submarine landslides during the process of natural gas hydrate exploitation at the field scale.

[0004] During the exploitation process of hydrate reservoirs in the South China Sea of our country, due to reasons such as large water depth, complex reservoir conditions, and difficult on-site operations, it is impossible to effectively monitor the formation settlement during the trial production process, resulting in difficulty in fully understanding the response characteristics of formation settlement during the process of natural gas hydrate exploitation. The problem of submarine landslides caused by formation settlement has become one of the main problems affecting the long-term safe and efficient exploitation of hydrates. Summary of the Invention

[0005] In order to overcome the deficiencies in the prior art, the present invention provides a method for evaluating the risk of submarine landslides during the process of hydrate exploitation, reveals the mechanical model and its coupling relationship with the thermal-fluid-phase change coupling model, establishes a multi-physical field coupling numerical simulation platform for hydrate exploitation at the field scale, can realize the quantitative analysis and prediction of potential submarine landslide areas during the production process, effectively monitor the formation settlement during the trial production process of natural gas hydrates, fully understand the response characteristics of formation settlement during the process of natural gas hydrate exploitation, and overcomes the defects of the prior art.

[0006] A method for evaluating the risk of submarine landslides during the process of hydrate exploitation proposed by the present invention can, based on the multi-physical field coupling numerical simulation platform for hydrate exploitation at the field scale, real-time observe the evolution characteristics of formation settlement of the hydrate-bearing reservoir during the field trial production process and provide a basis for the selection of stress monitoring areas; at the same time, it can quantitatively evaluate the risk of submarine landslides during the exploitation process according to the stress evolution curve in the monitoring area combined with the Mohr-Coulomb failure curve.

[0007] The technical solution of the present invention is as follows:

[0008] A method for evaluating the risk of submarine landslides during the exploitation of hydrates, comprising the following steps:

[0009] Step 1: Establish the coupling relationship between the mechanical model and the thermal-fluid-phase change coupling model, establish the thermal-fluid-mechanical-phase change coupling model, and construct a multi-physical field coupling numerical simulation platform for hydrate exploitation at the site scale, specifically including the following sub-steps:

[0010] Step 1.1: Establish the coupling relationship between the mechanical model and the thermal-fluid-phase change coupling model;

[0011] First, describe the coupling relationship between the formation effective stress and pore pressure according to the effective stress principle of unsaturated soil; then describe the stress-strain coupling relationship according to the elastoplastic constitutive equation of the soil; finally, establish the coupling relationship between the mechanical model and the thermal-fluid-phase change coupling model through the dynamic changes of porosity and permeability caused by effective stress to obtain the thermal-fluid-mechanical-phase change coupling model;

[0012] Step 1.2: Construct a multi-physical field coupling numerical simulation platform for hydrate exploitation at the site scale according to the thermal-fluid-mechanical-phase change coupling model;

[0013] Step 2: Conduct a quantitative analysis of the formation settlement behavior in the target area and select the stress monitoring area, and then quantitatively evaluate the risk of submarine landslides during the exploitation according to the stress evolution curve combined with the Mohr-Coulomb failure curve, specifically including the following sub-steps:

[0014] Step 2.1: Based on the geological conditions and exploitation conditions in the trial exploitation area, draw a map to establish a reservoir model of natural gas hydrate in the target area;

[0015] Step 2.2: Set the initial conditions and boundary conditions of the model: including temperature, pressure, saturation and stress, and conduct grid division on the reservoir model of natural gas hydrate in the target area;

[0016] Step 2.3: Input the main physical property parameters of the reservoir in the reservoir model after grid division into the multi-physical field coupling numerical simulation platform for hydrate exploitation at the site scale for calculation and solution to obtain the formation settlement evolution characteristics of the hydrate-bearing sediment at different simulation times, quantitatively analyze the formation settlement behavior according to the calculation results and select the stress monitoring area, and then quantitatively evaluate the risk of submarine landslides during the exploitation according to the stress evolution curve in the monitoring area combined with the Mohr-Coulomb failure curve.

[0017] Furthermore, the specific process of constructing the thermal-fluid-mechanical-phase change coupling model in Step 1.1 is as follows:

[0018] Step 1.1.1: Establish the force balance equation for the hydrate-bearing formation to express the coupling relationship between effective stress and pore pressure, as shown in formula (1):

[0019]

[0020] Where δ is the unit matrix; δ′ is the effective stress, in MPa; a B is the Biot coefficient; n a is the volume ratio of each phase; ρ a is the density of each phase, in kg / (m 3 ); g is the acceleration due to gravity, the unit is m / (s 2 );P p is the pore pressure;

[0021] Pore pressure P p The calculation of is shown in formula (2):

[0022]

[0023] Among them, S G and S W are gas and water saturation, respectively; P G and P W are the gas and water pressures, respectively, in MPa;

[0024] Step 1.1.2, establish the coupling relationship between stress and strain:

[0025] The relationship between displacement and strain is defined by the geometric equation, and its tensor form is shown in formula (3):

[0026]

[0027] where ε ij is the strain tensor; u is the displacement in m;

[0028] During the production of hydrate sediments, the reservoir skeleton will experience significant elastic-plastic deformation, the incremental form of which is dδ ij As shown in formula (4):

[0029] dδ ij =D ep dε kl (4)

[0030] Among them, D ep is the elastic-plastic coefficient matrix tensor, dε kl is the increment of the strain tensor;

[0031] Elastic-plastic matrix [D ep The calculation of ] is shown in formula (5):

[0032] [D ep = [D e - [D p (5)

[0033] Among them, [D e is the elastic matrix of the rock; [D p is the plastic matrix;

[0034] According to the elastoplastic associated flow rule, the plastic matrix is obtained as shown in formula (6):

[0035]

[0036] In the formula, f is the yield function;

[0037] Adopting the Drucker-Prager criterion, the material parameters are matched with the Mohr-Coulomb criterion. When the stress reaches the condition shown in formula (7), the material will enter the yield state:

[0038]

[0039] Among them, C is the cohesion with the unit of MPa; θ is the internal friction angle with the unit of °; I1 is the first invariant of the stress tensor with the unit of MPa; J2 is the second invariant of the stress deviator with the unit of MPa 2 ;

[0040] Among them, I1 and J2 are described as: I1 = δ1 + δ2 + δ3;

[0041] Step 1.1.3, based on the dynamic changes of porosity and permeability caused by the effective stress, establish the coupling relationship between the effective stress and the thermo-fluid-phase change coupling model:

[0042] The change in porosity caused by the effective stress is as shown in formula (8):

[0043]

[0044] Among them is the initial porosity of the sediment after the particle skeleton falls off; is the porosity of the sediment under high pressure; δ′ M is the mean effective stress with the unit of MPa, expressed as δ′ M = (δ′ x + δ′ y + δ′ z ) / 3; a is the stress sensitivity coefficient of porosity;

[0045] The variation of permeability k caused by effective stress is shown in Equation (9):

[0046]

[0047] where k0 is the intrinsic permeability of the reservoir; S H is the hydrate saturation; N is the Masuda permeability attenuation index; and b is the stress sensitivity coefficient of permeability.

[0048] Furthermore, in Step 2.3, the maximum seabed settlement area is selected as the stress monitoring area according to the seabed settlement evolution schematic diagram.

[0049] Furthermore, the specific expression of the Mohr-Coulomb failure curve in Step 2.3 is shown in (10):

[0050]

[0051] where δ1 is the maximum principal stress in MPa; and δ3 is the minimum principal stress in MPa.

[0052] The effect of the present invention is that, compared with the prior art, the present invention can, during the exploitation of natural gas hydrate resources, apply a thermal-fluid-mechanical-phase change coupling model and a reservoir model of a natural gas hydrate reservoir in the target area established based on on-site geological exploration data, calculate the seabed settlement evolution characteristics of the hydrate-bearing sediment formation during the trial production process and select the stress monitoring area, and then, based on the stress evolution curve in the monitoring area and combined with the Mohr-Coulomb failure curve, quantitatively analyze and predict potential seabed landslide areas during the production process, proving the effectiveness and practicality of a method for evaluating the risk of seabed landslides during the exploitation of hydrates. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] Figure 1 is a flowchart of a method for evaluating the risk of seabed landslides during the exploitation of hydrates in an embodiment of the present invention.

[0054] Figure 2 is a coupling relationship diagram of a mechanical model and a thermal-fluid-phase change coupling model and a thermal-fluid-mechanical-phase change coupling model diagram in an embodiment of the present invention.

[0055] Figure 3 is an example diagram of a reservoir model selected in an embodiment of the present invention.

[0056] Figure 4 is an example diagram of grid division in an embodiment of the present invention.

[0057] Figure 5 is a verification curve diagram of gas production in an embodiment of the present invention.

[0058] Figure 6Schematic diagram of formation settlement evolution in the embodiments of the present invention.

[0059] Figure 7 Schematic diagram of the stress evolution curve in the monitoring area and the selected Mohr-Coulomb failure curve in the embodiments of the present invention. Detailed implementation manners

[0060] To make the above objects, features, and advantages of the present invention more obvious and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific implementation manners.

[0061] Embodiment 1

[0062] In this embodiment, a method for evaluating the risk of submarine landslides during hydrate production is provided, as Figure 1 , and specifically includes the following steps:

[0063] S1: Describe the coupling relationship between the effective stress of the formation and the pore pressure according to the effective stress principle of unsaturated soil; describe the coupling relationship between stress and strain according to the elastoplastic constitutive equation of the soil; establish the coupling relationship between the mechanical model and the heat-fluid-phase change coupling model according to the dynamic changes of porosity and permeability, specifically as follows:

[0064] The force balance equation of the hydrate-bearing formation is expressed by formula (1):

[0065]

[0066] In the formula, δ is the unit matrix; δ′ is the effective stress, with the unit of MPa; a B is the Biot coefficient; n a is the volume ratio of each phase; ρ a is the density of each phase, with the unit of kg / (m 3 ); g is the acceleration of gravity, with the unit of m / (s 2 ); P p is the pore pressure;

[0067] The calculation of the pore pressure P p is shown in formula (2):

[0068]

[0069] In the formula, S G and S W are the gas and water saturations respectively; P G and P W are the gas and water pressures respectively, with the unit of MPa.

[0070] The geometric equation defines the relationship between the displacement and the strain, and its tensor form is shown in formula (3):

[0071]

[0072] where ε ij is the strain tensor; u is the displacement with the unit of m.

[0073] During the exploitation of hydrate sediments, obvious elastoplastic deformation occurred in the reservoir skeleton, and its incremental form dδ ij is shown in Equation (4) as follows:

[0074] dδ ij = D ep dε kl (4)

[0075] where D ep is the elastoplastic coefficient matrix tensor, and dε kl is the increment of the strain tensor;

[0076] The calculation of the elastoplastic matrix [D ep is shown in Equation (5) as follows:

[0077] [D ep = [D e - [D p (5)

[0078] where [D e is the elastic matrix of the rock; [D p is the plastic matrix.

[0079] The plastic matrix is obtained according to the associated flow rule of elastoplasticity, as shown in Equation (6):

[0080]

[0081] where f is the yield function.

[0082] Using the Drucker-Prager criterion to match the material parameters with the Mohr-Coulomb criterion, when the stress reaches the condition shown in Equation (7), the material will enter the yield state:

[0083]

[0084] where C is the cohesion with the unit of MPa; θ is the internal friction angle with the unit of °; I1 is the first invariant of the stress tensor with the unit of MPa; J2 is the second invariant of the stress deviator with the unit of MPa 2 ;

[0085] where I1 and J2 are described as: I1 = δ1 + δ2 + δ3;

[0086] Porosity caused by effective stress The change of is shown in formula (8):

[0087]

[0088] In the formula is the initial porosity of sediment after the particle skeleton falls off; is the porosity of sediment under high pressure; δ′ M is the average effective stress in MPa, expressed as δ′ M =(δ′ x +δ′ y +δ′ z ) / 3; a is the stress sensitivity coefficient of porosity;

[0089] The change of permeability k caused by effective stress is shown in formula (9):

[0090]

[0091] Where k0 is the intrinsic permeability of the reservoir; S H is the hydrate saturation; N is the Masuda permeability attenuation index; b is the stress sensitivity coefficient of permeability.

[0092] By leveraging the changes in permeability and porosity caused by effective stress, a coupling relationship between the mechanical model and the thermal-fluid-phase-change coupling model was established, completing the construction of the thermal-fluid-mechanical-phase-change coupling model. This thermal-fluid-mechanical-phase-change coupling model enables the calculation of gas production rate and volume, as well as the evolution of temperature and pressure, hydrate saturation, and mechanical behavior.

[0093] The model and mathematical equations constructed in S1 constitute the field-scale thermal-fluid-mechanical-phase change coupling model for hydrate extraction. The coupling relationship between the thermal-fluid-phase change coupling model and the mechanical model, as well as the ultimately established thermal-fluid-mechanical-phase change coupling model for hydrate extraction, are shown in Figure 1. Figure 2 shown.

[0094] S2: Using the PDE module and solid mechanics module provided by COMSOL, the thermal-fluid-mechanical-phase change coupling model is input to build a multi-physics coupling numerical simulation platform for hydrate mining at a site scale.

[0095] S3: Establish a natural gas hydrate reservoir model for the target area and perform initial boundary condition settings and grid division;

[0096] The hydrate reservoir in the second offshore trial production area of the South my country Sea was selected as the numerical simulation object to quantitatively evaluate the risk of submarine landslides during the long-term mining process in the second offshore trial production area. It is worth noting that the selection of simulation objects is not fixed.

[0097] The reservoir model selected in this embodiment is as Figure 3 shown. The thickness of the simulated target area is 326.2 m in total, including a hydrate reservoir with a thickness of 89.2 m, an overlying formation with a thickness of 207 m, and an underlying formation with a thickness of 30 m. The hydrate reservoir is located at 207 m to 296.2 m below the seabed surface. Based on the lithology distribution of the hydrate layer, the hydrate reservoir is divided into three layers with thicknesses of 45.6 m, 24.6 m, and 19 m respectively. The length of the model in the horizontal direction is set to 200 m. The main physical property parameters of each layer are shown in Table 1.

[0098] The upper, lower, and right boundaries of the model remain at a constant temperature and pressure during the hydrate production process. The upper boundary carrying seawater can move freely, the side boundaries have no displacement perpendicular to the boundary direction, and the lower boundary is a fixed constraint. The entire simulated target area is divided into 5,818 grids, as Figure 4 shown.

[0099] Table 1 Main physical property parameters of the reservoir

[0100]

[0101]

[0102] S4: Model calculation and result output: As Figure 5 shown, the cumulative gas production in the numerical simulation for 30 days is 8.614×10 5 m 3 , which is close to the total gas production during the trial production period (8.685×10 5 m 3 ), indicating that the established model can effectively reflect the process of the second trial production in the South China Sea of our country;

[0103] Figure 6 is the schematic diagram of the formation settlement evolution. It can be seen from the numerical simulation results that when measurable settlement appears on the seabed, the maximum seabed settlement always appears directly above the horizontal well. Therefore, this area is selected as the stress monitoring area.

[0104] Figure 7 is the schematic diagram of the stress evolution curve in the monitoring area and the selected Mohr-Coulomb failure curve evolution. The results show that the stress at the monitoring point does not reach the Mohr-Coulomb failure curve, indicating that the second trial production in the South China Sea of our country will not cause submarine landslide problems during the 5-year production process. Among them, the specific expression of the Mohr-Coulomb failure curve is as shown in (10):

[0105]

[0106] where δ1 is the maximum principal stress with the unit of MPa; δ3 is the minimum principal stress with the unit of MPa.

[0107] In addition, as Figure 2 shown, the established multi-physical field coupling numerical model for hydrate exploitation can also calculate the evolution characteristics of underground physical fields such as reservoir temperature, pressure, and hydrate saturation evolution.

[0108] In this paper, specific examples are used to illustrate the principle and implementation of the present invention. The description of the above embodiments is only used to help understand the method and its core idea of the present invention; at the same time, for those of ordinary skill in the art, according to the idea of the present invention, there will be changes in the specific implementation manner and application scope. In summary, the content of this specification should not be construed as a limitation on the present invention.

Claims

1. A method for assessing the risk of submarine landslides during hydrate mining, characterized in that: It includes the following steps: Step 1: Establish the mechanical model and its coupling relationship with the thermal-fluid-phase change coupling model, establish the thermal-fluid-mechanical-phase change coupling model, and construct a multi-physical field coupling numerical simulation platform for hydrate production at the field scale, which specifically includes the following sub-steps: Step 1.1: Establish the mechanical model and its coupling relationship with the thermal-fluid-phase change coupling model; First, describe the coupling relationship between the formation effective stress and pore pressure according to the effective stress principle of unsaturated soil; then describe the coupling relationship between stress and strain according to the elastoplastic constitutive equation of the soil; finally, establish the coupling relationship between the mechanical model and the thermal-fluid-phase change coupling model through the dynamic changes of porosity and permeability caused by the effective stress, and obtain the thermal-fluid-mechanical-phase change coupling model; Step 1.2: Build a multi-physical field coupling numerical simulation platform for hydrate production at the field scale according to the thermal-fluid-mechanical-phase change coupling model; Step 2: Conduct a quantitative analysis of the formation settlement behavior in the target area and select the stress monitoring area, and then quantitatively evaluate the submarine landslide risk during the production process according to the stress evolution curve combined with the Mohr-Coulomb failure curve; The specific process of constructing the thermal-fluid-mechanical-phase change coupling model in Step 1.1 is as follows: Step 1.1.1: Establish the force balance equation of the hydrate-bearing formation, representing the coupling relationship between the effective stress and pore pressure: Among them, is the unit matrix; is the effective stress, with the unit of ; is the Biot coefficient; is the volume ratio of each phase; is the density of each phase, with the unit of ; is the acceleration of gravity, with the unit of ; is the pore pressure; Pore pressure The calculation formula is: wherein, and are the gas and water saturations respectively; and are the gas and water pressures respectively, with the unit of ; Step 1.1.2: Establish the coupling relationship between stress and strain: Define the relationship between the displacement and strain by the geometric equation, and its tensor form is: in is the strain tensor; is the displacement in units of ; During the exploitation of hydrate sediments, obvious elastoplastic deformation will occur in the reservoir skeleton, and its incremental form is expressed as follows: Among them, is the elastoplastic coefficient matrix tensor, is the increment of the strain tensor; Elastic-plastic matrix The calculation formula is as follows: Among them, is the elastic matrix of the rock; is the plastic matrix; Obtain the plastic matrix according to the elastoplastic associated flow law: In the formula is the yield function; Adopt the Drucker-Prager criterion to match the material parameters with the Mohr-Coulomb criterion. When the stress reaches the following conditions, the material will enter the yield state: in is the cohesive force, in MPa; is the internal friction angle, in units of ; is the first invariant of the stress tensor, in MPa; is the second invariant of stress deflection, in MPa 2 ; in, , Described as: ; ; Step 1.1.3: Based on the dynamic changes of porosity and permeability caused by the effective stress, establish the coupling relationship between the effective stress and the thermal-fluid-phase change coupling model: Porosity caused by effective stress The change is defined as: where is the initial porosity of the sediment after the particle skeleton falls off; is the porosity of the sediment under high pressure; is the mean effective stress, expressed as ; is the stress sensitivity coefficient of porosity; Permeability due to effective stress The change is defined as: in, is the intrinsic permeability of the reservoir; is the hydrate saturation; is the Masuda permeability decay index; is the stress sensitivity coefficient of permeability.

2. The method for assessing the risk of submarine landslides during hydrate mining according to claim 1, wherein: Step 2 specifically includes the following sub-steps: Step 2.1: Based on the geological conditions and production conditions in the test production area, draw a map to establish the reservoir model of the natural gas hydrate reservoir in the target area; Step 2.2: Set the initial conditions and boundary conditions of the model: including temperature, pressure, saturation, and stress, and perform grid division on the reservoir model of the natural gas hydrate reservoir in the target area; Step 2.3: Input the main physical property parameters of the reservoir of the reservoir model after grid division into the multi-physical field coupling numerical simulation platform for hydrate production at the field scale for calculation and solution. According to the calculation results, quantitatively analyze the formation settlement behavior and select the stress monitoring area, and then quantitatively evaluate the submarine landslide risk during the production process according to the stress evolution curve of the monitoring area combined with the Mohr-Coulomb failure curve.

3. The method for assessing the risk of submarine landslides during hydrate mining according to claim 1, wherein: In Step 2.3, select the area with the maximum seabed settlement as the stress monitoring area according to the formation settlement evolution schematic diagram.

4. The method for assessing the risk of submarine landslides during hydrate mining according to claim 1, wherein: The specific expression of the Mohr-Coulomb failure curve in Step 2.3 is as follows: in is the maximum principal stress; is the minimum principal stress.

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