A prediction method for the sand production scale of reservoir during hydrate exploitation

Abstract

The present invention discloses a method for predicting the sand production scale of a reservoir during the hydrate exploitation process. First, a solid particle detachment and migration model is established. Subsequently, a sand production model for hydrate exploitation is established based on the coupling relationship between the thermal-fluid-mechanical-phase change coupling model and the solid particle detachment and migration model. Finally, a numerical simulation platform for sand production during hydrate exploitation at the field scale is established based on the sand production model for hydrate exploitation. By using the numerical simulation platform for sand production during hydrate exploitation at the field scale, a reservoir model of a natural gas hydrate reservoir in the target area is established based on on-site geological exploration data, and the sand production behavior is quantitatively analyzed according to the calculation results, so as to realize the quantitative prediction of the sand production scale of the reservoir during the hydrate exploitation process.

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 predicting the sand production scale of a reservoir during the hydrate production process. Background Art

[0002] The accelerated development of industrialization has led to a rapid increase in the demand for energy. However, the extensive use of traditional fossil fuels will release a large amount of greenhouse gases such as carbon dioxide, which will in turn bring a series of environmental pollution problems. At the same time, with the huge consumption and exploitation of fossil fuels, traditional fossil fuels are also facing shortages. Meanwhile, the action to find clean energy to replace traditional fossil fuels has also been rapidly launched. Natural gas hydrates are mainly distributed in terrestrial permafrost and deep-sea sediments in nature, and are one of the important clean energies. Their considerable reserves also make them one of the important alternative energies in the future.

[0003] At present, more than 10 natural gas hydrate production test projects have been carried out globally, providing valuable on-site experience for its commercial exploitation. In the prior art, by building an experimental device, using gases, liquids, solids, etc. to establish a reservoir model of the production area to simulate the production environment, and then conducting experimental simulation and analysis on the process of natural gas hydrate production at the laboratory scale.

[0004] However, looking at each hydrate production test project, almost all of them have encountered sand production problems, and even the early termination of the production test has been caused by a large amount of sand production. However, due to the complex diversity of the production environment, it is currently difficult to quantitatively analyze the sand production scale of the hydrate-bearing reservoir effectively at the field scale, which affects the evaluation of the long-term production safety of the hydrate-bearing reservoir and the selection of the location for the sand control device. Summary of the Invention

[0005] In order to overcome the deficiencies in the prior art, the present invention provides a method for predicting the sand production scale of a reservoir during the hydrate production process, reveals the coupling relationship between the thermal-fluid-mechanical-phase change coupling model and the solid particle detachment and migration model, comprehensively considers the combined action of the water flow velocity and plastic strain on the sand production process, proposes a hydrate production sand production model, and establishes a numerical simulation platform for hydrate production sand production at the field scale, which can realize the quantitative analysis and prediction of the sand production scale of the reservoir during the production process, and overcomes the defects of the prior art.

[0006] A method for predicting the sand production scale of a reservoir during the hydrate production process proposed by the present invention can observe the particle detachment and migration characteristics of the hydrate-bearing reservoir in real time based on the numerical simulation platform for hydrate production sand production at the field scale; and can quantitatively calculate the amount of framework particle detachment, the amount of liquefied sand in the reservoir, and the amount of sand production in the wellbore during the hydrate production process, which can quantify the sand production process of the hydrate production test and at the same time provide verification conditions for the model.

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

[0008] A method for predicting the sand production scale of a reservoir during the hydrate production process, comprising the following steps:

[0009] Step 1, establish the coupling relationship between the solid particle detachment and migration model and the thermal-fluid-mechanical-phase change coupling model, establish the hydrate production sanding model, and construct the numerical simulation platform for hydrate production sanding at the field scale, specifically including the following sub-steps:

[0010] Step 1.1, establish the particle detachment model of the hydrate-bearing formation according to the fluid erosion criterion and the critical plastic strain criterion, and combine the modified solute transport equation with the sand production volume and sand production rate equations to obtain the solid particle detachment and migration model;

[0011] Step 1.2, establish the coupling relationship between the solid particle detachment and migration model and the thermal-fluid-mechanical-phase change coupling model according to the dynamic changes of the sediment mechanical parameters and porosity, where the sediment mechanical parameters include bulk modulus, shear modulus, and cohesion, to obtain the hydrate production sanding model;

[0012] Step 1.3, construct the numerical simulation platform for hydrate production sanding at the field scale according to the hydrate production sanding model;

[0013] Step 2, conduct a quantitative analysis of the sanding behavior in the target area, and predict the sand production scale of the reservoir during the production process, specifically including the following sub-steps:

[0014] Step 2.1, based on the geological conditions and production conditions of the test production area, draw a map to establish the reservoir model of the natural gas hydrate reservoir 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 perform grid division on the reservoir model of the natural gas hydrate reservoir in the target area;

[0016] Step 2.3, input the main physical properties parameters of the reservoir of the reservoir model after grid division into the numerical simulation platform for hydrate production sanding at the field scale for calculation and solution, and quantitatively analyze the sand production behavior based on the calculation results to predict the sand production scale of the reservoir during the hydrate production process.

[0017] Furthermore, the particle detachment model in Step 1.1 is specifically expressed as:

[0018] The erosion of the rock skeleton is directly related to the local porosity and water flow velocity. The particle detachment model is as shown in formula (1):

[0019]

[0020] where m ssi is the mass of solid particle shedding, with the unit of kg / (m 3 ); t is the time, with the unit of s; is the mass of solid particle shedding per unit time, with the unit of kg / (m 3 ·s); λ is the sand production coefficient related to the formation plastic strain; is the local porosity; v w is the water flow velocity, with the unit of m / s; ρ s is the density, with the unit of kg / (m 3 );

[0021] In the formula, the sand production coefficient λ related to the formation plastic strain is shown in formula (2):

[0022]

[0023] where is the critical equivalent plastic strain, ε p is the equivalent plastic strain, ε p is expressed by formula (3):

[0024]

[0025] where ε pi (i = 1, 2, 3) are the plastic strains in three principal directions.

[0026] Furthermore, the corrected solute transport equation in step 1.1 is shown in formula (4):

[0027]

[0028] where c is the volume fraction of liquefied sand; v w is the water flow velocity, with the unit of m / s; t is the time, with the unit of s; m ssi is the mass of solid particle shedding, with the unit of kg / (m 3 ); ρ s is the density, with the unit of kg / (m 3 ).

[0029] Furthermore, the sand production amount and sand production rate equations in step 1.1 are shown in formulas (5) and (6):

[0030]

[0031]

[0032] where is the volume of skeleton particle shedding at time k, with the unit of m 3 ; V ck is the volume of sand grains contained in the formation at time k, with the unit of m 3 ; is the sand production at time k, with the unit of m 3 ; is the sand production at time k - 1, with the unit of m 3 ; is the sand production rate at time k, with the unit of m 3 / s.

[0033] Furthermore, the coupling relationship between the particle detachment and migration model and the thermal - fluid - stress - phase change coupling model established in step 1.2 is as follows:

[0034] The dynamic change of porosity is expressed by formula (7):

[0035]

[0036] where m ssi is the mass of solid particle detachment, with the unit of kg / (m 3 ); m ssi0 is the initial volume of the complete skeleton without particle detachment, with the unit of kg / (m 3 );

[0037] The strength of the sediment is proportional to the hydrate saturation and the volume of the complete skeleton, as shown in formulas (8), (9), and (10):

[0038]

[0039]

[0040]

[0041] where K SH1 and K SH0 are the bulk moduli when the hydrate saturation is 1 and when there is no hydrate, respectively, with the unit of MPa; G SH and G SH0 are the shear moduli when the hydrate saturation is 1 and when there is no hydrate, respectively, with the unit of MPa; C SH1 and C SH are the cohesive forces when the hydrate saturation is 1 and when there is no hydrate, respectively, with the unit of MPa; S H is the hydrate saturation.

[0042] The effect of the present invention is that, compared with the prior art, the present invention can, during the development of natural gas hydrate resources, apply a sand production model for hydrate mining, establish a reservoir model of natural gas hydrate reservoirs in the target area based on on-site geological exploration data, calculate the characteristics of sand grain shedding and migration, sand production volume, and sand production rate in the reservoir containing hydrates during the trial production process, realize the quantitative prediction of the sand production scale of the reservoir during the production process, and prove the effectiveness and practicability of a method for predicting the sand production scale during hydrate mining. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 It is a flowchart of a method for predicting the sand production scale during hydrate mining in an embodiment of the present invention.

[0044] Figure 2 It is a coupling relationship diagram between a thermal-fluid-mechanical-phase change coupling model and a solid particle shedding and migration model and a sand production model for hydrate mining in an embodiment of the present invention.

[0045] Figure 3 It is an example diagram of the reservoir model selected in an embodiment of the present invention.

[0046] Figure 4 It is an example diagram of grid division in an embodiment of the present invention.

[0047] Figure 5 It is a gas production and sand production verification curve diagram in an embodiment of the present invention.

[0048] Figure 6 It is a schematic diagram of the evolution of the shedding volume fraction of framework particles in an embodiment of the present invention.

[0049] Figure 7 It is a schematic diagram of the evolution of the liquefied sand volume fraction in the reservoir in an embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0050] In order 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 embodiments.

[0051] Embodiment 1

[0052] In this embodiment, a method for predicting the sand production scale during hydrate mining is provided, as Figure 1 , and specifically includes the following steps:

[0053] S1: Establish a particle detachment model for hydrate-bearing formations based on the fluid erosion criterion and the critical plastic strain criterion; describe the migration process of liquefied sand in the formation according to the modified solute transport equation (also known as the convection-diffusion equation); then obtain the sand production amount and sand production rate at different times according to the sand production amount and sand production rate calculation equations, and comprehensively obtain the solid particle detachment and migration model, as follows:

[0054] Establish a particle detachment model for hydrate-bearing formations, as shown in formula (1):

[0055]

[0056] In the formula, m ssi is the mass of solid particle detachment, with the unit of kg / (m 3 ); t is time, with the unit of s; is the mass of solid particle detachment per unit time, with the unit of kg / (m 3 ·s); λ is the sand production coefficient related to the formation plastic strain; is the local porosity; v w is the water flow velocity, with the unit of m / s; ρ s is the density, with the unit of kg / (m 3 );

[0057] In the formula, the sand production coefficient λ related to the formation plastic strain is specifically expressed as shown in formula (2):

[0058]

[0059] In the formula is the critical equivalent plastic strain, ε p is the equivalent plastic strain, ε p is expressed by formula (3):

[0060]

[0061] In the formula, ε pi (i = 1, 2, 3) are the plastic strains in three principal directions.

[0062] The modified solute transport equation (also known as the convection-diffusion equation) describes the migration process of liquefied sand in the formation, as shown in formula (4):

[0063]

[0064] In the formula, c is the volume fraction of liquefied sand; v w is the water flow velocity, with the unit of m / s; t is time, with the unit of s; m ssi is the mass of solid particle detachment, with the unit of kg / (m 3 ); ρ sis the density, with the unit of kg / (m 3 ).

[0065] The specific calculation processes of the sand production volume and sand production rate are shown in Formulas (5) and (6) as follows:

[0066]

[0067]

[0068] In the formula, is the volume of the framework particles shed at time k, with the unit of m 3 ; V c k is the volume of sand grains contained in the formation at time k, with the unit of m 3 ; is the sand production volume at time k, with the unit of m 3 ; is the sand production volume at time k-1, with the unit of m 3 ; is the sand production rate at time k, with the unit of m 3 / s.

[0069] The solid particle shedding and migration model can calculate the sand production volume and sand production rate, and simultaneously quantitatively describe the evolution of the shedding of framework particles and the migration evolution process of liquefied sand grains.

[0070] S2: The shedding of framework particles in the hydrate-bearing formation and the migration of liquefied sand in the pores will change the mechanical strength and porosity of the sediment. According to the dynamic changes of the sediment mechanical parameters (bulk modulus, shear modulus, cohesion, etc.) and porosity, the coupling relationship between the solid particle shedding and migration model and the thermal-fluid-mechanical-phase change coupling model is established as follows:

[0071] Porosity The dynamic change is expressed by Formula (7):

[0072]

[0073] In the formula, m ssi is the mass of the solid particle shedding, with the unit of kg / (m 3 ); m ssi0 is the initial volume of the complete framework without particle detachment, kg / (m 3 );

[0074] The strength of the sediment is considered to be proportional to the hydrate saturation and the volume of the complete framework, as shown in Formulas (8), (9), and (10):

[0075]

[0076]

[0077]

[0078] where K SH1 and K SH0 are the bulk moduli at hydrate saturation of 1 and without hydrates, respectively, in MPa. G SH1 and G SH0 are the shear moduli at hydrate saturation of 1 and without hydrates, respectively, in MPa. C SH1 and C SH0 are the cohesive forces at hydrate saturation of 1 and without hydrates, respectively, in MPa; S H is the hydrate saturation.

[0079] The thermal-fluid-mechanical-phase change coupling model affects the solid particle detachment and migration model through the water flow velocity and plastic strain; the solid particle detachment and migration model affects the thermal-fluid-mechanical-phase change coupling model through porosity, permeability, and mechanical strength.

[0080] The models and equations constructed in S1 to S2 constitute the field-scale hydrate production sanding model. The coupling relationship between the thermal-fluid-mechanical-phase change coupling model and the solid particle detachment and migration model, and the finally established hydrate production sanding model are as Figure 2 shown.

[0081] S3: Using the PDE module and solid mechanics module built in COMSOL, input the hydrate production sanding model to construct a field-scale numerical simulation platform for hydrate production sanding.

[0082] S4: Establish a reservoir model for natural gas hydrate reservoir in the simulated target area and perform initial boundary condition setting and grid division.

[0083] Select the Nankai Trough hydrate reservoir in Japan as the numerical simulation object, and quantitatively analyze the potential sanding areas and sanding scales during the 2013 offshore production test. It should be noted that the selection of the simulation object is not fixed.

[0084] The reservoir model selected in this embodiment is as Figure 3 shown. The total thickness of the simulated target area is 368 m, including a 62-m-thick hydrate reservoir, an overlying formation with a thickness of 276 m, and an underlying formation with a thickness of 30 m. The hydrate-bearing reservoir is located 276 m to 338 m below the sea floor. Based on the lithology distribution of the hydrate-bearing layer, the hydrate reservoir is divided into three layers with thicknesses of 14 m, 15 m, and 33 m, respectively. Considering the short actual production test time (6 days) and the limited influence range of pressure reduction, 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.

[0085] The upper, lower, and right boundaries of the model maintain constant temperature and pressure during the hydrate production process. The upper boundary carrying seawater can move freely, there is no displacement perpendicular to the boundary direction at the side boundaries, and the lower boundary is a fixed constraint. The entire simulated target area is divided into 12,200 grids: radially, the area is divided into 100 grids, and the grids near the pressure reduction port are densified; axially, the hydrate-bearing layer is evenly divided into 62 grids, and the underlying layer and overlying layer are divided into 50 grids and 10 grids respectively, as Figure 4 shown.

[0086] Table 1 Main physical properties of the reservoir

[0087]

[0088] S5: Model calculation and result output: As Figure 5 shown, the cumulative gas production in the numerical simulation for 6 days is 1.17×10 5 m 3 , which is 2.1% lower than the total gas production during the pilot production period (1.2×10 5 m 3 ); the cumulative sand production in the simulation for 6 days is 26.7 m 3 , which is close to the sand production of 27 m 3 during the pilot production, indicating that the established model can effectively reflect the first pilot production process of Nankai.

[0089] Figure 6 is the schematic diagram of the evolution of the volume fraction of skeleton particle shedding, Figure 7 is the schematic diagram of the evolution of the volume fraction of liquefied sand in the reservoir. It can be seen from the numerical simulation results that the upper surface area of the hydrate-bearing layer HBL1 is the position where solid particle shedding is more likely to occur. It is recommended to arrange the sand control device in this area to prevent a large amount of sand production in the wellbore.

[0090] In this article, specific examples are used to elaborate on 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, based on the idea of the present invention, there will be changes in the specific implementation and application scope. In summary, the content of this specification should not be construed as a limitation of the present invention.

Claims

1. A method for predicting the sand production scale of a reservoir during the hydrate exploitation process, characterized in that, It includes the following steps: Step 1: Establish the coupling relationship between the solid particle shedding and migration model and the thermal-fluid-mechanical-phase change coupling model, establish the sand production model for hydrate exploitation, and construct the numerical simulation platform for sand production during hydrate exploitation at the site scale, which specifically includes the following sub-steps: Step 1.1: Establish the particle shedding model for the hydrate-bearing formation according to the fluid erosion criterion and the critical plastic strain criterion, and combine the modified solute transport equation with the sand production volume and sand production rate equations to obtain the solid particle shedding and migration model; Step 1.2: Establish the coupling relationship between the solid particle shedding and migration model and the thermal-fluid-mechanical-phase change coupling model based on the dynamic changes of sediment mechanical parameters and porosity, where the sediment mechanical parameters include bulk modulus, shear modulus, and cohesion, to obtain the sand production model for hydrate exploitation; Step 1.3: Construct the numerical simulation platform for sand production during hydrate exploitation at the site scale according to the sand production model for hydrate exploitation; Step 2: Conduct a quantitative analysis of the sand production behavior in the target area and predict the sand production scale of the reservoir during hydrate exploitation, which specifically includes the following sub-steps: Step 2.1: Based on the geological conditions and exploitation conditions of 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 properties parameters of the reservoir of the reservoir model after grid division into the numerical simulation platform for sand production during hydrate exploitation at the site scale for calculation and solution, and quantitatively analyze the sand production behavior based on the calculation results to predict the sand production scale of the reservoir during hydrate exploitation; Among them, the modified solute transport equation in Step 1.1 is specifically expressed as: wherein is the volume fraction of liquefied sand; is the water flow velocity, with the unit of ; is the time, with the unit of ; is the mass of solid particle detachment, with the unit of ; is the density, with the unit of .

2. The prediction method for the sand production scale of a reservoir during the hydrate exploitation process according to claim 1, wherein The particle shedding model in Step 1.1 is specifically expressed as: Among them, is the mass of solid particle shedding, with the unit of ; is time, with the unit of ; is the mass of solid particle shedding per unit time, with the unit of ; is the sand production coefficient related to the plastic strain of the formation; is the local porosity; is the water flow velocity, with the unit of ; is the density, with the unit of ; The sand production coefficient related to the formation plastic strain in the formula is expressed as: wherein is the critical equivalent plastic strain, is the equivalent plastic strain: wherein are the plastic strains in three main directions.

3. The prediction method for the sand production scale of a reservoir during the hydrate exploitation process according to claim 1, characterized in that, The sand production volume and sand production rate equations in Step 1.1 are specifically expressed as: Among them is the volume of the shedding of the framework particles at the moment, with the unit of ; is the volume of sand grains contained in the formation at the moment, with the unit of ; is the sand production rate at the moment, with the unit of ; is the sand production rate at the moment, with the unit of ; is the sand production rate at the moment, with the unit of .

4. The prediction method for the sand production scale of a reservoir during the hydrate exploitation process according to claim 1, characterized in that, In Step 1.2, the process of establishing the coupling relationship between the particle shedding and migration model and the thermal-fluid-mechanical-phase change coupling model is as follows: Porosity The dynamic change of which is expressed as: Among them, is the mass of solid particle shedding, with the unit of ; is the initial volume of the complete skeleton without particle detachment, with the unit of ; The strength of the sediment is proportional to the hydrate saturation and the volume of the intact skeleton, and is expressed as: wherein, and are the bulk moduli at hydrate saturations of 1 and without hydrates respectively, with the unit of MPa; and are the shear moduli at hydrate saturations of 1 and without hydrates respectively, with the unit of MPa; and are the cohesion forces at hydrate saturations of 1 and without hydrates respectively, with the unit of MPa; is the hydrate saturation.

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