Method for predicting fluid production behavior after hydraulic fracturing of shale oil

Abstract

The invention belongs to the field of unconventional oil and gas resource development, and provides a method for predicting a fluid production behavior after shale oil hydraulic fracturing, which comprises the following steps of: on the basis of fluid characteristics in shale oil reservoir hydraulic fractures, a mass conservation equation of fluid flow, an energy conservation equation of fluid flow and a momentum conservation equation of fluid flow; establishing a heat transfer and flow model of the fluid nonlinear seepage behavior in the shale oil reservoir hydraulic fracture; the method comprises the following steps: constructing a reservoir model based on shale oil reservoir real geological data, setting initial conditions and boundary conditions of the reservoir model, and performing grid division on the reservoir model; and inputting the reservoir model into the heat transfer and flow model to obtain a spatio-temporal evolution result of fluid pressure and saturation in the shale oil reservoir recovery process, and predicting the fluid production behavior after hydraulic fracturing of shale oil according to the spatio-temporal evolution result of fluid pressure and saturation. The method can effectively capture the transient change of the fluid pressure and the accurate characteristics of the fluid saturation distribution, and significantly improves the prediction accuracy.

Description

Technical Field

[0001] The invention belongs to the field of unconventional oil and gas resource development, and specifically discloses a method for predicting fluid production behavior after shale oil hydraulic fracturing. Background Art

[0002] As an important unconventional energy source, shale oil has reservoir characteristics that are significantly different from conventional oil and gas reservoirs. The pore structure of shale oil reservoirs is complex, with small pores and poor connectivity, which makes the natural seepage capacity of shale oil low. In order to improve the recovery rate of shale oil, hydraulic fracturing technology is widely used to enhance the permeability of fluids by forming high-conductivity fracture channels in the reservoir. However, the flow rate of fluids in these fractures is high, and the traditional Darcy's law cannot accurately describe the actual flow of fluids when calculating the seepage velocity. Therefore, it is urgent to establish a nonlinear seepage model suitable for fracture seepage to more accurately simulate and predict the shale oil extraction process.

[0003] In the development process of shale oil, the fracture seepage characteristics are particularly important. Due to the high flow rate and complex flow paths in the fracture network, the traditional linear seepage model cannot accurately describe the actual situation, resulting in large errors in production prediction and scheme design. Therefore, it is necessary to establish a nonlinear seepage model. The nonlinear seepage model needs to take into account the high-speed flow characteristics of the fluid in the fracture and the possible non-Darcy effect, so as to more accurately describe the movement behavior of the fluid in the fracture network. This not only helps to optimize the hydraulic fracturing design, but also improves the recovery rate and production efficiency of shale oil, which is of great significance for in-depth understanding of the flow mechanism of shale oil reservoirs and improving development effects. Summary of the invention

[0004] In order to solve the problem of large errors between production prediction and actual production in the existing shale oil development process, the present invention proposes a method for predicting the production behavior of fluid after shale oil hydraulic fracturing.

[0005] The present invention provides a method for predicting the fluid production behavior after shale oil hydraulic fracturing, comprising the following steps:

[0006] S1. Based on the fluid characteristics in hydraulic fractures of shale reservoirs, the mass conservation equation of fluid flow, the energy conservation equation of fluid flow, and the momentum conservation equation of fluid flow, a heat transfer and flow model of nonlinear seepage behavior of fluid in hydraulic fractures of shale reservoirs is established;

[0007] S2. constructing a reservoir model based on real geological data of shale oil reservoirs, setting initial conditions and boundary conditions of the reservoir model, and meshing the reservoir model;

[0008] S3. Input the reservoir model constructed in step S2 into the heat transfer and flow model obtained in step S1 to obtain the spatiotemporal evolution results of fluid pressure and saturation during shale oil reservoir recovery, and predict the fluid production behavior after shale oil hydraulic fracturing based on the spatiotemporal evolution results of fluid pressure and saturation.

[0009] According to a method for predicting fluid production behavior after shale oil hydraulic fracturing in some embodiments of the present application, step S1 comprises the following steps:

[0010] S101. The Darcy law is modified by the fluid characteristics in the hydraulic fractures of shale oil reservoirs. The flow velocity u of the oil phase o As shown in formula (1):

[0011] As shown in formula (1) and formula (2):

[0012]

[0013] Among them, u o represents the flow velocity of the oil phase, k represents the permeability of the formation, and k ro Represents the relative permeability of the oil phase, μ o Indicates the viscosity of the oil phase, P o represents the pressure of the oil phase, represents the gradient operator, ρ o represents the density of the oil phase, g represents the acceleration due to gravity, and β represents the inertial resistance coefficient;

[0014] The flow rate of water phase u w As shown in formula (2):

[0015]

[0016] Among them, u w represents the flow velocity of the water phase, μ w represents the viscosity of the water phase, k rw Represents the relative permeability of the water phase, P w represents the pressure of the water phase, represents the pressure change of the water phase, ρ w represents the density of the water phase;

[0017] S102. Construct the mass conservation equation of the oil phase fluid, as shown in formula (3):

[0018]

[0019] in, Represents porosity, S o represents the saturation of the oil phase, and t represents the time;

[0020] The mass conservation equation of the water phase fluid is constructed as shown in formula (4):

[0021]

[0022] Among them, S w Indicates the saturation of oil and water phases;

[0023] S103. Construct the energy conservation equation in the fractured stratum, as shown in formula (5):

[0024]

[0025] Among them, ρ r represents the density of the solid skeleton of the stratum, c r represents the specific heat of the solid skeleton of the formation, c o represents the specific heat of the oil phase, c w represents the specific heat of the water phase, T represents the temperature, λ represents the thermal conductivity, and the thermal conductivity λ is shown in formula (6):

[0026]

[0027] Among them, λ r represents the thermal conductivity of the solid skeleton of the formation, λ w represents the thermal conductivity of the water phase, λ o represents the thermal conductivity of the water phase.

[0028] According to a method for predicting fluid production behavior after shale oil hydraulic fracturing in some embodiments of the present application, in step S2, the reservoir model includes a shale oil reservoir and hydraulic fractures.

[0029] According to a method for predicting fluid production behavior after shale oil hydraulic fracturing in some embodiments of the present application, step S2 also includes setting the height of the shale oil reservoir and the horizontal length of the shale oil reservoir.

[0030] According to a method for predicting fluid production behavior after shale oil hydraulic fracturing in some embodiments of the present application, step S2 also includes setting the horizontal length of the hydraulic fracture, the width of the hydraulic fracture and the number of hydraulic fractures.

[0031] According to a method for predicting fluid production behavior after shale oil hydraulic fracturing in some embodiments of the present application, in step S2, setting the initial conditions of the reservoir model includes: setting the saturation of shale oil, water saturation, initial pressure, initial temperature and production method in the reservoir model.

[0032] According to a method for predicting fluid production behavior after shale oil hydraulic fracturing in some embodiments of the present application, in step S2, setting the boundary conditions of the reservoir model includes: setting the left boundary pressure, upper boundary pressure, lower boundary pressure, right boundary pressure, upper boundary temperature, lower boundary temperature and right boundary temperature of the reservoir model.

[0033] According to a method for predicting fluid production behavior after shale oil hydraulic fracturing in some embodiments of the present application, in step S2, the grid division is performed using an unstructured triangular grid.

[0034] The present invention proposes a method for predicting the production behavior of fluid after hydraulic fracturing of shale oil, which can predict the production behavior of fluid after hydraulic fracturing of shale oil based on the spatiotemporal evolution results of fluid pressure and saturation. Compared with the prior art, the present invention can more truly reflect the actual flow state of the fluid under complex geological conditions after hydraulic fracturing by considering the nonlinear seepage behavior of the fluid in the high-conductivity fracture channel formed by hydraulic fracturing, and effectively capture the transient changes in fluid pressure and the precise characteristics of fluid saturation distribution, thereby significantly improving the accuracy of the prediction. In addition, the present invention can take into account the differences in geological characteristics of different shale reservoirs, and can be widely used in the prediction of fluid production behavior after hydraulic fracturing of various types of shale reservoirs, providing an effective method for solving the technical problems currently faced by shale oil exploitation. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 A schematic flow chart of a method for predicting fluid production behavior after shale oil hydraulic fracturing according to the present invention;

[0036] Figure 2 Schematic diagram of grid division of the reservoir model simulation domain used in Example 2 of the present invention;

[0037] Figure 3 (a) is a schematic diagram of the pore pressure of the shale oil reservoir after 200 days of exploitation calculated in Example 2 of the present invention, (b) is a schematic diagram of the water saturation after 200 days of exploitation calculated in Example 2 of the present invention, and (c) is a schematic diagram of the shale oil saturation after 200 days of exploitation calculated in Example 2 of the present invention. DETAILED DESCRIPTION

[0038] The following embodiments of the present invention are described in further detail in conjunction with the accompanying drawings and examples. The following examples are used to illustrate the present invention, but are not intended to limit the scope of the present invention.

[0039] Example 1: This example provides a method for predicting the production behavior of fluid after shale oil hydraulic fracturing. Figure 1 As shown, the following steps are included:

[0040] S1. Based on the fluid characteristics in hydraulic fractures of shale reservoirs, the mass conservation equation of fluid flow, the energy conservation equation of fluid flow, and the momentum conservation equation of fluid flow, a heat transfer and flow model of nonlinear seepage behavior of fluid in hydraulic fractures of shale reservoirs is established;

[0041] S2. Construct a reservoir model based on the real geological data of the shale oil reservoir, set the initial conditions and boundary conditions of the reservoir model, and grid the reservoir model;

[0042] S3. Input the reservoir model constructed in step S2 into the heat transfer and flow model obtained in step S1 to obtain the spatiotemporal evolution results of fluid pressure and saturation during shale oil reservoir recovery, and predict the fluid production behavior after shale oil hydraulic fracturing based on the spatiotemporal evolution results of fluid pressure and saturation.

[0043] Example 2: This example provides a method for predicting the production behavior of fluid after shale oil hydraulic fracturing, comprising the following steps:

[0044] S1. Based on the fluid characteristics in hydraulic fractures of shale reservoirs, the mass conservation equation of fluid flow, the energy conservation equation of fluid flow, and the momentum conservation equation of fluid flow, a heat transfer and flow model of nonlinear seepage behavior of fluid in hydraulic fractures of shale reservoirs is established;

[0045] Specifically, step S1 includes the following steps:

[0046] S101. The Darcy law is modified by the fluid characteristics in the hydraulic fractures of shale oil reservoirs. The flow velocity u of the oil phase o As shown in formula (1):

[0047] As shown in formula (1) and formula (2):

[0048]

[0049] Among them, u o represents the flow velocity of the oil phase, k represents the permeability of the formation, and k ro Represents the relative permeability of the oil phase, μ o Indicates the viscosity of the oil phase, P o represents the pressure of the oil phase, represents the gradient operator, ρ o represents the density of the oil phase, g represents the acceleration due to gravity, and β represents the inertial resistance coefficient;

[0050] The flow rate of water phase u w As shown in formula (2):

[0051]

[0052] Among them, uw represents the flow velocity of the water phase, μ w represents the viscosity of the water phase, k rw Represents the relative permeability of the water phase, P w represents the pressure of the water phase, represents the pressure change of the water phase, ρ w represents the density of the water phase;

[0053] S102. Construct the mass conservation equation of the oil phase fluid, as shown in formula (3):

[0054]

[0055] in, Represents porosity, S o represents the saturation of the oil phase, and t represents the time;

[0056] The mass conservation equation of the water phase fluid is constructed as shown in formula (4):

[0057]

[0058] Among them, S w Indicates the saturation of oil and water phases;

[0059] S103. Construct the energy conservation equation in the fractured stratum, as shown in formula (5):

[0060]

[0061] Among them, ρ r represents the density of the solid skeleton of the stratum, c r represents the specific heat of the solid skeleton of the formation, c o represents the specific heat of the oil phase, c w represents the specific heat of the water phase, T represents the temperature, λ represents the thermal conductivity, and the thermal conductivity λ is shown in formula (6):

[0062]

[0063] Among them, λ r represents the thermal conductivity of the solid skeleton of the formation, λ w represents the thermal conductivity of the water phase, λ o represents the thermal conductivity of the water phase;

[0064] S2. Construct a reservoir model based on the real geological data of the shale oil reservoir, set the initial conditions and boundary conditions of the reservoir model, and grid the reservoir model;

[0065] Specifically, setting the initial conditions of the reservoir model includes: setting the saturation of shale oil, water saturation, initial pressure, initial temperature and mining method in the reservoir model; setting the boundary conditions of the reservoir model includes: setting the left boundary pressure, upper boundary pressure, lower boundary pressure, right boundary pressure, upper boundary temperature, lower boundary temperature and right boundary temperature of the reservoir model. The reservoir model includes shale oil reservoirs and hydraulic fractures; it also includes setting the height of the shale oil reservoir and the horizontal length of the shale oil reservoir; setting the horizontal length of the hydraulic fractures, the width of the hydraulic fractures and the number of hydraulic fractures. Meshing is performed using unstructured triangular meshes. In this embodiment, if Figure 2 As shown in the figure, the height of the shale reservoir is 1000m, the horizontal length of the shale reservoir is set to 3000m, the horizontal length of the hydraulic fracture is 500m, the width of the hydraulic fracture is 0.01m, and the number of hydraulic fractures is 9. In the initial state, the saturation of shale oil in the entire reservoir model is 0.3, and the saturation of water is 0.7. The initial pressure is 15Mpa, the initial temperature is 388K, and the pressure reduction mining method is adopted. The left boundary pressure of the simulation domain of the reservoir model is 10MPa, the upper boundary pressure is 15Mpa, the lower boundary pressure is 15Mpa, the right boundary pressure is 15Mpa, the upper boundary temperature is 388K, the lower boundary temperature is 388K, and the right boundary temperature is 388K. The simulation domain of the reservoir model is divided into 7878 grids using unstructured triangular grids.

[0066] S3. Input the reservoir model constructed in step S2 into the heat transfer and flow model obtained in step S1 to obtain the spatiotemporal evolution results of fluid pressure and saturation during shale oil reservoir recovery, and predict the fluid production behavior after shale oil hydraulic fracturing based on the spatiotemporal evolution results of fluid pressure and saturation.

[0067] In this embodiment, if Figure 3 As shown in the figure, (a) is a schematic diagram of the pore pressure of the shale reservoir calculated after 200 days of production, (b) is a schematic diagram of the water saturation calculated after 200 days of production, and (c) is a schematic diagram of the shale oil saturation calculated after 200 days of production. As shown in the figure, the horizontal front of the low pressure propagation exceeds 500m of the fracture length, and the maximum reduction in shale oil saturation reaches 20%, achieving an effective characterization of fluid production behavior.

[0068] The embodiments of the present invention are given for the purpose of illustration and description, and are not intended to be exhaustive or to limit the invention to the disclosed forms. Many modifications and variations will be apparent to those of ordinary skill in the art. The embodiments are selected and described in order to better illustrate the principles and practical applications of the present invention and to enable those of ordinary skill in the art to understand the present invention and thereby design various embodiments with various modifications suitable for specific uses.

Claims

1. A method for predicting fluid production behavior after shale oil hydraulic fracturing, characterized in that: The steps include: S1. Based on the fluid characteristics in hydraulic fractures of shale reservoirs, the mass conservation equation of fluid flow, the energy conservation equation of fluid flow, and the momentum conservation equation of fluid flow, a heat transfer and flow model of nonlinear seepage behavior of fluid in hydraulic fractures of shale reservoirs is established; S2. constructing a reservoir model based on real geological data of shale oil reservoirs, setting initial conditions and boundary conditions of the reservoir model, and meshing the reservoir model; S3. Input the reservoir model constructed in step S2 into the heat transfer and flow model obtained in step S1 to obtain the spatiotemporal evolution results of fluid pressure and saturation during shale oil reservoir recovery, and predict the fluid production behavior after shale oil hydraulic fracturing based on the spatiotemporal evolution results of fluid pressure and saturation.

2. The method for predicting fluid production behavior after shale oil hydraulic fracturing according to claim 1, characterized in that: The step S1 comprises the following steps: S101. The Darcy law is modified by the fluid characteristics in the hydraulic fractures of shale oil reservoirs. The flow velocity u of the oil phase o As shown in formula (1): As shown in formula (1) and formula (2): Among them, u o represents the flow velocity of the oil phase, k represents the permeability of the formation, and k ro Represents the relative permeability of the oil phase, μ o Indicates the viscosity of the oil phase, P o represents the pressure of the oil phase, represents the gradient operator, ρ o represents the density of the oil phase, g represents the acceleration due to gravity, and β represents the inertial resistance coefficient; The flow rate of water phase u w As shown in formula (2): Among them, u w represents the flow velocity of the water phase, μ w represents the viscosity of the water phase, k rw Represents the relative permeability of the water phase, P w represents the pressure of the water phase, represents the pressure change of the water phase, ρ w represents the density of the water phase; S102. Construct the mass conservation equation of the oil phase fluid, as shown in formula (3): in, Represents porosity, S o represents the saturation of the oil phase, and t represents the time; The mass conservation equation of the water phase fluid is constructed as shown in formula (4): Among them, S w Indicates the saturation of oil and water phases; S103. Construct the energy conservation equation in the fractured stratum, as shown in formula (5): Among them, ρ r represents the density of the solid skeleton of the stratum, c r represents the specific heat of the solid skeleton of the formation, c o represents the specific heat of the oil phase, c w represents the specific heat of the water phase, T represents the temperature, λ represents the thermal conductivity, and the thermal conductivity λ is shown in formula (6): Among them, λ r represents the thermal conductivity of the solid skeleton of the formation, λ w represents the thermal conductivity of the water phase, λ o represents the thermal conductivity of the water phase.

3. The method for predicting fluid production behavior after shale oil hydraulic fracturing according to claim 1, characterized in that: In step S2, the reservoir model includes shale oil reservoirs and hydraulic fractures.

4. The method for predicting fluid production behavior after shale oil hydraulic fracturing according to claim 3, characterized in that: The step S2 also includes setting the height of the shale oil reservoir and the horizontal length of the shale oil reservoir.

5. The method for predicting fluid production behavior after shale oil hydraulic fracturing according to claim 3, characterized in that: The step S2 also includes setting the horizontal length of the hydraulic fracture, the width of the hydraulic fracture and the number of hydraulic fractures.

6. The method for predicting fluid production behavior after shale oil hydraulic fracturing according to claim 1, characterized in that: In the step S2, setting the initial conditions of the reservoir model includes: setting the saturation of shale oil, the saturation of water, the initial pressure, the initial temperature and the production method in the reservoir model.

7. The method for predicting fluid production behavior after shale oil hydraulic fracturing according to claim 1, characterized in that: In the step S2, setting the boundary conditions of the reservoir model includes: setting the left boundary pressure, upper boundary pressure, lower boundary pressure, right boundary pressure, upper boundary temperature, lower boundary temperature and right boundary temperature of the reservoir model.

8. The method for predicting fluid production behavior after shale oil hydraulic fracturing according to claim 1, characterized in that: In the step S2, the mesh division is performed using an unstructured triangular mesh.

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