A method for predicting fluid production behavior after hydraulic fracturing of shale oil

Abstract

The present application belongs to the field of unconventional oil and gas resource development, and proposes a method for predicting fluid production behavior after shale oil hydraulic fracturing, which comprises: based on the fluid characteristics in the shale oil reservoir hydraulic fracture, the mass conservation equation of fluid flow, the energy conservation equation of fluid flow and the momentum conservation equation of fluid flow, a heat and flow model of fluid nonlinear seepage behavior in the shale oil reservoir hydraulic fracture is established; based on the real geological data of the shale oil reservoir, a reservoir model is constructed, the initial conditions and boundary conditions of the reservoir model are set, and the reservoir model is divided into grids; the reservoir model is input into the heat and flow model to obtain the spatio-temporal evolution results of fluid pressure and saturation in the shale oil reservoir recovery process, and the fluid production behavior after shale oil hydraulic fracturing is predicted according to the spatio-temporal evolution results of fluid pressure and saturation. The present application can effectively capture the transient change of fluid pressure and the accurate characteristics of fluid saturation distribution, and significantly improve the accuracy of prediction.

Description

TECHNICAL FIELD

[0001] The application 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

[0002] As an important unconventional energy, shale oil has significant differences in reservoir characteristics compared with 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 efficiency of shale oil, hydraulic fracturing technology is widely used to form high-conductivity fracture channels in the reservoir and enhance the permeability of fluids. 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 production process of shale oil.

[0003] In the development process of shale oil, the fracture seepage characteristics are particularly important. Due to the high flow rate and complex flow path 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 consider the high-speed flow characteristics of fluids in the fracture and the possible non-Darcy effect, so as to more accurately describe the motion behavior of fluids in the fracture network. This not only helps to optimize the design of hydraulic fracturing, but also improves the recovery efficiency and production efficiency of shale oil, which has important significance for deepening the understanding of the flow mechanism of shale oil reservoirs and improving the development effect. SUMMARY

[0004] To solve the problem of large errors in production prediction and actual production in the development process of existing shale oil, a method for predicting fluid production behavior after shale oil hydraulic fracturing is proposed.

[0005] The application provides a method for predicting fluid production behavior after shale oil hydraulic fracturing, which comprises the following steps:

[0006] S1. Based on the fluid characteristics in the shale oil reservoir hydraulic fracture, 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 fluid nonlinear seepage behavior in the shale oil reservoir hydraulic fracture is established;

[0007] S2. Based on the real geological data of the shale oil reservoir, a reservoir model is constructed, the initial conditions and boundary conditions of the reservoir model are set, and the reservoir model is divided into grids;

[0008] S3. inputting the reservoir model built in step S2 into the heat transfer and flow model obtained in step S1 to obtain the spatio-temporal evolution results of fluid pressure and saturation in the shale oil reservoir recovery process, and predicting the fluid production behavior after the shale oil hydraulic fracturing according to the spatio-temporal evolution results of fluid pressure and saturation.

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

[0010] S101. correcting the Darcy law by fluid characteristics in the shale oil reservoir hydraulic fracture, 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] Wherein, u o represents the flow velocity of the oil phase, k represents the permeability of the formation, k ro represents the relative permeability of the oil phase, μ o represents 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 of gravity, and β represents the inertial resistance coefficient;

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

[0015]

[0016] Wherein, 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. constructing the mass conservation equation of the oil phase fluid, as shown in formula (3):

[0018]

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

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

[0021]

[0022] wherein S w represents the saturation of the oil phase and the water phase;

[0023] S103. The energy conservation equation in the fractured formation is constructed, as shown in equation (5):

[0024]

[0025] wherein p r represents the density of the formation solid skeleton, c r represents the specific heat of the formation solid skeleton, c o represents the specific heat of the oil phase, c w represents the specific heat of the water phase, and T represents the temperature.

[0026]

[0027] wherein l r represents the thermal conductivity of the formation solid skeleton, l w represents the thermal conductivity of the water phase, and l o represents the thermal conductivity of the water phase.

[0028] According to the method for predicting the fluid production behavior after the hydraulic fracturing of shale oil, in step S2, the reservoir model comprises a shale oil reservoir and a hydraulic fracture.

[0029] According to the method for predicting the fluid production behavior after the hydraulic fracturing of shale oil, in step S2, the height of the shale oil reservoir and the length of the shale oil reservoir in the horizontal direction are further set.

[0030] According to the method for predicting the fluid production behavior after the hydraulic fracturing of shale oil, in step S2, the horizontal length of the hydraulic fracture, the width of the hydraulic fracture, and the number of fracture strips of the hydraulic fracture are further set.

[0031] According to the method for predicting the fluid production behavior after the hydraulic fracturing of shale oil, in step S2, setting the initial conditions of the reservoir model comprises setting the saturation of shale oil, the saturation of water, the initial pressure, the initial temperature, and the production mode in the reservoir model.

[0032] According to the method for predicting fluid production behavior after shale oil hydraulic fracturing, the step S2 comprises: setting the boundary conditions of the reservoir model, including setting the left boundary pressure, the upper boundary pressure, the lower boundary pressure, the right boundary pressure, the upper boundary temperature, the lower boundary temperature and the right boundary temperature of the reservoir model.

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

[0034] The method for predicting fluid production behavior after shale oil hydraulic fracturing can predict the fluid production behavior after shale oil hydraulic fracturing according to the spatiotemporal evolution of fluid pressure and saturation. Compared with the prior art, the method can more truly reflect the actual flow state of fluid under complex geological conditions after hydraulic fracturing by considering the nonlinear seepage behavior of fluid in the high-conductivity fracture channel formed by hydraulic fracturing, effectively capture the transient changes of fluid pressure and the accurate characteristics of fluid saturation distribution, and thus significantly improve the prediction accuracy. In addition, the method can consider the differences in geological characteristics of different shale reservoirs and can be widely applied to the prediction of fluid production behavior after hydraulic fracturing of various types of shale reservoirs, thereby providing an effective method for solving the technical problems in shale oil exploitation. BRIEF DESCRIPTION OF DRAWINGS

[0035] Figure 1 FIG. 1 is a flowchart of the method for predicting fluid production behavior after shale oil hydraulic fracturing according to the present application;

[0036] Figure 2 FIG. 2 is a schematic diagram of grid division of the reservoir model simulation domain used in the example of embodiment 2 of the present application;

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

[0038] The embodiments of the present application will be further described in detail below with reference to the accompanying drawings and examples. The following examples are used to illustrate the present application, but cannot be used to limit the scope of the present application.

[0039] Embodiment 1 provides a method for predicting fluid production behavior after shale oil hydraulic fracturing, as shown in FIG. 1, comprising the following steps: Figure 1

[0040] ​S1. based on the fluid characteristics in the shale oil reservoir hydraulic fracture, 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 fluid nonlinear seepage behavior in the shale oil reservoir hydraulic fracture is established;

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

[0042] S3. the reservoir model constructed in step S2 is input into the heat transfer and flow model obtained in step S1, the space-time evolution results of fluid pressure and saturation in the shale oil reservoir recovery process are obtained, and the fluid production behavior after shale oil hydraulic fracturing is predicted according to the space-time evolution results of fluid pressure and saturation.

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

[0044] S1. based on the fluid characteristics in the shale oil reservoir hydraulic fracture, 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 fluid nonlinear seepage behavior in the shale oil reservoir hydraulic fracture is established;

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

[0046] S101. the Darcy law is modified by the fluid characteristics in the shale oil reservoir hydraulic fracture, the flow velocity u o As shown in formula (1):

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

[0048]

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

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

[0051]

[0052] Wherein, 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] wherein, represents the porosity, S o represents the saturation of the oil phase, t represents time;

[0056] Construct the mass conservation equation of the water phase fluid, as shown in formula (4):

[0057]

[0058] wherein, S w represents the saturation of the oil phase and the water phase;

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

[0060]

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

[0062]

[0063] wherein, λ r represents the thermal conductivity of the formation solid skeleton, 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 real geological data of a shale oil reservoir, set initial conditions and boundary conditions of the reservoir model, and divide the reservoir model into grids;

[0065] Specifically, setting the initial conditions of the reservoir model includes: setting the shale oil saturation, water saturation, initial pressure, initial temperature and production mode 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 reservoir and hydraulic fracture; also includes setting the height of the shale oil reservoir and the length of the shale oil reservoir in the horizontal direction; setting the horizontal length of the hydraulic fracture, the width of the hydraulic fracture and the number of fracture strips of the hydraulic fracture. The grid division adopts unstructured triangular grid for grid division. In this embodiment, as shown in Figure 2 the shale oil reservoir is 1000m, the horizontal length of the shale oil 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 fracture strips of the hydraulic fracture is 9. In the initial state, the shale oil saturation in the whole reservoir model is 0.3, and the water saturation is 0.7. The initial pressure is 15Mpa, the initial temperature is 388K, the mode of pressure reduction is used, 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 by using unstructured triangular grid.

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

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

[0068] The embodiments of the present application are given for illustration and description only, and are not exhaustive or limiting of the present application. Many modifications and variations will be apparent to those of ordinary skill in the art. The embodiments are chosen and described in order to best explain the principles of the present application and its practical application, and to enable others skilled in the art to understand the present application for various embodiments with various modifications as are suited to the particular use contemplated.

Claims

1. A method for predicting fluid production behavior after hydraulic fracturing in shale oil, characterized in that, Includes the following steps: S1. Based on the fluid characteristics, mass conservation equation, energy conservation equation, and momentum conservation equation of fluid flow in hydraulic fractures of shale oil reservoirs, a heat transfer and flow model for nonlinear seepage behavior of fluid in hydraulic fractures of shale oil reservoirs is established. S2. Construct a reservoir model based on real geological data of shale oil reservoirs, set the initial conditions and boundary conditions of the reservoir model, and perform grid division on 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 production, and predict the fluid production behavior after hydraulic fracturing of shale oil based on the spatiotemporal evolution results of fluid pressure and saturation. Step S1 includes the following steps: S101. Darcy's law is modified based on the fluid characteristics in hydraulic fractures of shale oil reservoirs, and the flow velocity of the oil phase is... As shown in formula (1): As shown in formulas (1) and (2): （1） in, Indicates the flow rate of the oil phase. Indicates the permeability of the formation. Indicates the relative permeability of the oil phase. Indicates the viscosity of the oil phase. Indicates the pressure of the oil phase. Represents the gradient operator. This indicates the density of the oil phase. Represents gravitational acceleration. Indicates the coefficient of inertial drag; Flow velocity of the water phase As shown in formula (2): （2） in, Indicates the flow velocity of the water phase. Indicates the viscosity of the aqueous phase. This indicates the relative permeability of the aqueous phase. Indicates the pressure of the aqueous phase. Indicates the pressure change in the aqueous phase. This indicates the density of the aqueous phase; S102. Construct the mass conservation equation for the oil phase fluid, as shown in formula (3): （3） in, Indicates porosity. Indicates the saturation of the oil phase. Indicates time; The mass conservation equation for the aqueous fluid is constructed as shown in equation (4): （4） in, Indicates the saturation of the oil and water phases; S103. Construct the energy conservation equation for fractured strata, as shown in formula (5): （5） in, The density represents the solid skeleton of the formation. Indicates the specific heat of the solid skeleton of the formation. Indicates the specific heat of the oil phase. Indicates the specific heat of the aqueous phase. Indicates temperature. Thermal conductivity represents the thermal conductivity. As shown in formula (6): （6） in, The thermal conductivity of the solid skeleton of the formation. This represents the thermal conductivity of the aqueous phase. This represents the thermal conductivity of the aqueous phase.

2. The method for predicting fluid production behavior after hydraulic fracturing of shale oil as described in claim 1, characterized in that, In step S2, the reservoir model includes shale oil reservoirs and hydraulic fractures.

3. The method for predicting fluid production behavior after hydraulic fracturing of shale oil as described in claim 2, characterized in that, Step S2 also includes setting the height of the shale oil reservoir and the horizontal length of the shale oil reservoir.

4. The method for predicting fluid production behavior after hydraulic fracturing of shale oil as described in claim 2, characterized in that, Step S2 further includes setting the horizontal length of the hydraulic fracture, the width of the hydraulic fracture, and the number of hydraulic fractures.

5. The method for predicting fluid production behavior after hydraulic fracturing of shale oil as described in claim 1, characterized in that, In 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 extraction method within the reservoir model.

6. The method for predicting fluid production behavior after hydraulic fracturing of shale oil as described in claim 1, characterized in that, 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.

7. The method for predicting fluid production behavior after hydraulic fracturing of shale oil as described in claim 1, characterized in that, In step S2, the mesh division is performed using an unstructured triangular mesh.

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