Shale oil-gas phase balance and flow capacity characterization method under water-containing condition

Abstract

The invention relates to the technical field of oil and gas development, and discloses a shale oil and gas phase balance and flow capacity characterization method under a water-containing condition, which comprises the following steps of: considering a water saturation condition, and obtaining a relation curve of oil and gas capillary force along with the change of gas saturation by utilizing rock fractal dimension and pore size and combining with the water saturation; an oil-gas phase balance model is established based on the curve, the gas saturation under the given temperature and pressure is solved, then the oil-gas capillary force is reversely solved, and distribution of oil gas in pores of different sizes is determined; meanwhile, pore size distribution is determined based on fractal dimensions, and the maximum value of the saturated water-containing pore size is calculated under the given water saturation; determining that the water phase is endowed with pores in a specific range of pore diameters, and the shale oil is endowed with pores in another specific range; the problem that an existing oil-gas phase change and flow capacity prediction technology cannot be applied to shale oil exploitation scenes related to hydraulic fracturing is solved.

Description

Technical Field

[0001] The present invention relates to the technical field of oil and gas development, and in particular to a method for characterizing shale oil and gas phase equilibrium and flow capacity under water-containing conditions. Background Art

[0002] During underground oil and gas production, the pore pressure within the reservoir gradually decreases as oil and gas are continuously extracted. This change in pore pressure significantly affects the phase state of the oil and gas, leading to phase transitions. Accurately predicting the phase transitions and flow capacity of oil and gas at different pore pressures is crucial for efficient oilfield development.

[0003] Chinese patent publication number CN 114252382 A, "A Method for Characterizing Oil-Gas Phase Equilibrium and Flowability in Porous Rock Media," is a technology for characterizing oil-gas phase transitions and flowability. This technology primarily analyzes the oil-gas phase equilibrium state in porous rock media and, combining relevant theoretical and experimental data, establishes a system for characterizing oil-gas phase equilibrium and flowability.

[0004] This method comprehensively considers the pore structure of porous rock media, the physical properties of oil and gas, and the impact of environmental factors such as temperature and pressure on oil and gas phase equilibrium and flow capacity. By establishing mathematical models and calculation methods, it can accurately predict the phase changes and flow characteristics of oil and gas under different conditions, providing technical support for oilfield development decisions.

[0005] However, hydraulic fracturing is currently widely used in shale oil production to increase oil and gas recovery. During hydraulic fracturing, water is injected into the pores of the reservoir, occupying a certain amount of pore volume. This process significantly alters the physical environment of the reservoir, significantly affecting the phase transitions of the oil and gas.

[0006] While existing technologies can characterize the phase equilibrium and flow capacity of oil and gas to a certain extent, they do not fully consider the situation under water saturation. Due to the differences in the physical properties and interactions between water and oil and gas, water's occupation of pore volume alters the distribution and flow of oil and gas within the pores, thereby affecting the phase transition process of the oil and gas. As a result, when applied to scenarios involving hydraulic fracturing, such as shale oil production, existing technologies often produce inaccurate predictions, failing to meet the demand for precise predictions of oil and gas phase transitions and flow capacity in actual production. Summary of the Invention

[0007] The present invention aims to provide a method for characterizing shale oil and gas phase equilibrium and flow capacity under water-containing conditions to solve the problem that existing oil and gas phase change and flow capacity prediction technologies cannot be applied to shale oil extraction scenarios involving hydraulic fracturing.

[0008] To achieve the above objectives, the present invention adopts the following technical solutions: a method for characterizing the oil-gas phase equilibrium and flow capacity of shale under water-containing conditions, wherein a relationship curve between the oil-gas capillary force and the gas saturation in the rock is obtained through the rock fractal dimension and pore size; based on the relationship curve of the gas saturation change, an oil-gas phase equilibrium model is established to obtain the gas saturation at a given temperature and pressure; based on the gas saturation at the given temperature and pressure and the curve of the gas saturation change with the capillary force, the oil-gas capillary force is inversely calculated to obtain the distribution of oil and gas in pores of different sizes; oil and gas phase flow models are respectively established through the fractal dimension and pore size, and the relative permeabilities of the oil and gas phases are respectively calculated based on the distribution of the oil and gas phases in pores of different sizes; The relationship curve between the capillary force of shale oil and gas and gas saturation is also obtained through water saturation; Combined with water saturation, the distribution of oil and gas in pores of different sizes is obtained; Based on the fractal dimension of the rock, the distribution of pore size is obtained; under a given water saturation, the maximum saturated water-containing pore size is calculated; the water phase exists in pores between the minimum pore diameter and , and shale oil exists in pores between the maximum saturated water-containing pore size and the maximum pore diameter.

[0009] The principles and advantages of this approach are as follows: Considering the water saturation conditions of shale under hydraulic fracturing, a curve is first derived that shows how the oil and gas capillary force changes with gas saturation based on rock parameters such as fractal dimension and pore size. This is then used to establish an oil-gas phase equilibrium model to determine gas saturation at a given temperature and pressure. Furthermore, a curve showing how the oil and gas capillary force changes with gas saturation is derived from water saturation. This is then combined with the fractal dimension to determine the pore size distribution, confirming that water resides in pores within a specific range of pore diameters (from the minimum pore diameter to the maximum pore size at saturation with water) and shale oil resides in pores within another specific range of pore diameters (from the maximum pore size at saturation with water to the maximum pore diameter). This allows the precise location of water in the pores to be determined, further narrowing down the location of the oil phase and laying the foundation for subsequent precise calculations.

[0010] Through the above-mentioned precise positioning of the water and oil phases, the fluid distribution in the pores can be more accurately considered when subsequently establishing oil and gas flow models, calculating relative permeability, and predicting oil and gas phase changes and flow capacity. This avoids the errors caused by traditional methods that ignore the influence of water, thereby significantly improving the accuracy of the final oil and gas phase change and flow capacity predictions, providing a more reliable decision-making basis for shale oil extraction.

[0011] Preferably, as an improvement, the distribution characteristics of the pore number are obtained by the distribution of pore size, and the pore volume of the porous medium shale is obtained based on the fractal dimension of the tortuosity and the straight-line distance between the two ends of the pore; The pore volume occupied by the oil phase is calculated based on the shale pore volume and the maximum value of the saturated water-containing pore size. Under given oil and gas capillary forces, the pore diameter range of the oil phase is between the maximum value of the saturated water-containing pore size and the critical maximum pore diameter occupied by the oil phase under oil and gas capillary forces.

[0012] The beneficial effect of this improvement is that in the process of calculating the pore volume occupied by the oil phase, since the pore diameter range occupied by the oil phase is clearly defined, the upper and lower limits of the integral can be accurately set to between the maximum value of the saturated water-containing pore size and the critical maximum pore diameter, making the integral calculation more targeted and accurate, and avoiding the calculation errors caused by inaccurate integration range in traditional methods.

[0013] Preferably, as an improvement, the gas saturation under given oil and gas capillary force is obtained based on the shale pore volume and the pore volume occupied by the oil phase, the gas saturation corresponding to different critical capillary forces is calculated, and the curve of gas saturation varying with capillary force under given water-containing conditions is obtained; based on the calculated curve of gas saturation varying with capillary force under given water-containing conditions, combined with the influence of capillary force on the oil and gas fugacity balance, the shale oil phase equilibrium model is established, and the gas saturation under given temperature and pressure is calculated.

[0014] The beneficial effect of this improvement is that it fully considers water saturation, a key factor, in the calculation of gas saturation, incorporating water saturation into the entire calculation system. By obtaining a curve showing how gas saturation changes with capillary force under given water conditions, the calculated results are more closely aligned with the complex hydraulic fracturing environment in actual shale oil production. This avoids the calculation errors caused by traditional methods that ignore water saturation, and improves the accuracy and reliability of gas saturation calculations.

[0015] Based on the calculated curve of gas saturation versus capillary force under given water-bearing conditions, and incorporating the influence of capillary force on the oil-gas fugacity balance, a more complete and accurate shale oil phase equilibrium model was established. This model more accurately reflects the phase equilibrium state of shale oil under water-bearing conditions, and the calculated gas saturation at a given temperature and pressure is more consistent with actual conditions, providing more accurate basic data for subsequent calculations of oil and gas flow capacities.

[0016] Preferably, as an improvement, when calculating the gas saturation under a given oil and gas capillary force from the total saturation, the given water saturation needs to be subtracted from the total saturation.

[0017] The beneficial effect of this improvement is that water saturation is taken into consideration as an important influencing factor in the formula for calculating gas saturation, allowing the formula to more comprehensively reflect the interaction between water, oil and gas phases in shale pores.

[0018] Preferably, as an improvement, based on the gas saturation and the updated oil and gas capillary force critical capillary force-saturation variation relationship curve, a linear interpolation method is used to calculate the oil and gas capillary force corresponding to the gas saturation; The critical pore size occupied by the oil phase is calculated based on the oil-gas capillary force and interfacial tension, and the oil and gas phase flow rates are calculated by continuously integrating the single pore flow rate over the pore size range occupied by the oil and gas phases.

[0019] The beneficial effect of this improvement is that by clarifying the pore size range occupied by the oil phase and the gas phase, and performing continuous integral calculations on the flow rate of a single pore, the flow conditions of the oil phase and the gas phase within the pore range they each occupy can be considered more comprehensively and accurately. This integral method avoids the problem of incomplete consideration of the flow conditions of the oil phase and the gas phase due to simplified processing in traditional methods, so that the calculation results can more realistically reflect the actual flow process. Compared with some traditional simplified calculation methods, the integral calculation method has higher accuracy and versatility. It can handle flow problems under complex boundary conditions and pore structures, and fully considers the non-uniform distribution and flow characteristics of the fluid in the pores, so that the calculation results are more in line with the actual situation.

[0020] Preferably, as an improvement, when calculating the elements of the critical capillary force change array, each element in the critical capillary force change array is based on the oil and gas capillary force value calculated based on the corresponding pore diameter range; the pore diameter range involved in the calculation is limited between the maximum pore diameter and the maximum value of the saturated water pore size.

[0021] The beneficial effect of this improvement is that when calculating the critical capillary force variation array elements, by narrowing the pore diameter range to between the maximum diameter and the maximum value of the saturated water-containing pore size, the pore range occupied by the water phase is excluded, making the calculation more focused on the pore region where the oil phase and gas phase are located. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 Flowchart of a method according to an embodiment of the present invention.

[0023] Figure 2 is the given interfacial tension of the embodiment of the present invention Curve showing the relationship between oil and gas capillary force and gas saturation under initial value conditions.

[0024] Figure 3 Schematic diagram of the oil-gas phase equilibrium calculation process in porous rock media according to an embodiment of the present invention. DETAILED DESCRIPTION

[0025] The following is further described in detail through specific implementation methods: Example Basically as attached Figure 1 As shown: This embodiment takes the components and phase parameters of shale oil reservoirs in my country as an example to provide a method for characterizing the phase equilibrium and flow capacity of shale oil under water-bearing conditions, including the following steps: S1. Using the shale pore fractal dimension, pore size, and water saturation, a curve showing the relationship between shale oil and gas capillary force and gas saturation is obtained. The main steps are as follows: Based on the shale pore fractal dimension , the distribution of the pore size is obtained: (1) Where, The value is 1.8, Indicates that the pore diameter is greater than The number of pores, is the pore diameter, is the maximum pore diameter, with a value of 2 Differentiating both sides of equation (1) yields: (2) Where -dN represents the pore size arrive The number of pores decreases with the increase of pore diameter. , the pore bending length L can be expressed as: (3) Where, is the fractal dimension of tortuosity, with a value of 1.05; is the straight-line distance between the two ends of the pore, which can be expressed as: (4) Where, is the rock sample porosity, which is 0.05. for .

[0026] At a given water saturation When the value is 0.2, the maximum value of saturated water pore size Calculated by the following formula: (5) in, is the minimum pore diameter, which is 5nm, and the maximum value of the saturated water pore size is calculated 0.86 The water phase exists in the pore diameter arrive The pores between the shale oil are located in the pore diameter arrive The gaps between them.

[0027] According to formula (2) and formula (3), the pore volume of porous media can be expressed as: (6) Oil and gas capillary force generated after oil and gas phase change It can be expressed as: (7) Oil-gas wetting angle Given as 0, the interfacial tension term The initial value is given as The maximum and minimum oil-gas capillary forces correspond to the oil-gas phase transitions at the smallest and largest pores, respectively. According to Equation (7), the smaller the pore size, the greater the oil-gas capillary force. Therefore, the oil phase precipitates first in small pores, and as the oil-gas capillary force decreases, the oil phase gradually precipitates in large pores. The critical capillary force change array elements are given by Equation (8): (8) Where, is the number of critical capillary force array elements, which is set to 200. Under the condition of oil phase precipitation, the critical maximum pore diameter for: (9) At this time, the pore diameter range occupied by the oil phase is the maximum pore size of the saturated water pore To the critical maximum pore diameter , so the pore volume occupied by the oil phase can be expressed as: (10) According to formula (10), given the oil and gas capillary force In this case, the gas saturation can be expressed as: (11) By calculating the gas saturation corresponding to different critical capillary forces according to equations (8) to (11), we can obtain the relationship curve between oil and gas capillary force and gas saturation, as shown in the attached figure. Figure 2 As shown in the figure, the gas saturation gradually increases and the oil-gas capillary force gradually decreases; in this process, the gas phase in the porous rock medium gradually increases and the oil phase gradually decreases, and the oil phase changes to the gas phase. .

[0028] S2. Based on the calculated curve of gas saturation changing with capillary force, and considering the influence of capillary force on oil and gas fugacity balance, an oil-gas phase equilibrium model is established to calculate the gas saturation at a given temperature and pressure. The main steps are as follows: (1) Given a shale reservoir temperature of T370K, two sets of pore pressures P (12 MPa, 30 MPa), and shale oil composition (see Table 1), the initial equilibrium constant is calculated using the Wilson equation: (12) Where, is the equilibrium constant of component i, is the critical temperature of component i, K; is the critical pressure of component i, Pa; is the eccentricity factor of component i, and the parameter values are shown in Table 1; Table 1 Gas composition parameters

[0029] (2) Calculate the gas phase mole fraction using the Rachford-Rice equation : (13) Where, is the mole fraction of component i, is the number of components, the components of the oil phase and the gas phase and Calculated by the following formula: (14) (15) 、 The relationship between can be expressed as: (16) (17) (18) (3) According to the fugacity balance principle, the fugacity balance of the gas phase and the liquid phase can be expressed as: (19) Where, , are the fugacity of oil phase and gas phase, respectively; , The oil phase pressure and gas phase pressure, Pa respectively; the gas phase pressure and oil phase pressure can be expressed by the oil and gas capillary force The conversion can be expressed as: (20) The initial value is set to 0, and the critical capillary force-saturation change relationship curve of oil and gas capillary force is used later. renew The fugacity of each phase of oil and gas can be measured by the fugacity coefficient To express: (twenty one) (twenty two) (twenty three) In the above formula, is the gas phase fugacity coefficient, The Peng-Robinson state equation is used to calculate the fugacity coefficient and compressibility factor of each phase of oil and gas. , gas phase compressibility factor : (twenty four) (25) The parameters in the formula can be expressed as: (26) (27) (28) Where A and B are constants of the state equation, , is the state equation constant of component i, is a binary action parameter, , , the values are shown in Table 2; for component i, and Can be expressed as: (29) (30) (31) Table 2 Peng-Robinson equation of state constants

[0030] According to the obtained oil phase compressibility factor , gas phase compressibility factor , and the components of the oil phase and gas phase calculated in step (2) and , use formula (32) and formula (33) to calculate the oil phase density and gas phase density .

[0031] (32) (33) Where, is the molar mass of each component, kg / mol; (4) Interfacial tension Calculated using the following formula: (34) Where, The isotonic specific volume of each component can be expressed as: (35) According to the interfacial tension calculated by formula (33), the interfacial tension value in formula (7) is updated, and the critical capillary force change array element after considering the interfacial tension change is further calculated according to formula (8). .

[0032] (5) According to formula (36)-formula (39) (36) (37) (38) (39) Gas saturation Calculated by the following formula: (40) (6) Based on the calculated gas saturation , based on the updated oil and gas capillary force critical capillary force-saturation change relationship curve in step (4) , the gas saturation is calculated using the linear interpolation method Corresponding oil and gas capillary force .

[0033] (41) Where, is the distance to gas saturation Most recent data point.

[0034] (7) Calculated according to step (6) The oil and gas fugacity is calculated according to formula (20)-formula (22). 、 , further calculation uses the following formula to calculate the relative error : (42) If the relative error Less than , output current oil and gas capillary force , gas saturation , oil phase and gas phase components and As the phase equilibrium calculation result. If the relative error is greater than 10-5, the gas equilibrium constants of each component are updated using the formula and the calculation process of steps (2) to (7) is repeated.

[0035] (43) The calculation process of S2 overall step is shown in the attached Figure 3 According to the S2 calculation process, the oil and gas capillary force at a pressure of 12 MPa is obtained. 1550.5Pa, gas saturation is 0.18, and the components of the oil phase and gas phase are shown in Table 3. Gas saturation at a pressure of 35 MPa When the pressure is 0, no oil-gas phase change occurs. The components at 12 MPa and 30 MPa are shown in Table 3.

[0036] Table 3 Oil-gas phase equilibrium calculation results at different pressures

[0037] S3. Based on the fractal dimension and pore size, the oil and gas flow models are established respectively. According to the distribution of oil and gas in pores of different sizes, the oil and gas permeabilities are calculated: (1) The oil and gas capillary force obtained by solving S2 at a pressure of 12 MPa and a temperature of 350 K 1550.5Pa and interfacial tension , calculate the critical pore size occupied by the oil phase: (44) According to formula (44), the critical aperture is calculated as , the gas flow rate in a single pore can be expressed as: (45) In the formula is the pressure difference across the pore, is the gas phase viscosity, is the first-order slip coefficient, with a value of 1.25, is the second-order slip coefficient, and its value is 0.23. is the average molecular free path of the gas, which can be expressed as: (46) According to formula (45), the average molecular free path of the gas is calculated to be 0.4 nm.

[0038] The flow rate of oil phase in a single pore can be expressed as: (47) In the formula is the liquid viscosity, is the slip length, which is 0.4nm. The pores between the oil phase and the pore size are The pores between the oil phase flow and oil phase flow It can be obtained by continuously integrating the single pore flow rate over the occupied pore size range: (48) (49) The oil phase flow rate is calculated according to the above formula and gas phase flow They are , .

[0039] In the case of single-phase flow, the oil phase flow rate and gas phase flow Can be divided into points The flow between is obtained, and according to formula (47) and (48): (50) (51) The oil phase flow rate is calculated according to the above formula and gas phase flow They are , .

[0040] According to Darcy's formula: (52) (53) In the formula , are the absolute permeability of gas phase and the absolute permeability of oil phase, respectively. , A is the cross-sectional area of the core, and its value is .

[0041] According to the generalized Darcy formula in the two-phase case: (54) (55) In the formula , are the gas phase effective permeability and the oil phase effective permeability, respectively, which can be expressed as: (56) (57) According to the above formula, the gas phase effective permeability and oil phase effective permeability are respectively , .

[0042] According to Equations (47)-(54), the gas phase relative permeability and oil phase relative permeability It can be expressed as: (58) (59) According to the above formula, the gas phase relative permeability and oil phase relative permeability are 0.108 and 0.78 respectively.

[0043] Note that in equations (56)-(59), the viscosity terms of the oil and gas phases and the pressure difference terms are offset after the flow ratio. Therefore, the effective permeability and relative permeability are only related to the pore structure parameters (maximum and minimum pore radius, fractal dimension), water saturation, and critical pore size.

[0044] The above is only an embodiment of the present invention, and the common knowledge such as the specific technical solutions and / or characteristics in the solution are not described in detail here. It should be pointed out that for those skilled in the art, without departing from the technical solution of the present invention, several variations and improvements can be made, which should also be regarded as the scope of protection of the present invention, and these will not affect the effect of the implementation of the present invention and the practicality of the patent. The scope of protection required by this application shall be based on the content of its claims, and the specific implementation methods and other records in the description can be used to interpret the content of the claims.

Claims

1. A method for characterizing the oil-gas phase equilibrium and flow capacity of shale under water-bearing conditions. This method uses the rock fractal dimension and pore size to obtain a curve showing the relationship between the oil-gas capillary force and the gas saturation in the rock. Based on the relationship curve of gas saturation change, an oil-gas phase equilibrium model is established to obtain the gas saturation at a given temperature and pressure; Based on the gas saturation at the given temperature and pressure, and the curve of gas saturation versus capillary force, the oil and gas capillary force is inversely calculated to obtain the distribution of oil and gas in pores of different sizes; oil and gas flow models are respectively established using the fractal dimension and pore size, and the relative permeabilities of the oil and gas phases are respectively calculated based on their distribution in pores of different sizes, characterized by: The relationship curve between the capillary force of shale oil and gas and gas saturation is also obtained through water saturation; Combined with water saturation, the distribution of oil and gas in pores of different sizes is obtained; Based on the fractal dimension of the rock, the distribution of pore size is obtained; under a given water saturation, the maximum saturated water-containing pore size is calculated; the water phase exists in pores between the minimum pore diameter and the maximum saturated water-containing pore size, and shale oil exists in pores between the maximum saturated water-containing pore size and the maximum pore diameter.

2. The method for characterizing shale oil and gas phase equilibrium and flow capacity under water-bearing conditions according to claim 1, characterized in that: The distribution characteristics of the pore number are obtained through the distribution of pore size, and the pore volume of porous shale is obtained based on the fractal dimension of tortuosity and the straight-line distance between the two ends of the pore. The pore volume occupied by the oil phase is calculated based on the shale pore volume and the maximum value of the saturated water-containing pore size. Under given oil and gas capillary forces, the pore diameter range of the oil phase is between the maximum value of the saturated water-containing pore size and the critical maximum pore diameter occupied by the oil phase under oil and gas capillary forces.

3. The method for characterizing shale oil and gas phase equilibrium and flow capacity under water-bearing conditions according to claim 2, characterized in that: Based on the shale pore volume and the pore volume occupied by the oil phase, the gas saturation under given oil and gas capillary forces is obtained, the gas saturation corresponding to different critical capillary forces is calculated, and a curve showing the change of gas saturation with capillary force under given water-containing conditions is obtained. Based on the calculated curve showing the change of gas saturation with capillary force under given water-containing conditions and combined with the influence of capillary force on the oil and gas fugacity balance, the shale oil phase equilibrium model is established, and the gas saturation under given temperature and pressure is calculated.

4. The method for characterizing shale oil and gas phase equilibrium and flow capacity under water-bearing conditions according to claim 3, characterized in that: When calculating the gas saturation under given oil and gas capillary forces from the total saturation, the given water saturation must be subtracted from the total saturation.

5. The method for characterizing shale oil and gas phase equilibrium and flow capacity under water-bearing conditions according to claim 4, characterized in that: Based on the gas saturation and the updated oil and gas capillary force critical capillary force-saturation variation relationship curve, the oil and gas capillary force corresponding to the gas saturation is calculated using a linear interpolation method; The critical pore size occupied by the oil phase is calculated based on the oil-gas capillary force and interfacial tension, and the oil and gas phase flow rates are calculated by continuously integrating the single pore flow rate over the pore size range occupied by the oil and gas phases.

6. The method for characterizing shale oil and gas phase equilibrium and flow capacity under water-bearing conditions according to claim 5, characterized in that: When calculating the elements of the critical capillary force variation array, each element in the critical capillary force variation array is based on the oil and gas capillary force value calculated for the corresponding pore diameter range; The range of pore diameters involved in the calculation is limited to between the maximum pore diameter and the maximum value of the saturated water-containing pore size.

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