Oil reservoir numerical simulation method considering oil-gas phase equilibrium change in micro-nano pores

Abstract

The invention discloses an oil reservoir numerical simulation method considering oil-gas phase equilibrium change in micro-nano pores. The method comprises the steps that stability testing is conducted on a phase equilibrium grid to be calculated to preliminarily judge whether an oil-gas mixing system represented by the grid is in a two-phase state or a single-phase state; for the oil-gas mixing system in the two-phase state, iteratively calculating phase separation considering capillary force to obtain a final two-phase equilibrium composition; the stability discrimination method comprises the following steps: calculating the current capillary force of the oil-gas mixing system to obtain corresponding gas-phase pressure and liquid-phase pressure, and performing the stability test by using the gas-phase pressure and the liquid-phase pressure; the problem that the capillary force effect cannot be considered in actual oil and gas reservoir model simulation of a conventional complex geological structure and a large-scale grid is solved, and the technical precision and efficiency can be ensured at the same time, so that the method can be directly applied to an actual digital-analog process.

Description

Technical Field

[0001] The present disclosure relates to the technical field of oil reservoir numerical simulation, and in particular to an oil reservoir numerical simulation method for shale oil and gas reservoirs that takes into account special oil and gas phase equilibrium changes in micro-nano pores. Background Art

[0002] Oil-gas phase equilibrium calculation is an important part of reservoir numerical simulation, also known as "flash calculation". The flash calculation is to calculate the proportion of the two phases and the distribution of each component in the two phases under the given system pressure P, temperature T and the composition of each component. The current mainstream reservoir numerical simulation software usually assumes that the oil and gas phase pressures are consistent when calculating oil-gas phase equilibrium, both of which are system pressure P, and ignores the capillary force P between the two phases. c In conventional reservoirs, this assumption is basically valid, but in shale oil and gas reservoirs, a large number of narrow micro-nanoscale pores will lead to a significant increase in oil and gas capillary forces. If the capillary force is ignored when calculating phase equilibrium, it will have a significant impact on phase diagrams, bubble point calculations, production predictions, etc.

[0003] Some current studies have also considered the impact of capillary force on phase equilibrium calculations, but the methods used in these studies are relatively simple, which is just a simple extension of the single-step flash algorithm, replacing the single pressure in the algorithm with the two-phase pressure. This simple extension based on the single-step algorithm has low calculation efficiency and can only be used to study phase diagram changes or perform conceptual model simulations. It is difficult to apply to actual oil and gas reservoir model simulation calculations with complex geological structures and large-scale grids. In addition, the traditional capillary force calculation formula used by traditional methods when calculating capillary force is: Where P c is the capillary force, σ is the oil and gas surface tension, θ is the contact angle, and r is the characteristic size of the pore throat. The oil and gas surface tension is calculated using the Macleod-Sugden empirical relationship. However, experiments show that when the pore volume is reduced to the micro-nano level, the oil and gas surface tension is also affected by the pore size. The surface tension value obtained by this relationship is low, which leads to low capillary force. Summary of the invention

[0004] In view of this, the present disclosure provides a reservoir numerical simulation method that takes into account the changes in oil and gas phase equilibrium in micro-nano pores, which solves the problem that the capillary force effect cannot be considered in the simulation of actual oil and gas reservoir models with conventional complex geological structures and large-scale grids, and can simultaneously ensure technical accuracy and efficiency, so it can be directly applied to actual digital modeling processes.

[0005] To achieve the above-mentioned purpose of the invention, the reservoir numerical simulation method considering the change of oil-gas phase equilibrium in micro-nano pores comprises:

[0006] Performing a stability test on the grid to be calculated for phase equilibrium to preliminarily determine whether the oil-gas mixture system represented by the grid is in a two-phase state or a single-phase state;

[0007] For the oil-gas mixture system in the two-phase state, iteratively calculating the phase separation to obtain the final two-phase equilibrium composition;

[0008] The stability determination method includes calculating the current capillary force of the oil-gas mixture system, obtaining the corresponding gas phase pressure and liquid phase pressure, and using the gas phase pressure and the liquid phase pressure to perform the stability test;

[0009] The fugacity ratio K in the solution formula of the iterative calculation is i The expression is:

[0010]

[0011] Where: y i represents the mole fraction of component i in the liquid phase, x i represents the mole fraction of component i in the gas phase; subscript i represents a certain component; P V Represents gas phase pressure; P L represents the liquid phase pressure; Represents the fugacity coefficient.

[0012] In the present disclosure and possible embodiments, the discriminant of stability is:

[0013]

[0014] Where: z represents the liquid phase properties, and y represents the gas phase properties.

[0015] In the present disclosure and possible embodiments, the capillary force is calculated by the J-function method.

[0016] In the present disclosure and possible embodiments, the capillary force calculation formula is:

[0017]

[0018] Where: S is the saturation of the wetting phase (i.e., oil phase), P c (S) is the capillary force at saturation S, σ is the surface tension of oil and gas, θ is the contact angle, J(S) is the rock J function determined experimentally and is a dimensionless number, k is the absolute permeability of the rock, and φ is the porosity of the rock;

[0019] The discriminant is solved using an iterative method.

[0020] In the present disclosure and possible embodiments, the system pressure is acquired; the liquid phase pressure is equal to the system pressure, and the gas phase pressure is equal to the sum of the liquid phase pressure and the capillary force.

[0021] In the present disclosure and possible embodiments, the phase separation calculation solution formula is:

[0022]

[0023] Where: z i represents the total mole fraction of component i, and α is the gas phase mole fraction.

[0024] The present disclosure has the following beneficial effects:

[0025] The disclosed reservoir numerical simulation method considering the changes in oil-gas phase equilibrium in micro-nano pores is based on a two-step flash evaporation algorithm. Since the influence of the capillary force effect is fully considered in the "stability test" and "phase separation calculation" processes, and the J function method is used to calculate the capillary force, the capillary force calculation formula is more accurate. Therefore, the method of the present invention can effectively guarantee the calculation accuracy, thereby more accurately calculating properties such as the phase diagram and bubble point pressure.

[0026] In addition, the stability test can be used to preliminarily determine whether the oil-gas mixture system is in a two-phase state or a single-phase state. If it is a single-phase state, there is no need to perform the time-consuming second step of "phase separation calculation", thus greatly saving the total flash calculation time. Moreover, the stability discriminant of the capillary force is considered. Since no new variables are introduced, the iterative method can still be used to solve it. At the same time, when performing the second step of "phase separation calculation", the K i It is the only variable in the equation, and the iterative method is still used to solve it, and the complexity of the solution is close to that before the expansion; therefore, after additionally considering the capillary force effect, the difficulty of the solution process of the method of the present invention is basically the same as that of the traditional method, and does not bring about a significant decrease in efficiency, and ultimately achieves efficiency even for complex actual oil and gas reservoir model simulations.

[0027] In summary, the method of the present invention is based on a two-step flash evaporation algorithm to expand the capillary force effect. Since it can ensure both accuracy and efficiency, it can be directly applied to actual digital simulation processes. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The above and other objects, features and advantages of the present disclosure will become more apparent through the following description of the embodiments of the present disclosure with reference to the accompanying drawings, in which:

[0029] Figure 1 is a flow chart of a method for numerical simulation of oil reservoirs considering changes in oil-gas phase equilibrium in micro-nano pores according to an embodiment of the present disclosure;

[0030] Figure 2 is an iterative calculation phase separation flow chart of an embodiment of the present disclosure;

[0031] Figure 3is a schematic diagram of the variation of the bubble point pressure of a reservoir oil sample with the pore throat size according to an embodiment of the present disclosure;

[0032] Figure 4 is a schematic diagram of the influence of the binding effect on the oil production according to an embodiment of the present disclosure;

[0033] Figure 5 is a schematic diagram of the influence of the binding effect on the cumulative oil production according to an embodiment of the present disclosure;

[0034] Figure 6 It is a schematic diagram of the influence of the binding effect on the cumulative oil production according to an embodiment of the present disclosure. DETAILED DESCRIPTION

[0035] The present disclosure is described below based on embodiments, but it is worth noting that the present disclosure is not limited to these embodiments. In the detailed description of the present disclosure below, some specific details are described in detail. However, for the parts that are not described in detail, those skilled in the art can also fully understand the present disclosure.

[0036] In addition, those skilled in the art should understand that the drawings are provided only to illustrate the purpose, features and advantages of the present disclosure, and the drawings are not actually drawn to scale. At the same time, unless the context clearly requires, the words "include", "comprise" and the like in the entire specification and claims should be interpreted as inclusive rather than exclusive or exhaustive; that is, "including but not limited to" meaning.

[0037] Figure 1 is a flow chart of a numerical simulation method for oil reservoirs considering the change of oil-gas phase equilibrium in micro-nano pores according to an embodiment of the present disclosure; Figure 1 As shown: The reservoir numerical simulation method considering the change of oil-gas phase equilibrium in micro-nano pores is specifically described as follows:

[0038] 1. Determination of capillary force in micro-nano pores:

[0039] The classic capillary force calculation formula used in traditional methods is:

[0040]

[0041] Where: P c is the capillary force, σ is the oil and gas surface tension, θ is the contact angle, and r is the characteristic size of the pore throat. The surface tension is calculated using the Macleod-Sugden empirical relationship. However, experiments show that when the pore volume is reduced to the micro-nano level, the surface tension is also affected by the pore size. The surface tension value obtained by this relationship is low, resulting in low capillary force. Therefore, shale reservoirs need to adopt a new method to calculate capillary force. The method of the present invention uses the J function method to calculate the capillary force as follows:

[0042]

[0043] Where: S is the saturation of the wetting phase (i.e., oil phase); the capillary force is no longer a single constant value, but is expressed as a function of the saturation S P c (S), and will be dynamically updated in the subsequent flash calculation and solution process; J(S) is the rock J function determined by experiment, which is a dimensionless number; k is the absolute permeability of the rock, and φ is the porosity of the rock. Since J(S) in the formula is determined by experiment and will be affected by the actual pore throat size, the new capillary force formula is more accurate and is affected by the actual saturation, so it is no longer a fixed value and is more reasonable.

[0044] 2. Phase equilibrium calculation method based on two-step flash evaporation algorithm and considering capillary force:

[0045] In the traditional flash algorithm, capillary force is not considered, and the oil and gas phase pressures are assumed to be the same, and both are system pressure P. When capillary force is considered, the oil and gas phase pressures are different, and the system pressure P is fixed to the oil phase pressure and the gas phase pressure P V The system pressure P and capillary force P c Therefore, if the capillary force is considered, the equations and solution process in the traditional flash evaporation algorithm need to be expanded and modified.

[0046] In the actual numerical simulation program, a more efficient two-step flash algorithm is used. These two steps are called "stability test" and "phase separation calculation", of which "phase separation calculation" is more time-consuming. In the actual numerical simulation process, most grids can be filtered through "stability test" without executing the "phase separation calculation" step, thus ensuring the calculation efficiency.

[0047] The following describes in detail how the present invention considers the influence of capillary force in these two steps, as follows:

[0048] (1) “Stability test” method considering capillary forces

[0049] The stability test is to preliminarily determine whether the oil-gas mixture system is in a two-phase state or a single-phase state. If it is a single-phase state, there is no need to perform the second time-consuming "phase separation calculation", so the total flash calculation time can be greatly saved. The traditional stability test formula is as follows. As long as the discriminant is established, it is a single-phase state:

[0050]

[0051] Where: z represents the liquid phase property, y represents the gas phase property; subscript i represents a component; f represents the phase or component fugacity; Represents the fugacity coefficient. In the traditional method, the same pressure P is used for both the gas and liquid phases.

[0052] If capillary forces are considered, the final form of the stability criterion is:

[0053]

[0054] Where: P V , P L Represent the gas phase pressure and liquid phase pressure respectively, both of which satisfy P V -P L =P c (S).

[0055] The solution to this extended equation is: Since the stability test criterion is derived under the assumption of a single-phase system, S remains unchanged during the iterative calculation process, so P c unchanged, so there is always P L =P,P V =P L +P c (S). Since no new variables are introduced into the equation considering the capillary force, the iterative method can still be used to solve it. Compared with before the expansion, the difficulty of solving the equation is the same, that is, the calculation efficiency is basically the same.

[0056] (2) “Phase separation calculation” method considering capillary forces

[0057] After stability testing confirms that the system is in two phases, it is necessary to further calculate the composition of the two phases to obtain the specific composition of each component in the two phases and the volume fraction of the two phases. This is the "phase separation calculation", which requires solving the following formula:

[0058]

[0059] Where: z i represents the total mole fraction of component i, α is the gas phase mole fraction, K i is the fugacity ratio of component i, which is expressed in the traditional method as:

[0060]

[0061] Where: y i represents the mole fraction of component i in the liquid phase, x i Represents the mole fraction of component i in the gas phase.

[0062] If the influence of capillary force is included, K needs to be corrected i expression:

[0063]

[0064] The solution to this extended equation is: K iis the only variable in the equation, and the iterative method is still used to solve it. The complexity of the solution is close to that before the expansion. Therefore, although the "phase separation calculation" step of the method of the present invention has been expanded, it has not affected the calculation efficiency; and because the influence of capillary force is taken into account, the calculation accuracy can be effectively improved. It should be noted that the specific details are slightly different from the traditional method when solving the traditional formula, that is, after each iteration, a new K i , thus obtaining a new α; when α changes, the saturation S also changes accordingly, and at this time, it is necessary to update and calculate the new P c , and the new P V In the next iteration, the new two-phase pressure needs to be used for calculation. The specific solution process is as follows: Figure 2 shown.

[0065] Application Examples

[0066] Using the method provided by the present invention, a simulation study was conducted on an actual block of a shale oil reservoir. For the oil samples in the study area, after considering the capillary force effect, the bubble point of the mixed fluid was reduced. Figure 3 As shown in Figure 2, the bubble point decreased by 14% when the median pore throat size was about 3 nm in the study area, indicating that the capillary force effect has a significant impact on the fluid phase equilibrium in micro-nano pores and cannot be ignored in the simulation.

[0067] To further study the impact of this effect on oil well productivity, simulations were carried out with and without considering capillary force. Bubble point pressure constant pressure production was selected during the simulation. Figure 4 and Figure 5 The results show that after considering the capillary effect, the daily and cumulative production can be increased by 10% to 20%. This is because the bubble point pressure is reduced due to the capillary effect, which enables depletion development to use a larger pressure difference for production.

[0068] Comparing the simulation time after considering the capillary force effect, the results are as follows Figure 6 As shown. On the one hand, after considering the capillary force effect, compared with the conventional flash time of 210h without considering the capillary force effect, the total simulation time of the method of the present invention is 213h, which is only increased by 1.4%, indicating that the expansion of the capillary force effect based on the two-step flash calculation process has almost no effect on the calculation efficiency. On the other hand, if in the actual model simulation, because the traditional method can only consider the capillary force in the single-step flash, the total simulation time will be greatly increased under the premise of using only the single-step flash method (that is, the traditional continuous elimination method in the figure) to calculate the expansion of the capillary force effect. In this test example, the time consumption increases by nearly 220%, indicating that the implementation method in the existing research is relatively inefficient and not suitable for actual digital simulation processes.

[0069] This application case shows that the method of the present invention can effectively simulate the phase equilibrium calculation under micro-nano pores and can effectively ensure accuracy and speed.

[0070] Based on the results of the above embodiments and application examples, it can be seen that the present invention provides a more accurate method for calculating the capillary force of shale reservoirs taking into account the dense pore effect, realizes the extended stability test taking into account the capillary force and the phase separation calculation taking into account the capillary force, and adds this method to the numerical simulation process so that it can be used in the actual numerical simulation process.

[0071] Therefore, the method of the present invention can be widely used in the calculation and analysis of oil and gas PVT phase states and numerical simulation processes in shale reservoirs and tight reservoirs, providing more accurate calculation of phase state parameters and recovery rate. At the same time, it has the advantages of low threshold for use, high similarity to traditional processes, and no adverse effects on calculation simulation, and can completely replace the current PVT calculation method in shale or tight reservoirs.

[0072] The above-described embodiments are only embodiments of the present disclosure, and the descriptions thereof are relatively specific and detailed, but they cannot be construed as limiting the scope of the present disclosure. It should be noted that, for a person of ordinary skill in the art, without departing from the concept of the present disclosure, several variations, equivalent substitutions, improvements, etc. may be made, all of which fall within the scope of protection of the present disclosure. Therefore, the scope of protection of the present disclosure shall be subject to the attached claims.

Claims

1. A reservoir numerical simulation method considering the change of oil-gas phase equilibrium in micro-nano pores, characterized in that: include: Performing a stability test on the grid to be calculated for phase equilibrium to determine whether the oil-gas mixture system represented by the grid is in a two-phase state or a single-phase state; For the oil-gas mixture system in the two-phase state, iteratively calculating the phase separation to obtain the final two-phase equilibrium composition; The stability determination method includes calculating the current capillary force of the oil-gas mixture system, obtaining the corresponding gas phase pressure and liquid phase pressure, and using the gas phase pressure and the liquid phase pressure to perform the stability test; In the solution formula of the iterative calculation, the fugacity ratio K i The expression is: Where: y i represents the mole fraction of component i in the liquid phase, x i represents the mole fraction of component i in the gas phase; The subscript i represents a component; P V Represents gas phase pressure; P L represents the liquid phase pressure; Represents the fugacity coefficient.

2. The reservoir numerical simulation method considering the change of oil-gas phase equilibrium in micro-nano pores according to claim 1 is characterized in that: The discriminant of the stability is: Where: z represents the liquid phase properties, and y represents the gas phase properties.

3. The reservoir numerical simulation method considering the change of oil-gas phase equilibrium in micro-nano pores according to claim 2 is characterized in that: The capillary force is calculated by the J-function method.

4. The reservoir numerical simulation method considering the change of oil-gas phase equilibrium in micro-nano pores according to claim 3 is characterized in that: The capillary force calculation formula is: Where: S is the saturation of the wetting phase (i.e., oil phase), P c (S) is the capillary force at saturation S, σ is the surface tension of oil and gas, θ is the contact angle, J(S) is the rock J function determined experimentally and is a dimensionless number, k is the absolute permeability of the rock, and φ is the porosity of the rock; The discriminant is solved using an iterative method.

5. The method for numerical simulation of oil reservoirs taking into account the change of oil-gas phase equilibrium in micro-nano pores according to claim 4, characterized in that: Get system pressure; The liquid phase pressure is equal to the system pressure, and the gas phase pressure is equal to the sum of the liquid phase pressure and the capillary force.

6. The reservoir numerical simulation method considering the change of oil-gas phase equilibrium in micro-nano pores according to any one of claims 1 to 5, characterized in that: The phase separation calculation formula is: Where: z i represents the total mole fraction of component i, and α is the gas phase mole fraction.

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