CN-121981003-A - Quantitative characterization method for evaluating displacement front stability in porous medium

Abstract

The invention relates to the field of gas injection development of compact oil, and provides a novel theoretical calculation method by adopting computational fluid dynamics simulation. Firstly, based on a user-defined function, the speed of all grids in the pore structure model and the volume fraction of the two phases of oil and gas are counted, and accordingly the weighted average speed of the two phases is solved. Then, based on the oil-gas two-phase speed difference, a set of quantitative evaluation standards for evaluating the stability of the displacement front based on the finger-advance coefficient are established. Finally, the system illustrates the inherent correlation between displacement efficiency and fingering coefficient. The invention further researches the influence of the displacement front stability on the displacement efficiency under the pore scale, provides an important theoretical basis for quantitatively evaluating the displacement front stability, and has important practical significance for optimizing injection and production parameters and improving the final recovery ratio in engineering.

Inventors

• Xie Huaxiao
• LIANG DONG
• YAN YOUGUO
• ZHANG JUN
• LI ZHEN
• WANG XIAO

Assignees

• 中国石油大学(华东)

Dates

Publication Date

20260505

Application Date

20260126

Claims (6)

1. 1. The invention provides a method for quantitatively characterizing the stability of a gas injection development compact oil displacement front in a porous medium by utilizing computational fluid dynamics simulation analysis. The method comprises the following steps: (1) Model construction, namely, rock is distinguished from pores by binarization image processing in Avizo software based on core slices obtained by CT scanning. Then, the binarized image is imported into Comsol software, the operation of extracting pore and rock boundary contour lines is carried out, and the image is converted into a pore structure model which can be used for simulating oil-gas seepage; (2) Computational fluid mechanics simulation based on UDF function development, namely, in Ansys Fluent software, seepage simulation of CO 2 displacement compact oil is carried out, and reservoir temperature is 354K. Reservoir pressure was 10 MPa under non-miscible conditions and 30 MPa under miscible conditions. Under the reservoir temperature and pressure condition, CO 2 is in a supercritical state, and C 12 H 26 is selected to represent a compact oil component. Simulating a non-miscible flooding process based on a VOF multiphase flow model of a tracking phase interface, simulating a miscible flooding process by a component transportation model, and analyzing the stability evolution behavior of a displacement front in the gas flooding process; (3) The quantitative evaluation method of the front edge stability is characterized in that the relation between fingering and displacement front edge stability and displacement efficiency is deeply studied by researching fingering behavior in the gas flooding process under the pore scale. And dividing grids for the two-dimensional pore structure model, traversing each grid through user-defined function (UDF) programming, obtaining the speed of the oil-gas two phases in each grid and the volume fraction of the grid occupied by each grid, and solving the speed weighted average of all grids according to the volume fraction, thereby obtaining the average speed of the two phases. Based on the oil-gas two-phase speed difference, the index coefficients of CO 2 non-miscible phase flooding and miscible phase flooding in the core porous medium are calculated, and the displacement efficiency under the two miscible phase conditions is analyzed, so that the displacement front stability is estimated. And by comparing the differences of the index coefficient and the displacement efficiency under the two miscible pressures, the system analyzes the correlation between the index coefficient and the stability and the displacement efficiency of the displacement front edge, and accordingly obtains the quantitative evaluation standard of the stability of the displacement front edge in the air displacement process.
2. 2. The displacement front stability quantitative evaluation method according to claim 1, characterized in that: In the step (1), the two-dimensional pore structure model construction method is a combination of methods such as binarization image processing, image boundary line extraction, image conversion into a pore structure model and the like.
3. 3. The displacement front stability quantitative evaluation method according to claim 1, characterized in that: In the step (2), in the process of simulating oil-gas seepage by computational fluid mechanics, the CO 2 non-miscible phase displacement is calculated by adopting a VOF multiphase flow model, and the CO 2 miscible phase displacement is calculated by adopting a component transportation model.
4. 4. The displacement front stability quantitative evaluation method according to claim 1, characterized in that: In the step (3), UDF function development is adopted, and the speed of each grid, the volume fraction of the oil phase and the gas phase in the displacement process are recorded in real time.
5. 5. The displacement front stability quantitative evaluation method according to claim 1, characterized in that: in the step (3), finger-feed coefficient calculation under different miscible phase conditions is obtained based on oil-gas two-phase speed difference.
6. 6. The displacement front stability quantitative evaluation method according to claim 1, characterized in that: in the step (3), the finger advance coefficient and the displacement efficiency are calculated under different miscible phase conditions, and a quantitative evaluation index is provided for the relation between the displacement front stability and the displacement efficiency.

Description

Quantitative characterization method for evaluating displacement front stability in porous medium Technical Field The application belongs to analysis and research of oil-gas multiphase flow in a porous medium, and particularly relates to a method for evaluating the stability of a displacement front in the process of displacing crude oil by computational fluid dynamics simulation analysis of CO 2. The method researches fingering behavior of crude oil in the gas injection displacement process, and evaluates evolution behavior of displacement front stability through quantification. Background In the gas injection development oil reservoir process, the oil reservoir is strong in heterogeneity and high in crude oil viscosity, so that the displacement front of injected gas is unstable, the viscous fingering phenomenon is serious, and the improvement of the crude oil recovery rate is restricted. The finger action refers to the breakthrough of injected gas (such as CO 2、CH4、N2 and the like) to an outlet position along a dominant channel, so that the gas channeling phenomenon occurs, and the swept volume and the recovery ratio are reduced. Thus, injection gas displacement front stability becomes a critical factor in controlling sweep efficiency and displacement efficiency. The injected gas displacement front has the oil gas migration viscosity difference and the concentration gradient caused inter-diffusion between the two oil gas phases. In a real reservoir, the stability of the injection gas displacement front is controlled by a number of factors, such as reservoir temperature, pressure, crude oil composition, injection pressure, etc. The traditional method (physical simulation and numerical simulation) focuses on phase interface evolution in the oil-gas displacement process, focuses on qualitative analysis of dynamic evolution characteristics of the gas-drive front, lacks a quantitative characterization method for the stability of the displacement front, and is difficult to establish a quantitative relation between a fingering coefficient and displacement efficiency. In recent years, a digital core technology is combined with a computational fluid dynamics simulation method to become a powerful means for researching oil-gas miscible displacement and multiphase seepage behaviors. Oil gas seepage characteristics, flow field distribution, interface evolution and displacement efficiency in the porous medium can be analyzed through computational fluid dynamics simulation. And establishing a displacement front stability quantitative characterization method based on a fingering coefficient on the scale of the porous medium, wherein the inherent correlation of the displacement efficiency and the front stability can be quantitatively analyzed. The theoretical method provides a theoretical basis for optimizing the gas injection strategy and formulating the channeling prevention measures in the actual mining field development, thereby being beneficial to improving the final recovery ratio. Disclosure of Invention The invention provides a method for quantitatively evaluating the front edge stability based on a fingering coefficient and establishes a corresponding relation between the front edge stability and displacement efficiency by utilizing a user-defined function algorithm in computational fluid dynamics simulation to study the front edge stability in the process of displacing crude oil by gas. The computer simulation software adopted by the invention is COMSOL, ANSYS SPACE CLAIM and Ansys Fluent. The research content adopted by the invention comprises two parts, namely: 1. construction of two-dimensional digital core model The rock core slice is obtained based on an X-ray computer tomography technology, and the rock is separated from the pore space through binarization image processing. In the binarized image, dark color areas represent rock particles and light color areas represent pores. And finding out the outline of the boundary line between the rock and the pore, extracting all boundary lines, converting the boundary lines into a DXF file, and importing the DXF file into a ANSYS SPACE CLAIM module. Rock particles are deleted and only the pore area for the flow of the oil and gas two-phase flow remains. Through connectivity analysis, a pore structure model which can be used for direct numerical simulation is obtained, and then subsequent flow simulation and displacement front stability analysis are carried out. 2. Establishment of quantitative evaluation method for displacement front stability in porous medium Based on the established two-dimensional pore structure model, through the VOF multiphase flow model and the component transportation model which can track the phase interface in Ansys Fluent, the flow migration behavior of the oil gas two phases in the pore structure under the conditions of CO 2 non-miscible phase and miscible phase is simulated, the evolution form and the fingering phenomenon of the displac