二氧化碳驱替甲烷过程中煤层气增产效果、储层渗透率的数值模拟分析

Numerical Simulation Analysis of CBM Production Increase Effect and Reservoir Permeability in the Process of Carbon Dioxide Displacement of Methane

摘要: 二氧化碳驱替煤层甲烷可以提高煤层气的采收率，有效利用煤层资源，减少二氧化碳排放和减少温室气体的影响，具有环保和经济双重效益。为了调查CO₂强化煤层气的采收效率，本文采用数值模拟的方法，基于Fick扩散定律、Darcy渗流定律和扩展朗格缪尔等方程，利用Comsol Multiphysics软件模拟分析了不同二氧化碳注气压力、注入井与生产井间距对二氧化碳驱替甲烷的影响。研究表明注CO₂对提高CH₄累计产出量具有积极的作用，注气压为4 MPa、6 MPa时的CH₄的产出量较2 MPa增长250%、351.92%。随着时间的抽采，生产井附近CH₄浓度最后均趋于一定值，抽采1800 d时，注入井与生产井两井孔水平间距为50 m时甲烷累计生产量最高，50 m为最优间距。生产井附近的压力主要由卸压作用决定，卸压降低了煤层的吸附能力和煤层的吸附变形，使煤层的孔隙度变大，导致煤层的渗透率高于初始渗透率，注入井煤层中CO₂与CH₄的竞争吸附作用引起煤体的膨胀和变形，降低了煤储层孔隙度进而降低其渗透率。

Abstract: Carbon dioxide displacement of coalbed methane can improve the recovery rate of coalbed methane, effectively utilize coal seam resources, reduce carbon dioxide emissions, and reduce the impact of greenhouse gases, with dual benefits of environmental protection and economy. In order to investigate the recovery efficiency of coalbed methane during CO₂-ECBM, the original text will use numerical simulation methods, based on Darcy flow, Fick diffusion, and extended Langmuir equations, and use Comsol Multiphysics software to simulate and analyze the effects of different carbon dioxide injection pressures and the spacing between injection wells and production wells on carbon dioxide displacement of methane. Research has shown that CO₂ injection has a positive effect on increasing the cumulative production of CH₄, with CH₄ production at 4 MPa and 6 MPa increasing by 250% and 351.92% compared to 2 MPa. With different spacing between two wells, the CH₄ concentration near the production well eventually tends to a certain value over time. When pumping for 1800 days, the cumulative methane production is highest when the horizontal spacing between the two wells is 50 meters, which is the optimal spacing. The pressure near the production well is mainly determined by the pressure relief effect, which reduces the adsorption capacity of the coal seam, thereby reducing the adsorption deformation of the coal seam, increasing the porosity of the coal seam, and causing the permeability of the coal seam to be higher than the initial permeability. For gas injection wells, the competitive adsorption effect of CO₂ and CH₄ will cause the expansion and deformation of the coal body, reduce the porosity of the coal reservoir, and thus reduce its permeability.