CO_(2)注入下煤层气产出及储层渗透率演化规律

Evolution of coalbed methane output and reservoir permeability under CO_(2) injection

为研究CO_(2)注入对煤层气(CBM)抽采及储层渗透率演变的影响,建立了考虑气体流动、竞争吸附、温度变化以及煤体变形的热-流-固耦合模型,通过对CBM和CO_(2)注入强化煤层气开采(CO_(2)-ECBM)过程的模拟,分析了煤层气产出及储层渗透率演变规律,提出了定量表征各参数影响程度的评价方法。结果表明CO_(2)注入3 600 d后煤层气累积产量(4.6×10^(4) m^(3))约为CBM抽采的5.75倍。CBM抽采过程中储层渗透率呈“降低-增加”趋势,其变化主要因CH解吸影响,相对影响系数约为0.57。其次因体积应变和孔隙压力影响,相对影响系数分别为0.2和0.23;CO_(2)-ECBM过程中储层渗透率呈“轻微增加-降低-显著增加-降低”趋势,其变化主要因CO_(2)吸附影响,相对影响系数为0.4,孔隙压力对渗透率的影响极小,其相对影响系数仅为0.02。

A hydraulic-mechanical-thermal coupling model considering gas flow, competitive adsorption, temperature change and coal deformation was established to study the effects of CO_(2) injection on coalbed methane(CBM) production and permeability evolution. Based on the simulation of CBM and CO_(2) injection enhanced CBM(CO_(2)-ECBM) processes, the law of gas production and permeability evolution was analyzed, and an evaluation method for quantitatively characterizing the influence of parameters is proposed. The results show that after 3 600 days of CO_(2) injection, the cumulative gas production(4.6×10^(4) m^(3)) is about 5.75 times than that by CBM. In the CBM mining process, the reservoir permeability shows a decrease-increase trend. Its variation is mainly affected by CHdesorption with the relative influence coefficient of 0.57, and it is also affected by the volume strain and pore pressure with the relative influence coefficient of 0.2 and 0.23, respectively. In the CO_(2)-ECBM process, the reservoir permeability shows a trend of slight increase-decrease-significantly increase-decrease, and its change is mainly affected by CO_(2) adsorption, with a relative influence coefficient of 0.4. Pore pressure has little influence on permeability, with a relative influence coefficient of only 0.02.