间歇注气抽采煤层气井注气量与采收率分析研究

水力压裂煤层、注气提高煤层气采收率等生产工艺过程中,煤层中流体的运动是多相多组分流体的扩散渗流。假定煤层为孔隙裂隙二重介质,孔隙中只存在吸附状态气相,裂隙是气、水共存空间,孔隙与裂隙之间气体的交换量是渗流场中的质量源。试验研究了质量源与时间和煤层结构的关系,测定了吸附置换速率的衰减系数。应用多孔介质中扩散渗流理论,再运用质量连续方程,导出了多相多组分流体的扩散渗流微分方程。忽略占煤层中气体总量百分比很小的游离状态气体在微分方程中随时间变化,从而求出了单井间歇注气抽采煤层气井在注气过程注气量的近似表达式。分析解表明注入气体流量与注入气体压力和煤层气初始压力之差、竞争吸附置换速率成正比;煤层渗透率越大、煤层气和注入气体的混合粘性越小,吸附竞争置换速率越小和注气抽采煤层气效果越差;注气抽采煤层气工艺适合低透气性煤层。

Research on the Injected gas Flux and Methane Recovery Ratio of Intermittent Injection Well

In hydraulic fracturing of coalbed and the exploitation of methane by gas injection,the fluid movement is diffusion and seepage flow of multiphase-multicomponent fluid in the coalbed.By supposing that coalbed is micropore-crevice double medium,there is only the adsorbed gas in micropore,the crevice is the space for gas and water coexistence,and the gas exchange quantity between the micropore and crevice is the mass source of seepage flow field,experimental study was carried out on the source change with time and coalbed structure,and measured the attenuation coefficient of the adsorption replacement velocity.By porous medium diffusion-seepage flow theory and the quality equation of mass continuity,the diffusion and seepage flow differential equations of the multiphase-multicomponent fluids were derived.The change of A little free gas with time was ignored in the differential equation,so that approximate expression of gas injection quantity was obtained in process of single-well intermittent injection gas.The expression indicates that the injected gas flux was directly proportional to difference between the injected gas pressure and the initial methane pressure and the competitive adsorption replacement velocity;when the coalbed penetration coefficient was bigger and gaseous mixture’s viscosity coefficient was smaller,the adsorption competitive replacement velocity was smaller and the effect t of gas injection was worse;and the gas injection craft adapted to the low permeable coalbed.