Investigation of gas displacement efficiency and storage capability for enhanced CH4 recovery and CO2 sequestration

Injecting gas into coal seam is an effective technique for enhanced coalbed methane recovery and CO2 sequestration. Displacement efficiency of various gas injections was investigated by experimental measurements. Experimental results show that any factor of adsorption amount, back pressure, displacement pressure and displacement time can have a significant influence on methane recovery. Displacement efficiency has a non-monotonic increase with the ascending equilibrium pressure, is enhanced by back pressure reduction and increasing displacement pressure, and tends to reach a higher degree by the mixture rich in nitrogen. Displacement pressure presents a significant influence on displacement efficiency and causes a rapid production response in early injection stage for the rich nitrogen mixture. Displacement dynamics of various gas injections was investigated by the numerical simulation. A good consistency of displacement efficiency by displacement experiment and numerical simulation is found. For the mixture rich in nitrogen, the higher displacement efficiency is obtained and the breakthrough time of N2 and CO2 can be both shortened. Spatial and temporal distribution of adsorbed gas for pure gas injection shows that the displacement and breakthrough time of pure CO2 are both delayed. Based on spatial and temporal distribution of mixed gas displacement, displacement efficiency is controlled by the composition and competitive sorption of the mixture. Gas transport is sensitive to the injection pressure and faster to reach the stable state with the ascending pressure. The result of this study is helpful to realize the mechanism of enhanced coalbed methane recovery by gas injection and design the scheme of gas injection.