Accumulation behaviors of methane in the aqueous environment with organic matters

Shale gas has received extensive attention and is becoming an important supplement to worldwide energy. Water is inevitable when developing shale gas with hydraulic fracturing. Thus it is necessary to understand the dynamic behavior of methane in water environments. Here, molecular dynamics simulations were performed to study the dynamic behavior of methane in the aqueous solutions with organic matters (OM). It is found that methane will accumulate at the OM-water interface and form a dense gas region spontaneously. The accumulation process is found to be little dependent on the gas saturation and the pressure within the simulating range, while it is closely related to the temperature and surface wettability. To explain the accumulation phenomenon of methane, the potential of mean force was calculated. We found that both the organic wall-gas interaction and the water-gas interaction are beneficial to the accumulation of methane and the former takes the leading role, while the gas-gas interaction impedes the accumulation. Our results imply the potentials of thermal exploitation and altering the surface wettability to enhance shale gas recovery. The dynamic behaviors of methane in aqueous solutions concerned in the present study are of great importance to estimate shale gas reserves, understand shale gas flow mechanisms and develop shale gas more efficiently.