Studies on gas behaviors in a shale matrix

Gas behavior in shale represents a critical scientific problem in the accurate simulation of shale gas production. Considering Darcy flow and Knudsen diffusion in inorganic matrix pores, surface diffusion in organic matrix pores, and the non-equilibrium desorption/adsorption mechanism (NEDAM) in inorganic and organic matrix pores, we studied the gas behavior in a shale matrix on both small (centimeters to decimeters) and larger (meters) scales. By employing three-dimensional modeling of matrix blocks on a small scale (centimeters to decimeters), we explain why production prediction using multi-continuum models deviates from actual production data obtained during production. When considering the pressure drop along the surface of matrix blocks at a larger scale (meters), the difference between the equilibrium desorption mechanism (EDM) and NEDAM is analyzed. It was discovered that the calculated decline rate of the adsorbed gas concentration in the EDM is higher than that of the NEDAM, and the calculated free gas concentration of the EDM is less, which violates the law of mass conservation. Therefore, the EDM is roughly accuracate and can lead to unreasonable results on the large scale. After further analysis of the NEDAM, it was discovered that establishing a relationship between the initial adsorbed gas concentration and the adsorption equilibrium constant is essential before studying the influence of adsorbed gas on shale gas flow.