Experimental investigation into simultaneous adsorption of water vapor and methane onto shales

Abstract

Methane is the main component of natural gas and a greenhouse gas. It usually coexists with water in geological formations. Methane adsorption onto shale has been studied extensively, but there is no report on the Simultaneous Adsorption of Water vapor and Methane (SAWM). This paper reports a new experimental method for studying SAWM onto shale. During the experiment, liquid water, water vapor and methane coexist in an adsorption cell. Water vapor is produced via evaporation of liquid water, and the processes of liquid water evaporation and water vapor and methane adsorption onto shale occur simultaneously. The rate of liquid water conversion into vapor and the free water vapor content in the gas mixture depend on Relative humidity (Rh). Changes in Rh are monitored with a humidity-sensor. In SAWM, the amount of water vapor adsorbed greatly exceeds that of methane. The adsorption of water vapor took longer to reach equilibrium than that of methane. The amount of water vapor adsorbed at equilibrium decreased with total pressure, while the opposite situation occurred with methane adsorption. Compared to Pure Methane adsorption onto Dry Shale (PMDS), the amount of methane adsorbed was lower by 10–59% in SAWM. The equilibrium time for methane adsorption was higher, and t50 and t80 (t50 and t80 represented the times required for adsorption of 50% and 80% of the equilibrium adsorption amount, respectively) increased 12.0–108.0 and 9.0–115.2 times, respectively. In the two experiments, the declines in the level of methane adsorption were equal when a critical threshold Equilibration Degree (Ed) was reached. Before reaching Ed, the rate of methane adsorption in SAWM was higher than that in PMDS. After reaching Ed, the phenomenon was reversed.