Three stages of methane adsorption capacity affected by moisture content

Methane adsorption capacity is a key factor in determining shale gas in place (GIP) – requiring that it is determined under in situ moisture conditions. Current methods may be insufficient to investigate these exact characteristics when applied to actual reservoirs with high or variable moisture contents. We propose a heating and cooling (HC) method to prepare shale samples to arbitrary moisture contents (M_c up to 10%). A series of CH₄ adsorption experiments on two different types of shale are conducted as a function of M_c at 35 °C, 45 °C, and 55 °C, and at a CH₄ pressure of up to 10 MPa. Experimental results indicate that the methane sorption capacity versus moisture content curves exhibit a linear decreasing stage, a flat stage and a convex decreasing stage, separated by two threshold moisture contents. The lower moisture content threshold (M_fc) represents coverage of the entire hydrophilic surface by a monolayer of water. The upper moisture content threshold (M_sc) is the point at which no methane is adsorbed on the surface of the clay pores and adsorption capacity is further reduced as moisture content is increased. The linear stage with M_c up to the M_fc is mainly dominated by the competition between water and methane for adsorption sites on the surface of clay pores. Slope value of this stage are affected by pressure, temperature and shale compositions. The flat stage represents that the moisture content has negligible effect on shale adsorption capacity for M_c in the range M_fc to M_sc. Methane adsorption capacity decreases in a convex manner above M_sc, suggesting water condensation in organic pores as the surface area for methane adsorption is reduced by water blocking. A conceptual Bi-Langmuir model is presented to represent the crucial effects of moisture content on methane adsorption capacity including accurate estimations of original GIP under different reservoir conditions.