Effect of Wettability on Adsorption and Desorption of Coalbed Methane: A Case Study from Low-Rank Coals in the Southwestern Ordos Basin, China

It is important to quantitatively study the effect of wettability on low-rank coals and further analyze the mechanism of moisture on methane adsorption/desorption for the production 16 technology of coalbed methane (CBM) on low-rank coals and its reservoir evaluation. In this work, a typical long-flame coal sample was chosen to perform methane adsorption/desorption experiments and water wettability tests. The results indicated that the application of surfactant can effectively enhance the water wettability of a coal matrix, and then promote methane desorption; it can improve the critical desorption pressure and theoretical recovery efficiency of CBM. The wettability reversal agent modified coal properties, which is conducive to gas (methane) wettability. Depressurized desorption capacity, desorption efficiency, and desorption hysteresis showed a logarithmic decrease with the increasing pressure. Compared with the relatively low pressure stage, the specific pressure drop in the relatively high pressure stage has no significant effect on methane desorption. High pressure restrained the growth of desorption hysteresis; however, low pressure did the opposite. Different coal samples had different desorption hystereses due to the intrinsic differences in wettability. Wettability characteristics and pore-fracture development affected moisture distribution in coal matrix. The wetting migration as liquid water and the diffusion of gaseous water (vaporization) were the two main forms of water migration. Additionally, the mechanism of depressurized desorption was only the external consequence for replacement desorption; inversely, the wettability effect and phase transformation of water may have been the essence of the replacement desorption of CBM.