Wettability modification and its influence on methane adsorption/desorption: A case study in the Ordos Basin, China

Abstract

Wettability modification of coal rock may cause serious change in the methane adsorption/desorption behaviors. Thus, the contact angle measurement and isothermal adsorption/desorption experiment of raw coal and samples treated with surfactants (STS) were conducted to investigate the wetting properties and methane adsorption/desorption characteristics. The results indicated that the long-flame coal (LFC) has the good hydrophilicity itself, and there was no evident sign of saturated adsorption for different samples at relatively low-pressure (<8 MPa) stage. The sample treated with the wettability reversal agent (G502) had the larger methane adsorption capacity than the wetted samples (LAS, JFC, and 6501), with the latter facilitating the methane desorption. Additionally, the adsorption capacity of the air-dried samples was stronger than that of the water-containing samples, including the moisture-equilibrated, LAS, JFC, and 6501, reflecting that the better the hydrophilicity, the weaker the methane adsorption. Besides, the desorption hysteresis rate of the same sample was mainly controlled by pressure, i.e. the desorption hysteresis indicated a logarithmic decrease with the increasing pressure, but the STS clearly showed the desorption hysteresis to varying degree due to the wettability differences. The pressure drop in the low-pressure stage was difficult to drive the adsorbed methane to immediately desorb from the matrix surface, whereas even caused further adsorption or re-adsorption, meaning that the coal required a greater depressurization to enter the effective desorption stage. Finally, the positive implications of wettability modification including enhancing CBM recovery, as well as coal and methane outburst prevention and coal dust control in the mining industry were discussed.