Effects of pore structure and wettability on methane adsorption capacity of mud rock: Insights from mixture of organic matter and clay minerals

Organic matter content and pore structure are two most important parameters to evaluate methane adsorption capacity of mud rock. However, the applicability of pore structure parameters has not received much attention in previous studies. In this paper, artificial samples consisting of mixture of organic matter (anthracite, type II kerogen) and clay minerals (kaolinite, montmorillonite) are used to investigate the effects of pore structure and wettability on methane adsorption capacity of mud rock. Methane adsorption isotherms from the mixed samples are in accord with Langmuir model and the methane adsorption capacity of organic matter is higher than that of clay minerals. From nitrogen adsorption isotherm analyses, pore structure parameters including specific surface area and pore volume of organic matter are lower than that of clay minerals showing that the methane adsorption capacity of mixed samples is primarily controlled by organic matter content and are inversely related to the specific surface area and pore volume. This makes the organic matter content a more reliable parameter than pore structure to evaluate methane adsorption capacity. The result of wettability for organic matter and clay minerals shows that organic matter is hydrophobic and clay minerals are hydrophilic. Methane as an organic fluid, is more preferentially to be adsorbed on organic matter instead of clay minerals. During methane adsorption, well developed pores in clay minerals are not as effective as those in organic matter, which leads to an inevitable methane adsorption competition between organic matter and clay minerals in mixed samples. From this viewpoint, wettability plays a more important role than pore structure in competitive methane adsorption at subsurface where moisture is always existing.