An adsorption model for evaluating methane adsorption capacity in shale under various pressures and moisture

Shale generally contains water that has a significant effect on the adsorption of methane in shale. Since methane in shale is mostly stored in an adsorbed state, understanding the effect of moisture on shale adsorption behavior will play a vital role in shale gas development. In this paper, an adsorption model has been proposed to consider the effects of moisture to describe the methane adsorption behavior in wet shale. The model modified the actual methane gas pressure in reservoirs, which to evaluate accurately the methane adsorption levels in the reservoir. Moreover, the model also quantitatively described the degree of influence that moisture in shale had on the amount of shale methane adsorbed. Published data have been used to further verify the applicability of the model. Results have shown that, during a change in gas pressure, the methane adsorption amount in shale revealed a typical three-stage change law. With an increase in moisture, the methane adsorption level decreased linearly owing to the inhibitory effect of water molecules on methane adsorption. In terms of test data matching, the calculated results of the model were in approximate agreement with the measured values, indicating that the model could better describe the adsorption behavior in aquifers. Additionally, the classic Langmuir model, the Langmuir-Freundlich model, and the D-A model have been used as comparison with the proposed model in this paper. The comparison results showed that the model proposed in this paper could better quantify the effects of moisture on methane adsorption. And it possessed advantages over models, such as the Langmuir one, under high pressure conditions. In addition, because of the hydrophilic nature of shale, methane could only be adsorbed at a lower energy adsorption site, which indicated that the adsorption heat on wet shale was significantly lower than that in dry shale.