Dynamic behaviors of methane adsorption on partially saturated shales

Abstract

The dynamic behavior of methane adsorption onto shales plays an important role in many resource and environmental problems. Previous studies have mainly analyzed gas diffusion, but the studies have also shown that the adsorption process consists of multiple forms of gas movement. In fact, water always exists in shale, but the process of methane adsorption onto partially saturated shales has received little attention. To study the movement behavior of methane in the adsorption process and determine how water influences the dynamic behaviors of methane adsorption, a dynamic adsorption model (DAM) was proposed. In this study, dynamic adsorption experiments involving three shale samples with four water saturation levels were conducted. Based on the experiments, 48 adsorption process curves were obtained. The dynamic adsorption process was divided into three stages based on the curves. The first stage occurred when the adsorption amount reached 50% of the equilibrium adsorption amount, and the corresponding time range was t < t50; the second occurred when the adsorption amount increased from 50% to 80%, and the corresponding time range was t50 < t < t80; and the third occurred when the adsorption amount increased from 80% to 100%, and the corresponding time range was t80 < t < t100. These stages were defined as the gas flow control stage, transition stage and interface interaction stage, respectively. The basis for this determination was that the adsorption rate was mainly controlled by the gas flow, gas flow and interface interaction, and interface interactions during these stages, respectively. The experimental data were fitted with the unipore diffusion model (UNM) based on Fick's second law and the modified unipore diffusion model (MM). The results showed that the UNM was better fit to the gas flow control stage but cannot describe the entire adsorption process. The MM was well fit to the dry shale methane adsorption process, although the wet shale fitting effect was inferior to that of the DAM. Analysis of the effect of water on the adsorption rate in each stage revealed that the inhibition of water acted against the methane gas flow more strongly than against the interface interaction. The effective pore volume (the fraction of pores not occupied by water molecules that could be reached by methane molecules) had a strong positive correlation with the adsorption rate in the early stage of the adsorption process, whereas the correlation was weak in the later stage. Under low water saturation conditions, the reduction of the adsorption rate due to the increase in the unit water saturation was larger, whereas this effect was smaller under high water saturation conditions. Gas flow was greatly affected by water in the clay matrix, whereas the interface interaction was less affected. The pattern was diametrically opposite in the organic matrix.