Methane absorption of coal-measure shales with and without pore water from the Qinshui Basin, North China: Based on high-pressure methane absorption experiments

Abstract

The coal-measure shale in the Qinshui Basin, North China has a high thermal maturity and a huge shale gas resource, but studies related to its gas-bearing property (GBP) are lack. In this paper, shale samples taken from the carboniferous coal-measure strata in the Yangquan block of Qinshui Basin were mainly investigated on their pore water contents and occurrence characteristics, and their high-pressure methane adsorption capacities under the dry and moist conditions. The results show that the pore water in the shale is in an ultra-low saturation state, occurring mainly in clay minerals (91% on average), significantly decreases its effective pore structure parameters, especially the nonmicropore specific surface areas (by 70.21% on average), and reduces its methane adsorption capacity (by 33% on average at 30 °C), with the adsorbed methane occurring mainly in micropores and controlled by the TOC content. The adsorption of methane on the shale is an exothermic process, and both the shale adsorption capacity and adsorbed methane density are reduced with increasing temperature. Pore water not only has a competitive adsorption with methane, but also weakens the adsorbed methane inter-molecular interaction forces, decreasing the methane adsorption capacity, especially under the low temperature and pressure conditions. The GBP models of a selected shale under the dry and moist conditions further indicate that pore water also decreases the adsorbed gas percentage to some extent, especially at a burial depth < 1500 m, and the adsorbed gas is dominant in the shale. These GBP characteristics of coal-measure shale should pay a great attention in the future exploration and development of shale gas in the Qinshui Basin.