Characteristics of methane isothermal adsorption of deep shale from Lower Silurian Longmaxi Formation in southeastern Sichuan Basin and its geological significance

Abstract

Breakthrough of the exploration of deep shale gas in the Sichuan Basin, Southwest China, has been achieved for decades, there are still some controversies about the adsorption performance and occurrence state of shale gas in deep strata. Isothermal adsorption experiments were carried out under high temperature and high pressure with typical shale samples from deep stratum of the Lower Silurian Longmaxi Formation in Fuling and Dingshan areas in the southeastern Sichuan Basin. Results show that there is a big difference between absolute adsorption and excess adsorption, thus, it is proposed that the excess adsorption capacity should be corrected to the absolute adsorption capacity when evaluating the adsorption performance of deep shale gas. After correction, the variation of absolute shale adsorption with pressure appeared to have three stages including "rapid rise", "slow rise" and "steady rise". The absolute adsorption capacity of deep shale samples from Fuling and Dingshan areas is higher than 2.0 m3/t at 110℃ and 70 MPa, and combined with the adsorption and free gas ratios of deep shale, the total gas content is more than 4.0 m3/t, indicating that deep shale may still have good adsorption performance under high temperature and high pressure. The evaluation of deep shale adsorption capacity can provide reliable parameters for the study of shale gas content and the evaluation of resource potential and reserve for shale gas. Considering the difference of ratio of free gas to adsorbed gas between deep shale and shallow-medium shale, different exploration and production methods can be adopted to effectively improve the production capacity and production cycle of shale gas.

Keywords

• isothermal adsorption
• absolute adsorption capacity
• deep shale gas
• longmaxi formation
• lower silurian
• southeast sichuan basin