Hydration Impact on Pore Structure and Methane Adsorption in the Lower Silurian Longmaxi Shale of the Sichuan Basin

Abstract

ABSTRACT: For shale gas reservoirs in the Sichuan Basin of China, renowned for low water saturation and complex pore structures rich in clay minerals, exhibit significant fluid imbibition post-hydraulic fracturing. This study delves into the Longmaxi Formation shale in the Sichuan Basin, China. Employing quantitative isothermal adsorption techniques, it elucidates pore structure alterations pre- and post-hydration. Quantification of the impact of fluid imbibition on different pore sizes was conducted. It was found that the influence on organic matter pores and macropores larger than 50 nm was minor, whereas the effect on clay inorganic matter pores in the range of 2-30 nm was most significant. Additionally, the influence of water saturation on methane adsorption capacity was quantified. With increasing water saturation, methane adsorption exhibited a nonlinear decreasing trend. An empirical formula was established to predict the adsorbed gas content evolution based on pore structures ranging from 0 to 20 nm under shale imbibition conditions. The reservoir methane adsorption capacity was calculated based on different soaking times, indicating that the reservoir's adsorbed gas content exhibited the highest desorption efficiency after 6 days of soaking. 1. INTRODUCTION Shale oil and gas originate from organic-rich hydrocarbon source rocks characterized by low porosity, low permeability, integrated source-reservoir systems, and developed bedding structures (Jarvie et al., 2003). In the development of shale oil and gas, horizontal well multi-stage fracturing technology is commonly employed to enhance production artificially (Curtis, 2002; Zhao et al., 2018). Furthermore, due to hydrocarbon generation pressure and gasification-driven liquid carrying effects in shale reservoirs, they often exhibit extremely low water saturation characteristics (Bennion et al., 2000; Bennion & Thomas, 2005). Moreover, shale composition and pore systems are highly complex, consisting of micro-nano pores in organic matter and inorganic pores in clay minerals and brittle minerals (Han et al., 2020; Xie et al., 2022), leading to strong water absorption capabilities in shale (Minardi et al., 2018; Zeng et al., 2021).