From Micropores to Macropores: Investigating Pore Characteristics of Longmaxi Shale in the Sichuan Basin

Pore characteristics are crucial in the occurrence, aggregation, migration, and potential for CO2 sequestration in shale gas reservoirs. We employed N-2 adsorption/desorption, CO2 adsorption, mercury intrusion porosimetry (MIP), focused ion beam-scanning electron microscopy (FIB-SEM), and three-dimensional reconstruction of FIB-SEM images to characterize the pore characteristics in the Longmaxi Formation of the Sichuan Basin, southwestern China. These methods allowed for the detailed study of microstructure, porosity, and permeability down to the micropore scale (<2 nm). Meanwhile, N-2 and CO2 isotherm analyses revealed a range of pore sizes, including micropores, mesopores (2-50 nm), and macropores (>50 nm). Micropores significantly contribute to the specific surface area, while mesopores and macropores predominantly contribute to pore volume. MIP results indicated extremely high pore tortuosity and connected porosity less than 1.43%. FIB-SEM and its three-dimensional reconstructions showed significant pore distribution within organic matter. At the FIB-SEM resolution (5 and 10 nm), pore connectivity is notably poor, with many large pores several hundred nanometers in diameter, undetected by the N-2 isotherm method. Permeabilities estimated by FIB-SEM are 1-2 orders of magnitude lower than those measured by MIP, exhibiting anisotropy. Assessments of gas in place and CO2 storage capacity suggest that porosity evaluations via MIP may underestimate the quantifiable gas content in shale formations. The combined use of N-2 adsorption/desorption, CO2 adsorption, MIP, and FIB-SEM techniques for integrating pore size characteristics offers a holistic perspective of the pore size spectrum in shale gas reservoirs, effectively addressing the limitations inherent in each individual method.