Optimal selection of inversion method for gas-adsorption pore characterization of shales in Wufeng and Longmaxi Formation, Sichuan Basin

Abstract

The pore space in gas shale spans multiple scales, ranging from nanometers to micrometers with an extremely complex structure and strong microscopic heterogeneity. Existing low-temperature gas adsorption (LTGA) pore size distribution (PSD) analysis models are based on the assumption of a one-dimensional tubular model and there is a lack of sufficient basis for selecting the most appropriate inversion model among the various types available. In this study, a comprehensive characterization of numerous samples from the Wufeng and Longmaxi Formation in the southern Sichuan Basin was conducted using different methods and inversion models. Based on multiple dimensions, such as fitting errors, the correlation between different methods including low-temperature CO2 adsorption (LTCA), nitrogen adsorption (LTNA), and large-scale mosaic Scanning Electron Microscopy (LAM-SEM), the most suitable one-dimensional models for LTCA and LTNA PSD analysis for the studied shale samples are determined. The simulation of gas condensation processes on LAM-SEM images yielded pore volume (PV) in the range of 32–48 nm comparable to the PV characterized by LTNA. Furthermore, the differences observed in PSDs between the gas condensation simulation procedure and the equivalent circular area diameter method indicate that irregular corner regions within larger pores are attributed to the volume of smaller-sized pores during gas adsorption characterization. It is revealed that the correlation between stitched surface area (SA) and maximum methane adsorption is stronger than individual characterizations, indicating that both micropores and mesopores contribute significantly to adsorption, and both characterization methods need to be employed to obtain SSA that influences methane adsorption. The research findings are helpful to the improvement of the multi-scale pore characterization level and evaluation of shale reservoirs.