Interpreting Pore Dimensions in Gas Shales Using a Combination of SEM Imaging, Small-Angle Neutron Scattering, and Low-Pressure Gas Adsorption

Abstract

Permian shales of Barakar formation in India were investigated to study their pore structure to understand their potential for natural gas production and possible CO2 sequestration. The studied shale samples with variable clay content were of early mature stage and contained low (<2%) total organic carbon. Initially, a combination of small-angle neutron scattering (SANS) and low-pressure gas adsorption (LPGA) was used to identify the pore sizes and fractal dimensions of Indian shales. It was found that the quenched surface density functional theory model in the LPGA method gave better pore size distribution (PSD) estimates over the nonlocal density functional theory model. The micropores and smaller mesopores contribute the most to the total pore volume and the surface area of the studied shale samples. The average pore size decreased with an increase in pore volume. The fractal studies using SANS reveal that all studied shales possess similar fractal dimension despite being different in mineralogy, maturity, and total pore volume. The PSD and its possible relation with the mineral composition and the accessibility of the pores in terms of gas storage have been elucidated. Pore morphology was analyzed using image analysis of field emission scanning electron microscopy and low-pressure adsorption, corroborated by SANS results. The effects of dissolution and deposition probability on the fractal dimension of the shale were interpreted using the Monte Carlo-based computer modeling. The fractal dimension was higher in the case of shales that underwent simultaneous dissolution and deposition processes.