Pore Evolution during Combustion of Distinct Thermally Mature Shales: Insights into Potential In Situ Conversion

Organic-rich shales are marked by the presence of complex pore structures and extremely low permeabilities, which present challenges while extracting hydrocarbon from them. With the potential negative environmental impacts of hydraulic fracturing, recent research has focused on alternative techniques such as in situ combustion/pyrolysis for enhancing the permeability of shales. In this study, low-temperature combustion was used to evaluate the evolution of pore structures in shales for contrasting thermal maturities and organic matter type from the Raniganj and Bikaner-Nagaur basins of India. The heating temperatures were decided on the basis of the combustion behavior of the samples observed from thermogravimetric analysis (TGA). Results from low-pressure N₂ and CO₂ gas adsorption indicate that mesopore and micropore structures in shales are significantly altered due to thermal treatment at higher temperatures. In general, for both of the shales, initially, when treated at lower temperatures, with respect to the raw shales, the mesopore surface area and fractal dimensions were observed to increase with lowering of pore sizes and vice versa. Similar to the mesopore trend, the increase-decrease trend of microporosity with heating was observed to be consistent for both of the shales. The oil-window mature shales showed a significant increase in micropores compared to the thermally immature shales. Microstructural investigations using high-resolution imaging also indicated a dramatic alteration of visible porosity with thermal treatment.