A technical review of in situ conversion technologies for oil shale

The global depletion of conventional oil and gas resources has led to the exploration of alternative energy resources. Among these resources, oil shale is a fine-grained sedimentary rock with a large amount of kerogen that has attracted increasing attention from the hydrocarbon industry and researchers due to the enormous global reserves of oil shale, which are larger than those of conventional crude oil. However, in situ conversion technologies (ICTs) constitute a practical and environmentally friendly technology for extracting hydrocarbons from oil shale formations without traditional surface mining. This study provides a comprehensive analysis of the primary ICTs developed and evaluated in this field to date. Convection-based approaches, such as steam injections and modified thermal injection (MTI), conduction-based heating approaches, such as Shell’s in situ conversion process (ICP) and ExxonMobil’s Electrofrac™, and radiant heating techniques that utilize radio frequency and microwave energy, are critically evaluated in this study. Advanced innovations, such as near-critical water extraction, nitrogen-assisted in situ conversion process (NAICP), and terahertz spectroscopy (a diagnostic tool for reservoir evaluation), highlight the advanced direction of this field. Mechanistic insights into pore–fracture network formation, kerogen pyrolysis, and their function in improving permeability and hydrocarbon flow have also been analyzed. Challenges related to energy efficiency, operating costs, and environmental impacts still remain despite these technological advancements. Addressing these limitations by maximizing thermal delivery, catalytic enhancement, and integrated modeling will be crucial for realizing the large-scale viability of ICTs. Future research should focus on increasing the efficiency of energy recovery while maintaining environmental sustainability and economic competitiveness.