The importance of physiochemical processes in decarbonisation technology applications utilizing the subsurface: A review

The Earth’s subsurface not only provides a wide range of natural resources but also contains large pore volume that can be used for storing both anthropogenic waste and energy. For example, geothermal energy may be extracted from hot water contained or injected into deep reservoirs and disused coal mines; CO2 may be stored within depleted petroleum reservoirs and deep saline aquifers; nuclear waste may be disposed of within mechanically stable impermeable strata; surplus heat may be stored within shallow aquifers or disused coal mines. Using the subsurface in a safe manner requires a fundamental understanding of the physiochemical processes which occur when decarbonising technologies are implemented and operated. Here, thermal, hydrological, mechanical and chemical perturbations and their dynamics need to be considered. Consequently, geoscience will play a central role in Society’s quest to reduce greenhouse gas emissions. This contribution provides a review of the physiochemical processes related to key technologies that utilize the subsurface for reducing greenhouse gas emissions and the resultant challenges associated with these technologies. Dynamic links between the geomechanical, geochemical and hydrological processes differ between technologies and the geology of the locations in which such technologies are deployed. We particularly focus on processes occurring within the lithologies most commonly considered for decarbonisation technologies. Therefore, we provide a brief comparison between the lithologies, highlighting the main advantages and disadvantages of each, and provide a list of key parameters and properties which have first order effects on the performance of specific rock types, and consequently should be considered during reservoir evaluation for decarbonising technology installation. The review identifies several key knowledge gaps that need to be filled to improve reservoir evaluation and performance prediction to be able to utilize the subsurface efficiently and sustainably. Most importantly, the biggest uncertainties emerge in prediction of fracture pattern development and understanding the extent and timescales of chemical reactions that occur within the decarbonising applications where external fluid or gas is cyclically injected and invariably causes disequilibrium within the system. Furthermore, it is clear that whilst geoscience can show us the opportunities to decarbonise our cities and industries, an interdisciplinary approach is needed to realize these opportunities, also involving social science, end-users and stakeholders.