Effects of CO₂ Exposure on Unconventional Reservoir Rock Microstructure

In efforts to reduce atmospheric carbon emissions, increasing attention has been focused on permanent and safe geological storage of CO₂. Depleted unconventional reservoirs have been considered as potential sites for permanent storage because of their low permeability, preventing CO₂ from escaping the formation once injected. However, for the storage to be feasible long term, the integrity of the microstructure of the reservoir and/or seal rocks exposed to CO₂ at reservoir temperature and pressure, and with the presence of remaining reservoir fluids, needs to be understood. These unconventional reservoirs rocks have highly heterogeneous microstructures and varying mineralogy that make each reservoir unique in its potential for permanent CO₂ storage.

Samples from a variety of different unconventional reservoirs were selected and prepared for scanning electron microscopy (SEM) imaging by ion milling. Mineralogical characterization was performed by Fourier Transform Infrared (FTIR) spectroscopy. In addition, Young’s modulus of each sample was determined by nanoindentation pre- and post-exposure to CO₂. SEM imaging was performed to characterize the microstructure and energy dispersive spectroscopy (EDS) was used to determine spatial mineralogy prior to CO₂ exposure. Samples were then saturated with brine solutions and exposed to supercritical CO₂ at a pressure of 3000 psi and temperature of 65 °C for up to 3 weeks. Samples were then re-imaged in the same areas imaged prior to CO₂ exposure. Results showed significant alteration in microstructure at the surface in contact with the CO₂. Carbonates were dramatically altered and wholly removed in some areas. Iron sulfide was observed to be altered with some areas appearing to form a layer of iron oxide where the iron sulfide was. Changes in Young’s modulus for some samples were observed. This combined with the microstructural observations suggest a weakening of the rock samples.

For long term storage of CO₂ in unconventional reservoirs to be realized, the effects of CO₂ on the microstructure of these reservoirs need to be fully understood. Our results show that CO₂ alters the microstructure and mechanical behavior of the reservoir rocks on relatively short time scales. The heterogeneous microstructure of unconventional reservoirs and the presence of reservoir fluids complicates the understanding, requiring thorough study of rocks from individual reservoirs under various conditions.