CO2†induced chemo†mechanical alteration in reservoir rocks assessed via batch reaction experiments and scratch testing

The injection of carbon dioxide (CO2) into geological formations results in a chemical re†equilibration between the mineral assemblage and the pore fluid, with ensuing mineral dissolution and re†precipitation. Hence, target rock formations may exhibit changes of mechanical and petrophysical properties due to CO2 exposure. We conducted batch reaction experiments with Entrada Sandstone and Summerville Siltstone exposed to de†ionized water and synthetic brine under reservoir pressure (9–10 MPa) and temperature (80°C) for up to four weeks. Samples originate from the Crystal Geyser field site, where a naturally occurring CO2 seepage alters portions of these geologic formations. We conducted micro†scratch tests on rock samples without alteration, altered under laboratory conditions, and naturally altered over geologic time. Scratch toughness and hardness decrease as a function of exposure time and water salinity up to 52% in the case of Entrada and 87% in the case of Summerville after CO2†induced alteration in the laboratory. Imaging of altered cores with SEM†EDS and X†ray microCT methods show dissolution of carbonate and silica cements and matrix accompanied by minor dissolution of Fe†oxides, clays, and other silicates. Parallel experiments using powdered samples confirm that dissolution of carbonate and silica are the primary reactions. The batch reaction experiments in the autoclave utilize a high fluid to rock volume ratio and represent an end member of possible alteration associated with CO2 storage systems. These types of tests serve as a pre†screening tool to identify the susceptibility of rock facies to CO2†related chemical†mechanical alteration during long†term CO2 storage. © 2017 Society of Chemical Industry and John Wiley & Sons, Ltd.