Stress state and stress path evaluation to address uncertainties in reservoir rock failure in CO₂ sequestration in deep saline aquifers: An experimental study of the Hawkesbury sandstone formation

Injecting CO₂ into aquifer pore fluid (high salinity brine) in deep saline aquifers during the sequestration process causes the chemico-mineral structure to be altered through complex chemically-coupled mechanical deformations. This is as yet poorly understood in the field. The authors conducted a series of tri-axial strength tests on Hawkesbury sandstone under in-situ stress and temperature conditions to characterise the behaviour of reservoir rock upon exposure to super-critical CO₂ (ScCO₂₎ to determine this chemically-coupled mechanical behaviour. According to the findings, injection of CO₂ into a brine-saturated reservoir rock mass may cause a considerable strength reduction, probably due to the rock's mineralogical alteration-induced mechanical weakening of grain contacts. This was confirmed by SEM analysis, according to which the mineral dissolution process upon exposure to ScCO₂ is significant, and considerable quartz and calcite dissolution were noticed in the tested samples. Importantly, this rock mineral dissolution may alter the reservoir's natural pore geometry. This eventually affects the effective stress patterns acting on the rock matrix. In addition, the slip tendency of brine+CO₂-reacted reservoir rock is increased with increasing injection pressure, revealing the fate of the resulting pore pressure-dominant effective stress field through the CO₂ injection process. The results were then incorporated in the effective stress field model. This model can be used to predict the possibility of mechanical failure of reservoir rock upon CO₂ injection into saline aquifers.