AbstractWe have investigated the effect of CO2 on the mechanical stability of the reservoir-caprock system. Castlegate and Bentheimer sandstones were used as analogues for reservoir rock. Pierre shale was utilized as an analogue material for a typical cap rock. The effect of CO2 on carbonate rocks was studied by carrying out Brazilian tests on Lixhe and Austin chalks. The tensile strengths of both salt water and CO2-salt water exposed samples were observed to decrease with sample porosity. There was a positive correlation with tensile strength and p-wave velocity. The tensile strength of sandstone, shale and chalk is not markedly affected by the presence of CO2 in our tests. This observation has important implications for modeling fracture growth due to the injection of CO2 on geological formations because geomechanical models require tensile strength as an input parameter. Future experimental work should quantify the effect of CO2 on the entire failure envelope by using preserved core material and a triaxial test setup that mimics the in-situ stress and temperature conditions at a storage site.