CO2 injection into limestone reservoirs is a typical process in enhanced oil recovery operations, and it is also suggested for carbon geo-storage. However, CO2, together with water, is acidic and creates medium strength acid (pH value 3-4) at HP/HT (High Pressure/High Temperature) conditions. At the same time, it is well known that carbonates react and dissolve when exposed to acid. It can, therefore, be expected that the limestone properties change significantly during CO2 injection.
We thus hypothesized that limestone dissolution results in a substantial reduction of mechanical strength of the rock, with potential subsequent (mechanical) collapse of the rock – which would represent a major geohazard and could lead to land subsidence. We therefore conducted HP/HT core flooding tests on Savonnières limestone plugs; the plugs were thoroughly characterised with various experimental techniques (NMR-T2 response, porosity, dynamic permeability, acoustic response, x-ray computed tomography and rock-mechanical tests) before and after acid injection, i.e. exposure to supercritical CO2.
We indeed measured a significant dissolution of the rock and associated substantial mechanical weakening of the rock. Differently shaped wormholes formed, which strongly influenced the mechanical behaviour. Furthermore, a significant permeability increase was observed (up to 42.3% increase after injection), consistent with wormhole formation. We conclude that CO2 injection may pose a geohazard if the geo-mechanical strength of the reservoir is compromised.