Influence of CO2 on the long-term chemomechanical behavior of an oolitic limestone

Abstract

In order to study the long-term mechanical and petrographical evolutions of a carbonate rock (oolitic limestone) during storage of CO2, CO 2 injection tests were performed in triaxial cells at temperature and mechanical stresses (isotropic and deviatoric) corresponding to the depth of the Dogger carbonate reservoirs of the Paris basin (∼800 m). We used a specific "flow-through" triaxial cell which allowed us to measure very low strain rates in both axial and lateral directions, while ensuring the sealing of the samples during the injection of CO2. Under isotropic loading, neither the dynamic percolation (i.e., flow-through tests) of dry supercritical/gaseous CO2, nor the diffusion of CO2, into initially saturated samples was shown to produce significant axial compaction and calcite dissolution. Indeed, even though the interstitial aqueous fluid becomes acidic, the progressive increase in dissolved species induces the H 2O-CO2-calcite re-equilibrium. The dynamic injection of CO2-saturated solution induced significant axial compaction due to the dissolution of calcite at the sample/piston interface only under open flow conditions (i.e., the injected acidic solution is continuously renewed). Under closed flow conditions (i.e., acidic solution recirculation or no-flow conditions) which reproduce the physicochemical conditions of CO2 storage at the field scale better, the rapid H2O-CO 2-calcite re-equilibrium inhibits calcite dissolution. Under deviatoric loading and closed conditions, the diffusion of CO2 induced a very small increase in the PSC (pressure solution creep) process which was stopped by the H2O-CO2-calcite re-equilibrium inside the sample. Therefore, a significant compaction of limestone samples was obtained only under open conditions and is mainly due to a purely chemical mechanism (calcite dissolution), while the contribution of the chemo-mechanical mechanism (PSC) was found to not be of any great importance. In the context of massive injection of CO2 at the field scale, if the reservoir can be considered as a closed system from a hydrodynamic point of view (i.e., the brine circulates in the aquifer but is not renewed by any groundwater), CO 2 will not play a significant role in the chemistry of carbonate reservoirs due to the H2O-CO2-calcite re-equilibrium and will not induce reservoir compaction and affect its long-term storage capacity, whatever the stress state (isotropic or deviatoric). Copyright 2011 by the American Geophysical Union.