Influence of CO₂on the long-term chemomechanical behavior of an oolitic limestone

Abstract

[1] In order to study the long-term mechanical and petrographical evolutions of a carbonate rock (oolitic limestone) during storage of CO 2 , 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 CO 2 . Under isotropic loading, neither the dynamic percolation (i.e., flow-through tests) of dry supercritical/gaseous CO 2 , nor the diffusion of CO 2 , 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 2 O-CO 2 -calcite re-equilibrium. The dynamic injection of CO 2 -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 CO 2 storage at the field scale better, the rapid H 2 O-CO 2 -calcite re-equilibrium inhibits calcite dissolution. Under deviatoric loading and closed conditions, the diffusion of CO 2 induced a very small increase in the PSC (pressure solution creep) process which was stopped by the H 2 O-CO 2 -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 CO 2 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 H 2 O-CO 2 -calcite re-equilibrium and will not induce reservoir compaction and affect its long-term storage capacity, whatever the stress state (isotropic or deviatoric).

The effect of CO2 on the mechanical properties of the Captain Sandstone: Geological storage of CO2 at the Goldeneye field (UK)

“…As a result, compaction and compaction rates increase (Grgic, 2012;Le Guen et al, 2007;Liteanu et al, 2012;Vanorio et al, 2011), while a decrease in elastic properties (Madland et al, 2006;Vialle and Vanorio, 2011) and rock strength (Madland et al, 2006) is observed.…”

“…Studies on the effect of CO 2 -saturated fluids on mechanical and transport properties of carbonate rocks and aggregates (Grgic, 2012;Le Guen et al, 2007;Liteanu et al, 2012;Madland et al, 2006;Vialle and Vanorio, 2011) show dissolution of the carbonate phases, leading to the creation of flow channels (Le Guen et al, 2007;Madland et al, 2006) and a significant increase in porosity and permeability (Le Guen et al, 2007;Vanorio et al, 2011;Vialle and Vanorio, 2011). As a result, compaction and compaction rates increase (Grgic, 2012;Le Guen et al, 2007;Liteanu et al, 2012;Vanorio et al, 2011), while a decrease in elastic properties (Madland et al, 2006;Vialle and Vanorio, 2011) and rock strength (Madland et al, 2006) is observed.…”

“…We will proceed to present the results for the Ca 2+ concentration measurements, as [Ca 2+ ] can be used as an indicator for calcite dissolution (Grgic, 2012;Pokrovsky et al, 2009). As can be seen in Fig.…”

“…Using the effluent data obtained during the CO 2 -stage and assuming all Ca 2+ comes from calcite dissolution according to the reaction CaCO 3 + CO 2 + H 2 O → Ca 2+ + 2HCO 3 − (Grgic, 2012;Pokrovsky et al, 2009), mass balance calculations can be made to determine the amount of calcite that has been dissolved. For both experiments, it amounts to ∼0.3 wt% of calcite, which is in accordance with the amount predicted by the XRD measurement on the starting material.…”

“…However, it is difficult to evaluate the effect of such processes as little data exists on the rates of the creep processes relevant for subsurface reservoirs (e.g. see Grgic, 2012;Hangx et al, 2010;Le Guen et al, 2007;Liteanu et al, 2012) and no constitutive laws to predict the chemical-mechanical response to CO 2 injection have been developed. Therefore, for each individual site considered for CCS, it is currently necessary to evaluate the effect of CO 2 /brine/rock chemical interactions on rock properties, under in situ conditions, in order to provide long-term numerical simulation models with appropriate input parameters.…”