There have been extensive studies of the kinetics of pristine carbonate minerals in acidified media including (CO2 + H2O) systems at elevated temperature and pressure conditions pertinent to carbon storage. However, most of those studies have not considered the several complexities that occur in real reservoirs. The goal of this study was to investigate some of these complexities and their impacts on reaction rates under reservoir conditions. The variables investigated in this study include: aqueous chemistry and ionic strength, saturation state, surface contaminants and chemical heterogeneity in reservoir minerals. The majority of the reaction rates reported in this study are from batch reactor experiments implementing a form of the rotating disk technique, which is chosen to eliminate mass transport effects.
Calcite (CaCO3) dissolution kinetics were investigated in (CO2 + H2O + NaCl), (CO2 + H2O + NaHCO3), (CO2 + H2O + Na2SO4) and (CO2 + H2O + Mixed Salts) systems. These studies were carried out at temperatures ranging from (323 to 373) K and pressures ranging from (6 to 10) MPa. A minor increase in the dissolution rates as a function of ionic strength was observed in every system except for (CO2 + H2O + NaHCO3), where a marked reduction in dissolution rates was measured. These observations are consistent with the predicted changes in the pH of the aqueous system. The influence of saturation states in the dissolution kinetics of calcite was investigated in the (CO2 + H2O) system at a temperature T of 373 K and a pressure p of 6 MPa. Consistent with previous studies, the measured dissolution rates deviate from the classical transition state theory (TST) model which was developed for elementary reactions in homogeneous media. A modified TST expression was subsequently proposed. Next, the dissolution kinetics of three chemically heterogeneous carbonate reservoir rocks were investigated in (CO2 + H2O) system at T = 323 K and p = 10 MPa. For a single carbonate mineral in the heterogeneous matrix, the measured dissolution rates were found to be comparable to those of a chemically homogeneous system under similar experimental conditions.
Finally, the impact of surface alterations (including adsorbed biofilms and crude-oil films) on calcite dissolution kinetics was investigated in (CO2 + H2O) systems at temperatures ranging from (325 to 333) K and pressures up to 10 MPa. Some of these films made a minor difference in reaction rates and the effects were found to be dependent on temperature, pressure, exposure time and reactor configurations. In this study, extensive characterizations were performed on both fluid and solid phases, and geochemical simulations were implemented in the PHREEQC software. Further, preliminary insights from Lattice Boltzmann modelling and reactive core flooding studies are presented.