Modelling CO2-induced fluid-rock interactions in the Altensalzwedel gas reservoir. Part II: coupled reactive transport simulation

Injection of CO2 into gas reservoirs for CO2-enhanced gas recovery will initiate a series of geochemical reactions between pore fluids and solid phases. To simulate these conditions, the coupled multiphase flow and multicomponent reactive transport simulator OpenGeoSys-ChemApp was extended to take into account the kinetic nature of fluid/mineral reactions. The coupled simulator is verified successfully for the correctness and accuracy of the implemented kinetic reactions using benchmark simulations. Based on a representative geochemical model developed for the Altensalzwedel compartment of the Altmark gas field in northeastern Germany (De Lucia et al. this issue), the code is applied to study reactive transport following an injection of CO2, including dissolution and precipitation kinetics of mineral reactions and the resulting porosity changes. Results from batch simulations show that injection-induced kinetic reactions proceed for more than 10,000 years. Relevant reactions predicted by the model comprise the dissolution of illite, precipitation of secondary clays, kaolinite and montmorillonite, and the mineral trapping of CO2 as calcite, which starts precipitating in notable quantities after approximately 2,000 years. At earlier times, the model predicts only small changes in the mineral composition and aqueous component concentrations. Monitoring by brine sampling during the injection or early post-injection period therefore would probably not be indicative of the geochemical trapping mechanisms. One-dimensional simulations of CO2 diffusing into stagnant brine show only a small influence of the transport of dissolved components at early times. Therefore, in the long term, the system can be approximated reasonably well by kinetic batch modelling.