Numerical Modeling of Mineral Dissolution of Carbonate Rocks During Geological Co2 Sequestration Processes

Abstract

Abstract Carbonate formation is an ideal candidate for geological CO2 sequestration (GCS) because of its large storage capacity. One of the important issues is the CO2 leakage through highly conductive pathways. During a GCS process, the dissolved CO2 can form a weak acid in brine that can dissolve carbonate rocks by various geochemical reactions. Carbonate rocks are composed of a variety of minerals, including calcite, quartz, clay, etc. Such dissolution process may enhance the existing natural fracture system to eventually form highly conductive pathways for possible CO2 leakage. In this paper, we have developed a numerical model that couples the Stokes-Brinkman equation instead of the Darcy's Equation and a reactive transport equation, and applied for modeling of the coupled process consisting of fluid flow, solute transport, and chemical reactions. Compared to the Darcy's equation, the Stokes-Brinkman equation is a unified approach for modeling fluid flow in both porous media and free flow regions, which is an ideal candidate for modeling of porosity alteration and fracture enhancement due to mineral dissolution. The nonlinear reactive transport equations are derived for primary species from mass balance equations. In the numerical model, the Stokes-Brinkman equation and the transport reactive equations are solved by a mixed finite element method and the control-volume finite difference method, respectively, in a sequential fashion. The numerical model is validated using a CO2-saturated brine flooding experiments from the existing publications. Good agreements of effluent concentrations of aqueous species can be found between our simulation results and experimental observations. The numerical simulation study focuses on core-flooding scenarios with different mineral volume fractions and different injection rates in fractured rocks composed of multiple minerals. The preliminary results demonstrated that the mineral volume fractions have significant impact on the porosity alteration and fracture propagation. The calcite dissolution is preferred in acidic fluids over less reactive minerals including quartz and clay, and the rock properties are altered accordingly. The competitive coupling between the flow and chemical reaction rates is another important factor for mineral dissolution in our simulation study. In addition, the simulation results demonstrated that mineral dissolution processes can be altered by controlling the injection rates because the chemical reactions in the GCS processes are reversible. This work presents a mathematical model allowing us to simulate the dynamic behavior of natural fracture evolution during the GCS processes, and provides some important guidelines for the GCS implementation. Currently, we are trying to apply the simulation technology for solving some real-world problems.