Modelling of geochemical reactions during CO₂ WAG injection in carbonate reservoirs

Abstract

In this thesis calcite dissolution and precipitation are investigated during injection of CO₂ WAG (water alternating gas) in limestone oil reservoirs. First, the equilibrium between calcite and the carbonic acid system is studied in a static environment to understand how variations in chemical composition, temperature and pressure affect the mineral reactions. Then, four models of CO₂ solubility are presented (PHREEQC, CMG GEM, Duan & Sun and Diamond & Akinfiev) and compared against experimental data from the literature. After that, reactive transport simulations are performed using PHREEQC and GEM. The injection of carbonated water in a limestone reservoir is simulated with PHREEQC to assess the behaviour of calcite reactions. The obtained results are explained and also observed in a similar model using GEM. Additional simulations are performed in GEM concerning single-phase injection (seawater and pure CO2 injections) and their analyses are used to assist in the interpretation of the more complex CO2 WAG and CO2 SWAG (simultaneous water and gas injection). Different WAG slug sizes are simulated and simple relationships between the WAG ratio (volumetric ratio between injected water and injected gas at reservoir conditions) and the dissolved calcite are determined. Sensitivity of the porosity change and scale deposition is assessed during grid refinement. A dissolution zone around the injector wellbore is obtained for the WAG process that is dependent on the WAG scheme. Later, reactive transport simulations performed in GEM are extended to 3-phase non-isothermal flow in 2D and reanalysed. The impact of heat exchange on the dissolution is investigated for different time step sizes. More WAG scenarios are simulated and a relationship between the WAG scheme and dissolution similar to 1D simulations is obtained. The final model is completed by adding layers with different properties to investigate how the communication between layers affects the reactions. The dissolution zone and porosity increase are determined based on the flow capacities and front velocities of the layers, while the depth of more sever scale risks are located by considering also the gravity segregation of injected fluids. Finally, different geological scenarios, well operations and initial reservoir conditions are simulated.