Modelling porosity and permeability alteration during CO₂ wag injection in carbonate oil reservoirs

The alternate injection of CO₂ gas and water (CO₂ WAG) adds more complexity to the development of carbonate oil reservoirs. First, the switching between water and gas cycles causes oscillations in the pressure and flow rate because of variable fluid mobilities. Second, gravity segregation can occur since the CO2 is less dense than water (and oil) under most reservoir conditions. Moreover, since the injected fluids are generally cooler than the initial reservoir temperature, heat exchange is constantly happening because the injected fluids (especially water) extract heat from the reservoir rock as they propagate from cooler to warmer regions. These changes in temperature disturb the flow behaviour not only by modifying the physical properties of the fluids, such as densities and viscosities, but also by enhancing calcite dissolution and subsequent pore space alterations. In this work, we simulate multiphase flow coupled with heat exchange and mineral reactions to model the porosity and permeability changes during CO2 WAG injection in carbonate oil reservoirs. A multilayered model of a limestone reservoir is initialised with 500 bar to calculate calcite volume changes in a more reactive environment comparable with the Brazilian pre-salt, as compared to reported (carbonate) field cases and simulations. Porosity changes are derived from volume change of calcite at each time step, while permeability is updated following the formulation of Carman-Kozeny. Dissolution zones around the injectors are calculated as well as their impact on permeability and injectivity increase. Special considerations are made regarding the mass of calcite that can potentially precipitate in the production system (i.e. scale management). This contibution extends the knowledge about the impacts of calcite dissolution and re-precipitation on CO2 WAG operations in deep carbonate reservoirs. The novelty lies on the investigation of the effects of cross-flow between layers and gravity segregation on the scale deposition. Moreover, this model extends our previous work on twodimensional simulation by including the vertical heterogeneity. We show how reactions proceed in layer of different properties and what are the risks of scale deposition at well perforations for a given vertical profile of initial porosity and permeability.