CO2 Injection in Geological Formations: Determining Macroscale Coefficients from Pore Scale Processes

Carbon dioxide (CO₂) injections in geological formations are usually performed for enhanced hydrocarbon recovery in oil and gas reservoirs and storage and sequestration in saline aquifers. Once CO₂ is injected into the formation, it propagates in the porous rock by dispersion and convection. Chemical reactions between brine ions and CO₂ molecules and consequent reactions with mineral grains are also important processes. The dynamics of CO₂ molecules in random porous media are modeled with a set of differential equations corresponding to pore scale and continuum macroscale. On the pore scale, convective–dispersive equation is solved considering reactions on the inner boundaries in a unit cell. A unit cell is the smallest portion of a porous media that can reproduce the porous media by repetition. Inner boundaries in a unit cell are the surfaces of the mineral grains. Dispersion process at the pore scale is transformed into continuum macroscale by adopting periodic boundary conditions for contiguous unit cells and applying Taylor-Aris dispersion theory known as macrotransport theory. Using this theory, the discrete porous system changes into a continuum system within which the propagation and interaction of CO₂ molecules with fluid and solid matrix of the porous media are characterized by three position-independent macroscopic coefficients: the mean velocity vector , dispersivity dyadic , and mean volumetric CO₂ depletion coefficient .