CO₂ injection is a widely used enhanced oil recovery technique. It’s typically applied to recover the last third of oil in a reservoir.
CO2 reduces oil viscosity, so it flows more easily. It's typically used in formations of highly porous and permeable rock like sandstone. Sandstone and carbonate rocks differ mainly in how they form and how fluids move through them. Sandstones have relatively uniform, predictable pore networks where porosity and permeability correlate well. Carbonates, by contrast, have highly variable textures with multiple pore types often resulting in permeability that doesn’t match porosity. Because of this, sandstone reservoirs tend to be more homogeneous and easier to model, while carbonate reservoirs are far more heterogeneous and complex.
