Explicit method: solves node by node, unstable when time of each step increases
Implicit method: solves all nodes at one time with matrix, unconditionally stable
Finite Difference (corner-point grids) is simpler and easier to code than Finite Element (unstructured grids) and FE has material balance errors (flow going from big cell to a small one), which is probably why FEHM has volume control.
Advection: transport mechanism of solute by bulk flow
Diffusion: transport mechanism of solute from high concentration to low concentration
Supercritical CO2: when density of the gas phase and density of the liquid phase become so close to one another, the liquid and gas phase mix into one. The new fluid phase has properties of a gas but the density of a liquid.