1. The problem statement, all variables and given/known data Hello All, I'd please like some help in deriving a 2-phase 3-dimensional partial differential flow equation from first principles. 2. Relevant equations I know that the starting point is Darcy's flow equation. 3. The attempt at a solution A key step is substituting the Darcy flow equation into the material balance equation, but I don't know how to get from there to the final equation for a 2-phase 3-dimensional flow. Many thanks for helping out.