3. Write the governing flow equation(s) in partial differential form for the 2-D, two-phase flow (gas-water) reservoir with heterogeneous and isotropic permeability distribution. There is depth gradient in the system; however, gravitational force for gas phase can be ignored. Porosity is changing throughout the production period and the solution of gas in water exists. Also, there is one producing well in the system. Capillary pressure between gas and water can be expressed as: Pcgw = P.-P