1. A method to determine fluid flow in a subsurface reservoir, comprising: dividing the reservoir into discrete volume elements; determining, for one or more fluid phases comprising the fluid, the potential for each volume element; ordering, for one or more of the fluid phases, the volume elements according to their potentials; determining a local, fully coupled time-step sequence using a local conservation solution based on the potential ordering of one or more of the fluid phases; and determining the fluid flow using a global conservation solution based on the local, fully coupled time-step sequence.