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 local, fully coupled time-step sequences using local conservation solutions 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 sequences wherein the determining the fluid flow comprises computing, using a computing device, a fully coupled solution using unknowns resulting from refined local discretizations in time associated with the local time-step sequences along with unknowns associated with a final time-step.