Practical Control of Timestep Selection in Thermal Simulation

Abstract

Summary We present a new technique for estimating accumulating time truncation error in thermal reservoir simulation. The calculation uses only current reservoir information, thereby avoiding difficulties encountered by some past formulations, and is integrated easily into fully implicit reservoir simulators. We describe an error-controlled timestep selector based on this estimate as well as provisions for limiting the calculated timestep based on the imminent appearance or disappearance of a gas phase. We also provide a new technique for estimating the solution of certain Newtonian iterations. The algorithm is capable of calculating timesteps at the beginning of a simulation and when major well rate changes are specified. The resulting timestep selector enhances simulator accuracy without increasing computational work and affords greater reliability through the reduction of failed timesteps.