Modified Sequential Fully Implicit Method for Geothermal Reservoir Simulation

Abstract

This study presents a new sequential coupling strategy for geothermal reservoir simulation – modified sequential implicit (m-SFI) method. The m-SFI method was implemented and tested using the new GENeral Implicit Coupling framework (GENIC) within the Automatic-Differentiation General Purpose Research Simulator (AD-GPRS). This framework allows for the rapid prototyping and consistent testing for this new m-SFI method. In this work, we demonstrated how the m-SFI method performs for several challenging examples where a standard sequential formulation fails. In particular, we showed that for complex flow regimes, the m-SFI method takes a similar number of time steps as the fully implicit method (FIM). It was found that for cases where accurate front prediction is possible and the two-phase region is limited, such as strictly injection or production problems, the m-SFI approach outperforms FIM. However, for a reservoir with mostly two-phases, the overall performance of the m-SFI method is more expensive than FIM due to the increased cost of the sequential iterations.