A reservoir fluid flow simulator for the study of advanced subsurface processes is described. It is based on the volume balance formulation for implicitly determining the fluid pressure. A set of component conservation equations are used to determine the molar masses of each component, and an energy conservation law is used to determine the system emperature. Fluid property calculations are clearly separate from flow computations, and this is illustrated by a brief discussion on a compositional black-oil formulation. As well-flow is not of primary interest, only some very simple source and sink treatment has been implemented. Different degrees of implicitness have been tested. To try to keep the formulation simple, we have preferred to use a sequential approach in which the different equations are solved in sequence. Then for each equation, and implicit or explicit solver may be used. For the pressure, only implicit solvers have been used, while for the molar masses, explicit and different types of implicit discretization have been tried. However, we were not able to develop an implicit molar mass solver which yielded higher performance than a simple explicit solver. This appeared to be due to large decoupling errors and the increased time to solve the additional implicit equations. Consequently, the simulator is using an implicit pressure solver followed by a temperature and explicit molar mass solvers. Another aspect of the simulator is its use of general unstructured grids. On those grids, it is often necessary to apply a multi-point flux approximation scheme (MPFA). As many commercial simulators experience performance degradation using MPFA rather than a two-point flux approximation (TPFA), we provide some examples which show that in our case, MPFA is not considerably slower than TPFA on unstructured meshes. This is partly due to the choice of not using a high degree of implicitness, which reduces the size of the linear systems. Finally, the simulator parallelization strategy is outlined. A domain decomposition preconditioner is used as a solver, and the its performance is ascertained using a varying number of subdomains.<br>


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