Compositional Modelling of Petroleum Reservoirs and Subsurface CO2 Storage with the MUFITS Simulator

Abstract

Summary We present recent extensions of the academic reservoir simulator MUFITS (www.mufits.imec.msu.ru) for modelling multicomponent flows in porous media. The extended simulator capabilities include a new fluid properties module for the compositional and thermal reservoir simulations in which a cubic equations of state (EoS) is used for prediction the PVT properties. Besides the standard three-parameter EoS, e.g. Peng-Robinson or Soave-Redlich-Kwong EoS with the volume correction, the simulator is supplemented with a built-in library of coefficients for a more general cubic EoS proposed by Brusilovskii (2002 ) which allows for improved prediction of phase compositions and densities without the volume-shifts. Unlike some other codes, MUFITS allows for compositional modelling of thermal processes in porous media. The phase enthalpies are predicted with EoS supplemented by the correlations of Passut & Danner (1972 ) for the ideal-gas enthalpy. In order to simplify the simulator usage, an extended library of hydrocarbons, carbon dioxide, nitrogen, water, and other components is built into the simulator, and additional components can be characterized and loaded into the library. The standard iterative algorithms for the calculation of phase equilibria can stall near the critical condition, if two phases are almost identical, or in the case of multi-phase equilibria, e.g. VLL-equilibria. To model the fluid transport at such conditions, the simulator is supplemented with another thermal module which utilises our new method SDPE (Simplex Decomposition for Phase Equilibria) for the fluid properties prediction. The SPDE method is based on the decomposition of phase diagram into the simplices, i.e. multidimensional tetrahedra. At constant pressure and temperature, the space of total concentrations is triangulated, i.e. divided into simplices. The thermophysical parameters are approximated by linear functions in each simplex, and any boundary between equilibria of different type must be the boundary between two simplices. Thus, the set of simplices is a triangulated look-up table for the properties of multiphase fluid. Consequently, SPDE is a non-iterative method, what results in accelerated prediction of the fluid properties. We demonstrate the method applications for modelling the flows of simple 3-component hydrocarbon mixtures and the CO2-H2O-NaCl mixture in the problems of CO2 storage. We acknowledge the funding from Russian Science Foundation under grant # 19-71-10051.