“…This paper is a continuation of the previous statutory papers of the Department of Hydrocarbon Reservoir and UGS Simulation (Miłek et al, 2013;Szott and Gołąbek, 2014;Szott, 2015a, 2015b;Gołąbek and Szott, 2017) within the framework of which a reservoir simulator was built (through the modification of an open code BOAST reservoir simulator (Fanchi et al, 1982) correctly taking into account the phenomenon of mixing of gases. Modifications of the simulator concerned implementation of the hybrid method of minimizing numerical dispersion (mobility with multi-point weighing in the direction of inflow (Tood et al, 1972) + double discretization grid (Audigane and Blunt, 2003) and extension of standard saturation equations with additional term describing physical dispersion and control by specific parameters (Redlich and Kwong, 1949;Soave, 1972;Peaceman, 1977;Kreft and Zuber, 1978;Shrivastava et al, 2005). These changes were tested on simplified simulation models (in which fixed size of blocks, homogeneous reservoir parameters and stationary flow of reservoir fluids were assumed) and the real formation model (selected natural gas reservoir model).…”