Due to the scarcity of fossil fuel with a simultaneous rising in global energy demand, it is important to gain access to other energy sources. Geothermal energy holds great potential, and has therefore been studied increasingly in recent years. Within the construction of a geothermal plant, a fluid is introduced via a borehole into the initially gas-filled porous rock. Due the rising pressure gradient, the fluid distributes, displaces the gas and escapes through a second borehole. The modelling approach of these processes in a heterogeneous subsurface proceeds from the Theory of Porous Media (TPM) including an elastically deformable solid, an incompressible fluid, and a gaseous phase [1]. To solve the initial-boundary-value problem, the governing primary variables of the strongly coupled three-phase model are spatially approximated by mixed finite elements, whereas the time-discretisation is carried out by an implicit Euler time-integration scheme. The goal of the presented numerical simulations is to study the specific flow characteristics in a heterogenous subsurface. (© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)