Modeling studies of heat transfer and phase distribution in two-phase geothermal reservoirs

Abstract

Phase distribution as well as mass flow and heat transfer behavior in two-phase geothermal systems have been studied by numerical modeling. A two-dimensional porous-slab model was used with a non-uniform heat flux boundary condition at the bottom. Steady-state solutions are obtained for the phase distribution and heat transfer behavior for cases with different mass of fluid (gas saturation) in place, permeabilities, and capillary pressures. The results obtained show very efficient heat transfer in the vapor-dominated zone due to the development of heat pipes and near-uniform saturations. The phase distribution below the vapor-dominated zone depends on permeability. For relatively high-permeability systems, single-phase liquid zones prevail, with convection providing the energy throughput. For lower permeability systems, a two-phase liquid-dominated zone develops, because single-phase liquid convection is not sufficient to dissipate heat released from the source. These results are consistent with observations from the field, where most high-temperature liquid-dominated two-phase systems have relatively low permeabilities (e.g. Krafla, Iceland; Olkaria, Kenya; Baca, New Mexico). The numerical results obtained also show that for high heat flow a high-temperature single-phase vapor zone can develop below a typical (240°C) vapor-dominated zone, as has recently been found at The Geysers, California, and Larderello, Italy.