Geopressured geothermal fluids at Croatian reservoirs contain energy in three different forms such as: dissolved gases and minerals, high pressure and relatively high temperatures. Natural gas must be separated for the optimal utilization efficiency, which simultaneously maximizes the conversion process due to the wellhead temperature. Binary Rankine cycle uses a secondary working liquid, because the temperatures of the most geothermal fluids are usually too low to be used directly. However, behavior of representative secondary fluids is strongly dependent on characteristics of the geothermal fluid. Superheating degree in secondary cycle could be correlated to the molecular properties of each working fluid which is determined by the geothermal wellhead temperatures. Optimization of the geothermal energy production process includes minimization of all energy losses from the reservoir conditions to the end user. As the huge amount of the available field energy is mostly spent in the borehole and in the surface equipment, process optimization requires scientifically-research access, based upon the physical-chemical conditions of flow and on construction of production equipment.