There are two mechanisms for the production of superheated steam from geothermal wells. The first one is that the steam generated by the boiling of immobile liquid trapped within micro fractures in rock matrix of geothermal reservoirs flows through fractures of high permeability and eventually flows into wellbores as superheated steam (Mechanism 1). The second one is that two-phase fluid flashes and dries-out during the flow within in-flow zones around wellbores. The superheated steam then eventually flows into wellbores (Mechanism 2). Mechanism 1 is from a microscopic viewpoint, while Mechanism 2 is from a macroscopic viewpoint. The largest difference between the two is the presence or absence of a flow of two-phase mixture. Mechanism 1 is involved in the generation of HCl gas in superheated steam. The mechanisms have been described by Truesdell and White (1973) and Grant (1979). However, based on advances in our understanding in recent years, we need to add explanations to these models in terms of relative permeability. Also, the interrelationships and the differences in thermodynamic processes between these two mechanisms have never been described. In this paper, they are discussed from a viewpoint of geothermal reservoir engineering.

Geothermal wells at Sumikawa are divided into two groups: one produces fluids of vapor dominated and another of water dominated. This is probably because of the presence of vapor-dominated zone formed partly in the reservoir of water single-phase. A numerical three dimensional reservoir model was developed for simulations both natural state and history matching of production and reinjection for about 25 years since the start of the power generation at Sumikawa in 1995. A relative permeability curve of Corey type was employed with residual saturations of 0.23 and 0.16 for water and steam, respectively. Simulation results of specific enthalpy history of produced fluids at Wells SA-1 and SA-3 showed good agreement with measured values in the range of 1500- 2500 kJ/kg. A horizontal narrow layer of high permeability in the model was assigned in the area connecting high temperature zone in the south and the grid where Well SA-1 presents for describing high specific enthalpy values and its gradual decrease with time.