An unsaturated three-dimensional model of fluid flow and heat transfer in NW Sabalan geothermal reservoir

This paper presents a 3D numerical model for the north-west (NW) Sabalan geothermal system, including the unsaturated vadose zone overlying the system, based on a conceptual model derived from data gathered from 10 deep exploration wells. To achieve the goal, the EOS3 (water-air state equation) module of the Tough2 simulator was utilized to develop the model. The model was constructed from a rectangular prism, which is 11.5 km long, 8 km wide, and variable depths ranging from 3.8–5.11 km. The 21 horizontal layers with a thickness range of 100−1000 m were expanded from the maximum height of 4110 to -1000 masl. A total of 22 rock types were distributed within the model based on the availability of rock units and geological structures derived from the exploration wells. The permeability of these rock types varied from 1.0 × 10⁻¹⁷ to 9.0 × 10^-13 m². The model was initially run to match the natural state of the reservoir. A close agreement was obtained between the measured data from the exploration wells, and the model results for subsurface temperatures and pressures. In the calibrated model, a high temperature upflow zone was required in the southeast part of the area (below the exploration sites D and E). This flow rises to the land surface through permeable zones, faults, and fractures, and finally appears on the earth's surface as hot springs in the northwestern part of the area. This model was then used as the initial model to predict the reservoir performance for the 50 MWe scenario. The results showed that the reservoir can generate about 45 MWe by assigning two makeup wells and remain at this level for more than 30 years.