The Heber Geothermal Field, California: Natural state and exploitation modeling studies

Using numerical simulation techniques and an axisymmetric model of the Heber geothermal field, the natural (preexploitation) state of the system and its response to fluid production are analyzed. The results of the study indicate that the Heber geothermal anomaly is sustained by the upflow of hot water through a central zone of relatively high permeability. The best model suggests that in its natural state the system is recharged at depth by a 15-MWt (megawatts thermal) (reference temperature 0°C) convective heat source. The existence of an axisymmetric convection pattern, whose axis coincides with the center of the Heber anomaly, is also suggested. At the lower part of the ascending hot water plume the deep recharge water mixes with colder water moving laterally toward the axis of the system. In the upper part the rising plume spreads radially outward after reaching the bottom of the cap rock, at about 550 m depth. The model results suggest that the so-called cap rock is quite permeable (5×10−15 m2) with Convection controlling its temperature distribution. The results also show reduced permeability (10×10−15 m2) of the upper zones in the outer region of the system that may be due to mineral precipitation. In modeling the exploitation of the field the generation rate is allowed to build up over a period of 10 years; after that, 30 years of constant power production is assumed. Full (100%) injection of the spent brines is considered; the fluids being injected 2250 m (“near injection”) or 4250 m (“far injection”) from the center of the system. The study shows that a maximum of 6000 kg/s (equivalent to approximately 300 MWe (megawatts electric)) of fluids may be produced for the near injection case but only 3000 kg/s (equivalent to approximately 150 MWe) for the far injection case. The results indicate that the possible extraction rates (generating capacity) are generally limited by the pressure drop in the reservoir. The average temperature of the produced fluids will decline 10°–18°C over the 40-year period.