ABSTRACT

The gas and other solute concentrations have been estimated for the aquifer fluid of the Oku-aizu geothermal system prior to discharge-induced boiling. The model takes into consideration the excess enthalpy (i.e. two-phase) reservoir conditions which developed at production depths following depressurization. The model consists of two end-member types of boiling. One is a “high flow-low temperature drop type” and the other is a “low flow-high temperature drop type”. The first is a process which could be present in an aquifer with high permeability, and is characterized by a large total discharge, accompanied by a small temperature and pressure drop around the well. The fractionation of gases into vapor phase in the downhole feed zone may be smaller than predicted for equilibrium conditions, due to single step steam separation under dynamic conditions. The second type is a process which could occur in an aquifer with low permeability, and is characterized by a low flow rate and a high temperature and pressure drop. The net gas fractionation into vapor phase in the feed zone is very large, resulting in almost all the gases fractionating into the vapor phase, due to multi-step steam separation. Based on this model, the concentration of gases and other solutes are estimated for the reservoir liquid of the Oku-aizu geothermal system. Calculated ranges of gas concentrations in the reservoir liquid (prior to flashing) are 0.3 to 1.0 wt% for CO₂ and 150 to 250 mg/kg for H₂S. The estimates of gas concentrations in the original (pre-boiled) fluid are necessary to calculate mineral-fluid equilibria and to estimate such development-related factors as the potential for scaling.