Wet-Steam Well Discharges. II. Assessment of Aquifer Fluid Compositions

Abstract

The discharge enthalpy of wells drilled into liquid-dominated high-temperature geothermal reservoirs is often elevated, i.e. higher than that of steam saturated water at the aquifer temperature. Sometimes wells drilled into such reservoirs discharge steam only. The discharged steam largely forms by depressurization boiling, but a fraction may form by conductive heat transfer from the reservoir rock to the fluid in the aquifer or be present initially in the reservoir. Two processes are considered to be responsible for generating well discharge enthalpy which is higher than that of the parent fluid. They are phase segregation and the mentioned conductive heat transfer. Phase segregation may occur in the depressurization zone around discharging wells as a consequence of the effects of capillary forces, relative permeability and the difference in the flow properties of water and steam. Production of elevated discharge enthalpy by heat transfer from the aquifer rock does not cause the chemical composition of the total well discharge to deviate from that of the initial aquifer fluid. By contrast, when phase segregation occurs, the total well discharge chemical composition differs from that of the initial aquifer fluid. Methods are outlined, which permit calculation of initial aquifer fluid compositions of wet-steam wells with elevated discharge enthalpy from analytical data on water and steam samples collected at the wellhead.