Estimation of Initial Steam Fraction in Boiling Geothermal Reservoirs

Abstract

The gas content of steam from wet-steam wells has been used to estimate the initial steam to water ratio in the producing aquifers. For this estimation there are essentially two approaches. One assumes various gas-gas equilibria and that the total well discharge composition is the same as that of the initial reservoir fluid. The other approach considers various mineral-gas equilibria and takes into account possible segregation of the flowing water and steam in the aquifer. Such segregation will cause the discharge composition to differ from that of the parent reservoir fluid. The results obtained by the two approaches often yield very different results. There are considered to be two main reasons for the difference. One is that phase segregation may occur in the producing aquifer. The second reason relates to the assumption of specific chemical equilibria in the undisturbed reservoir fluid. In order to study whether or not specific gas-gas or mineral-gas equilibria are closely approched in geothermal reservoirs it would be of interest to study liquid enthalpy well discharges and preferably those where the first level of boiling is in the well. Under such circumstances the initial reservoir fluid comosition can be assessed with confidence.