Assessment of boiling effects on δD and δ18O composition of geothermal reservoir fluids and their suitability on use as tracers of origin of geothermal fluids

Abstract

A geothermal fluid can boil adiabatically (at constant enthalpy) as the fluid rises to the surface or at constant temperature/pressure (isobaric) due to addition of heat from the reservoir rocks to the fluid. These two modes of boiling cause isotope fractionation between liquid water and the water vapour formed. As a result, the isotope composition of the original fluid is modified and this increases the complexity in interpretation of isotope composition and their use as tracers for the origin of geothermal fluids, especially when the two phases separate in their ascent to the surface. This study evaluates the effects of the two boiling processes for Olkaria geothermal reservoir fluids and their effect on the deuterium and oxygen18 isotope composition of the geothermal fluid. Two well samples 17-KEN-1 (OW42) and 17-KEN-14 (OW-732) with a reservoir quartz geothermometer temperature of 236 and 294°C, respectively, and both sampled at 11 bar were simulated for isobaric and adiabatic boiling. The boiling was carried out at temperature and enthalpy steps and at each step, fractionation of deuterium and oxygen 18 assessed. From the results of the model, adiabatic boiling fractionates deuterium and oxygen18 to a higher extent compared to isobaric boiling. The results of the modelling are presented together with the isotope compositions of wells plotted along the Kenya Rain Line (KRL) in order to give a glimpse into the complexity caused by these reservoir processes in the use of stable isotopes of water as tracers for the origin of geothermal fluid.