Modelling the geochemical effects of geothermal fluid injection in the Olkaria geothermal field, Kenya

Abstract

Reinjection of used geothermal waters has become an integral part for sustainable geothermal utilization in the Olkaria geothermal field. The location of the field inside the Hell’s Gate National Park requires an environmentally friendly way of disposing of separated brine and condensate. Additionally, increased production capacity has been proceeded over the years with increased reinjection. Despite the benefits mentioned, one of the major issues associated with reinjection is the possible loss of permeability due to scaling. In this study, the potential for scale formation in several hot reinjection wells in the Olkaria geothermal field was evaluated. The first approach assessed the potential of scale formation in reinjection wells prior to production using the WATCH speciation program to obtain the baseline data. The second approach using the PHREEQC geochemical model, simulated mixing of injected fluids and aquifer fluids to assess the scaling potential of the mixed fluid. The results from the two approaches were then compared for calcite, amorphous silica and anhydrite saturation in the geothermal fluid. The saturation state of the injected fluid with respect to calcite, amorphous silica and anhydrite at the temperature of reinjection was also assessed in the PHREEQC program using different databases. The results from this study did not only show both positive and negative geochemical effects of fluid injection in the Olkaria geothermal field, but they also showed some inconsistencies in the mineral thermodynamics properties in different PHREEQC databases. Despite these limitations, the confidence level for using the PHREEQC program in geothermal fluids was identified.