Re-injection 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 separated brine and condensate. Additionally, increased production capacity has been realized over the years with increased re-injection. Despite the benefits mentioned, one of the major issues associated with re-injection is the possible loss of permeability due to scaling. In this study the potential for scale formation in several hot re-injection wells in the Olkaria geothermal field was evaluated. The first approach assessed the potential of scale formation in re-injection 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 mixed fluid composition was then input in the WATCH speciation program to simulate boiling and degassing. The results from the two approaches were then compared for calcite, amorphous silica and anhydrite in the geothermal fluid. The results from this study showed both positive and negative geochemical effects of fluid injection in the Olkaria geothermal field and gives an indication on how to mitigate the undesirable geochemical effects arising from injection into this field.

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