Evaluation of factors controlling the concentration of non-condensible gases and their possible impact on the performance of wells in Olkaria, Kenya

Abstract

The Olkaria geothermal field is located in the Kenyan Rift valley about 120 km from Nairobi. Development of geothermal resources in a high temperature field in the Olkaria area started in the early 1950s. In the following years numerous developments have been made with additional power plants being installed. These include a binary plant at Olkaria South West (Olkaria III) in 2000, a condensing plant at Olkaria North East (Olkaria II) in 2003, another binary plant at Olkaria North West (Oserian) in 2004 and finally two more condensing plants in the year 2014 within the East production field (EPF) and Olkaria Domes (OD) areas. The total generation from the field is about 730 Mw. In this study samples from four producing wells located in three fields in the Olkaria geothermal area (OW-44 in the Olkaria East, OW-724A in the Olkaria North East and OW-914, and OW-915 in the Olkaria Domes field) were studied. The chemical analysis was carried out using SOLVEQ. The equilibrium state of the system was determined, the reservoir temperatures and the total moles were run through the CHILLER program. The CHILLER runs analysed the processes that have been proven to be occurring in the Olkaria field, i.e. boiling and condensing processes, fluid-fluid mixing rocks and titration resulting from water-rock interaction. The effects on gas evolution were evaluated based on the recalculated gas pressures.

The results indicate that the gas species are not in equilibrium with the mineral assemblages. The CHILLER evaluation shows boiling as the major process leading to evolution of gases. Well OW-44 had the smallest gas concentrations resulting from the reservoir processes in question due to degassing and near surface boiling. Gas breakout is most likely in well OW-914 and least likely in well OW-44. The study proposes different reservoir management strategies for the different parts of the Olkaria geothermal field. The first would be by increasing hot reinjection in the eastern sector around well OW-44. In the part of the reservoir around well OW-914, the wells should be operated at a minimum flow rate, or chemical inhibitors to prevent calcite scaling could be used.