Integrated geological assessment and numerical simulation for Olkaria's East and Southeast geothermal fields

Abstract

Olkaria East and Southeast geothermal fields are located in the East African Rift System and are approximately 120 km from Nairobi, Kenya. It was the first geothermal field in the country to start power production in 1981 Unit I (15 MWe). It has steadily increased its capacity from Olkaria I-VI units to 278.3 MWe (2022). Despite the steady increase in power generation, there has been a notable pressure decline of 10–15 bars. To ensure optimum resource utilization, a frequent appraisal is necessary to track productivity and management practices that will guide future investments. Reservoir simulation using TOUGH 2 computational code was employed to assess natural geo-controlling structures for optimal productivity and prediction of future developments in the geothermal field. A summarized review of geological and geochemical features was used to determine input parameters for the numerical model and assigned fault and reservoir rock properties were computed using temperature-dependent fluid movement equations which reduce the ambiguity involved in the trial-and-error approach used to construct a conceptual model. The initial state model was calibrated using measured temperature and pressure profiles from 16 wells. Subsequently, the production and reinjection model was set up to investigate reservoir performances before and after reinjection. Different water reinjection scenarios were tested with changing well location (infield and outfield), well depth and recharge rate. It was discovered that declining pressure trends up to 19 bars after a period of 29 years when 1656 t/hr mass is extracted while 42% water reinjection occurs. The extraction of 717 t/hr from new wells caused a further decline in production life to 13 years. The optimum reinjection program depended on recharge rate, well location and depth interval to mitigate reservoir pressure decline and thermal breakthrough. This study suggested hot and condensate water reinjection of 1530 t/hr (74% of extracted fluid) south of Olkaria East field.