Evaluation of Tracer Test Results In Olkaria Northeast Geothermal Field, Kenya, and Its Implications On Resource Management

Reservoir management is a vital key to having a sustainable production field in an environment where geothermal energy is utilized. A robust re-injection strategy is one of the essential reservoir management techniques used in developed geothermal fields. Such a strategy is not only environmentally friendly but also, by maintaining the subsurface pressure, goes a long way in ensuring that the geothermal reservoir is not being depleted. To develop a good re-injection system, it is necessary to understand the geothermal field´s reservoir fluid chemistry and flow patterns, to assist in proving/disapproving the wells´ connectivity within or around it. This can be done by undertaking tracer tests. Tracer testing started in the early 90´s in the Olkaria´s East (OEPF) and Northeast (ONEPF) production fields though geothermal utilization has been undertaken in the Greater Olkaria Volcanic complex for the last 40 years, starting with the commissioning of Olkaria 1 Unit I in June 1981. The latest tracer injection of sodium fluorescein in ONEPF was done in December 2020 by injection of the tracer into Well OW-703. The tracer recovery was thereafter monitored for about six months in most of the production wells within the vicinity. This study evaluates the effects of injection into the well and on the production wells in the vicinity by looking at the injection process, production wells´ sampling, tracer analysis, and data interpretation. The tracer had a breakthrough in several wells in the field, including wells OW-714 (2.0%) and OW-730A (1.2%) implying connections with OW-703. These connections were modeled using the TRINV software which showed that although the re-injected brine is, optimistically, at 150°C, thermal breakthrough would not be significant in the next 10 years in other wells. However, it was noted that the tracer used, sodium fluorescein, usually decays at high temperatures thus the cooling predictions are likely to be somewhat optimistic. Therefore, it is recommended that tracer testing be repeated using other, more suitable tracers.