Results of injection and tracer tests in Olkaria East Geothermal Field

This paper presents results of a six month Injection and Tracer test done in Olkaria East Geothermal Field The Injection tests show that commencement of injection prior to onset of large drawdown in the reservoir leads to greater sustenance of well production and can reduce well cycling which is a common feature of wells in Olkaria East Field. For cases where injection is started after some drawdown has occurred in the reservoir, injection while leading to improvement of well output can also lead to increase in well cycling which is a non desirable side effect. Tracer tests reveal slow rate of fluid migration (< 5 m/hr). However estimates of the cumulative tracer returns over the period of injection is at least 31% which is large and reveals the danger of late time thermal drawdown and possible loss of production. It is shown in the discussion that the two sets of results are consistent with a reservoir where high permeability occurs along contact surfaces which act as horizontal "fractures" while the formations between the "fractures" have low permeability. This type of fracture system will lead to channeled flow of injected fluid and therefore greater thermal depletion along the fractures while formations further from the fracture would still be at higher temperature. In an attempt to try and achieve a more uniform thermal depletion in the reservoir, it is proposed that continuous injection be done for short periods (~2 years) and this be followed by recovery periods of the nearly the same length of time before resumption of injection again