Evaluation of Reservoir Production Trends in Olkaria Geothermal Field

Abstract

Geothermal wells are requisites for monitoring changes within a geothermal reservoir. Some report an increment in production whereas majority have declined resulting in changes in chemistry, enthalpy and pressure, boiling and drying up of the reservoir and the destruction of production equipment which caused a drop in production and economic losses. The study area was the Olkaria Geothermal Field located in the Kenyan Rift Valley. Due to its location along the rift axis, it is associated with a high geothermal gradient with the heat source occurring below the volcanoes. Since 1981 when production wells were drilled in Olkaria, there has been decline in steam production which was managed by reinjection of brine and drilling of make-up wells. However, decline still occurs at a gradual rate due to the fact that the causes have not been well studied. Objectives included: assessment of trends of retired geothermal wells before their decline, determination of trends of production geothermal wells and identification of declining production wells and prediction of future fluid production of the production geothermal wells. Data from seven production and two retired geothermal wells from East and North-east fields (2005-2015, 1985-2000 and 1990-2005) were subjected to time series and trend curve analysis of geochemical as well as physical parameters, correlation and decline curve analysis using R Studio, Grapher, and MS Excel. Through the use of trend graphs, key reservoir parameters were monitored during the production history thus enabling the determination of the causes of the trends of these parameters and identification of mitigation measures to reduce the decline of production wells. Furthermore, the graphs also helped to narrow down potentially declining geothermal wells which were subjected to further study to determine their future performance. Retired geothermal wells trends displayed repeated cycles in all their parameters apart from Mg (stable with peaks). All the North-east production wells’ trends displayed repeated cycles whereas the East production field wells had varying trends: Well Head Pressure (WHP)- rising; enthalpy- rising (26), declining (29) and repeated cycles (30); Cl- declining (26 and 30) and rising (29); SiO2- repeated cycles and Mg- stable (26), declining (29) and repeated cycles (30). Changes in production wells were attributed to sub-surface processes such as boiling, influx of cool waters, adiabatic cooling, silica scaling and recharge of hot geothermal fluids. Relationships existing between chosen wells varied from weak to very strong but no two production wells were identical whereas parameters revealed moderate correlation between WHP and enthalpy and Enthalpy and Mg. Trends revealed one declining production well ( OW-26) with an annual decline rate of 3.66% which is expected to reach its economic limit in 2047 whereas OW-709, 713, 720 and 728 had constant production and OW-29 and 30 had rising trends of production. Major challenges to production are mixing and silica scaling which can be mitigated through hot reinjected brine while its decline can be attributed to overproduction and the inflow of cooler waters. Decline curves are thus suitable for the prediction of the future production and rate of decline of the production wells through use of normalized steam flow rate and trend analysis.