Geochemical reservoir monitoring - a case study of Olkaria Northeast field in Kenya

Abstract

The results of the geochemical bi-annual reservoir-monitoring program, of the Olkaria Northeast Field (ONEF) for the second half of 2007 are presented. The chemical changes in the reservoir have been evaluated in terms of the changes in the concentration of total discharge chloride, silica, sulphate, discharge enthalpy, calculated geo-thermometer temperatures and the variations in the gas composition of the discharge fluids. The regions of high chloride concentration are around wells OW-714 and OW-716 to the east, OW-709 and OW-727 to the west and OW-726 at the center of the Olkaria Northeast field. The three regions indicate upflow zones within the field. The area in the vicinity of wells OW-R2, OW-710, OW-711 and OW-724 are recharge areas with low mineralized fluids. The Olkaria fault zone contributes significantly to the reservoir fluid movement in the Olkaria Northeast field. Recharge from the Ol Olbutot fault may be encroaching the reservoir around well OW-711. This will be confirmed in the next monitoring periods.

The Na/K geothermometer temperature distribution in 2nd half of 2007, indicate three low temperature (less than 250°C) centers around wells OW-708, OW-720 and OW-724. The rest of the area lies between 270 and 340°C. The probable cause of heating around the wells to the southwest is boiling while the increase around well OW-727 may in addition, be due to incursion of separated re-injection fluids. The CO2 geothermometer temperature mark out wells that have received the separated reinjected fluid from wells OW-720 through OW-728 to well OW-714 & OW-716 in a NE-SW elongated structure.