A preliminary update of natural state numerical model of Olkaria geothermal system, Kenya

Abstract

A field-wide 3-D natural state numerical model of the entire Olkaria geothermal system was developed in 1987 by G.S.Bodvarsson and K.Pruess. The model simulated the geothermal system to be recharged at a total rate of 600 kg/s from two major distinct upflow zones located in Olkaria Northeast and Olkaria West. The upflow zone in OlkariaNortheast recharged the system at 250 kg/s of 1290 kJ/kg water and the one in Olkaria West at 350 kg/s of 1090 kJ/kg of water. The recharge from the two upflow zones mixed near well OW-201 and discharged at 260.5 kg/s to the south and 175.1 kg/s to the north through Ololbutot fault and Olkaria fracture, respectively. Steam loss from the system amounted to 126 kg/s. Between 1987 and 2000, 46 new additional wells were drilled andtested in the greater Olkaria geothermal system. While data from some of the newly drilled wells have agreed quite well with those calculated by the 1987 model, several other data, especially from the Olkaria Central wells, have shown big variations. An attempt has therefore been made to recalibrate the 1987 model with the new data acquired from the 46 additional wells. The same numerical grid has been used and theresults have indicated a slight variation in recharge rates and enthalpies. The upflow in Olkaria West in the updated model is 245 kg/s of 1200 kJ/kg water and that in the East (both Northeast and east field) is 320 kg/s of 1290 kJ/kg water. The discharge to the south in the updated model is 304.3 kg/s and to the north is 134.4 kg/s. Steam loss from the system is 128 kg/s.