Interpretation of drawdown test data from Olkaria Domes geothermal field - a case study of OW-907B, OW-912B and OW-917

Abstract

The greater Olkaria geothermal field, located within the rift valley province of Kenya, is probably the most explored geothermal system in Africa. It is bounded by volcanic activities and features that form a part of the geothermal manifestations in the region. The Greater Olkaria geothermal area (GOGA), under study here, covers close to 204 km2 and the Olkaria Domes production field is a part of this area. Quite intense exploitation as well as exploration drilling has been ongoing in the Domes production field since late 2009. The field is a high-temperature geothermal field with most of its wells producing a two-phase fluid, as established during the wells production testing processes. The completion tests data i.e. temperature and pressure downhole profiles, were cross-examined and critically analysed to establish where the major aquifers/feed zones occur, considering the three observation wells under study, OW-907B, OW-912B and OW-917.This includes an in-depth analysis of their reservoir characteristics that rendered them to be used as monitoring wells in the Domes area.

The main objective of this report is to critically analyse and interpret the pressure monitoring data from the three observation wells in order to determine the extent of drawdown occurring within the Olkaria Domes production field bearing in mind the upcoming Olkaria V 140 MWe power station. The program Lumpfit was used in the simulation. The three-tank closed model obtained from the lumped parameter modelling was used to predict an expected pressure drawdown for three different net production scenarios of 750, 1000 and 1250 kg/s, equivalent to an electric power generation capacity of 76, 176 and 230 MWe, respectively. The study is mainly aimed at helping Kenya Electricity Generating Company (KenGen) to estimate the future trends of the entire Olkaria reservoir and, thus come up with the best and most economical resource management strategies in order to ensure a more sustainable production.