Borehole geology and hydrothermal alteration of well OW-912B, Olkaria geothermal field, Kenya

Abstract

OW-912B is a directional well in the Olkaria Domes geothermal field. It was drilled to a depth of 2998 m in a N90°E direction and the production casing was set at 846 m depth. A maximum formation temperature of 306°C was measured at 2980 m depth. The injectivity index, calculated after well testing, was 47 lpm/bar, which is among the lowest values recorded in the Domes field. Rock types identified in the well include pyroclastics, rhyolites, tuffs, trachytes and basalts. Rhyolitic dyke intrusions were encountered at 1068 and 2990 m. Changes in the rate of penetration were noted at depths where intrusives were penetrated. Parameters controlling hydrothermal alteration in the well are temperature, rock type, fluid chemistry and permeability. The first appearance of epidote occurred at 950 m and four alteration zones were identified based on the alteration mineral assemblages. These are a zeolite-illite zone, a chlorite-illite zone, an epidote-chlorite-illite zone and an actinolite-epidote-chlorite-illite zone.

The top 136 m is unaltered. Nine minor feed zones were encountered in the well. Shallow permeable zones are associated with loss of circulation zones and stratigraphic boundaries while deeper feed zones are identified from heat-up temperature profiles and changes in the intensity of alteration. Thirty one fluid inclusions analysed from quartz crystals at depths of 756 and 1438 m indicated homogenization temperatures ranging from 160 to 300°C. Boiling conditions were encountered between 1000 and 1180 m, associated with high permeability. The elevated temperatures at this depth resulted in the formation of garnet between 1048 and 1068 m. By comparing the current formation temperature, alteration mineralogy and fluid inclusion measurements, the indications are that the area around the well is in a state of equilibrium. The well is drilled in the direction of an up-flow zone encountering high temperature but low permeability, resulting in the well being a poor producer.