Borehole geology and alteration mineralogy of well OW-205, Olkara geothermal field, Kenya

Abstract

Well OW-205 is an appraisal well drilled to open the Olkaria Central Field as well as to cater steam for the Olkaria VII power plant. Permeability in the well is controlled by the NW-SE fault in the west and the Ololbutot fault in the east. These faults facilitate near lateral movement of cold fluids in 1400-2000 m and are responsible for temperature reversals at this depth. Abundance of haematite, fluorite and sphene from 800 m to the bottom of the well (3000 m) has been associated with cooling. Possible processes proposed for causing the cooling conditions include mixing of brine and ca-rich meteoric fluids or mixing of two brines with different chemical composition. Temperature evolution around this area is associated with the NW-SE fault and the rejuvenated Ololbutot fault. In the area around well OW-205 two temperature regimes are found; relatively high paleo-temperatures of about 285°C and a lower temperature regime (225°C). With low temperatures around this well, it may not be suitable for production as it was planned initially. Alternative uses such as reinjection are an option since the well is intersecting with a deeper flow path of cooler fluids.