Characterization of Subsurface Permeability of the Olkaria East Geothermal Field

Abstract

Subsurface permeability refers to the conditions that favours a productive well. Permeability forms an important aspect of a productive geothermal well. The notable permeability parameters of a geothermal well include intense loss zones, veining, fracturing and micro faulting. Fault and fracture identification is essential in defining subsurface permeability. Mineralogical sequencing in a geothermal well such as pyrite dissemination, calcite precipitation among others are essential in characterizing its permeability. The study area (Olkaria Geothermal field) is located in the Eastern Rift-Valley of Kenya. It is divided into East, North East, West, Domes, North West and Central areas. It is structurally controlled, dominated by faults, fissures, volcanic centres, Domes among others. The main tectonic structures in the Olkaria volcanic complex include fractures, faults, the Ol’Njorowa gorge and the ring structure. The main faults are - the ENE-WSW Olkaria fault and the N-S Ololbutot fault, NNESSW, NW-SE and WNWESE trending faults. Surface manifestation features such as fumaroles, hot-grounds, geothermal grass; sulphur deposits are found in the study area. The Olkaria East Geothermal field forms a section of the seven divisions of the Olkaria field. The study approach includes modelling of existing well stratigraphies, geological cross-sections, mapping of fault boundaries among others. Correlation of loss zones and alteration intensity was also done. The knowledge will be essential in siting of new geothermal wells and updating of the existing conceptual model.