Joint Geophysical Imaging of Fluid-Filled Fracture Zones in Geothermal Fields in the Kenya Rift Valley

摘要

We present the application of joint geophysical imaging (JGI) and interpretation of seismic shear-wave splitting and electrical resistivity polarization due to aligned, fluid-filled fracture zones as targets of drilling exploration and production wells. The method included coupling of physical properties as well as geological structures. The joint imaging scheme has been used to obtain the orientation of fracture zones in two geothermal areas in Kenya using microearthquake (MEQ) recordings, Transient Electromagnetic (TEM) and magnetotelluric (MT) soundings. Joint imaging results show two main polarization directions aligned both with younger fractures and tectonically activated older fracture systems. The complex interconnection between these fracture zones causes heterogeneity and anisotropy which determines geothermal fluid movement. The JGI results have been used to successfully target higher production geothermal exploration and production wells. Our experience from the geothermal fields in Kenya suggests that the best targets for drilling geothermal production wells can be found on fracture zones that are aligned with the old tectonically activated fractures. Intersections of NE and NW trending fractures also form important fluid up-flow channels. The alignment of fractures has been used to successfully determine the direction of drilling deviated wells. This result shows that due to heterogeneity and anisotropy, it is difficult to determine and predict fluid flow paths and therefore routinely drilling across interpreted surface fractures does not always guarantee success. The most successful wells should be drilled either along NW trending fractures or intersections of fractures which form zones of enhanced permeability.