Identification of Permeability Controls in a Geothermal System Using Gravity Method, Menengai Case Study

Surface studies done in Menengai field have indicated a huge geothermal potential and has led to the drilling of several wells. However, some of these wells have not produced as earlier projected and there are questions as to whether this field has good permeability. The purpose of this study was to employ gravity method to understand density variations in the Menengai field by developing structural models and establishing whether there is reliable permeability. Raw data collected by Kenya Electricity Generating Company- Kengen and Geothermal Development Company- GDC was reduced to complete Bouguer data by subjecting it to all reduction procedures and gravity models developed using a Golden software program called Surfer. These models were then subjected to specific filters that helped in identification of structures responsible for permeability. These filters are: Band pass filter; applied to remove certain wavelengths and horizontal derivative filter; applied to image drastic changes. Density inversion from results of complete Bouguer anomaly readings were generated by 3D inversion Grablox1.6 programme that calculated synthetic gravity anomaly of a 3D block model. Comparison of gravity and resistivity models indicates a good resemblance of the magmatic intrusion and the Tectono-Volcanic Axis-TVA’s. From this analysis, permeability controls for Menengai geothermal field were identified as follows: caldera rim faults that contribute mostly to deep vertical recharge, NNE-SSW faults along Solai graben, NNW-SSE faults along Molo axis, the southern fault extending towards Lake Nakuru and the uplifting dome in the central part of the caldera which enhance further fracturing within the caldera. This has led to a conclusion that there is good permeability in Menengai geothermal field. The research and its findings therefore have shown that gravity should be used as a key technique rather than a preliminary tool in geothermal exploration. The practice has mainly been to use gravity for deformation monitoring or as a preliminary tool. However, failure by other techniques in identifying permeability controls forms the basis of this study and from the results; it has been found that if well utilized gravity method could solve the many challenges encountered in geothermal exploration. The results from this study therefore would be of high importance to GDC if adopted and used in locating sites where geothermal wells can be drilled.

Keywords: Menengai, permeability, gravity, geothermal.