Geothermal exploration of the Menengai geothermal field

Abstract

In geothermal resource assessment a multidisciplinary approach is adopted. The aim is to ascertain and map the size of the reservoir, determine reservoir temperature and chemical composition of the reservoir fluids. Ultimately, a conceptual model and sites for exploration drilling are produced. This paper describes findings and current status of Menengai geothermal exploration and resource assessment respectively. Deep drilling geothermal program in Menengai field has so far resulted in 31 geothermal wells within the Menengai Caldera. Geoscientific data acquired are continually reviewed and integrated to bring out an updated geothermal model of the field. Current geoscientific data shows that Menengai caldera has been volcanically active in recent geological time. In regards to fluid flow, the most significant structures are mainly the young NNE-SSW, this based on measured temperature contours. Other regional structures are those oriented to N-S and E-W. The N-S are older regional structures while the E-W are local structures are as a result of near field stresses related to magma uplifting within the centre of the caldera. Assessment from the surface geology indicates that the most active area is the central (summit) and south of caldera owing to young structures and widespread eruption. Borehole geology data shows that a syn-caldera tuff marker horizon between 300-400 m CT is present in all wells. Moreover high temperature alteration minerals like actinolite are present in wells drilled within the summit area indicating zones of contact metamorphism related to system of hot dike intrusions.

These dike systems can also be inferred from surface data that shows a majority of lava eruption are within the summit area. In addition syenitic intrusive have been encountered from the wells within the caldera summit area as deduced from borehole geology data and measured temperature. Measured temperature contours shows a marked NNE-SSW anomaly pattern inferring fluid flow pattern. Gravity survey presents an anomaly at the centre of the caldera which is most likely related to a magmatic forming the dike intrusion. Furthermore, the aforementioned summit area exhibits shallow seismic movements confirming shallow magmatic activity. Resistivity interpretations have some ambiguity but it is certain that the reservoir is around 30ohm-m. The discharge chemistry of wells shows that the discharge is Na-HCO3 fluid with a high pH and moderately high chloride concentrations (> 400 mg/kg). The reservoir fluids exhibit marked variations whereby some wells discharge one phase (i.e. steam) while the others discharge two phase. It can be concluded from the fluid and gas chemistry of the discharged wells and temperature contours, it is evident that the wells around the small portion of the summit area is within the up-flow thus displaying low NCG contents, low CO2 and high temperature