Temperature and well test analysis of selected wells of Aluto Langano geothermal field, Ethiopia

Abstract

Aluto Langano geothermal area is one of the 22 high-temperature geothermal prospects in the Ethiopian main rift. It has a potential of 70 MWe and has confirmed temperatures greater than 200°C at a depth of 1000 m. Of the 10 deep wells drilled in the area, two are directional, drilled in 2015 to a measured depth of 1921 and 1950 m. The remaining eight wells are all vertical, drilled in the 1980s. The objective of this report is to analyse the temperature and pressure in wells LA-4 and LA-6 in order to estimate the formation temperature of the geothermal system, as well as estimate the reservoir parameters of the field based on well test analysis for wells LA-4 and LA-7. Temperature and pressure data from the warm up period and injection tests are limited. The formation temperatures of the wells were estimated using the Horner method, from which the estimates are usually lower than the actual value. Comparing the latest measurements from 2016 and 2017 and combining with the Horner plot results, gave a more realistic formation temperature. The temperature reaches over 300°C in well LA-6 and 230°C in well LA-4.

The main feed zones of well LA-4 are located at 1460 and 1900 m, as indicated by pivot points in pressure logs and confirmed by temperature changes at the same depths. Pivot points in the pressure logs are at 1900 m in LA-6, which indicates a main feed zone confirmed by a slight temperature change at the same depth, but an already defined feed zone at 1600 m from previous studies, was also indicated by temperature change. The injection test was processed in Welltester software, which was developed at ISOR to estimate reservoir and well parameters. The data from the injection tests were too scarce to use directly in Welltester, so manually adding data where there were gaps, followed by interpolation in Welltester, gradually gave good results with respect to the quality of the data. Storativity was of the order of magnitude 10-8, and transmissivity 10-8 to 10-9, where the latter one is rather low compared to Icelandic high-temperature geothermal wells. The skin factor was generally negative, which indicates that the permeability in the closest surroundings of the well is higher than farther away. The injectivity index is rather low, between 1.3 and 2.2 (L/s)/bar, which can explain partly, why the wells have not been good producers.