Geothermal exploration in Uganda

Abstract

A secure and sustainable energy mix is one of the key challenges which Government of Uganda (GoU) as a nation faces in the years ahead, as the world responds to the challenges of climate change, energy security and economic competitiveness. As a strategy to mitigate energy security and climate change, GoU decided to increase power generation in short term but also diversify its power generation sources in the long term. This was to include developing its geothermal energy resources. The timeline for geothermal exploration in Uganda has stretched too far dating back in early 1950’s when swallow wells were drilled in Buranga. There is need to apply breakthrough techniques and technology in order to fast track geothermal development in Uganda. Key elements of successful geothermal energy development include institutions, policies, reliable resource information and finance. Each of these four elements represents a factor that directly affects the outcome of the geothermal project. Geothermal systems in Uganda are deep circulation amagmatic systems which typifies other fault-controlled Rift valley geothermal fields that are driven by deep circulation of ground meteoric waters. The heat source is ascribed to extension and thinning which resulted in high heat flow. Normally, these are deep reservoirs which can be assessed by deep penetrating measurements like magnetotelluric (MT) in combination with reflective seismic and Transient Electromagnetism. Electromagnetic (EM) and magnetotelluric (MT) methods are typically used to map resistivities at depths greater than 500 meters.

Magnetotelluric (MT) measurements were tested in Kibiro and Panyimur and inversion of the data revealed a deep, subvertical conductor presumed to be geothermal reservoir. The interpretation of reflective seismic surveys formed a highly detailed and reliable picture of the subsurface structure, with resolution unattainable by most other geophysical methods. Exploration models have been developed for Kibiro and Panyimur geothermal sites. This will be tested by drilling deep exploration wells. Soil-gas and gas-flux measurements at Kibiro revealed active main faults presumed to control fluid flow. The 3He/4He ratios of geothermal fluids from Buranga geothermal systems were measured to determine if a deep mantle signature was present. Results indicated elevated 3He/4He ratios. These elevated 3He/4He ratios were believed to be evidence of deep permeability and possibly deeper, higher-temperature fluid reservoirs. The results could be used to identify extensional faults with deep permeability. It is important to have and follow a strategy to minimize cost and maximize success in exploring for and evaluating geothermal resources. Without a good understanding of the geology of a prospect area, exploration is merely guesswork.