Title: Thermo-Economic Optimization of Slim Well Power Cycles: A Case of Ngozi Geothermal Project-Tanzania

Abstract

Ngozi geothermal field located in Mbeya Tanzania, is the main geothermal project that the
country focuses on, aiming at producing clean energy in term of both electricity and heat. The
project is an exploration drilling phase.
The present study focuses on developing an optimized strategy in terms of wellbore size selection
and power plant technology to be used, for the development of Ngozi geothermal field in Tanzania.
To obtain this, a comparative thermo-economic analysis of slim wells against full size wells is
done for a selection of power plant cycles. To obtain a realistic estimate of the wellbore output,
as required for the thermo-economic analysis, well bore data from Pritchett
[1] is used as basis for the analysis. The geothermal field is assumed to be liquid dominated.
Results show that, binary cycle has the highest first and second law efficiencies, produces the
highest net-work output, and offers the lowest total cost per unit Megawatt compared to single and
double flash power cycles. The binary technology would therefore be the most suitable power plant
technology to be employed in Ngozi geothermal prospect. Apart from the capability to produce the
highest amount net-work output and lowest cost per unit megawatt compared to the other slim
wellbore diameters, the well can support testing.
If a geothermal field is liquid dominated and low enthalpy, slim wells would be more suitable for
the power production instead of full-size wells. It can be concluded that in liquid dominated, low
enthalpy geothermal fields, where a decision is to be made to drill slim wells, 15 cm well bore
diameter slim wells are more suitable. It has also been shown that it takes less time to break even
when slim wells are used to produce power compared to full size wells.
The results from this work are expected to equip developers including TGDC with adequate
information to approach exploration drilling with a holistic and strategic view of minimizing the
total investment cost and using exploration slim holes more productively compared to the
traditional way. The aim is to improve energy efficiency and prove the business case for start- up
geothermal projects with production taking place after drilling and testing successful slim wells
instead of just plugging them. Moreover, to use slim well for production, proper casing design must
be used to allow the flow of adequate quantity of geothermal fluids for production
of electricity.