Design of slim well drilling programme for geothermal exploration : Case of Ngozi, Tanzania

Abstract

This report deals with the design of future exploration slim wells for Tanzania Geothermal Development Company (TGDC) at the Ngozi geothermal prospect in Mbeya, in the south-western part of Tanzania. The area offers a different challenge in terms of drilling infrastructure since the terrain is very mountainous and has a unique feature of a caldera that hosts Lake Ngozi. This provides challenges in terms of accessibility of the area. However, the prospect is suggested to be of high temperature, which makes it a great prospect for production of electricity. The designing process of a geothermal well starts with the collection of comprehensive geoscientific data. These include the stratigraphy of the area, temperature gradient, boiling point depth curve and expected drilled depths. The calculation of the minimum casing setting depth, cementing quantities expected and expected wellhead pressure follows.

This paper has a proposed slim well design for a new site like Ngozi in Tanzania. The proposed design will contain a 17 ½´´ surface hole, an intermediate 12 ¼´´ hole, 8 ½´´ hole for the production casing and 6 1/8´´ hole drilled for the last section to install perforated liners. A special design for the top casing that anchors the wellhead has been particularly looked at due to its role in containing the high pressures and temperature that may build up when the well is shut off. The paper also looks at key activities such as the selection of the rig suitable for slim well drilling, and the casing depths, casing material and cementing quantities have been discussed in detail keeping a keen eye on minimizing the costs that are involved in the drilling of the wells. Coring options for geothermal exploration have also been discussed showing further reduction in costs of exploration drilling. This is mainly due to smaller rigs used in exploration using coring technology.