The basis for well design and drilling programme for geothermal exploration in Kinigi, Rwanda

Abstract

The western branch of the East African Rift System is expected to flourish in geothermal resources as it is the case on the eastern branch. However, unlike the eastern branch, these resources are not yet developed. Even though exploration has been undertaken recently in countries like Rwanda and Uganda several setbacks were encountered and the worst was the deep exploration drilling in Karisimbi, Rwanda in 2013-2014. An intensive geoscientific exploration process is being performed and ultimately drilling will be carried out and therefore, a drilling programme specific to the area needs to be prepared. The African Union has provided the code of practice, the African Union Code of Practice for Geothermal Drilling, to guide engineers in the design of drilling programmes and this report is about the basis of the design of the casing programme, the cementing programme and the mud programme for future Kinigi exploration drilling. The casing programme is made up of 3 cemented strings of casings, namely: surface casing (100 m), anchor casing (450 m) and production casing (1230 m). A slotted liner is hung from the production casing from 1200 m depth down to the bottom at 3000 m. The choice for the design of the 3000 m deep well at Kinigi is based on minimum design factors and possible worst load scenarios.

Drilling fluids play a key role for the success of the drilling process, and well cleaning together with well stability are top considerations. Poor selection of drilling fluids either in quality or in quantity may result in well collapse, a stuck drill string or worse. While designing the drilling fluids programme for exploration wells in Kinigi, a number of factors has to be considered including the formation pressure, temperature, and expected loss zones in the well. Apart from those technical consideration, the cost also must be considered because some fluids e.g. the air drilling package required for pressure balance drilling with aerated drilling fluids is expensive. The design for the cementing programme focused mainly on the estimated volume of cement slurry necessary for the annular volume to be cemented plus the excess of 150%. Therefore, the estimated volume of the slurry is 180 m3 per well and considering the planned density of the slurry of 1.87 kg/l, the amount of dry cement required for each well is estimated at 250 tonnes. The success of getting a sound cement around the casing at the end of the cementing process was the priority and the inner string cementing method was chosen. The slurry pumping and displacement time also were explored and the calculated pumping time is less than 3 hours per well