Casing design for high-temperature geothermal well using the African Union code of practice for geothermal drilling : A case study of Well OW 736a in Olkaria, Kenya

Abstract

Exploration drilling is the key step to verify the existence of a geothermal resource at the end of the exploration phase. All the processes connected to drilling operations should adhere to high standard concerning safety and the use of approved materials since the pressures involved are very high especially in high-temperature geothermal fields. This report presents the casing redesign for well OW 736A in Olkaria geothermal field, Kenya using The African Union Code of Practice for Geothermal Drilling. The well was drilled in the year 2014 following mostly the New Zealand code of practice for deep geothermal wells (NZS 2403:1991), now superseded by the New Zealand standard of 2015 (NZS 2403:2015) and adopted by the African Union Standard. During redesigning of the casing the main factors considered are the lithology of the Olkaria area and wireline logging data, namely pressure and temperature in well OW 736A. Using the methodology of the standard, the redesigned well casing depths were set to 440 m for the surface casing, 790 m for anchor casing and 1470 m for the production casing. The grades, sizes and weights of different casing strings were determined by analysing axial and hoop stresses during casing running, cementing operation and well production operation to make sure that they meet minimum design factors for different scenarios. The final redesigned casings selected for well OW 736A are 20'' 94 lb/ft K55 surface casing, 13⅜'' 54.5 lb/ft K55 anchor casing, 9⅝'' 47 lb/ft L80 production casing and 7'' 26 lb/ft L80 perforated liner. The recommended top two segments of the anchor casings, which are normally of higher strength, are of size 13⅜'' OD, weight 68 lb/ft and grade T95. The master valve recommended in the redesigned well is ANSI class 1500.