Geothermal drilling rigs, like oil and gas drilling rigs, employ the hook load capacity as the main discriminating factor when deciding which rig size to use. Hook load capacity becomes important for deep wells, as is the case for Enhanced Geothermal Systems (EGS). For all drilling rigs, the depth of the planned well determines basic rig requirements like hoisting capacity, power system, mud circulation system and the pressure control system. When selecting the most cost-efficient geothermal drilling rig, the use of a compact design would save money by reducing mobilization costs, fuel consumption and make geothermal resources more cost competitive. Also, environmental concerns such as rig ‘footprint’ make smaller rigs more attractive. This paper summarises the investigation of existing compact rigs used for drilling geothermal wells. Criteria to aid the selection of optimum geothermal drilling rigs are proposed. Case studies are presented where tailored drilling rigs, inclusive of the latest technology, have been implemented. Finally, the paper shows a comparative study of drilling utilisation factor based on various drilling rigs and different types of geothermal resources (i.e. EGS vs. hydrothermal).

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