Main technical issues regarding problems when drilling geothermal wells

Abstract

Geothermal well drilling is one of the most expensive activities of geothermal projects; well costs represent 35–50% of the total capital costs of a typical geothermal project. This report describes drilling of geothermal wells and the main technical issues regarding drilling problems. Drilling information comprises both drilling rig data and well logging, used to study and explore geothermal wells. Data is collected on or around the drilling rig and downhole data is retrieved by some type of logging tool that is either lowered into the borehole or forms a part of the bottom hole assembly. Some down–hole logs are specially made as diagnostics for drilling problems. This report mainly describes general technical issues about geothermal drilling, not specific practical case histories of boreholes drilled in Iceland and Ethiopia. This report includes a description of the most common problems encountered in geothermal drilling: loss of circulation, getting stuck, twist off, slow drilling, bit wear and cementing. Drilling records from the digital rig instrumentation system and from downhole logging are of great aid in handling these issues Methods of cementing zones with large losses of circulation are briefly dealt with at the end of the report.