Drilling performance, injectivity and productivity of geothermal wells

•The risk of geothermal drilling in proven fields is smaller than perceived.•Drilling wells of 1338″ or 958″ took the same time, the wider ones gave 35% more steam.•Predictions of steam-flow from injectivity must consider reservoir conditions.•It might be feasible to sidetrack the production section of up to 40% of the wells.•Power output per drilled well amounts to 5.9 MWe but 7.5 MWe per productive well.

Drilling performance of 77 high-temperature production and reinjection wells in the Hengill Area in Iceland was analyzed. The results demonstrate that the perceived high risk of drilling in a proven field is less than commonly thought. No difference was found in the time required to drill holes of 1338″ or 958″ production casing but the wider wells delivered 30–40% more steam. The average power output per drilled well amounts to 5.9 MWe but 7.5 per productive well. To predict steam mass flow on the basis of the Injectivity Index one must consider reservoir conditions and enthalpy of the expected inflow into wells.