FORGE well 21-31 was drilled and completed to a total depth of 8,139 ft from 5 February to 31 March 2018. Specifying the surface location, drilling, and testing of the 21-31 deep exploration well was a primary activity of Fallon FORGE Phase 2B. While the primary objective for drilling the well was to assess that subsurface conditions of the Fallon FORGE site, drilling the well offered opportunities to test the performance of drilling technology in the geothermal conditions of the Fallon geothermal resource, which included comparative analysis of bit performance for the polycrystalline diamond compact (‘PDC’) bits used to drill the 21-31 well versus the tricone bits used to drill the nearby Fallon 82-36 well.

PDC bits are routinely used by the oil and gas industry for drilling medium to hard rock but continue to see limited application in the geothermal industry, largely due to previous reliability issues and higher purchase costs. Design improvements have led to reliability improvements and, as evidenced by the case study of Well 21-31’s drilling performance, when correctly applied (with adequate attention given to bit selection and drilling parameters), PDC bits can present a drill-time reduction advantage that can offset their higher purchase cost. Specific steps that may be taken during planning and execution of well drilling to further improve and realize the time and cost benefit that PDC bits can provide are described herein.