An Engineered Geothermal System (EGS) pilot project was commissioned to prove the economic viability of an industrial scale geothermal heat plant in Finland. The project aims to generate 40 MW of emission free heat energy, supplying up to 10% of the city of Espoo’s district heating needs. Two wells of 6400 m MD and 6213 m MD (measured depth) were drilled through formations of hard, abrasive granitic gneiss with maximum measured 560 MPa UCS (unconfined compressive strength). Typical dull conditions of lost and worn cutting structure and gauge diameter wear of between 3/16-in to ¼-in contributed to excessive torque, stuck incidences, low rate of penetration (ROP) and difficulties achieving build rate. To address these drilling challenges, this paper explores the interplay between new cemented carbide compact technology, drill bit design, and drilling parameter road mapping. The directional section of the first well was drilled with an average ROP below 2 m/hr and run length averaging 56 m per bit. The well took 246 drilling days and 44 BHAs (bottom hole assemblies) to achieve TD (total depth). Between the first and second well an application specific drill bit design package and step-wise parameter program were implemented. Design enhancements included improved gauge protection, bit hydraulics for minimizing cone erosion and subsequent TCI (tungsten carbide insert) compact loss. Novel hybrid TCI materials technology was introduced having a 100% improvement in wear resistance and durability as compared with conventional grades, to drill these hard and abrasive granitic formations. New BHAs and drilling plan were selected based on the bit design selection to reduce wear on BHA components, improve directional control and reducing drilling dysfunctions. Once these factors were under control, a low risk approach to extending the bit revolution limits (krev) for the roller cone sealed bearings could be implemented based on downhole parameters and previous bit dulls, leading to longer run lengths. The combination of bit design and material enhancements with a properly selected BHA and drill plan increased run lengths and ROP. The second well’s 8.5-in directional section was drilled with a 13% increase in average ROP and a 69% increase in average run length without exceeding krev limits. Well on well, a 77 day reduction in AFE (authorization for expenditure) was realized. We demonstrated the combination of oil and gas bit and BHA design, drilling plan, and new cutting material capabilities can reduce EGS well construction costs in order to make these renewable energy sources economical.

The oil-and-gas drilling industry has been utilizing modified PDC cutter geometry for improving performance. One of the first reinforcements of beliefs in positive effects of edge geometry changes was in report by Lin et al. in 1992 from conducting laboratory experiments on cutters. Currently, certain geometries are being favored based on limited testing and analysis of the design space. The detailed effect of geometry on attributes such as strength, cutting, wear and the tie to performance has not been fully understood. This paper reports results of laboratory studies, numerical models, and field runs towards building such understanding. Single cutter edge-loading tests, pressurized cutting and wear tests were carried out alongside full-scale PDC bit tests and field runs. Improved analysis techniques were utilized to extract meaningful information from the tests. Numerical models were used to exploit the design space for the geometry. The optimized geometry was selected with greater than 50% strength improvement as indicated by the numerical model. For the optimized geometry, pressurized single cutter lab tests indicated greater than 25% performance improvement in several aspects. Of the field runs in various applications, the largest dataset was in Norway with an ROP increase of 18% among greater than 187 bits run. This study is part of a holistic approach to understanding the thermo-mechanical behavior of PDC cutter drilling as it applies to improving design as demonstrated and drilling practices. The improved cutter geometry has helped customers drill further and faster, which has helped reduce the cost of drilling. There are still improvements that can be made to cutter geometries that will push performance even further. The methods discussed in this paper will help expedite the learning process and help build better insights.