The Engineer

Potter creates hard-rock drilling system

News

Engineers at Potter Drilling have developed a new method of drilling that could help create a whole new generation of geothermal power plants.

The drilling technology currently used in the oil and gas industry has been optimised over the past century for soft sedimentary rocks where carbon deposits are found.

Even though this technology works well for extracting fossil fuels, it is not well suited for the crystalline hard-rock environments where man-made engineered geothermal system (EGS) power plants will be developed.

Now, engineers at Potter Drilling have developed a spallation-based technology that removes rock without making contact with it - an approach to hard-rock drilling that they claim has significant cost savings and performance advantages over conventional drilling technology.

The process starts by applying a high-intensity fluid stream to a rock surface to expand the crystalline grains within the rock. When the grains expand, micro-fractures occur in the rock and small particles called spalls are ejected. The process is accelerated by several factors including inherent stress in the rock formation.

Potter Drilling is not the first company to develop spallation drilling technology. Air spallation drilling was used commercially from 1947 through 1961 for ore mining and was adapted to geothermal drilling by the Department of Energy in the 1970s. Air spallation demonstrated impressive drilling performance, producing 8in to 12in boreholes to depths of 1,100ft at rates faster than 50ft/hr in solid granite.

But the company’s technology differs from those air-based techniques in that it uses hot fluid rather than air to spall the rock. Because spallation occurs in a water-filled borehole, Potter Drilling’s technology can be used to drill to depths required for EGS plants (12,000 to 30,000ft).

Tests of two different laboratory prototype drills at Potter Drilling have proven that the hydrothermal spallation can be effective in a wide range of rock types from the surface to deep borehole conditions. Penetration rates approaching 30ft/hr - five to 10 times higher than is achievable using conventional rotary contact bit technology - have been demonstrated.

Potter Drilling is now proposing to construct a complete prototype drilling system and to conduct field trials to determine the technology’s real-world performance.