Quench-Spallation Drilling: A Novel Drilling Head Design for Routine Heat Mining Above the Brittle-Ductile Transition

Reaching high temperatures in the crust (400-600 C) for heat mining is optimal for a number of reasons, but the primary one may be the extraction of fluids in a supercritical state. We present here a new patent for a drilling head (Holtzman, US 2022/0235612 A1, pending) that is designed to function at temperatures into and above the brittle-ductile transition, to take over where mechanical drilling is no longer practical. The process is similar in theory to standard hydrothermal spallation drilling in which flame or hot fluid jets applied to colder rock cause thermal expansion cracking and spallation. In our “quench-spallation” drilling (QSD) method, we flip the sign of the temperature difference to produce spallation by quenching. Cold fluids jetted onto hot rock will cause thermal contraction stresses and cracking, essentially converting the thermal energy of the rock to mechanical energy that drives cracking. A simple demonstration of this process at ambient pressure shows a clear effect of quenching on spallation rates: a water jet (commercial pressure washer) applied to a cold granitic rock shows negligible spallation (<1 mm/minute); repeating the same jetting after 1.5 hours at 480 C causes rapid spallation (~30 mm/minute). Further experiments on other rock samples show a variety of behaviors. At deep crustal conditions, in addition to this thermoelastic stress, decompression near the drilling face can drive further cracking, as well as hydraulic stresses in cracks and hydrodynamic stresses from the fluid jetting. In its simplest form, the QSD head applies multiple jets to a rock face; in more complex designs, a spallation chamber is created to control fluid pressure at the rock face, such that local scale fluid pressure and solid stress gradients can be controlled to optimize spallation rates. Highly directional drilling should also be possible with our designs. Further experiments and modeling of this quench spallation drilling process are underway.