Hydrothermal spallation drilling addresses some of the limitations encountered in flame-jet spallation drilling. In particular, it enables drilling in aqueous media at the high borehole pressures encountered in deep geothermal drilling. In this study, an electrically heated hydrothermal jet was impinged on the surface of cylindrical Barre Granite samples (basement rock) contained in a hydrothermal autoclave reactor. Comminution of the rock samples’ surface was achieved at supercritical water temperatures ranging from 535°C to 560°C in a 22.5-27 MPa pressure environment to simulate deep wellbore conditions. Preferential removal of quartz grains from the rock matrix was observed. In the cases examined experimentally, it was found that comminution cannot be attributed to erosion by either the jet’s momentum, or by differential pressure forces. Additionally, the rate of silica removal was greater than can be attributed to dissolution alone, implying a secondary comminution mechanism associated with hydrothermal spallation. However, experimentally determined heat flux and surface temperature measurements indicated that hydrothermal comminution occurred below the empirically determined minimums for the onset of continuous thermal spallation, from low density flame jets or laser heating at atmospheric conditions in air. Successful chemically enhanced hydrothermal spallation drilling experiments reduced the temperature and heat flux threshold for spallation. In these tests sodium hydroxide was introduced into the hydrothermal jet to weaken the rock matrix by increasing the dissolution rates of the constituent minerals.

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