Dissolution and Thermal Spallation of Barre Granite Using Pure Water Hydrothermal Jets

Spallation induced by rapid hydrothermal heating was investigated as a possible method of drilling rock. In this study, an electrically-heated hydrothermal jet was impinged on the surface of cylindrical Barre Granite specimens (basement rock) contained in an autoclave reactor to induce localized thermal stress. Comminution of the rock surfaces was achieved at supercritical water conditions, temperatures from 535 to 580 °C and pressures of 22.5–27 MPa. These conditions simulate those encountered in drilling deep, water-filled wells at depths greater than about 2300 m. Preferential removal of quartz grains from the rock matrix was observed. This comminution cannot be attributed to erosion by either the jet’s momentum, or by differential pressure forces. Additionally, silica removal (primarily from quartz grains) was observed at rates greater than those that could be attributed to dissolution of quartz alone. This implied a secondary comminution mechanism associated with spallation caused by the local thermal stresses from the impinging hydrothermal jet. The experimentally determined heat flux and surface temperature measurements indicated that hydrothermal drilling occurred below the empirically determined minima for the onset of continuous thermal spallation reported in earlier studies for low-density, high-velocity and high-temperature flame jet drilling at temperatures in excess of 1000 °C jets impinging on rock surfaces at near-atmospheric pressures.