Chemical Dissolution Drilling of Barre Granite Using a Sodium Hydroxide Enhanced Supercritical Water Jet

Abstract

This experimental study focused on evaluating the technical feasibility of chemically enhancing rock comminution during supercritical hydrothermal jet drilling by increasing pH with additives. Comminution of the crystalline rock samples occurred under supercritical conditions at temperatures and pressures ranging from 535–580 °C and 22.5–27 MPa, respectively, using hydrothermal jets with sodium hydroxide concentrations ranging from 0.0725 to 0.345 mol/kg. Impinging hydrothermal jets induced thermal spallation and rock removal from 21 Barre granite specimens examined. These experimental conditions replicate those that would be encountered in drilling water-filled wells at depths greater than ~ 2300 m. The combination of accelerated mineral dissolution due to the presence of hydroxide ions, high solution temperature increasing reaction rates, and thermal stresses resulted in rates of rock removal up to 5.16 g in 10 min at 410 °C and 0.345 mol/kg NaOH. Comparison with published empirical quartz dissolution rates at high pH suggests granite mass removal primarily resulted from weakening of the rock matrix as a result of the accelerated dissolution of quartz minerals. Experimentally determined heat flux and surface temperature measurements indicated that the rock comminution occurred below the empirically determined minimum levels for the onset of continuous thermal spallation resulting from impinging low-density flame jets or high energy laser heating.