Field test of a Combined Thermo -Mechanical Drilling technology. Mode I: Thermal spallation drilling

Accessing hydrocarbons, geothermal energy and mineral resources requires more and more drilling to great depths and into hard rocks, as many shallow resources in soft rocks have been mined already. Drilling into hard rock to great depths, however, requires reducing the effort (i.e., energy), time (i.e., increasing the rate of penetration) and cost associated with such operations. Thus, a Combined Thermo-Mechanical Drilling (CTMD) technology is proposed, which employs a heat source (e.g., a flame jet) and includes two main drilling modes: (I) Thermal spallation drilling, investigated here as a field test and (II) Flame-assisted rotary drilling, investigated as a field test in the companion paper. The CTMD technology is expected to reduce drilling costs, especially in hard rocks, by enhancing the rock penetration rate and increasing the bit lifetime. Mode I of the CTMD technology (thermal spallation drilling) is investigated here by implementing the concept on a full-scale drilling rig to investigate its feasibility and performance under realistic field conditions. During the test, the successful thermal spallation process is monitored, employing a novel acoustic emission system. The effects of thermal spallation in the granite rock are analyzed to provide conclusions regarding the rock removal performance and the application potential of the technology. The field test shows that thermal spallation of the granitic rock can be successfully achieved even when a liquid (water) is used as the drilling fluid, as long as the heat source is appropriately shielded by compressed-air jets. Thermal damage of the surrounding rock is investigated after the spallation test, employing micro-computer tomography imaging and modeling the stability of the cracks, generated by the spallation field test. This study shows that thermally induced damage is mainly confined within a narrow region close to the rock surface, suggesting that thermal spallation only marginally affects the overall mechanical stability of the borehole. Thus, this confirms that, as part of the Combined Thermo-Mechanical Drilling (CTMD) technology, thermal spallation drilling is a promising mode that has a high potential of facilitating the drilling of deep boreholes in hard rocks.