Widespread adoption of geothermal energy will require access to deeply buried resources in granitic basement rocks at high temperatures and pressures. Exploiting these resources necessitates novel methods for drilling, stimulation, and maintenance, under operating conditions that are often difficult or impossible to reproduce in laboratory settings. Physically rigorous numerical modeling tools are vital to highlight potential risks, guide process optimization and reduce the uncertainties involved in developing new technologies for these environments. Lawrence Livermore National Laboratory has developed, and is constantly improving, several multi-physics solid/structural mechanics, fluid dynamics, chemistry, and discrete element codes. Integration of the LLNL simulation tools into a coherent simulation environment will provide a predictive capability for the thermomechanical response - in particular the spall and fracture - of basement rocks at high temperatures and pressures useful for drilling and other geothermal applications. This paper outlines a modeling effort investigating the processes involved in hydrothermal spallation drilling. These include interconnected phenomena on several length and time scales: from system-scale fluid dynamics and heat transfer of the high temperature jet to the rock face to the grain-scale thermomechanics of spallation. Three models are described to capture these different scale processes: a grain-scale model to investigate the onset of spallation; a particulate fluids model to simulate the transport of the produced spalls; and a borehole-scale model to represent the integrated system behavior.