Widespread adoption of geothermal energy will require access to deeply buried geothermal sources in granitic basement rocks at high temperatures and pressures. Exploiting these resources necessitates novel methods for drilling, stimulation, and maintenance, under operating conditions difficult or impossible to test in laboratory settings. Physically rigorous numerical modeling tools are vital to highlight potential risks, guide process optimization and reduce the uncertainties involved in these developing technologies. In this presentation, we discuss a numerical modeling effort investigating the multiscale mechanics of thermal spallation drilling (TSD) - a technique in which rock is fragmented into small flakes by a high temperature fluid jet. This process encompasses interconnected phenomena on several length and time scales: from system-scale fluid dynamics to grain-scale thermomechanics of spallation. Here we describe how these disperate scales are simulated using GEODYN, a parallel Eulerian compressible solid and fluid dynamics code with adaptive mesh refinement (AMR) capabilities. GEODYN is able to simulate materials under extremely large deformations, resolve details of wave propagation within grains, and uses a continuum damage mechanics approach to represent fracture. We will present results from both system- and grain-scale simulations describing the transfer of heat from the high temperature jet to the rock face, and the effect of grain-scale properties such as incipient flaw distribution, grain size and grain size distribution, heat flux, applied temperature and material heterogeneity on the onset of spallation. Detailed computer modeling helps to address several of the uncertainties surrounding TSD: 1) What rock compositions are drillable with TSD? 2) How do grain size and grain size distribution affect TSD and drilling rates? 3) What combination of macroscopic (Poisson ratio, heat capacity and thermal conductivity) and microscopic (flaw distribution, tensile strength) characteristics favor thermal spallation? 4) How well do experimental results under lab conditions (relatively low confinement pressure and low ambient temperature) extrapolate to deep underground reservoir conditions? 5) Can rock debris in the bottom of the hole significantly alter heat transfer from the high temperature jet to the rock surface?
|Number of pages||1|
|Publication status||Published - 2011|
|Event||Fall Meeting of the American-Geophysical-Union 2011 - San Francisco, United States of America|
Duration: 5 Dec 2011 → 9 Dec 2011
|Conference||Fall Meeting of the American-Geophysical-Union 2011|
|Country||United States of America|
|Period||5/12/11 → 9/12/11|