Stimulating Granites: from Synchrotron Microtomography to Enhancing Reservoirs

Abstract

We present a fresh approach for stimulating granites based on a new multiscale workflow that starts with: (1) an analysis of a fossil geothermal system to assess long time scale processes; and (2) with a time-lapse Synchrotron heating experiment monitored with high-speed microtomographic imaging to analyse the short time scale micromechanisms. (1) The fossil long time scale analysis is based on identification of the thermo-chemo-hydro-mechanical processes leading to formation of fluid transfer zones in granites. These are cast into an upscaled continuum calculation capable of modelling deformation on millions of years time scale. (2) For the short time scale analysis, the propagation of observed fractures through the granite in the Synchrotron heating experiment is reproduced in a numerical experiment to derive the multiphysics processes leading to formation of microcracks. In a final step the calibrated computational model is used to design the stimulation protocol for a reservoir. The aim of this step is to control the coupled propagation of permeability in a virtual experiment prior to any costly field trials. This final step allows the assessment of the physical time and length scales relevant for a reservoir and its surrounding tectonic environment and can be used to optimize the injection strategy. We show a specific example of the workflow applied to granite in Central Australia. Using the above microtomography-based approach, we identify new physics that has previously been overlooked in the formation of the permeability system at depth. This resulting fracture network stems from longer time-scale processes than previously considered. Of particular interest for the injection strategy is that our model predicts a specific depth from which reservoir-scale material response is strongly controlled by thermochemically activated creep processes. These cause fractures that are well known as creep fractures in ceramics. The new physics-based method is promising for characterising and stimulating many other reservoir materials. It may, for instance, also be applicable to unconventional gas plays: clay minerals creep at very low temperatures and can show thermally activated dewatering/degassing-reactions, which generate ductile shear- or compaction- and dilation-aligned porosity.