Using the migration of the induced seismicity as a constraint for fractured Hot Dry Rock reservoir modelling

The challenge for Hot Dry Rock technology is to develop a heat exchanger in deep hot rock masses and to circulate a fluid in order to extract its energy to be used at the ground surface. The present day strategy is to take advantage of natural fractures that pre-exist at these depths and to improve their hydraulic properties. The extension of the area with enhanced properties must then be evaluated so that the best locations for further boreholes can be proposed. To date, this development procedure is based on fluid injection at high rates, forcing hydro-mechanical interactions to take place along pressurised fractures. These pore-pressure-driven mechanisms are accompanied by seismic activity. Assuming the validity of poro-elastic theory in the fractured host rock, some authors have derived the virgin hydraulic diffusivity of the fractured reservoir from the analysis of the spatio-temporal growth of the induced seismicity. The present work is aimed at verifying this approach using a numerical code to solve directly for hydro-mechanical interactions in random fracture networks. Our approach assumes that the seismic activity is controlled by a Coulomb shear criterion and we show how the interpretation of spreading rate of the modelled shear activity in a given network coincides with the upscaled virgin hydraulic diffusivity of the same fracture network, calculated from an independent numerical procedure at the reservoir scale. Therefore, it is shown that the direct analysis of the seismicity migration is appropriate to give reliable estimates of virgin hydraulic and mechanic parameters. These parameters can then be used for performing any further quantitative analysis of a reservoir open to the far field. This is of importance, as fluid mass-balance in multi-well exploitation systems will include the contribution from areas surrounding the stimulated zone.