Fluid flow in sparse fracture systems, prior to and after fault slip

Abstract

Many geothermal pilot projects worldwide have been forced to shutdown in the last fifteen years after triggering earthquakes by fluid injections into critically-stressed faults. This reveals that our capacity to control the migration of injected fluids is still poor and shows the importance to better understand how faults control flow to improve the safety of hydraulic stimulations. Little is known also about how deep fluid injections affect heat transport and, ultimately, the performance of enhanced geothermal systems. Here, I investigate how downscaled controlled hydraulic stimulations performed at the Grimsel Rock Laboratory, Switzerland, enhanced subsurface flow and heat transport in a highly-monitored reservoir analog. The installation of 15 boreholes to monitor fluid pressure, temperature and rock deformation in a previously unexplored area of the Grimsel Rock Laboratory, from 2015 to 2016, provided a unique opportunity to carry out experiments and observe flow and heat transport in situ at unprecedented resolution. I first investigate how the internal structure of shallow crustal faults affects flow and pore-fluid diffusion. Starting at borehole scale, I systematically test the permeability of the host rock, single fracture and fracture networks and show how brittle damage induces a power-law decay in permeability with lateral distance from faults. Building on this knowledge, I investigate cross-hole scale flow interactions to understand the nature of spatial dimensions amenable to flow. By applying a generalized flow model, I show that fractional flow dimensions systematically emerge, converging to a dimension of $n$=1.3, due to a pressure diffusion slowdown. This slowdown is inconsistent with normal diffusion and suggests that anomalous diffusion is an important flow process in shallow crustal faults, and that their hydrologic structure is fractal. For the faults tested, I obtain a fractal dimension of $d_f$=2.23, which may reflect network-scale flow channeling. Characterizing the hydrological state of these faults after stimulation shows that their permeability and internal flow structure sustained significant, multiscale changes. Locally, changes in permeability measured in stimulation wells, where high-pressure injections took place, were found to be inversely proportional to the initial permeability and scale positively with seismic magnitudes and rupture areas. This suggests a relation between permeability enhancement and induced seismicity. Several locations in the near field became also less permeable. This shows, for the first time, that permeability reduction may be an overlooked but important near-field process following stimulation. Globally, stimulated faults became more permeable and better connected hydraulically compared to their pre-injection state. Faults with poor to moderate natural connectivity experienced the largest increase in connectivity. Hence, I propose that such faults constitute better stimulation targets. Finally, heat injection experiments confirmed that the heterogeneous flow structure of faults strongly affects subsurface heat transport. Thermal breakthroughs were observed at multiple locations, yet confined to the most permeable faults. These breakthroughs were found to be better explained by heat transport through a parallel plate fracture, instead of tube-like channels. Comparing then pre- and post-stimulation thermal transfer time distribution showed that high-pressure fluid injections enhanced, but also delayed, advective heat transport. The thermal response to shear stimulation of shallow crustal faults appears, therefore, to be more complex than previously described.