Speaker: Yves Guglielmi, Lawrence Berkeley National Laboratory
Designing effective and reliable strategies to prevent and/or manage fluid-injection induced seismicity of concern while minimizing economic impact depend on an in-depth understanding of the physics of the earthquake source and how it is influenced by the changes in subsurface fluid pressures, stresses, and geochemistry resulting from injection and their interactions. Fault and fractures dynamics studies face two key problems (1) the up-scaling of laboratory determined properties and constitutive hydromechanical laws to the reservoir/crustal scale which is not straightforward when considering faults and fractures heterogeneities, (2) the difficulties to control both the induced seismicity and the stimulated zone geometry when a fault is reactivated. Using instruments developed to measure coupled pore pressures and deformations downhole and a surrounding seismicity sensors network, we conducted field academic experiments to characterize fault hydromechanical dynamic behavior during the earthquake nucleation process. We show experiments where different fault zones geologies under contrasted state of stresses were explored in three different underground research laboratories (IRSN-Tournemire-France, LSBB-Apt-France, Mt-Terri-Ste-Ursanne-Switzerland) where experimental conditions can be optimized. Experiments consisted in pressurizing intervals in different fault zone facies (core, fractured damage zone, etc.) to induce changes in effective stresses high enough to produce measurable fault movements and seismicity. Shear and normal displacements respectively of 0.05 to 1.5 10-3m were measured at velocities of 0.1 to >10 micrometer per seconds. The induced seismic events computed magnitude ranged between -4.3 and ~-2.5. Their spatio-temporal distribution, compared with the measured displacement at the injection points, shows that most of the deformation induced by the injection is aseismic, and that there is a similarity in the way faults of contrasted geologies activate under effective stress variations. As no seismicity is observed in the close vicinity of the injection areas, the presence of fluid seems to prevent seismic slips, because of a high “mostly” poroelastic dilatant effect. Therefore, the seismic behavior seems to be strongly sensitive to the structural heterogeneity, including permeability of the fault zone, which leads to an heterogeneous stress response in and propagating away from the pressurized volume.