Lithology, stress or pressure control of the seismicity in shale? Insights from a controlled experiment of fluid-induced fault reactivation

Abstract:

Shale materials are important components in petroleum systems (e.g. sealing and source rocks) and play an important role in earthquake faults. However, their mechanical behavior is still poorly understood, as it can vary from plastic to brittle. The role of fluid pressure in seismicity triggering is also not fully understood. A unique, decametric scale experiment, which aims at reactivating a natural fault by high pressure injection, was performed in shale. The injection interval was surrounded by a dense monitoring network of pressure, deformation and seismic sensors, in a well-characterized geological setting. 37 seismic events, with very small magnitude (about -4) were recorded during five series of injections within the fault zone. The spatio-temporal distribution of these events, compared with the measured displacement at the injection points, shows that most of the induced deformation is aseismic. The locations of the microseismic events are strongly asymmetrical, with most events distributed East of the fault and South of the injection area. Focal mechanisms are predominantly consistent with shear slip on a family of calcified fractures. The shale mineralogy appears to be a critical parameter governing the seismogenic character of deformation, as only calcified structures slipped seismically. As no seismic event occurs in the close vicinity of the injection, high-fluid content seems to inhibit seismic slip. Consequently, stress changes induced by this highly-pressurized volume should more likely influence the spatial distribution of seismicity. Finally, the main fault, acting as a fluid flow and/or stress barrier, strongly modifies the stress transfer. From the monitoring point of view, this experiment reflects the difficulty in quantitatively following fluid propagation in a shale formation, as seismicity will be strongly influenced by the presence of calcified fractures and by the interaction between stress transfer and lithological heterogeneities.