Brittle failure and fracture reactivation in sandstone by fluid injection

Abstract

We performed laboratory experiments on sandstone specimens to study brittle failure and the reactivation of an experimentally produced failure plane induced by pore-pressure perturbations using constant force control in high compressive stress states. Here, we focus on the shear failure of a dry sample and the later on induced fracture plane reactivation due to water injection. Acoustic Emission (AE) monitoring has been used during both experiments. We also used ultrasonic wave velocities to monitor pore fluid migration through the initially dry specimen. To characterise AE source mechanisms, we analysed first motion polarities and performed full moment tensor inversion at all stages of the experiments. For the case of water injection on the dry specimen that previously failed in shear, AE activity during formation of new fractures is dominated by tensile and shear sources as opposed to the fracture plane reactivation, when compressive and shear sources are most frequent. Furthermore, during the reactivation of the latter, compressive sources involve higher compressive components compared to the shear failure case. The polarity method and the moment tensor inversion reveal similar source mechanisms but the latter provides more information on the source components.

Keywords:

• water injection
• reactivation of pre-existing fractures
• polarity method
• moment tensor inversion

Acknowledgments

The authors would like to particularly thank Stefan Gehrmann (GFZ) for the specimen preparation. Helen Lewis, Antonio Claudio Soares and two more anonymous reviewers are gratefully acknowledged for their fruitful comments. This work has been funded by the EU-GEISER Project (FP7-ENERGY-2009) and GFZ German Research Centre for Geosciences.