Role of drainage conditions in deformation and fracture of porous rocks under triaxial compression in the laboratory

Abstract

[1] In order to investigate the role of drainage conditions in deformation and fracture behaviors of porous rocks, the authors carried out a series of rock fracture tests under triaxial compression in the laboratory. The detailed space-time distribution of acoustic emission due to microcracking was used to examine pre-failure damage and failure behavior in Berea sandstone, which has a porosity of ∼20% and a permeability of ∼100 mD. The pore pressures or flow rates at the ends of the test sample were precisely controlled to simulate different drainage conditions. Experimental results indicate that drainage conditions play a governing role in deformation and fracture. The well-established dilatancy-hardening effect can be greatly suppressed by dilatancy-driven fluid flowing under good drainage conditions. Fast diffusion of pore pressure leads to a significant reduction in rock strength and stabilization of the dynamic rupture process. Furthermore, good drainage conditions have the potential to enlarge the nucleation dimension and duration, thereby improving the predictability of the final catastrophic failure. In addition, compaction bands, which were observed in porous rocks under higher confining pressure, were also observed at low confining pressure (corresponding to a depth of ∼1 km) in undrained tests. These results are particularly important for research fields in which fluid migration or pore pressure diffusion is expected to play a role, such as hydrocarbon reservoirs, enhanced geothermal systems, geological storage of CO2.