The brittle deformation regime of water-saturated siliceous sandstones

Abstract

SUMMARY We present here new experimental data on the mechanical behaviour of water-saturated Bentheim and Fontainebleau sandstones deformed in the brittle failure regime. Bentheim sandstone samples were stressed at room temperature and subjected to confining pressures PC ranging from 12 to 120 MPa and pore pressures PP ranging from 1 to 70 MPa. For all samples the evolution of the volumetric strain first shows compaction eventually reversing to dilation of the pore volume when approaching the peak stress. All samples failed localized on a single shear zone. Critical stresses that is onset of dilatancy and peak stress, can be uniquely defined as a function of the effective pressure Peff = PC – PP. The failure curve parameters, when fitted with the Hoek–Brown criterion or a parabolic envelope, are consistent with previous values obtained on similar material. When compared to data obtained on dry samples of the same rock, the present data show no notable effect of the presence of water on the critical stress levels. The same conclusion holds for the stress–strain curves when dry and water-saturated experiments under equivalent effective confining pressures are compared. When compared to previously published data obtained on various quartzose rocks, which show a quite variable water-weakening effect, our results lead to the conclusion that the quasi-exclusive presence of quartz grains bonded together by a quartzose cement combined with the absence of clayey minerals may explain the absence of this effect in Bentheim sandstone. Moreover, this conclusion is supported by complementary results obtained by Fontainebleau sandstone with similar microstructural and compositional characteristics. A previously developed micromechanical model based on the interaction of pore cracks has been used and modified to take into account the presence of water. When compared to the experimental data, the results of the model show good agreement for low to intermediate effective confining pressures. Critical stresses as well as deformation curves are well reproduced as was the case for the dry rock provided that effective pressure is introduced. At higher effective pressures, a mix of mechanisms including shear-enhanced compaction may explain the progressive discrepancy between the modelled and the experimental curves when approaching the apex of the failure envelope.