Dilatancy, failure, and fluid flow in basalt: Implications for geothermal reservoirs

Abstract

Understanding how the strength of volcanic rocks varies with stress state, pressure and microstructural attributes is fundamental to understanding the dynamics of geothermal reservoirs. In this study we investigated the micromechanics of deformation and failure in basalts. We performed uniaxial and triaxial compression experiments samples covering a porosity range between 5 and 16%, at effective pressures up to 200 MPa. Dilatancy and brittle faulting were observed in all samples with porosity of 5%. Shear-enhanced compaction was observed at effective pressures as low as 80 MPa in samples of 8% porosity and more. Microstructural data revealed the complex interplay of microcracks and pores on dilatant failure and inelastic compaction in basalt. The micromechanics of brittle failure was controlled by wing crack propagation under triaxial compression in low porosity end members, and by pore-emanated cracking in the more porous samples. Permeability of basalt was found less sensitive to porosity then mechanical strength.