A threshold stresses-based permeability variation model for microcracked porous rocks

Abstract

The dependence of permeability on stress is an important property of rocks related to their microstructural alterations. Justified by the distinct regions on the stress–strain curves under mechanical loading, this study proposed a threshold stresses-based permeability variation model for microcracked porous rocks by separating voids of rocks into a more-rounded pore system and a flat crack system. In the proposed model, the permeability reduction caused by progressive closure of pre-existing cracks is characterised by a negative exponential function of the effective mean stress, while the enhancement of permeability induced by the initiation and growth of new cracks is correlated to the cumulative acoustic emission counts described with a normal cumulative distribution function. The permeability induced by the elastic compaction of the less-compressible pores is assumed to decrease linearly with effective mean stress, typically with a rather small slope, which lower bounds the permeability of rocks when the pre-existing cracks are sufficiently closed while new cracks are not yet initiated. The proposed model has advantages of clear physical meaning, easy laboratory parameterisation and low computational cost and reproduces well the main features of permeability variation observed in the laboratory for sedimentary rocks under hydrostatic loading and granitic rocks under triaxial loading.