Model for (De)Compaction and Porosity Waves in Porous Rocks Under Shear Stresses

Abstract

An understanding of instantaneous and long-term compaction of porous rocks is important for reservoir engineering and Earth sciences. Experience from laboratory triaxial compression tests and from subsurface operations indicates that shear and volumetric deformations are interdependent. Their mutual dependence results in shear-enhanced compaction and shear-induced dilation under short-term and long-term loading. Using a classical averaging approach, we consider the evolution of a single fluid-filled pore in a solid elastoplastic or viscoplastic matrix under combined pressure and shear loading to introduce a new failure envelope and 3-D constitutive relations for both rate-dependent and rate-independent deformation of porous rocks. Our model provides a simple description of rock behavior under a wide range of strain rates. The model predictions agree well with experimental data from triaxial instantaneous and creep tests. Analytical and numerical solutions for solitary porosity wave propagation in viscoplastic rocks in the presence of shear were obtained. New solutions show that new rheological laws have serious implications for porosity waves. Plasticity onset leads to compaction-decompaction asymmetry and the formation of elongated channel-like porosity waves. Shear-induced dilation facilitates porosity wave propagation at fluid pressures below the lithostatic stress. This makes porosity waves a viable mechanism in the formation of focused fluid flow structures in crustal rocks.