Abstract
AbstractAn 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.
Highlights
Many engineering and natural processes in the Earth involve coupled rock deformation and fluid flow (Cai & Bercovici, 2013; Connolly & Podladchikov, 2015; Keller et al, 2013; Petrini et al, 2020; Yarushina et al, 2013)
Models describing fluid flow in deformable porous rocks can be based in part on the principles of irreversible thermodynamics (Yarushina & Podladchikov, 2015)
Viscous deformation and strong coupling between fluid flow and geomechanical deformation can eventually lead to the formation of focused fluid flow, often evidenced in the Earth as dikes, veins, volcanic diatremes, or seismic chimneys (Räss et al, 2014; Yarushina, Podladchikov, et al, 2015; Minakov et al, 2017)
Summary
Many engineering and natural processes in the Earth involve coupled rock deformation and fluid flow (Cai & Bercovici, 2013; Connolly & Podladchikov, 2015; Keller et al, 2013; Petrini et al, 2020; Yarushina et al, 2013). Models describing fluid flow in deformable porous rocks can be based in part on the principles of irreversible thermodynamics (Yarushina & Podladchikov, 2015). Shear‐enhanced compaction induces considerable permeability and porosity reduction, affecting fluid flow (Xiao et al, 2006; Zhu et al, 1997) It is especially pronounced in weak rocks, which are prone to creep, resulting in many engineering problems such as underestimated subsidence and loss of stability of underground constructions (Tsai et al, 2008). While conservation equations were derived based on the principles of irreversible thermodynamics, closure relations were obtained based on the effective media theory by looking at the effective behavior of rocks containing idealized cylindrical or spherical pores. The influence of shear stresses on fluid flow is studied based on the geological system with propagating porosity waves
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