Specific storage as a poroelastic coefficient

A definition for the specific storage coefficient S s is given which is unambiguous for general isotropic three‐dimensional aquifer elasticity. In every representative elementary volume, S s is the fluid volume released from storage per unit decline in hydraulic head, per unit bulk volume, under conditions such that there is no strain in two orthogonal directions, and the total normal stress in the third orthogonal direction is constant. The specific storage coefficient is a point property of the aquifer and is defined independently of problem domain stress and head boundary conditions. The expression for S s in terms of aquifer and fluid compressibilities is identical to the familiar forms obtained assuming zero horizontal strain and constant overburden in an aquifer, although it is not restricted to these conditions. As a point property of the fluid‐saturated material, the specific storage coefficient is one of four constants in the general constitutive poroelastic equations relating three‐dimensional aquifer stress and strain to fluid pressure and dilatation. Written in terms of S s , these equations show that pore fluid mass diffusion is governed by a diffusivity equal to the ratio of hydraulic conductivity to specific storage under arbitrary boundary conditions. It is shown that S s controls slow compressional body wave velocity in the low frequency limit and that the uniaxial aquifer compressibility α is not necessarily related to the vertical direction.