Effects of Plastic Deformation on Poroelastic Pore Pressure Coefficients in Pierre II Shale.

Skempton's parameters B_S and A_S describe the undrained pore pressure response to stress state changes in porous media. In our study we examined the relationship between two poroelastic coefficients, (equation) and A, equivalent to Skempton's parameters within given elastic region and plastification of a transversely isotropic (TI) medium. We used a triaxial apparatus to perform a series of experiments on ten Pierre II shale samples drilled at various angles with respect to rock's bedding and saturated with brine equivalent to their original pore medium. The tests consisted of stress- and strain-driven loading and unloading cycles carried out in undrained conditions, during which stresses, strains, pore fluid pressure and temperature were measured. We determined the poroelastic parameters and estimated axial and radial plastic strains for each of the unloading steps and investigated the relationship between them. We documented a transition of unloading-induced pore pressure change from negative to positive values in the constant mean stress conditions, as well as gradual deviation from purely elastic TI medium symmetry. We found the ratio between elastic and total axial strains to correlate well with A changes and identified it as a potentially useful parameter for modelling of pore pressure response in shales in the direct proximity of faults.

Stress-dependent pore pressure response in lowpermeable soils and rocks has been researched and successfully quantified for several decades (Skempton, 1954, Henkel, 1960, Henkel and Wade, 1966, Janbu et al., 1990). One of the very first studies, described by Skempton (1954), introduced two parameters capturing the impact of the principal stress changes on the undrained pore pressure response in triaxial conditions (i.e. equal horizontal stresses). Skempton's coefficient B_S describes the effect of the mean stress and coefficient A_S of the deviatoric stress changes, as shown in Eq. 1:

(equation)

where p_F is the pore pressure, σ₃ and σ₁ are the minimum and maximum principal total stresses, respectively.