ABSTRACT
A medium-to-high porosity St. Peter sandstone was subjected to hydrostatic confining pressure P c (= S) under fully-drained conditions, simulating the stressed rock in situ during depletion or injection. We recorded variations of static strain and dynamic velocities with confining pressure and pore pressure to estimate the corresponding effective stress coefficient. For the static deformation data, α is clearly less than unity, ranging between 0.3 and 0.7 for any tested stress conditions. The effective stress coefficient is dependent on P p during depletion but not so during injection. Given the same stress condition, the effective stress coefficient during injection is consistently higher than during depletion. The dynamic effective stress coefficient for Vp is generally close to unity when σ is less than 20 MPa. During depletion, α for Vp first decreases with σ when the latter is up to 30 MPa. However σ jumps to unity for σ≥30 MPa. The effective stress coefficient for V S increases significantly with σ, regardless of the loading path. It is important to note that the effective stress coefficient with respect to velocities is different from that of static deformation, in both the magnitude and the dependency on P c and P p.
