Deformation of Wilcox Shale: Undrained Strengths and Effects of Strain Rate

ABSTRACT: Mechanical and fluid transport properties of brine-saturated illite shale from the Wilcox Formation have been investigated to examine the effects of strain rate on fracture strength and to determine the conditions over which rates of deformation and internal fluid flow compete. Permeability (k) measurements predict time scales for fluid pressure equilibration that are long relative to those of deformation experiments; values of k are small even at relatively low effective pressures P_e (assumed ? P_c P_f) and they drop below 10^-20 m² at pressures of = 5 MPa. Failure strengths (s₁-s₃) of Wilcox shale . specimen s saturated (as nearly as can be achieved) with brine and shortened at strain rates of 2.8x10^-7 = e = 8.2x10^-3_s^-1 fit an exponential law e = A exp{a(s₁-s₃)} of the same form as determined for Wilcox shale specimens deformed at much higher pressures in the absence of pore fluids. The positive slope and comparable value of a (= 0.29 MPa^-1) to that determined for specimens without pore fluids a = 0.3 MPa^-1) suggest that Wilcox shale exhibits undrained behavior at strain rates > 10^-7_s^-1 and that strain rate effects associated with pore pressure perturbations and pore fluid transport occur at much lower strain rates.