Effect of Water on Fracture Mechanical Properties of Shales

Abstract

Recognition of the potential for subcritical fracture growth of rocks is essential for the evaluation of long-term stability of fluids trapped in geologic structures. In shales, subcritical fracture growth may control migration of hydrocarbons in source rock and unconventional reservoirs and of sequestered CO2 across top seals. Using the double torsion method, we investigated the mode-I fracture mechanical properties of three shales of different composition, namely, Woodford Shale, Mancos Shale, and Marcellus Shale, under water-saturated conditions, in ambient air, and in dry CO2 gas conditions to systematically investigate the effect of water on the fracture growth properties of shales. The clay-rich (illite with minor kaolinite) Woodford and Mancos Shales demonstrate a strong water-weakening effect despite their low smectite content, with the fracture toughness reduced by up to 54% and the subcritical fracture growth index by up to 77% under water-saturated conditions compared to tests in ambient air. Carbonate-rich Marcellus Shale, in contrast, displays negligible water-weakening effects. The clay-CO2 interaction is minor for the three shales. Specimens treated with a hydrophobic surface coating were also tested to limit water-weakening effects to the fracture tip. Data from these tests reveal rate-dependent stress intensity factor-velocity curves for clay-rich Woodford and Mancos Shales suggesting a competition between mechanical fracture growth and physicochemical water-rock interactions. Our results suggest that subcritical failure by water-enhanced subcritical fracture growth could significantly increase brittle failure in clay-rich shales.