Experimental investigation of the influence of strain rate on strength; failure attributes and mechanism of Jhiri shale

The modern improved engineering technologies in the field of rock mechanics and the successful identification of the hydrocarbon potential of gas shales have turned the tight shale formations as a profitable resource for the natural gas. In the current study, Jhiri shale was tested for its strength; deformational failure attributes and mechanism at different strain rates in order to understand the dependence of the deformation rate upon various geomechanical properties. The rock samples were subjected to varied strain rates during loading and the resultant geomechanical properties such as uniaxial compressive strength (UCS), tensile strength (σ_t), Young's modulus (E), failure strain (ε_f), mode I and mode II fracture toughness (K_IC and K_IIC) and brittleness index (B₁ and B₂) were determined in each case. The stress-strain behaviour of the Jhiri shale was estimated at four different strain rates that varied from 1.7 × 10⁻² s⁻¹ to 7.9 × 10⁻⁵ s⁻¹. It was found that all of the mechanical parameters of the rock that are mentioned above, except for the failure strain, increased with increasing strain rates. Such behaviour of the rock due to the strain rates may be due to stress redistribution during grain fracturing. At a strain rate of 7.9 × 10⁻⁵ s⁻¹, UCS, tensile strength, mode I fracture toughness and mode II fracture toughness of Jhiri shale were found to be 25.45 MPa, 7.71 MPa, 0.171 MPa m^1/2 and 0.083 MPa m^1/2, respectively, which increased up to 50.57 MPa, 13.06 MPa, 0.565 MPa m^1/2 and 0.467 MPa m^1/2, respectively, at a strain rate of 1.7 × 10⁻² s⁻¹. Critical and appropriate empirical equations have been proposed to evaluate the strain-rate dependence of the mechanical properties of the rock.