Laboratory estimates of normal and shear fracture compliance

Laboratory estimates of the normal (Bn) and shear (Bt) compliance of artificial fractures in samples of Jurassic and Carboniferous limestone under wet and dry conditions are presented. The experiments were performed over a range of confining pressures (from 5 MPa up to 60 MPa), at ultrasonic frequencies in a Triaxial Hoek cell, using the pulse-echo reflection technique. The results of this study confirm that the Bn/Bt ratio of a fracture is dependent on the fluid fill. A value of Bn / Bt of less than 0.05 was obtained for our wet (honey saturated) sample which is consistent with the prediction that this ratio should be close to zero for fluid saturated fractures. Values of Bn/Bt for the dry sample are significantly higher and increase with confining pressure from 0.2 to 0.5. It is suggested that a Bn/Bt ratio of 0.5 is probably a more representative value to use in modelling studies of gas filled fractures than the common assumption that Bn ≈ Bt.