The effect of notches on breakdown pressure during hydraulic fracturing at various stress regimes

Abstract

High stress environments are becoming more common as the trend and need to develop deeper block cave mines economically and safely continue. Hydraulic fracturing (HF) is considered to be a powerful tool in the success of hard rock block cave mines as it assists with caveability, economic recovery, and seismic risk mitigation. Therefore, determining critical design parameters such as breakdown pressure and the impact of notching on this parameter is paramount for designing and implementing a successful preconditioning campaign. To better understand the impact of notches on the breakdown and propagation pressure, a series of lab experiments were undertaken with variations in stress regime, stress magnitudes and the presence and number of notches were evaluated. This enabled us to explore the impact of confining pressure and spacing of notches on the breakdown and fracture propagation pressures. Various types of samples including Adelaide Black Granite, Engineered plastic (PMMA) were tested to give ranges and balance for homogeneity and real-world mining applications. We observed the fracture initiation and propagation through breakdown pressures along with acoustic emission measurements. The experimental results confirm that increasing confining stress on a sample increases the breakdown pressure. The works also validate the non-impact of axial stresses and the variation of tensile strength on a sample on the breakdown pressure.