Laboratory measurement and interpretation of the fracture toughness of formation rocks at great depth

The fracture toughness of the underground rock material at great depth is an important parameter in the numerical simulation of hydraulic fracturing treatments of reservoir pay-zones in petroleum industry, and its value is usually determined under laboratory conditions. In this paper, based on a large number of elaborate experiments, finite element analysis (FEA) is employed to investigate the width influence of pre-fractures on stress intensifying factors (SIF). Also, the influence of the confining pressure on the toughness is studied in a similar way. Works in this paper lead to the relationship between dimensionless stress intensifying factor and the fracture tip radius, which is the complementary part to the analytical procedures of ideal situations. It is proven experimentally that the analytical procedures can be used to calculate the fracture toughness only when the fracture tip radius is small, due to their ineffectiveness in dealing with problems beyond fracture mechanics. It is required that the failure pressures, required for fracture toughness calculations, correspond to the real magnitude of fracture tip radius. And it is found that the influence of fracture width on the toughness is obvious, and that of confining pressure is great and linear. The conclusions drawn provide theoretical grounds as well as reliable experimental procedures for correctly determining the fracture toughness.