Determining the micro-fracture properties of Antrim gas shale by an improved micro-indentation method

Abstract Knowledge of the fracture properties of gas shale rocks is of key importance for more reliable and economical recovery of natural gas via hydraulic fracturing. This paper presents a new micro-indentation technique to probe the elasticity and fracture properties of Antrim gas shale. With the explicit consideration of the holding-load-constant stage at the peak load, new models with simple expressions and clear physical mechanisms were formulated. The load-displacement data of loading and unloading can be characterized by energy terms and constants. Three different displacement levels were used in the indentation tests. The elastic modulus and hardness of Antrim shale were analyzed by the new method. The fracture toughness was evaluated by the crack length-based and improved energy-based methods. Results show that both the elastic modulus and hardness decrease with increasing the displacement, which follows a homogenization process. The values of fracture toughness are relatively small and vary with the adopted data-processing methods due to the intrinsically different underlying principles. The indentation technique, together with appropriate interpreting methods, may pave paths to the accurate determination of the mechanical properties of gas shale at the nano and micro scales.