Control of fracture toughness of kerogen on artificially-matured shale samples: An energy-based nanoindentation analysis

Abstract

Understanding controls on mechanical properties of organic-rich shale is essential in exploration and successful production from hydrocarbon reservoirs. In previous works, most researchers have focused on the influence of thermal maturity and organic type on elastic and viscoelastic properties of organic matter. However, fracture toughness, an important parameter in engineering design, has been less investigated. In this study, shale samples with different thermal maturity were obtained by programmed heating. Then, the fracture toughness of organic matter in shale samples was measured using an energy-based nanoindentation method, and the influence of geochemical features was determined. Organic matter was classified as inertinite, vitrinite and bitumen, and the fracture toughness of tested organic matter was in the range of 0.023–0.081 MPa*m0.5. The results indicated that the fracture toughness of vitrinite increased with thermal maturity. Additionally, both the reduced modulus of vitrinite and bitumen increased, while the viscosity of all types of organic matter decreased, with higher levels of thermal maturity. Moreover, a comparison between mechanical parameters of artificially-matured samples, and those of natural samples, was conducted. The fracture toughness of organic matter obtained in this study is crucial for parameter estimation of fracture propagation and homogenization models.