Application of nano-scratch technology to identify continental shale mineral composition and distribution length of bedding interfacial transition zone - A case study of Cretaceous Qingshankou formation in Gulong Depression, Songliao Basin, NE China

Bedding is the smallest macroscopic layer in a sedimentary sequence, usually containing different mineral compositions and ranging in thickness from a few tens of micrometers to a few millimeters. The microscale mineral composition and mechanical properties of the bedding interfacial transition zone (BITZ) have a significant influence on the crack propagation mode, which further leads to changes in hydraulic fracturing efficiency. At present, most of the identification of the shale mineral composition and the distribution length of the bedding interface transition zone relies on scanning electron microscope, but this method can only obtain the surface morphology and cannot further evaluate the mechanical properties. In this paper, the mechanical parameters such as friction coefficient and fracture toughness of shale samples from Songliao Basin were measured by nano-scratching technique. The damaged shapes and mineral composition in the scratched area were analyzed by scanning electron microscope. Indentation depth and fracture toughness data obtained from scratch tests were fitted with a Gaussian function to determine mineral composition. The slope of the d2-L curve is closely related to the mechanical behavior of minerals and is considered an effective parameter for identifying mineral composition. Based on the fluctuation characteristics of friction coefficient and fracture toughness curve, a quantitative method of shale BITZ distribution length based on linear regression is proposed, and the upgrade from single-parameter to multi-parameter combination is realized by using the method of dimensional analysis. The results of the study show that plastic minerals can effectively absorb energy and convert it into volume deformation and the rapid release of strain energy from brittle minerals results in the collapse of rock debris. The identification of mineral composition based on fracture toughness is a more accurate and effective method compared to the scratch depth and the slope of the d2-L curve. As the influence of mechanical performance was not considered, the length of BITZ distribution determined by SEM was significantly smaller than the analysis results of mechanical parameters. Compared with the friction coefficient method and the fracture toughness method, the dimensionless parameter method integrates several mechanical parameters and has high accuracy and applicability in identifying the distribution length of the interfacial transition zone of shale bedding. The results will help study the mechanical properties of different mineral interfaces of shale at the microscopic scale using the nano-scratch technique.