Optical visualization of strain development and fracture propagation in laminated rocks

Abstract

Shales are commonly anisotropic in nature due to layering and natural fractures which consequently impact the mechanical behavior of shale in terms of stress-strain distribution, fracture initiation, propagation and fracture pattern. The classical method of strain measurement with strain gauges can not reveal the complex process of strain accumulation and fracture propagation in such anisotropic rocks. Therefore, we applied optical techniques along with image processing to measure strain on the surface of laminated Green River shale samples with various lamination angles under indirect tensile testing conditions. Full-field strain development was monitored over time and fracture patterns were identified utilizing digital image correlation (DIC) technique. A load-strain nature were established as a function of lamination angles incorporated with DIC generated horizontal, vertical and shear strain maps which results in better understanding of sequences in strain development and fracture pattern due to laminations. Variation in indirect tensile strengths are evaluated and reported with applied energy which shows that sample with higher tensile strength required higher applied energy to fail it. Fracture pattern obtained from DIC visualization are characterized which show that shear failure is dominant when failure occurred through laminations and tension for central failure samples. Finally, the measured axial strain from load frame is compared with DIC and achieved a considerable agreement in both measurement. The knowledge of this study can help in evaluating fracture behavior of laminated formations which can be important for energy-related activities including fracturing, fluid injection and extraction.