Investigating the Meso-Mechanical Anisotropy and Fracture Surface Roughness of Continental Shale

Continental shale is abundant with unconventional oil and gas and has considerable prospect for exploitation. However, due to deposited environment differences, continental shale presents unique characteristics in terms of structure and mineralogy compared to marine shale. The mechanical property is elemental parameter for reservoir production, but relevant studies of continental shale are rarely reported. In this study, several groups of mesoscale (0.1-10 mm) uniaxial compression tests were conducted on continental shale with various bedding angles, sourced from the Yanchang Formation in the southern Ordos Basin, China. A new integrated real-time loading-observation-acquisition system, specifically, a miniature tensile instrument-light microscope (MTI-LM) system, was utilized, which allowed us to visualize the failure process and simultaneously record the stress-strain curves, providing snapshots for investigation of fracture initiation, propagation, and fracture surface features. The results demonstrate that as the bedding angle relative to the maximum compression direction increases, the uniaxial compressive strength (sigma(c)) and the percentage of bedding-parallel fractures (F-p) fluctuate reversely, namely, higher sigma(c) corresponds to lower F-p. Additionally, a good consistency is revealed among the variations of sigma(c), failure time and fracture surface roughness with bedding angles, suggesting stronger shale having rougher fracture surfaces and longer failure time at a certain loading rate. The meso-mechanical anisotropy of shales is supposed to have an origin in heterogeneous component and microtexture, as indicated by different bridges connecting the opposite facies of fractures. The findings help reveal the mechanism underlying mechanical anisotropy and facilitate fracability evaluation of shallow shale gas reservoir. Plain Language Summary Shale formed in a continental environment is abundant with unconventional resources, characterized by a layering structure and varied mechanical behaviors with different layer orientations. This feature of shale, referred to as mechanical anisotropy, directly affects the design of fracturing for efficient exploitation of shale gas/oil, yet studies on this property of continental shale have rarely been published. In this paper, mesoscale (0.1-10 mm) laboratory tests were performed on organic-rich Chang 7 shale from the Yanchang Formation with a combined MTI-LM system to visualize and investigate the compressive behavior of shale with various layer orientations relative to the loading axis. The results indicate a reverse fluctuation in the uniaxial compressive strength (sigma(c)) and the percentage of bedding-parallel fractures and a good consistency among the variations in sigma(c), failure time, and fracture surface roughness with layer orientations, namely, the stronger shale has rougher fractures surfaces and longer failure time at a certain loading rate. The uneven composition and microtexture is supposed to produce different bridges connecting the opposite sides of fractures and result in anisotropic mechanical behavior. These findings are helpful for understanding the origin of mechanical anisotropy of shales and serve for the evaluation of shallow shale gas reservoir.