Study on the influence of natural cracks on the mechanical properties and fracture mode for shale at the microscale: An example from the Lower Cambrian Niutitang Formation in northern Guizhou

A large number of natural cracks exist in shale reservoirs, and the presence weakens integrity shale, which is an important factor governing effectiveness gas extraction. In this paper, shales from Lower Cambrian Niutitang Formation northern Guizhou were scanned by electron microscopy, their microstructures selected for digital image processing, uniaxial compression numerical tests conducted on models containing different crack dips using rock fracture process system RFPA2D-DIP to study effects mechanical properties patterns at microscopic scale. The shows that peak strength elastic modulus increase with increasing inclination angle. modes scale can be roughly divided into four categories: similar I-type fractures (0°), oblique (15°, 45°, 60°, 75°), folded line (30°), V-type (90°). Natural within are found have a significant effect distribution stress. Acoustic emission reflect stress change rupture dip angles has AE energy fractal dimension. magnitude increases level reaches maximum value 90°, while dimension zigzag upwards because jointly influenced both newborn native cracks.