Initiation and propagation of small fatigue cracks in shales containing inclined bedding planes

Abstract

The inclined bedding plane is one of the most common geological characteristics in shale reservoirs. Due to the low permeability of shales, hydraulic fracturing (HF) is widely used in shale reservoirs, but the complexity of the crack network is limited by the inclined bedding planes. A method to increase the number of cracks and enhance their connectivity is needed for shale reservoirs. This study proposes a fatigue loading method to fracturing the shale samples with different dip angles (the complementary angle of the angle between the bedding planes and the prefabricated notch in shale samples), whose effectiveness was validated by the in-situ SEM three-point bending tests. The experimental results showed that the average peak tensile stress for the samples containing bedding plane at a dip angle of 30°, 60° and 90° is only 40.67%, 13.74% and 8.79%, respectively, of the peak tensile stress observed for the samples with a dip angle of 0°. Comparing the initiation and propagation characteristics of small cracks (SCs) under monotonic and fatigue loadings, the fatigue loading method increases the number of artificial cracks by five times. Additionally, the fatigue loading method was useful for enhancing the connectivity between adjacent cracks due to more damage induced along the maximum shear stress direction. Finally, the SC surface morphologies under different loading methods were compared and it is found that the fatigue shear stress along the bedding plane decreases the roughness of the wavy SC surface. This study sheds light on understanding the influence of fatigue loading on SC initiation and propagation, which may thus be used for facilitating HF treatments in shale oil/gas reservoirs containing inclined bedding planes.