Influences of the stress ratio and local micro mineral aggregates on small fatigue crack propagation in the shale containing bedding planes

Due to the existence of bedding planes in shale oil reservoirs, the complexity of crack networks created by conventional hydraulic fracturing (HF) technology is limited, resulting in low oil production. In this paper, a fatigue loading method was proposed to increase the complexity of the cracking network. The propagation behaviors of small cracks were investigated and compared under both monotonic and fatigue loading conditions by using SEM in-situ three-point bending tests. In addition, the influences of local micro mineral aggregates and stress ratios (R = 0.1, 0.3 and 0.5) on the propagation behavior of small fatigue cracks (SFCs) in shale were quantitatively evaluated. The SEM in-situ fatigue tests demonstrated the effectiveness of fatigue loading in enabling SFC to propagate through the bedding plane and increase crack network complexity from a micro perspective. A fitting exponential formula of the stress intensity factor range threshold ΔK_th and stress ratio R was obtained. This study assists us in deeply understanding the influence of micro mineral aggregates and stress ratio on the SFC propagation mechanism and its implications for HF design in shale oil reservoirs.