Due to the combined effects of complex in-situ stress states and geological characteristics of the Longmaxi shale formation, the hydraulic fracture height growth geometry and propagation exhibited great differences at different burial depths. In this paper, through many true triaxial fracturing experiments of deep and medium-deep shale outcrops, the hydraulic fracture propagation height behavior of shale at different burial depths was summarized, and the main influencing factors were obtained. Moreover, considering the effects of two dominant influence factors, namely the bonding strength and the frictional characteristics of shale bedding planes, a three-dimensional numerical model to describe the interaction mechanism between the hydraulic fracture and the beddings was established. The effects of interface strength and in-situ stress on fracture penetration behavior were evaluated quantitatively, and then a comprehensive chart was proposed. Results showed that according to the intersection relationship between the hydraulic fracture and the bedding planes, five basic types of the hydraulic fracture initiation and propagation near the wellbore in shale were obtained: ① Hydraulic fracture initiated and propagated perpendicular to the bedding planes; ② Hydraulic fracture initiated and propagated paralleled to the bedding planes; ③ Hydraulic fracture initiated and propagated perpendicular to the bedding planes. During fracture propagation, a fishbone-like fracture network was induced by diverging from and bypassing the weak bedding planes; ④ Hydraulic fracture initiated and propagated paralleled to the bedding planes. During the fracture propagation, penetration behavior occurred as the bonding strength of the bedding plane was larger while arrest or swerve behaviors occurred as the bonding strength of the bedding plane was smaller; ⑤ Hydraulic fracture initiated and propagated simultaneously from a few natural fractures near the initiation point, and then diverted into a different propagation path by the bedding planes. The hydraulic fracture network in the vertical direction gradually changed from a small horizontal sweep type to a large horizontal sweep type as the depth increased. The final fracture pattern for the medium-deep shale was a fishbone fracture network with transverse fractures as main fractures, while for the deep shale the stepped fracture network with horizontal fractures was the main fracture pattern. The bedding cementing strength and vertical stress difference coefficient determined the intersection mode between the hydraulic fracture and beddings, thus controlling the final fracture height morphology of shale formation with different depths. The findings obtained in this paper could provide an insight for understanding the geometry and behavior of shale fracture networks and guide the fracturing treatment.

The first Geo-Energy Frontier Forum with the theme of “opportunities and challenges for geo-energy exploration and development” was successfully held in Wuhan, recently. The forum included 32 sessions, mainly focused on four directions: geo-energy development and reserve, petroleum geophysical exploration, oil and gas geology, and field development engineering. This paper summarizes the key findings in the 22^nd session titled “Reservoir stimulation for unconventional oil and gas resources”. A total of 17 experts and scholars participated in the presentations, covering a wide range of topics in unconventional oil and gas resources development. This research collectively highlighted the significance of reservoir stimulation techniques in unconventional oil and gas resource development, including research progress in fracture network modeling techniques, fluid pressure, rock mechanics, fracture propagation, and proppant migration in hydraulic fracturing.

Hydraulic fracturing is crucial for extracting shale oil and gas. This technique involves creating fractures in rock formations to enhance reservoir development efficiently. Due to the complexity of shale rock, it is important to conduct multiscale investigations into the fracturing process. Despite extensive research, the technology for deep-underground shale hydraulic fracturing continues to advance as it moves deeper underground. This paper explores the existing technical challenges of shale fracturing, review the current status of physical experiments and numerical simulations, and highlight the importance of multiscale numerical simulation methods. Meanwhile, an integrated approach to optimizing fracturing designs for field cases is introduced. Finally, this paper summarizes the challenges and opportunities in shale hydraulic fracturing, aiming to provide fresh insights into the advancements of hydraulic fracturing technology.