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.

There are fracture-cavity systems with different shapes and sizes distributed in the fracture-cavity carbonate formation in Tahe Oil Filed, China, while they are also the main oil and gas reservoirs. Due to the uneven distribution of fracture-cavity systems in the carbonate reservoirs, hydraulic fracturing is used to communicate with the fracture-cavity systems to establish the flow channels between the fracture-cavity systems and the wellbore. So it is worth studying the propagation rule of hydraulic fracture in fracture-cavity carbonate formation, which determines the effect of reservoir reconstruction. In this paper, based on the discontinuous discrete fracture model, we built a hydraulic fracture propagation model in the fracture-cavity carbonate formation. First, we built the fluid-solid coupled stress field model for the fracture-cavity reservoir and then used the discrete fracture model to construct the hydraulic fracture. The model allowed the hydraulic fracture to expand along the initially divided grid, and the minimum strain energy density criterion was used to determine the propagation path. According to the different fracture-cavity distribution rules, we set three fracture-cavity reservoir models with different characteristics in this paper. Based on the simulation result of different fracture-cavity formation characteristics, we found that: In fracture-cavity carbonate reservoirs, the hydraulic fracture would be deflected by the local stress field disturbed around the caves. The larger the cave was, the more pronounced the disturbance was. According to the different relative positions, the disturbance could divide into two cases, frontal repulsion and lateral attraction, which were not conducive to the communication between the hydraulic fracture and the caves. However, when there were nature fractures around the caves, the communication probability was raised as the hydraulic fracture could easily intersect these nature fractures around the cave. By increasing the hydraulic fracture’s net pressure, the leading role of the hydraulic fracture could enhance when it intersected with the fracture-cavity systems so that it was able to break through the repulsion of the cave to communicate with the fracture-cavity systems in the direction of the principal stress. When the hydraulic fracture penetrated the fracture-cavity systems, the injection pressure was mainly controlled by the flow energy loss within the hydraulic fracture and the fracture-cavity systems’ fluid loss rate. Optimizing the fracturing fluid performance could reduce the penetration pressure and improve the propagation range of hydraulic fractures. The results of this paper could provide a reference for the fracturing evaluation of the fracture-cavity carbonate reservoir.