Differences and causes of fracture height geometry for Longmaxi shale with different burial depths in the Sichuan basin

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.