Effect of Shale Transverse Isotropy on Hydraulic Fracture Propagation

Abstract Hydraulic fracturing is a critical technology for the efficient development of shale reservoirs. Induced by factors such as the preferred orientation of clay minerals, bedding planes, and natural fractures, shale exhibits pronounced transverse isotropy. The interaction between hydraulic fractures and these natural weak planes typically induces complex propagation behaviors. However, conventional studies often idealize shale reservoirs as isotropic media, neglecting the significant influence of transverse isotropy on fracture morphology. This study performs triaxial compression tests on Longmaxi formation shale to quantify mechanical anisotropy. Subsequently, a numerical model based on the global cohesive zone method is established to investigate fracture propagation across shale matrix, laminated shale, naturally fractured shale, and multicluster fracturing scenarios. Results demonstrate that Longmaxi shale exhibits strong mechanical anisotropy. Numerical simulations reveal that the transverse isotropy ratio ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" alttext="k" display="inline" overflow="scroll"> <mml:mi>k</mml:mi> </mml:math> ) acts as a primary geometric control: increasing <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" alttext="k" display="inline" overflow="scroll"> <mml:mi>k</mml:mi> </mml:math> significantly promotes vertical fracture height growth while simultaneously suppressing width opening due to the enhanced horizontal stiffness. This deformation behavior is accompanied by intensified horizontal stress disturbance and diminished vertical stress disturbance. The transverse isotropy characteristics and weak interfaces (bedding planes and natural fractures) jointly control the fracture propagation morphology. Laminae and natural fractures alter the hydraulic fracture propagation path, while the transverse isotropy ratio can regulate the propagation direction of the hydraulic fracture. When the ratio is between 1.5 and 2.0, the differences in horizontal and vertical stress disturbances are small, and hydraulic fractures easily communicate with laminae and natural fractures, forming a complex fracture network. Regarding multicluster fracturing, competitive propagation is primarily governed by cluster spacing and modulated by <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" alttext="k" display="inline" overflow="scroll"> <mml:mi>k</mml:mi> </mml:math> . Cluster spacing determines the fundamental intensity of interference, whereas the anisotropic stress regime compels the restricted central fracture to redirect energy toward vertical extension, enabling it to attain a vertical height comparable to that of the side fractures. These findings offer theoretical insights for the optimization of hydraulic fracturing designs in shale reservoirs.