Fracture Morphologies of Shale in Brazilian Tests Based on A Thin-Layer Model

Effect of Shale Transverse Isotropy on Hydraulic Fracture Propagation

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 ( k ) acts as a primary geometric control: increasing k 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 k . 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.

Fracture Propagation Characteristics and Influencing Factors in Cross-Layer Fracturing of Interlayered Shale Reservoirs

Hydraulic-fracture transmission across lithologic interfaces governs fracture-height growth and reservoir connectivity in shale–sandstone interbedded reservoirs, yet the coupled effects of mechanical contrast, interface properties, layer geometry, and operational parameters remain insufficiently quantified. A two-dimensional plane-strain hydraulic-fracturing model was developed in ABAQUS by coupling Biot poroelasticity, cohesive-zone damage, and fracture-fluid flow. The model explicitly represents thin, alternating continental shale–sandstone layers, lithology-dependent in situ stress and stiffness, and cohesive interfaces; its implementation was assessed against the KGD solution and published layered-rock fracture morphologies. Under the simulated conditions, increasing the shale-to-sandstone elastic-modulus ratio from 0.4–0.5 to 0.6–0.8 reduced the number of penetrated layers from eight to six. Increasing tensile strength from 4 to 16 MPa reduced the number of penetrated layers from ten to six and the final fracture length from 32 to 21 m, while increasing the maximum aperture from 6.75 to 9.02 mm. A sandstone interlayer thickness of approximately 3 m marked a transition in the present parameter set rather than a universal threshold. Sandstone-centered perforation and higher injection rates promoted vertical connectivity, whereas very high fluid viscosity increased near-wellbore aperture but restricted long-distance fracture-height growth. These results provide a mechanics-based framework for optimizing perforation placement and stage-specific fluid design in continental interbedded shale reservoirs.

Anisotropic progressive shear damage and fracture mechanisms of fractured shale based on acoustic-optical-mechanical methods

Bedding planes and natural fractures are key factors controlling the anisotropic mechanical properties of shale reservoirs, exerting a critical influence on underground engineering. However, their shear deformation and fracture evolution mechanisms remain poorly understood. In this study, synthetic shales containing bedding planes and fractures were prepared using a natural mineral reduction−stepwise compaction−mechanical similarity method. Subsequently, the evolution characteristics of macroscopic mechanical behavior, acoustic emission (AE) signals, and strain fields during the shear process of fractured shale were explored using acoustic-optical-mechanical multiphysics monitoring. Finally, the anisotropy of damage deformation, failure modes, and fracture mechanisms in shale were revealed. The test results indicate (1) synthetic shales exhibit strong similarity to natural shale, making them suitable for investigating variations in mechanical behavior; (2) cohesion and peak shear strength reach their maximum and minimum values at bedding angles (α) of 45° and 0°, respectively, while the friction coefficient and residual strength reach the two at α = 90° and 0°, respectively; (3) as α increases, a localized high-strain zone forms along the bedding planes and shear plane, leading to irregular shear fracture zones with jagged surfaces; (4) the shear fracture mechanisms in artificial shale include bedding shear fracture, bedding tensile fracture, matrix shear fracture, and matrix tensile fracture; at α = 45°, the bedding direction aligns with the σ1 direction, causing shear fracturing jointly controlled by the matrix and bedding planes; and (5) the shear failure process of fractured shale exhibits a well-coupled acoustic-optical-mechanical effect. The deformation energy density, in particular, demonstrates a strong response to the energy accumulation and release patterns. This study provides a novel experimental framework and mechanistic insights that are critical for the efficient development of deep shale gas resources, as well as for the prediction and mitigation of geological hazards.

Research on the Stability of Arc-Fractured Rock–Concrete Composites: Mechanical Response and Energy Evolution

To mitigate the violent movement of overlying strata in goaf areas, rock–concrete composite support systems are widely utilized. However, the mechanical behavior of such systems under the influence of complex pre-existing defects, such as arc-shaped fractures, remains insufficiently understood. This study aims to clarify the failure mechanisms and the evolution of stability in these composites by evaluating the influence of fracture inclination angles. A synergistic methodology was adopted, combining laboratory uniaxial compression tests with discrete-element method simulations. Based on energy dissipation theory and the strain equivalence hypothesis, a statistical damage constitutive model was established to bridge the gap between microscopic damage and macroscopic mechanical response. The results demonstrate that fracture inclination significantly dictates the energy partitioning and crack propagation patterns within the composite. The established constitutive model, validated by numerical results ( R 2 > 0.999), effectively quantifies how increasing inclination angles enhance energy absorption efficiency and retard structural damage progression. Due to the high toughness of the concrete component, the composite maintains substantial residual bearing capacity, preventing instantaneous failure. These findings provide a robust theoretical framework and practical guidance for optimizing support designs in deep underground excavations with intricate geological defects.

Fracture behavior of synthetic sandstone under hydro-mechanically coupled dual-period compressive torsion

Differential Propagation Laws and Mechanisms of Hydraulic Fractures Controlled by Reservoir Structural Effects

Coal-measure gas co-production is a critical strategy for enhancing the single-well productivity of unconventional natural gas. However, the pronounced vertical heterogeneity and complex combinations of co-existing reservoirs create substantial asynchronous propagation behaviors during hydraulic fracturing, fundamentally limiting the accurate prediction of multi-reservoir stimulation outcomes. This study employs numerical simulation to investigate fracture development, using the reservoir combinations of the Linxing area on the northeastern margin of the Ordos Basin as a geological model. Our results show that the thickness ratio and mechanical properties of individual rock layers are primary controls on fracture propagation. Specifically, a higher coal seam thickness ratio reduces fracture half-length but increases width, while a greater sandstone layer thickness ratio decreases width and increases height. We further propose the novel concept of the fracture propagation coefficient to characterize the heterogeneity of the fracturing process. It is found that fracture development is closely related to the distance from the injection point, the physical properties of rock layers, and the mechanical property differences between adjacent strata. The distribution of fractures is governed by the coupling effect between injection point location and reservoir mechanical properties. The reservoir–fracture response relationships established in this study provide a scientific basis for optimizing reservoir selection and fracturing parameters in coal-measure gas development.