Key Takeaways & Executive Findings
- •• The propagation path of hydraulic fractures at coal-measure rock interfaces is jointly controlled by interface strength coefficient (η), interface inclination angle (θ), and vertical stress difference coefficient (k). • When fractures propagate from soft to hard rock, interface strength dominates; larger η promotes interface penetration, whereas from hard to soft rock, vertical stress difference controls the path, with larger k favoring crossing. • Interface inclination angle θ influences the vertical stress component; smaller θ facilitates penetration, while larger θ leads to fracture propagation along the interface. • Fracture network complexity increases with higher k and θ, but the complexity of fracture morphology shows a non-monotonic trend, initially decreasing then increasing with rising k and θ.
Abstract
This study integrates true triaxial hydraulic fracturing experiments with finite-discrete element method (FDEM) numerical simulation to systematically investigate the control mechanisms of interface strength and inclination angle on hydraulic fracture propagation in coal measure strata under different in-situ stress conditions. The results indicate that the fracture propagation path at the rock interface is jointly controlled by the interface strength coefficient (η), the interface inclination angle (θ), and the vertical stress difference coefficient (k). When fractures propagate from soft rock to hard rock, the interface strength coefficient (η) plays a dominant role. The larger the η is, the more likely the hydraulic fracture is to penetrate the interface along the direction of vertical stress. Conversely, when fractures propagate from hard rock to soft rock, vertical stress primarily controls the propagation path. A larger vertical stress difference coefficient promotes interface crossing, while a smaller coefficient tends to cause the fracture to extend laterally along the interface. The interface inclination angle influences the magnitude and direction of the vertical stress component along the interface. A smaller θ facilitates interface penetration by hydraulic fractures, whereas a larger θ leads to fracture propagation along the interface. The complexity of the hydraulic fracture network increases with higher k and θ . Moreover, the complexity of hydraulic fracture morphology exhibits a non-monotonic trend, initially decreasing and then increasing with rising k and θ. This research provides an important theoretical basis for the design and control of hydraulic fracturing in coal measure strata.
1. Introduction
The sudden rupture of a hard roof stratum, or a hard roof composed of multiple rigid rock layers, overlying a coal seam can release elastic energy, which is prone to inducing coal mine rock bursts [1, 2]. These events result in severe casualties and economic losses [3, 4]. Hydraulic fracturing technology, which alters the integrity and reduces the strength of hard roof strata by artificially creating fractures, is widely used in the prevention and control of coal mine rock bursts [5, 6]. When hydraulic fractures propagate through a hard roof composed of multiple rigid rock layers and encounter a stratum interface, their propagation trajectory may undergo abrupt changes due to the sudden variation in rock properties and the characteristics of the interface [7, 8].
The interfacial characteristics of multi-lithologic composite rock layers primarily refer to the cementation strength and the dip angle of the interfaces. JIANG et al [9] found that abrupt changes in interfacial strength are the main cause of hydraulic fracture trajectory deflection. ZHAO et al [10] investigated the mechanical model describing the interaction between hydraulic fractures and rock interfaces. They provided a mathematical formula that represents the correlation between interlayer stress variations and the vertical distance over which hydraulic fractures propagate after crossing an interface. Their findings indicated that, under similar conditions, larger stress differences across the interface result in greater fracture propagation distances following penetration. Similarly, JIANG et al [11] emphasized the significant role of fracture toughness in rock interfaces in influencing the geometry of hydraulic fractures. High fracture toughness at the interface tends to encourage the branching and deflection of hydraulic fractures, which in turn leads to the creation of multiple secondary fractures. ZHAO et al [12] found that shear slip and fluid leakage-induced tip blunting at rock interfaces can inhibit hydraulic fracture penetration. However, when hydraulic fractures extend from a high-elastic-modulus layer to a low-elastic-modulus layer with a significant modulus contrast and under high vertical stress, the inhibitory effect of interfacial strength on fracture penetration is reduced, allowing fractures to cross the interface under vertical stress. TAN et al [13] showed that hydraulic fractures are more likely to penetrate the interface when stress differences are significant and the angle between the interface and the vertical is small.
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MA Jun-qiang, WEI Si-yuan, LI Xue-hua, DONG Guo-wei, YAO Qiang-ling, YUAN Yu-xin, WANG Hong-sheng (2026). True triaxial experiment and FDEM simulation on the controlling effect of coal-measure rock interfaces on hydraulic fracture propagation. Journal of Central South University. https://doi.org/10.1007/s11771-026-6256-8
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Frequently Asked Questions
What is the main objective of this study?
The study integrates true triaxial hydraulic fracturing experiments with FDEM numerical simulation to systematically investigate the control mechanisms of interface strength and inclination angle on hydraulic fracture propagation in coal measure strata under different in-situ stress conditions.
How does interface strength affect hydraulic fracture propagation?
When fractures propagate from soft to hard rock, a larger interface strength coefficient (η) promotes fracture penetration across the interface along the vertical stress direction. Conversely, when propagating from hard to soft rock, vertical stress difference coefficient (k) becomes more dominant, with larger k favoring interface crossing.
What role does the interface inclination angle play?
The interface inclination angle (θ) influences the magnitude and direction of the vertical stress component along the interface. A smaller θ facilitates fracture penetration, while a larger θ tends to cause fracture propagation along the interface.
How does the complexity of hydraulic fracture networks change with stress conditions?
The complexity of the hydraulic fracture network increases with higher vertical stress difference coefficient (k) and interface inclination angle (θ). However, the complexity of fracture morphology shows a non-monotonic trend, initially decreasing and then increasing with rising k and θ.
What is the practical significance of this research?
This research provides an important theoretical basis for the design and control of hydraulic fracturing in coal measure strata, particularly for preventing coal mine rock bursts by optimizing fracture propagation through layered rock masses.
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