True triaxial hydraulic fracturing experiments and FDEM simulation study of coal-measure rock strata

To understand the variation in fracture propagation trajectories caused by lithological abrupt changes when hydraulic fractures encounter stratum interfaces, this study investigates the propagation behavior of hydraulic fractures in coal-measure strata under different lithological combinations through true triaxial hydraulic fracturing experiments and finite-discrete element method (FDEM) simulations. The influence mechanisms of in-situ stress and lithological strength differences on hydraulic fracture propagation trajectories are revealed. The results indicate that the morphology of hydraulic fractures in coal-measure strata exhibits significant asymmetry, with seven distinct patterns: single, cross, “T” shaped, “+"shaped, “工” shaped, “干” shaped, and complex fractures. The hydraulic fracturing pressure curve can be divided into four stages: rapid increase to fracture initiation pressure, sudden pressure drops, stable fluctuation, and pressure decline after pump shutdown. When hydraulic fractures encounter natural fractures or stratum interfaces, the pressure curve exhibits noticeable fluctuations, and the magnitude of the sudden pressure drop is negatively correlated with the complexity of the hydraulic fractures. The control of in-situ stress on hydraulic fractures is related to the orientation of the maximum principal stress and the vertical stress difference coefficient. When the vertical stress is the maximum principal stress, a larger vertical stress difference coefficient facilitates the vertical propagation of hydraulic fractures across interfaces. The influence of lithological strength differences on hydraulic fracture propagation trajectories is manifested as follows: when hydraulic fractures initiate and propagate from soft rock to a stratum interface, they tend to arrest or propagate along the interface; conversely, when hydraulic fractures initiate in hard rock, a larger lithological strength difference (ΔS) promotes vertical propagation across the interface, while a smaller ΔS favors propagation along the interface. This study provides insights into the complex behavior of hydraulic fractures in coal-measure strata, offering a theoretical foundation for optimizing hydraulic fracturing designs in multi-lithological layered formations.