Multi-Factor Coupling Simulation and Mechanism Investigation of Hydraulic Fracture Propagation in Hard Roof

Numerical Simulation Study of the Permeability Evolution of Sandstone during the Loading and Failure Process Based on FDEM

Rock failure under hydromechanical coupling is a complex process that has attracted considerable attention in deep underground engineering. In this study, a hydraulic coupling analysis method for rock loading and failure is developed based on the finite–discrete-element method (FDEM) combined with the grain-based model, which integrates the dual-medium seepage–stress coupling theory to simultaneously capture pore and fracture seepage. Numerical simulations of sandstone under varying confining pressures were performed and validated against laboratory experiments, and the effects of confining pressure and weak joint content on the permeability evolution were systematically investigated. The results demonstrate that the permeability evolution during loading is governed by the competitive interplay between matrix seepage and fracture seepage, exhibiting a nonlinear pattern. Higher confining pressures prolong the stage of slow permeability growth, whereas an increase in the weak joint content reduces strength and stiffness, leading to an earlier occurrence of the permeability inflection point and an enhanced dominance of fracture flow in controlling macroscopic permeability. These findings provide new insights into the mechanisms of hydromechanical coupling in fractured porous rocks and offer theoretical support for predicting and controlling hydraulic hazards in deep rock engineering.

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.

Multi-Field Coupled Fracture Propagation Mechanisms of Supercritical CO2 Fracturing in Gulong Shale and Tight Sandstone

Strong heterogeneity in unconventional reservoirs leads to complex fracture propagation and challenges in quantitative stimulation evaluation. This study integrates true triaxial fracturing experiments, three-dimensional CT reconstruction, multi-field coupled numerical simulation, and multiple linear regression to investigate the fracture behavior of Gulong shale (Q1, Q9) and tight sandstone under supercritical carbon dioxide (SC-CO2) fracturing. A fracture complexity index (FCI) that incorporates fractal dimension, spatial uniformity, and aperture distribution is proposed as a quantitative metric. The results show that SC-CO2 significantly reduces breakdown pressure and increases fracture complexity compared to water. For Q9 shale, SC-CO2 gives a breakdown pressure of 32.91 MPa (10.46% lower than water), a fractal dimension of 2.41, and an FCI of 8.92. In tight sandstone, the SC-CO2 breakdown pressure is 34.12 MPa, whereas water increases it to 44.50 MPa; the fractal dimension and FCI are only 2.05 and 3.40, respectively, lower than those of shale fractured with water. Multiple linear regression quantifies contribution weights: lithological weak-plane development dominates fracture complexity (41.6%), far exceeding the brittleness index. The injection rate mainly controls stimulation scale: the damage area ratio rises from 1.79% to 2.90% when the rate increases from 10 to 40 mL/min. The horizontal stress difference is key to complexity enhancement: the fractal dimension increases from 1.9230 to 1.9901 as the stress difference rises from 0 to 4 MPa. The numerical simulations further reveal the coupled thermal-hydraulic-mechanical effects. The proposed FCI-based evaluation and regression models provide a quantitative framework for optimizing SC-CO2 fracturing design in heterogeneous unconventional reservoirs.

Numerical Analysis of Hydraulic Fracture Propagation Behaviors in Ultra-Deep Lattice-like Fractured Reservoirs

Ultra-deep lattice-like fractured carbonate reservoirs, formed by multi-period tectonic movements, feature strong heterogeneity, multi-scale fracture nesting, and anisotropic in situ stress. However, hydraulic fracture (HF) propagation behaviors within these complex formations remain poorly understood. In this study, using an unstructured fracture network approach, we simulated HF propagation in two typical fault-controlled lattice-like structures: compressive-torsion and pull-apart overlap zones. The performance of commingled, staged, and temporary plugging fracturing was evaluated, alongside sensitivity analyses of wellbore orientation, plugging timing, pump rate, and fluid viscosity. Results indicate that HFs in compressive-torsion zones exhibit long, straight geometries with local tensile activation points. Conversely, pull-apart overlap zones promote step-shaped, multi-branched fractures with superior lateral connectivity. The optimal timing for temporary plugging exhibits a delayed trend with increasing natural fracture density, ranging from 50% to 70% of the fracturing process in compressive-torsion zones, whereas an earlier implementation is preferred in pull-apart overlap zones, occurring at 33–65% of the fracturing process. Furthermore, HFs in compressive-torsion zones are less sensitive to viscosity and pump rate. To optimize stimulated volume, a moderate viscosity of 50–60 mPa·s is universally recommended. Regarding pump rates, 8–10 m3/min is ideal for balanced connectivity in pull-apart overlap zones, whereas >12 m3/min is required for compressive-torsion zones. These findings provide critical theoretical and engineering guidelines for differentiated fracturing strategies in ultra-deep reservoirs.

Mechanical response and failure mechanisms of sandstone under water–rock interaction: a multi-scale study and constitutive modeling

Propagation and Fracture Mechanisms of One‐Dimensional Explosive Stress Waves Regulated by Cavity Structures

The propagation characteristics of explosive stress waves in rock masses are fundamentally influenced by the coupling conditions between the charge and the surrounding medium. This study systematically investigates the effect of annular cavity structures on one‐dimensional explosive stress wave propagation and rock fracture behavior through a custom‐built one‐dimensional loading experimental system. Eight groups of comparative tests were carried out with decoupling coefficients ranging from 0 to 0.875, and the time‐domain and frequency‐domain responses of rock specimens were obtained via ultra‐dynamic strain measurement, digital image correlation (DIC), and Hilbert–Huang transform (HHT). The results show that rock fracture patterns evolve through four stages with increasing decoupling coefficient, and a critical decoupling coefficient of 0.75 is identified, corresponding to the most significant asymmetric spalling effect. Frequency‐domain analysis indicates that the cavity structure acts as a mechanical filter, suppressing high‐frequency components (30–80 kHz) while enhancing low‐frequency energy (below 20 kHz), which transforms the loading regime from impulsive shock to combined impact‐quasi‐static gas expansion. This study establishes the correlation between spectral evolution and fracture mechanisms and provides theoretical support for the optimization of decoupled charge designs in rock blasting engineering.