Mechanical and hydraulic properties of fractured Bentheim sandstone at different laboratory-simulated depths

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.

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

Lithology-Dependent Fracture Propagation in Ultra-Large True-Triaxial Hydraulic-Fracturing Experiments

Tight reservoirs commonly exhibit low permeability and pronounced lithological heterogeneity, resulting in complex interactions among far-field stress, local structural weakness, and fluid-driven fracture propagation. In this study, four non-replicated 2 m × 2 m × 1 m physical-model specimens representing tight glutenite, tight sandstone, and No. 3 coal rock from the Huabei Oilfield were investigated using an ultra-large true-triaxial hydraulic-fracturing system. Surface-fracture observations, microseismic monitoring, and high-frequency wellhead-pressure measurements were integrated to compare fracture responses under lithology-specific combinations of injection rate, fluid viscosity, perforation configuration, and stress state. The tested glutenite cases exhibited branched or localized fracture patterns depending on the combined treatment configuration; the sandstone case was dominated by a throughgoing main fracture approximately aligned with the principal-stress direction; and the coal-rock case showed extensive participation of bedding and cleat systems. These morphological differences were accompanied by distinct pressure and microseismic signatures, indicating different pathways of hydraulic-energy redistribution and fracture activation. For the two glutenite cases, the combined change from a single-perforation configuration at 0.5 m3/min to three helical perforations at 120° and 0.7 m3/min was associated with a 42.2% larger microseismic-derived stimulated reservoir volume (SRV). Taken together, these responses indicate a shift from stronger far-field-stress-controlled localization in the comparatively uniform sandstone to progressively greater local structural control by heterogeneous interfaces in glutenite and by bedding/cleat discontinuities in coal rock. Because each configuration was represented by a single specimen and several experimental variables changed simultaneously among cases, the observed differences are interpreted as case-specific mechanistic trends rather than statistically established universal relationships. The results show the value of combining fracture morphology, microseismic spatial evolution, and pressure dynamics for interpreting lithology-dependent fracture propagation in ultra-large physical models and for developing qualitative, lithology-adapted hydraulic-fracturing concepts.

Influence of microstructural characteristics and boundary conditions on the viscoelastic properties of carbonate rocks

Carbonate formations host hydrocarbon resources and are a focus of deep and ultra-deep oil and gas exploration and development. Carbonate rocks exhibit complex pore structures, which influence the rock framework and govern fluid distribution, thereby affecting their viscoelastic properties. Furthermore, the boundary conditions of carbonate rocks (drained or undrained) affect pore-fluid pressure dissipation and viscoelastic characteristics. Most poroelastic rock-physics models assume closed boundaries and neglect open boundary states, limiting their applicability to viscoelasticity. In this work, six dolomite and five limestone samples were collected from carbonate reservoirs in the Sichuan Basin, China. Thin-section analyses, physical-property measurements, and ultrasonic and low-frequency experiments were conducted. Frequency-dependent variations in elastic-wave velocities were evaluated for fractured, fractured-vuggy, and pore-cavity samples. The fractured-vuggy samples showed greater dispersion than other samples. A boundary opening index was introduced to improve the equivalent inclusion average stress (EIAS) model. The model was combined with the standard linear solid (Zener) model to develop a full-frequency-band EIAS–Zener boundary model incorporating open boundary conditions. The model was validated using low-frequency and ultrasonic experimental data. Relationships among boundary opening degree, pore structure properties, and elastic-wave dispersion and attenuation were elucidated. A higher degree of boundary opening, corresponding to a lower boundary opening index, was associated with a lower P-wave velocity. The boundary opening index was more sensitive to effective pressure than the crack aspect ratio and crack volume ratio. Crack parameters and the boundary opening index correlated more strongly with permeability than with porosity, indicating that the boundary opening index may serve as a quantitative indicator of reservoir permeability.

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.

Analytical Model of Seepage Pressure Effect on Dynamic Compressive Fractures Caused by Microcrack Growth in Brittle Rocks

The dynamic compression fracture of brittle rock under seepage pressure is a critical issue for deep underground engineering. It directly influences the stability and safety of the surrounding rock during blasting or seismic loading. However, research on the mechanisms of microcrack evolution under these coupled conditions is still lacking. The relationship between microcrack evolution and macroscopic mechanical properties also remains poorly understood. This study develops a micro–macrofracture model grounded in the wing microcrack propagation framework, integrating both mechanical and chemical interactions between free water and rock. Mechanically, it incorporates seepage pressure, dynamic Stefan force, and dynamic fracture toughness. Chemically, it accounts for the effects of saturated water on rock mechanical parameters. This model characterizes the total stress–strain constitutive behavior of brittle rock under varying seepage pressures during dynamic compression failure, encompassing both strain-hardening and strain-softening phases. This result is validated against experimental data. It accounts for the influence of seepage pressure on both the initial crack and the newly formed wing crack. The seepage pressure weakens the wedging force F W on the initial crack while enhancing the seepage tensile force F P on the wing crack, which constitutes the seepage pressure-driven crack growth mechanism. Furthermore, under the combined effects of dynamic loading and free water, the dynamic Stefan force F S and the dynamic fracture toughness K ICD serve as the mechanism for inhibiting crack growth. The combined influences of seepage pressure, confining pressure and initial crack characteristics on the dynamic mechanical behavior of brittle rock under seepage pressure are discussed.