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

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

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

The propagation behavior of hydraulic fractures in deep hard roofs is governed by the multi-factor coupling of injection rate, in situ stress, and fluid viscosity. Taking the Cuimu coal mine as the engineering background, this study systematically investigates the effects of injection rate, lateral pressure coefficient, and fluid viscosity on fracture propagation through numerical simulation. Theoretical derivations based on the KGD (Kristianovich-Geertsma-de Klerk) model are further integrated and validated by field tests. The results indicate a critical injection rate of 6 × 10−8 m3/s, above which the marginal increase in acoustic emission events declines significantly. Increasing the lateral pressure coefficient from 1.0 to 3.5 shifts the fracture pattern from relatively simple to increasingly complex and interwoven, accompanied by a logarithmic increase in fractal dimension from 1.38 to 1.80. The total acoustic emission count rises to a peak of 125,910 as viscosity increases from 0.001 Pa·s to 0.5 Pa·s, then drops to 44,061 at 1.0 Pa·s, showing a unimodal trend. Theoretical analysis shows that during the propagation stage, the fracture length follows L∝Q1/2, and the maximum fracture opening follows wmax∝Q1/3. The lateral pressure coefficient controls the complexity of the fracture network through the directional distribution of stress intensity factors. Field tests at the Cuimu coal mine adopted a combination of stepwise injection rate and low-viscosity fluid, together with borehole densification and interval-skipping fracturing sequences. The effective fracturing radius reached 25~30 m, roof convergence was reduced by 31%, and the proportion of high-energy microseismic events decreased from 12% to 4%. This study establishes a complete theoretical framework from initiation theory to propagation dynamics and then to multi-crack competition, providing both a theoretical basis and engineering example for optimizing fracturing parameters in hard roofs under high stress anisotropy.

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

Understanding how rock properties change with depth is crucial for a variety of geoengineering applications. Even rocks that are homogenous at both micro and macro scales, such as Bentheim sandstone, lose this characteristic once fractured. While recent studies have shown how concomitant changes in stress, temperature and pore pressure affects the evolution of intact sample permeability at depths, an equivalent study on fractured material is missing. Therefore, by combining a multi-methodological approach consisting of rock deformation experiments simulating depth conditions up to 4 km, thin section analysis and fluid composition analysis of water samples, the evolution of permeability of fractured Bentheim sandstone is investigated in this study. Results suggests that fine particles produced by the fracturing and the movements along these fractures play a crucial role in permeability evolution. When these particles are removed, the fracture constitutes a preferential pathway and, together with the chemical processes occurring on the rock–fluid system, lead to a 3–7 times reduction in permeability followed by a complete recovery of it after a simulated burial and exhumation path. On the contrary, when these particles are still present within the fracture zone, they impede fluid flow. This causes a slightly reduction of permeability during the burial path followed by almost constant values of permeability throughout the exhumation path. These findings provide crucial information for georeservoir applications and the transfer of results from laboratory experiments to in situ conditions for a correct prediction of hydraulic properties.

Numerical Investigation of Time-Dependent Deformation and Fracture Evolution in Flawed Sandstone

Discrete element simulation and dimensionless parameter-based optimization of near-wellbore fracture propagation during perforation fracturing of tight sandstone.

Perforation fracturing is essential for stimulating unconventional reservoirs such as shale and tight sandstone. However, field monitoring techniques has revealed that many perforation clusters contribute little to post-fracturing production, indicating inefficient fracture initiation and limited stimulation effect. This study conducts a sensitivity analysis of key controlling engineering parameters for perforation fracturing based on the discrete lattice method, and introduces theoretically derived dimensionless parameters to bridge the gap between numerical simulation and multi-parameter engineering optimization. Single-cluster simplified perforation fracturing models under different perforation patterns are established to describe mesoscopic cross-scale fluid-solid coupling during fracture initiation and propagation, and are validated against near-wellbore simulation results and experimental observations. After model validation, a single-stage multi-cluster fracturing model is developed based on the validated model to reveal the competitive propagation mechanisms and geometrical evolution patterns of multiple fractures. The study found that helical perforation can produce the highest breakdown pressure and the largest volume of initiated crack compared to directional and fixed-plane perforation. Increasing the helical perforation diameter and perforation depth while reducing the perforation density can reduce breakdown pressure, increase stimulated reservoir volume (SRV), and reduce SRV standard deviation. For a fixed stage length, cluster length, cluster spacing, and cluster number have limited effects on breakdown pressure, whereas increasing cluster spacing and cluster length can reduce SRV standard deviation. Through quantitative statistics of simulation results and theoretical analysis, this study clarifies the relationships among dimensionless engineering parameters for the target sandstone reservoir and develops a construction optimization scheme, providing technical guidance and theoretical support for perforation fracturing in tight sandstone horizontal wells.

Mechanical Behaviour and Flow Characteristics of Reservoir Sandstone Under Deep Triaxial Stress Conditions

The development of deep oil and gas reservoirs has become a key focus in the exploration and exploitation of hydrocarbon resources. To elucidate the mechanisms governing the flow evolution of reservoir sandstone under deep, high-triaxial stress conditions, this study takes the deep reservoir sandstone of a specific block as its subject. Using a true triaxial rock mechanics–seepage coupled testing system to simulate the in situ high-triaxial stress environment. Synchronous testing of the sandstone’s mechanical and seepage properties was conducted, and a staged permeability model for the elastic and damaged zones of the sandstone was developed and experimentally validated. The results indicate that, under deep, high-triaxial stress conditions, the total stress–strain behaviour of sandstone is divided into compaction, elastic deformation, yield and post-failure stages. The mechanical behaviour exhibits a ‘compression–unloading–failure’ pattern, whilst permeability follows a U-shaped evolution characterised by an ‘exponential decline–sudden increase–stabilisation’ trend; furthermore, the influence of damage on flow properties is irreversible. The intermediate principal stress exerts a significant strengthening effect on the mechanical properties of sandstone; as it increases, the peak strength and residual strength of the sandstone rise, whilst post-peak brittleness decreases and ductility increases. The intermediate principal stress is a key factor regulating the evolution of permeability. During the elastic stage, the initial permeability of the sandstone decreases as the stress increases, and the rate of exponential decay accelerates; during the damage stage, the post-peak permeability decreases as the stress increases, and the irreversible closure of pores caused by true triaxial stress makes it difficult for the post-peak permeability to recover to its initial value. The coefficient of determination for the permeability model fitted to the elastic stage is greater than 0.9, whilst that for the damage stage ranges from 0.76 to 0.85. These models effectively characterise the quantitative relationship between permeability and strain under different stress conditions, demonstrating good reliability and applicability. The research findings provide experimental evidence for elucidating the coupled relationship between mechanics and flow in sandstone under deep, high-triaxial stress conditions, as well as for predicting flow behaviour in deep reservoirs and designing development schemes.