Search Results (474)

Ultra-low-permeability tight oil reservoirs, including the Chang 6 and Chang 8 formations in the Changqing Oilfield and the Fuyu reservoir in the peripheral Daqing Oilfield, are characterized by poor reservoir properties and limited waterflooding efficiency. Conventional areal waterflooding either fails to establish effective displacement or results in rapid local water breakthrough, leading to rapid production decline and a recovery degree of less than 10%. Large-scale refracturing combined with modification of the water injection strategy has, therefore, become an important approach for improving single-well productivity. However, long-term injection–production imbalance may cause substantial formation-energy depletion and an increase in horizontal stress contrast, which are unfavorable for the development of complex fracture networks during refracturing. To investigate the mechanism and optimize the design of pre-fracturing energy enhancement, rock-mechanics experiments were first conducted on cores from the Fuyu reservoir in the Daqing Oilfield. The resulting pore pressure and stress responses were interpreted based on poroelastic coupling and the effective-stress principle. A three-dimensional coupled reservoir–geomechanical model was subsequently established for a Chang 6 tight oil block in the Changqing Oilfield using formation-specific geological, petrophysical, geomechanical, and production data, and field performance was further used for validation. The laboratory results show that pre-fracturing water injection increases pore pressure, reduces effective confining stress, and decreases the horizontal principal-stress difference, thereby promoting a transition in rock failure from isolated shear fractures toward intersecting fracture patterns. The laboratory-derived mechanical trends were transferred to the Chang 6 model primarily at the mechanistic level, while quantitative parameters were recalibrated using reservoir-specific data. By establishing the relationship between the energy enhancement ratio, defined as the ratio of injected fluid volume to cumulative produced fluid volume, and formation pressure recovery, and further considering sensitivity, economic feasibility, and operational constraints, the optimal energy enhancement ratio was determined to be 0.8–1.0. These results clarify the geomechanical mechanism and key design parameters of pre-fracturing energy enhancement and provide practical guidance for refracturing design in ultra-low-permeability tight oil reservoirs. Full article

Diabetic refractory wounds are a prevalent and severe complication of diabetes, whose pathological progression is jointly mediated by multiple factors, including oxidative stress imbalance, chronic inflammation, impaired angiogenesis, bacterial infection, and biofilm formation. Current clinical hydrogel dressings generally suffer from drawbacks such as single-function performance, potential toxicity of nano-components, static networks incompatible with dynamic wound conditions, and the absence of bionic repair signals. Therefore, they cannot simultaneously satisfy the dual repair requirements of complex pathological microenvironments and dynamic mechanical properties for diabetic wounds. In this study, a multi-functional dynamically responsive composite hydrogel (MC group) with high-efficiency antioxidant, antibacterial, and pro-angiogenic capacities was fabricated. Using SDS-C₁₈ micelles as hydrophobic units, a rigid–flexible dual-network framework was constructed with polyvinyl alcohol (PVA) and methacrylated hyaluronic acid (HAMA). Manganese dioxide nanozymes were introduced to scavenge reactive oxygen species (ROS) and mitigate oxidative stress. Calcium-ion-mediated dynamic micelle reconstruction was adopted to regulate the hydrophilic–hydrophobic balance, while achieving antibacterial effects and facilitating tissue regeneration. In vitro experiments verified that the MC hydrogel possesses mechanical properties well-matched to human soft tissues (fracture stress: 25 kPa) and excellent biocompatibility (cell viability > 100%, hemolysis rate: only 0.13%). It also exhibits prominent antioxidant activity (DPPH radical-scavenging rate: 36.95%), antibacterial performance (>99.86% bactericidal rate against Staphylococcus aureus, survival rate of Escherichia coli reduced to 15.95%), and cell-migration-promoting activity (endothelial cell migration rate of 83.72% and mouse fibroblast migration rate of 90.88% within 24 h). In the full-thickness skin defect model of diabetic mice, the wound-healing rate reached 99% on day 16. Moreover, it promoted ordered collagen deposition, skin appendage regeneration, and functional microvascular reconstruction, thereby accomplishing high-quality tissue repair. This design synergistically intervenes in multiple pathological links of diabetic wounds, overcomes several key limitations of existing dressings, and provides an innovative strategy for developing smart dressings. Full article

Lost circulation in fractured formations remains a critical challenge in petroleum engineering, significantly hindering drilling efficiency and increasing operational risks. To overcome the limitations of conventional plugging materials under high-temperature and complex fracture conditions, a nanomaterial L-modified thermally expandable thixotropic gel system was developed. By incorporating a nanomaterial with strong thixotropic characteristics and multifunctional monomers, multiscale network regulation and interfacial enhancement were achieved. Systematic rheological and performance evaluations were conducted to optimize the formulation, enabling a balanced combination of flowability, structural recovery, and plugging performance. Experimental results demonstrate that the gel exhibits pronounced shear-thinning behavior and excellent thixotropic recovery, ensuring superior injectability under high shear and strong structural integrity under low-shear conditions. The system also maintains good thermal stability below 150 °C and achieves effective plugging across varying fracture scales. In addition, moderate salinity enhances network strength, whereas excessive salt concentration weakens structural stability and recovery capability. This study provides a novel design strategy for nanomaterial-regulated thermally expandable gels and offers a promising solution for lost circulation control in complex fractured formations. Full article

Conventional Rate Transient Analysis (RTA) models, based on homogeneous fracture assumptions, are inadequate for characterizing flow in complex fracture networks of heterogeneous unconventional reservoirs. This study develops a fractal-based RTA (FD-RTA) workflow integrating lithofacies analysis, microseismic fracture interpretation, and post-fracturing production data from the Yingxiongling shale oil field in the Q’aidam Basin. The workflow is applied to eight horizontal wells completed in layered and laminated dolomites. Results show that the two lithofacies exhibit distinct fractal flow behaviors. Layered dolomite tends to develop preferential flow pathways, characterized by rapid initial depletion followed by declining supply capacity, with the half-flow dimension (δ) decreasing from 0.299 to 0.074 during production. Laminated dolomite displays stronger fracture-matrix interaction and sustained production performance, with δ increasing from 0.469 to 0.678 as multi-scale fractures are progressively activated. The FD-RTA workflow effectively links fracture complexity with production behavior, providing a dynamic characterization tool for evaluating hydraulic fracturing effectiveness in shale oil reservoirs. Full article

Lacustrine shale oil has become an important target for increasing unconventional oil and gas reserves and production in China. However, compared with shallow to medium-depth shale oil systems, ultra-deep shale oil systems have experienced more complex burial evolution, diagenetic alteration, and fluid activities, and their large-scale enrichment mechanisms remain poorly understood. This study focuses on the ultra-deep shale oil system of the fourth member of the Paleogene Funing Formation (E₁f₄) in the Gaoyou Sag, Subei Basin. Core observations, mineralogical analyses, organic geochemical tests, scanning electron microscopy (SEM), pore structure characterization, and laser confocal microscopy were integrated to investigate shale lithofacies, organic matter characteristics, and shale oil occurrence mechanisms, and to identify the key factors controlling shale oil enrichment. The results show that five major lithofacies are developed in the E₁f₄, including laminated calcareous–dolomitic shale and laminated clay-rich mixed shale. Among them, carbonate-rich laminated lithofacies exhibit the highest organic matter abundance and the best reservoir quality. Laser confocal microscopy further reveals that heavy hydrocarbons are mainly retained within organic-rich clay laminae, whereas light hydrocarbons preferentially migrate into and accumulate within adjacent carbonate-rich and felsic laminae, forming an efficient source–reservoir association at the microscale. The integrated results indicate that abundant algal organic matter and thick high-quality source rocks provided sufficient hydrocarbon-generation potential. Differential source–reservoir associations controlled by laminated structures, together with multi-scale pore–fracture networks, promoted short-distance hydrocarbon migration and effective accumulation. In addition, thick mudstone roof and floor seals, a weak faulting background, and an overpressure system ensured long-term hydrocarbon preservation. This study reveals the enrichment mechanism of ultra-deep mixed shale oil in faulted lacustrine basins of eastern China, improves the understanding of lacustrine shale oil accumulation, and provides a theoretical basis for sweet spot evaluation and favorable area selection in similar basins. Full article

Human activities such as intensive groundwater abstraction and mine dewatering can profoundly disrupt the natural hydrological functioning of karst aquifers. The Transdanubian karst aquifer in Hungary represents one of Central Europe’s most prominent examples, where decades of coal-mine dewatering lowered groundwater levels by more than 40 m and fundamentally altered the natural recharge–discharge regime. Understanding and forecasting recovery in such complex karst systems remain challenging because of heterogeneous conduit–fracture networks, strong climate sensitivity, incomplete monitoring records, and uncertainty in long-term predictions. This study presents an integrated end-to-end machine learning framework for groundwater characterization and climate-constrained probabilistic forecasting. Monthly groundwater-level records (1970–2026) from five monitoring wells were first reconstructed using a hybrid Moving Average–Random Forest gap-filling approach, achieving high reconstruction accuracy (R² = 0.87–0.98). Self-Organizing Maps subsequently identified four hydrogeological states representing the dewatering, transition, recovery, and near-equilibrium phases, while inter-well weight-plane correlations (>0.95) confirmed strong basin-scale hydraulic connectivity. A Bootstrapped Random Forest model forced by bias-corrected COSMO-CLM precipitation projections under the SSP2-4.5 climate scenario generated probabilistic groundwater forecasts through 2030, achieving high predictive performance (NSE > 0.80; RMSE = 0.10–0.35 m). Forecast results indicate that the basin as a whole is approaching hydraulic equilibrium by 2030, with distinct well-specific trajectories including mild steady decline and near-stable water level. The proposed framework provides a robust and transferable methodology for groundwater characterization and long-term forecasting in complex karst and fractured aquifer systems under changing climatic conditions. Full article

Understanding the failure behavior of cylindrical-hole hard rocks is essential for rockburst prevention in deep underground engineering. In this study, fractal–statistical analysis was combined with true triaxial testing and discrete element modeling to quantify the rate-dependent failure and crack-network evolution of holed granodiorite. The results showed that, with an increasing loading rate, the peak axial stress increased from 143 to 190 MPa, the peak axial strain decreased from 1.24% to 0.86%, and the post-peak brittleness index increased from 0.83 to 1.19. The final failure pattern evolved from multi-crack tension–shear coupled failure to localized dominant fracture and intense hole-wall exfoliation. The mass fractal dimension of rockburst fragments increased with loading rate, reflecting a transition toward finer and more dispersed fragmentation. To extend the experimentally observed hole-wall failure mechanism to adjacent openings, a calibrated PFC3D double-hole model was further established. The numerical results revealed that crack interaction was governed by stress-concentration superposition and progressive rock-bridge damage, and the hole-spacing ratio controlled the connectivity and complexity of the crack network. As S/2R increased from 1.25 to 2.00, the dominant fracture-band inclination increased from 27° to 54°, reflecting a transition from steep inter-hole coalescence to more inclined and spatially dispersed fracture development. Full article

Shale gas is a major unconventional energy resource in China. Its low porosity and permeability require large-scale volumetric fracturing to create conductive fracture networks. However, most induced fractures are propped discontinuously because shale reservoirs are geometrically complex. Fracture conductivity and proppant placement efficiency therefore directly control stimulation performance. Following SY/T 6302-2009, this study used linear flow-through experiments and a large-scale visual fracture simulation system to investigate the effects of proppant particle-size distribution, injection sequence, flow rate, and closure pressure on fracture conductivity and placement. The results show that the 20/40:40/70 mesh dual-particle-size combination at a 3:2 ratio provides the best overall performance. A fine-particle content of no more than 16.7% limits conductivity loss and improves the match between particle size and fracture aperture. Multilayer placement at fracture corners distributes high-stress loading and maintains conductivity. Injecting 70–140 mesh fine proppant before 40–70 mesh coarse proppant at 3.6 m³/h improves transport distance, coverage, and placement uniformity. The optimized scheme maintains stable conductivity at closure stresses of 10–80 MPa and achieves at least 95% propped-area coverage. These findings provide experimentally supported parameters for discontinuous propping and can inform shale gas fracturing design. Full article

Fracture evolution of rock–backfill composites (RBCs) under complex loading–unloading and dynamic disturbances is critical for stope stability in deep backfill mining. Using PFC3D, this study constructs numerical models of RBCs to investigate this process, considering burial depths (500, 1000, 1500, 2000 m), interface angles (IA: 60°, 90°), and cement–tailings ratios (CTR—1:4, 1:8), while replicating true triaxial paths and blasting impacts. Systematic analysis of mesoscopic crack quantity, spatiotemporal distribution, and multiscale fracturing reveals that shear cracks dominate damage, with crack counts evolving in stages as strain increases. With greater depth, the number of propagation stages and growth rate inflection points shift systematically. During mining–filling disturbance, crack quantity negatively correlates with depth but turns positive during late static loading beyond 70% peak stress. Spatial crack distribution is synergistically controlled by IA, CTR, and depth. For IA 60°, shear crack angles spread broadly yet concentrate at 50–70°; for IA 90°, they are near-axial, concentrated at 80–90°. The synergistic process progresses through microscopic initiation, mesoscopic accumulation, and macroscopic instability. In terms of failure modes, IA 60° exhibits shear failure along the cemented interface plus tensile fracturing in rock, while IA 90° shows combined diagonal shear and axial tension. Higher CTR yields more extensive fracture networks in backfill, indicating superior synergistic bearing capacity. Full article

Shale gas reservoirs are characterized by low permeability, nanoscale pore structures, and complex fracture networks. Multistage fractured horizontal wells are an important technology for commercial shale gas development. However, many shale gas productivity models primarily emphasize gas transport within the reservoir and fracture system, while pressure variations caused by frictional losses along the horizontal wellbore are often simplified or treated separately. To address this issue, this study develops a fully coupled multiscale dual-porosity numerical model that integrates the shale matrix, hydraulic fractures, and horizontal wellbore within a unified simulation framework. The model incorporates key physical mechanisms governing shale gas transport, including Knudsen diffusion, Langmuir adsorption–desorption, stress sensitivity, and non-Darcy flow in fractures. Meanwhile, the Darcy–Weisbach equation is introduced to describe wellbore frictional pressure losses. The reliability of the proposed model is validated through history matching with field production data from the Changning shale gas reservoir. The results demonstrate that neglecting wellbore friction losses leads to a 30–50% overestimation of horizontal well productivity, indicating that wellbore friction has a significant impact on fracture flow distribution and productivity prediction. Furthermore, an exponent factor r is introduced to characterize and evaluate non-uniform fracture placement patterns. The results show that toe-dense fracture placement can increase cumulative gas production by approximately 37.8% compared with uniform fracture placement when r = 1.10, which yields the highest cumulative gas production among the tested cases. However, the additional production benefit becomes substantially smaller after the initial increase and remains relatively stable as r further increases. This study improves the understanding of friction-induced heel-to-toe effects and provides an effective numerical approach for productivity prediction and fracture placement design in shale gas horizontal wells. Full article

Natural fractures dominate fracturing effects and well production. Conventional fracturing fails to fully activate multi-scale fractures, and most simulations adopt homogeneous rock assumptions, lacking systematic analysis on fracture propagation and seepage evolution in heterogeneous fractured formations, while the natural fracture activation mechanism of pulsed fracturing remains unclear. This work constructs a pulsed fracturing model for heterogeneous fractured rock to simulate fracture growth and permeability evolution in intact rock and formations with various fracture attitudes, revealing the coupled laws of fracture propagation and seepage change. Results show rock mechanical heterogeneity determines fracture network complexity in intact rock; pulsed loading slows main fracture breakthrough and stimulates microcracks, creating a near-well dense and far-well sparse fracture distribution. Single-orientation fractures drive directional asymmetric fracture extension following near-weak-zone priority, with matrix heterogeneity merely causing local fracture deflection. Multi-orientation fractures display layered activation: low-angle and near-well fractures initiate first, and cross-fracture interactions raise network complexity and coverage. Fracture growth is jointly governed by weak bedding, pulse fatigue damage and matrix properties. Pulsed fracturing achieves remote non-contact activation of natural fractures, with fracture-permeability evolution showing strong spatiotemporal coupling; main fracture breakthrough triggers abrupt permeability growth. Serving as both mechanical weak planes and preferential flow paths, natural fractures build composite seepage systems of main channels and micro flow zones. This study provides theoretical support for parameter optimization and efficient permeability improvement in fractured reservoirs. Full article

Surface fissures induced by shallow coal seam mining create interconnected pathways for ambient air leakage, significantly aggravating coal spontaneous combustion (CSC) risks in goafs. However, the spatiotemporal evolution of these fractures and the quantitative dynamics of air leakage under repeated mining conditions remain poorly understood. This study investigates the evolutionary laws of mining-induced cracks and air leakage behaviors through laboratory physical similarity simulations and field tracer gas testing. The results demonstrate that during repeated extraction, vertical fractures in the goaf boundaries undergo an expansion-to-stabilization process with significantly increased widths, whereas fractures in the central region experience a process from expansion to closure and stabilization. Crucially, the fracturing of the inter-seam key stratum marks a vital milestone where the upper and lower goafs merge into a complex goaf, precipitating a sudden, sharp surge in air leakage volume. Field observations categorize surface cracks into graben type, collapse type, and tensile type. Graben-type and collapse-type cracks act as the principal pathways for surface air infiltration, collectively forming a rectangular distribution network across the goaf. These findings provide a critical theoretical framework and practical guidance for predicting and controlling surface air leakage disasters in close-distance shallow seam mining. Full article

To clarify how shale-reservoir heterogeneity constrains hydraulic-fracturing effectiveness in complex structural areas, this study analyzes exploration well X in the Mugan–Shoushan area, Yunnan Province, using organic geochemistry, petrology and mineralogy, reservoir-property, and rock-mechanical data from the Wufeng–Longmaxi formations. The results show pronounced vertical engineering-geological differentiation. Average clay content decreases from 42% to 8%, Average carbonate minerals increase from 16% to 50%, and quartz is anomalously enriched in the Longyi 1-1 layer of the Longmaxi Formation (Longyi 1-1; 76%). The Longyi 1-3 layer of the Longmaxi Formation has the highest porosity (9.37%) but low matrix permeability (0.013–0.019 mD); the Longyi 1-2 layer of the Longmaxi Formation is highly brittle and tight; and the Longyi 1-4 layer of the Longmaxi Formation is highly ductile and water-rich. Accordingly, four engineering geological facies are defined: Type I, organic-rich, moderately brittle, and moderately ductile composite facies; Type II, organic-rich, highly brittle, tight, and strongly stress-sensitive facies; Type III, organic-poor, highly ductile, water-rich, and strongly water-sensitive facies; and Type IV, highly brittle, fracture-developed, and high-adsorption facies. Implications for fracturing are proposed for each facies, including mixed-fluid network stimulation, acid pretreatment with controlled flowback, interval avoidance, and coordinated stimulation with adjacent main reservoirs. Full article

Deep shale reservoirs in the tectonically complex margin of the southern Sichuan Basin have experienced multistage deformation, resulting in strong spatial heterogeneity of the present-day geostress field. However, the influence of stress heterogeneity on multiscale pore structure evolution and reservoir quality remains poorly constrained. Here, we integrate in-situ stress measurements, overburden porosity and permeability experiments, CO₂/N₂ adsorption, high-pressure mercury intrusion, SEM-MAPS (Scanning Electron Microscopy-MAPS) pore imaging, stress well profile interpretation, and multifractal analysis to quantify the controls of present-day geostress heterogeneity on pore structure evolution in deep Longmaxi Formation shale. The results show that the present-day stress regime is characterized by a strike-slip pattern (σ_H > σ_v > σ_h), with significant variations among different structural deformation zones. Increasing structural deformation results in enhanced differential stress, increasing by 30–80% from gentle structures to tight folds and fault-affected zones, accompanied by a 60–70° rotation of the maximum principal stress orientation. Differential stress, effective stress, differential stress coefficient, and stress structure index exhibit strong negative correlations with porosity, whereas permeability decreases nonlinearly with increasing stress, indicating progressive pore-throat compression and connectivity degradation under heterogeneous stress conditions. Multifractal analysis reveals that pore-size domains exhibit different sensitivities to stress heterogeneity. The macropore fractal dimension (DN3) shows the strongest response, followed by mesopores (DN2), whereas micropores (DN1) exhibit relatively limited variations. Fault-affected zones and strongly deformed regions display higher DN3 values (>2.8), reflecting enhanced complexity of macropore and fracture networks. In contrast, gentle structural zones characterized by curvature values <0.10 km⁻¹ and distances >500 m from faults exhibit relatively low and stable fractal dimensions (<2.73), indicating more homogeneous pore structures. Increasing stress heterogeneity induces the transformation of organic matter pores from regular subcircular shapes to flattened and slit-like morphologies, accompanied by pore-size migration toward smaller scales (<15 nm) and enhanced pore heterogeneity (Df > 1.35). These findings reveal that present-day geostress heterogeneity governs shale pore fractal evolution and promotes the transition from micropore-dominated to heterogeneous macropore–fracture systems. This study provides quantitative insights into stress-controlled pore evolution and reservoir quality evaluation in deep shale reservoirs under complex tectonic settings. Full article

To elucidate the structural evolution of fracture networks around gas drainage boreholes and their controlling mechanisms on grouting sealing performance, a coal seam in the Zhongmacun Mine (No. 2-1 coal seam) was selected as the engineering background. An integrated approach combining borehole camera observation, gray-level co-occurrence matrix (GLCM) texture analysis, and fractal theory was adopted to systematically characterize the development behavior, spatial heterogeneity, and fractal evolution of fracture networks under different stress zones surrounding the borehole. Furthermore, the quantitative relationship between fracture structure characteristics and grouting parameters was explored. Results indicate that three axial stress-related zones are formed, including a stress-relief zone (0–4 m), a post-peak stress concentration zone (4–20 m), and a pre-peak stress concentration zone (>20 m), with fracture development strongly dependent on stress state. Quantitative analysis based on GLCM parameters and their coefficients of variation reveals a progressive transition from highly complex and strongly heterogeneous fracture structures in the stress-relief zone to simpler and weakly heterogeneous characteristics in the pre-peak stress concentration zone. The fractal dimension (D) decreases from 1.847–1.907 to 1.676–1.713 across these zones, consistent with the evolution trends of GLCM metrics. Based on the relationship between fractal dimension and fracture connectivity, a prediction model for the equivalent permeability of fracture networks based on fractal dimension was established, and the quantitative relationship between grouting pressure and fractal dimension, slurry viscosity, and diffusion radius was derived, providing a theoretical method for analyzing the correlation between fracture structure characteristics and grouting parameters. On this basis, a zonal differentiated grouting sealing optimization scheme was proposed, and a theoretical calculation method for grouting pressure and sealing section length based on fractal parameters was established, providing theoretical references and technical support for precise sealing of gas drainage boreholes. Full article