Search Results (238)

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. Full article

Natural-fracture connectivity substantially influences hydraulic-fracture interaction with pre-existing discontinuities, but its quantitative role in fracture-network propagation remains insufficiently constrained. In this study, a coupled LEFM–cohesive-zone hydraulic-fracture propagation model was developed by combining crack-tip deflection criteria, traction-separation damage evolution and fluid–solid coupling. True triaxial hydraulic-fracturing experiments were conducted on artificial fracture networks with I-, V-, Y- and X-shaped connectivity elements to evaluate the model response. The results show that connected natural fractures redirect hydraulic fractures under low horizontal stress differences, producing deflection angles of 30–50 degrees. When the stress difference exceeds 4 MPa, fracture growth becomes more strongly aligned with the maximum principal stress direction. In the true triaxial tests, the total number of connected natural fractures increased from 14 in the I-shaped network to 17 and 21 in the Y- and X-shaped networks, corresponding to increases of 21.4% and 50.0%, respectively. X-shaped networks showed the strongest sensitivity to stress difference and injection rate, while higher elastic modulus reduced fracture width and promoted longer, narrower fractures. Scale-normalized comparisons based on image-derived experimental measurements showed that the predicted propagation length, fracture width and connected-fracture number followed the experimental trend from I-shaped to Y-shaped and X-shaped networks, with relative errors within 7.1% and a mean absolute percentage error of 4.8%. These findings suggest that fracture topology strongly influences pressure transmission and multidirectional activation in the tested models, whereas field-scale extrapolation requires three-dimensional validation and transport analysis. Full article

Deep coalbed methane (CBM) resources are abundant and represent a critical component of future energy supply and carbon reduction strategies. However, deep coal seams are characterized by well-developed cleat systems and high Poisson’s ratios, rendering the mechanisms of hydraulic fracture initiation, propagation, and complex fracture network development insufficiently understood. In this study, deep coal rock at a burial depth of 2700 m is investigated. A finite element–based hydraulic fracturing model incorporating complex face-cleat and end-cleat networks is established by explicitly representing cleat geometry, mechanical properties, fluid leak-off behavior, and hydraulic loading conditions. Using this model, the effects of cleat inclination angle, horizontal stress difference, and displacement on fracture evolution are systematically analyzed. The results indicate that when face cleats are orthogonal to the maximum horizontal principal stress, fractures preferentially penetrate cleats and propagate along the maximum stress direction. In contrast, when face cleats form acute angles with the maximum horizontal stress, pronounced branching fractures develop along both face and end cleats, with propagation increasingly dominated by face cleats as the angle decreases. Increasing horizontal stress difference suppresses fracture branching, leading to simpler fracture networks but greater total fracture length and maximum fracture width. Moreover, under identical injection pressures, the equal-pressure fracture length increases, indicating enhanced fracture propagation capacity. With increasing displacement, fracture networks evolve from simple to complex patterns, accompanied by accelerated propagation and enlarged fracture widths; however, excessive displacement intensifies fluid leak-off, ultimately reducing the equal-pressure fracture length. Full article

Direct core-scale evidence remains insufficient for evaluating hydraulic fracture–natural fracture coupling and stimulation effectiveness in tight sandstone oil reservoirs. In this study, post-fracturing full-diameter cores from the Chang 8₁ tight oil reservoir in the Xi 119 well block, Xifeng Oilfield, Ordos Basin, were investigated using core observation, computed tomography (CT) scanning, fracture-source evidence and three-dimensional fracture-network reconstruction. A total of 87.56 m of core from 11 core runs was scanned at a voxel size of 50.62 μm. Natural fractures, hydraulic fractures and engineering-induced fractures were identified and distinguished based on fracture-surface features, CT expression, spatial continuity, proppant/tracer evidence and their relationship with bedding and lithological boundaries. The results show that lithological structure exerts a first-order control on hydraulic-fracture surface morphology. Massive sandstone tends to generate straight and continuous high-conductivity main fractures, argillaceous laminated sandstone promotes bedding-controlled discontinuous fractures with limited connectivity, and cross-bedded sandstone favors fracture diversion, branching and natural-fracture activation. Based on fracture assemblage, spatial connectivity and seepage behavior, three hydraulic fracture–natural fracture coupling types were classified: single hydraulic-fracture type, single main fracture–diverted fracture–natural fracture type, and dual main fractures–diverted fractures–natural fractures type. Their equivalent permeability increases stepwise from 155 mD to 345 mD and 586 mD, respectively, indicating a positive relationship between fracture-network complexity and seepage capacity. A CT-derived stimulation-effect evaluation framework was further established by integrating pore–fracture structural modification, fracture volume increase, aperture improvement and seepage-capacity enhancement. The dual main fractures–diverted fractures–natural fractures type shows the strongest stimulation response, with the largest reduction in small-aperture pore/fracture proportion, the greatest lamina-fracture aperture enlargement and the most significant permeability improvement. These results provide direct core-scale evidence for understanding fracture-network formation in continental tight sandstone reservoirs and support more targeted hydraulic-fracturing design and stimulation-effect evaluation. Full article

Reservoir-triggered seismicity (RTS) is commonly associated with stress perturbations induced by reservoir loading and pore-pressure diffusion within fractured rock masses. Shortly after the initial impoundment of the Irapé Hydropower Plant (Minas Gerais, Brazil), a sequence of induced earthquakes was recorded, providing an opportunity to investigate the compatibility between reservoir loading and the observed seismic response. This study presents a first-order hydromechanical assessment integrating finite element modelling, analytical elasticity solutions, and pore-pressure diffusion theory to evaluate the spatial distribution of stress perturbations and the characteristic diffusion times associated with reservoir impoundment. A two-dimensional elastic model was developed to simulate stress redistribution induced by the maximum reservoir load, while a parametric diffusion analysis was performed for representative hydraulic diffusivities and hypocentral depth scenarios between 1 and 6 km. Numerical results showed excellent agreement with the analytical elasticity solution (RMSE = 14.36 kPa, MAE = 11.08 kPa, mean relative error = 1.38%, and R² = 0.999), supporting the reliability of the numerical model. The simulations indicate that vertical stress perturbations decrease from approximately 1.8–2.0 MPa immediately beneath the reservoir to about 0.01–0.1 MPa at kilometer-scale depths, where the recorded seismicity is presumed to occur. The diffusion analysis indicates that pore-pressure propagation to these depths generally requires substantially longer times than the interval between reservoir filling and the onset of seismic activity. Nevertheless, owing to uncertainties in hydraulic diffusivity, fracture connectivity, and hypocentral depth estimates, the diffusion results are interpreted as a first-order sensitivity analysis rather than a site-specific prediction. Overall, the results support the temporal compatibility and physical plausibility of rapid elastic stress redistribution as a potential triggering mechanism, while recognizing that the available geological and seismological data are insufficient to establish a direct causal relationship or demonstrate fault reactivation. Full article

Floor Ordovician karst confined water inrush severely restricts safe exploitation of lower coal seams across North China-type coal basins. Surface directional drilling regional grouting serves as the dominant aquiclude reconstruction technology for water hazard mitigation, yet existing research lacks quantitative decoupling and hierarchical sensitivity quantification of medium intrinsic attributes and controllable grouting parameters. To resolve this knowledge gap, this work delineates five core governing variables: porous medium permeability, matrix porosity, injection pressure, slurry dynamic viscosity and slurry bulk density. A coupled Darcy–Bingham two-phase flow numerical framework based on the COMSOL Multiphysics fluid–solid interaction module is constructed—combined with L₂₅(5⁶) orthogonal experimental design to quantitatively characterize the gradient response law of slurry effective diffusion volume against multi-factor perturbation. Variance analysis (ANOVA) demonstrates a hierarchical control sequence: porous medium permeability > matrix porosity > grouting pressure > slurry dynamic viscosity > slurry bulk density. Medium permeability, porosity and injection pressure dominate slurry migration behavior with diffusion volume perturbation amplitudes ranging 2–191%; whereas, rheological and density parameters exert secondary marginal effects limited within 1–8%. Fracture hydraulic theoretical interpretation reveals permeability acts as the primary groutability discriminant index, and injection pressure exhibits prominent marginal diminishing effect with an efficiency threshold of 8 MPa. This study establishes a quantitative parameter optimization framework for Ordovician top aquiclude reconstruction engineering, providing theoretical support for targeted grouting parameter regulation and risk reduction in blind high-pressure injection. Full article

This study focuses on the Chang 8 Member tight sandstone reservoir in the Xifeng Oilfield. Integrated approaches including 3D reconstruction of CT digital cores, seepage simulation, and multi-parameter coupling analysis were employed to systematically classify lamina types, quantitatively characterize lamina-associated fractures, and elucidate the pore–lamina fracture coupling seepage mechanisms. The results show that laminae in the study area can be categorized into three types: banded, inclined, and cross-laminated. Lamina-associated fractures are dominated by bedding-parallel fractures, with a small number of cross-bedding fractures having developed in cross-laminated intervals. The development of lamina-associated fractures is jointly controlled by sedimentary heterogeneity and mechanical differences between interlaminar layers. A significant positive correlation exists between the volume proportion of interlaminar layers and fracture porosity (R² = 0.8913). Cross-laminated intervals exhibit the optimal fracture parameters, with fracture porosity reaching up to 3.6% and maximum fracture volume exceeding 3500 mm³, facilitating the formation of three-dimensional interconnected fracture networks. Distinct pore–lamina fracture coupling patterns are observed in different lamina types: independent development with weak coupling in banded laminae, segmented synergy with heterogeneous coupling in inclined laminae, and strong synergy with network coupling in cross-laminated intervals. In cross-laminated sections, matrix porosity and fracture porosity evolve synchronously, yielding the highest permeability (up to 7.3 mD) and superior storage–permeability performance. Seepage simulations confirm that permeability in tight reservoirs is not directly controlled by matrix porosity; instead, lamina-associated fractures act as the dominant fluid migration pathways. Cross-laminated fractures form multi-directional high-efficiency seepage networks, while banded and inclined laminae exhibit unidirectional linear seepage and segmented dominant channeling characteristics, respectively. These findings provide critical geological insights for sweet spot evaluation, horizontal well trajectory optimization, and hydraulic fracturing design in tight oil reservoirs. Full article

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 $L\propto Q^{1/2}$, and the maximum fracture opening follows $w_{\max }\propto Q^{1/3}$. 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. Full article

This study presents an integrated workflow to characterize and optimize hydraulic fracturing operations in horizontal shale reservoirs using passive wellhead pressure monitoring (PWPM). Pressure data from offset wells in the Horn River Shale Basin were analyzed to identify passive pressure responses and distinguish between direct hydraulic communication and stress-induced behavior, providing insight into fracture dynamics and inter-well connectivity. A multivariate sensitivity analysis was performed to evaluate how key fracture and reservoir mechanical properties, fracture orientation, and in situ stresses govern passive pressure signatures. A fully coupled hydro-mechanical model, implemented using a distinct element formulation, was developed based on the observed passive pressure and microseismic data to generate a physics-based representation of fracture propagation and fluid migration. The modeling framework enables forward prediction of passive pressure responses during future stimulation stages, supporting improved treatment design, real-time operational adjustments, and more reliable refracturing strategies under evolving subsurface conditions. By enhancing fracture network characterization and complementing microseismic monitoring, PWPM demonstrates strong diagnostic value for supporting safer and more efficient injection practices in unconventional reservoir development, as well as broader sustainable energy applications. Full article

With the increasing construction of ultra-long tunnels, complex and highly variable geological conditions along tunnel alignments pose major challenges to geological investigation, design, and construction. Conventional vertical drilling mainly provides discrete vertical geological profiles and is limited in its ability to continuously characterize lithological variations, fracture zones, and groundwater conditions along the tunnel axis. To overcome this limitation, this study proposes an integrated investigation approach based on horizontal directional drilling (HDD) for continuous along-axis geological exploration. Using the Tianshan Shengli Tunnel as the geological setting, the technical advantages of HDD for tunnel investigation—including ultra-long reach, ultra-high accuracy, high penetration rate, and strong adaptability—are first summarized. An integrated investigation method is then developed by combining HDD with targeted borehole coring, hydraulic fracturing, comprehensive borehole logging, and borehole TV imaging. A 2271 m long investigation borehole was drilled along the tunnel axis from the portal section. As a result, precise directional control limited the maximum deviation between the HDD borehole trajectory and the tunnel axis to only 6.32 m. Meanwhile, the lag distance between cuttings was determined through theoretical calculations to reconstruct the true borehole positions corresponding to the collected cuttings. Based on XRD mineralogical analysis, macroscopic observations, and preliminary investigation results, the lithology of the tunnel surrounding rock was delineated with high resolution. In addition, daily borehole inflow was monitored, and tunnel inflow during construction was predicted using the groundwater dynamics method and an empirical railway relationship, yielding an expected normal inflow of 4016.6 m³/d and a maximum inflow of 12,049.8 m³/d; furthermore, borehole TV footage was used to accurately locate inflow points and intervals with well-developed joints and fractures within the surrounding rock. Highlights This study proposes an HDD–downhole geophysics method for tunnel investigation, classifies surrounding-rock lithology from cuttings and cores, and predicts tunnel construction inflow from HDD borehole inflow monitoring data. Full article

To improve the stability of gob-side entry in the mining of extra-thick coal seams, we examined the field practice of reserving a narrow coal pillar with a width of 8 m in a 17 m thick coal seam. A combination of roof directional hydraulic fracturing and pouring concrete to consolidate the coal pillars was proposed and applied to reduce the deformation of the gob entry. First, roof directional hydraulic fracturing inhibits the vertical stress transmission to the key roof strata, and it transfers more vertical stress to the gob and reduces the lateral abutment stress in the roof. Subsequently, the narrow coal pillar is strengthened with a reinforced concrete wall, forming a strong–weak coupled bearing structure capable of bearing the overburden load, and the higher stiffness of the reinforced concrete wall effectively resists the lateral deformation of the coal pillar into the roadway. Additionally, the feasibility of this application was verified by comparing the results with the roadway convergence for a 43 m coal pillar on site. The results show that the maximum convergence, stability time, and advanced influence range were all significantly decreased by the proposed method, and the gob-side entry stability was improved. The proposed method and the results obtained provide a valuable reference for mining in similar conditions. Full article

Efficient heat extraction from fractured geothermal reservoirs is strongly controlled by fracture network heterogeneity, well-layout design, and injection operations. In this study, a two-dimensional discrete fracture network (DFN) model incorporating thermo-hydraulic processes was established to investigate the effects of well layout and injection rate on the long-term heat extraction performance in fractured geothermal reservoirs. Two well-layout orientations, specifically 45°/135° and 0°/90°, were designed according to their geometric relationship with the dominant fracture orientations. For each orientation, four well configurations were considered, including one-injection–one-production, one-injection–two-production, one-injection–three-production, and one-injection–four-production schemes. Three injection rates of 1.0, 1.5, and 2.0 kg/s were then assigned to each well layout, resulting in 24 simulation cases. The spatiotemporal evolution of the temperature field, average outlet temperature, total production mass flow rate, and heat-output power were systematically analyzed. The results show that injected cold water preferentially migrates along connected fractures, and that cooling-front propagation is jointly controlled by fracture connectivity, well spacing, and injection–production alignment. When the well alignment is consistent with the dominant fracture orientation, nearly direct preferential flow channels are more likely to form, leading to a faster outlet-temperature decline and a higher thermal-breakthrough risk. Multi-production-well layouts can activate more fracture pathways and improve the total production mass flow rate, but their enhancement effect is limited by competitive flow diversion and local fracture connectivity. Increasing the injection rate enhances early-stage heat-output power but shortens fluid residence time and accelerates thermal breakthrough, thereby reducing long-term thermal stability. Overall, the 0°/90° multi-production-well layouts exhibit better long-term heat-output performance, while a lower injection rate is more favorable for maintaining a stable outlet temperature and heat-output power. These findings provide useful guidance for well-pattern optimization and injection-scheme selection in fractured geothermal reservoirs. Full article

Hydraulic fracturing in naturally fractured rock is governed by complex interactions between fluid flow, rock deformation, fracture propagation, and induced seismicity. In this study, a fully coupled hydro-mechanical framework based on the FDEM is developed to investigate fracture evolution and seismic responses during fluid injection in fractured rock masses. Three representative horizontal stress ratios (R = 1.0, 1.5, and 2.0) were considered to investigate the influence of stress anisotropy on fracture propagation and induced seismicity. The results demonstrate that stress anisotropy exerts a dominant control on fracture propagation patterns, fluid pressure diffusion, and induced seismicity. Under low stress ratios, fracture propagation is diffuse and strongly influenced by pre-existing fractures, whereas higher stress ratios promote localized, directional fracture growth controlled primarily by the stress field. Fluid pressure becomes increasingly concentrated with increasing stress ratio, leading to higher injection pressures and more pronounced pressure fluctuations. The spatial and temporal evolution of mean stress and volumetric strain closely follows that of fluid pressure, indicating that fluid pressurization directly controls effective stress reduction and associated deformation. Seismic analysis reveals a systematic decrease in the Gutenberg–Richter b-value with increasing stress ratio, indicating a transition from distributed micro-fracturing to more coherent fracture reactivation and larger seismic events. Under quasi-steady injection pressure conditions, fracture propagation is found to be episodic and unstable, as evidenced by pronounced positive and negative spikes in the fracture volume change rate and associated pressure fluctuations; these are accompanied by intermittent fracture opening and closure, stress redistribution, and temporary reductions in cumulative seismic moment. These findings provide new insights into the coupled mechanisms governing hydrofracturing-induced seismicity and have important implications for the assessment and mitigation of seismic risks in subsurface engineering applications. Full article

To address severe stress concentration, excessive convergence, and instability of the roadside backfill body (RBB) in gob-side entry retaining (GER) under thick and hard roof conditions, this study investigates the control mechanism of main roof fracture position on surrounding rock stability, using the 3⁻¹01 working face of Huoluowan Coal Mine as a case study. A combined approach integrating theoretical analysis, numerical simulation, and field investigation is adopted. A statically indeterminate mechanical model based on masonry beam theory is established to characterize the lateral roof fracture behavior. The deflection and bending moment distributions are derived, and a criterion for fracture position determination is developed based on the maximum bending moment condition. The theoretical results indicate that the natural fracture position is located approximately 9.4–11.2 m inside the gob boundary. Numerical simulations using UDEC Trigon under different fracture positions (−2 m, 1 m, 5 m, and 9 m) show that fracture location significantly affects the mechanical response of GER. Fractures occurring above the roadway or RBB induce large deformation levels and more extensive plastic zones, while gob-side fracture conditions correspond to relatively lower disturbance levels and improved structural stability. The RBB exhibits shear-dominated failure characteristics, and the displacement distribution is non-uniform along height, with larger deformation in the middle-to-upper region. To improve stability, a coordinated control strategy combining anchor cable reinforcement and directional long-distance hydraulic fracturing (HF) is proposed to regulate the main roof fracture position through the formation of artificial weak planes. Field monitoring results show that the maximum displacements of the roof, floor, and ribs are 558 mm, 233.5 mm, and 71.3 mm, respectively, with a convergence ratio of 19.8%. Borehole imaging confirms the development of hydraulic fractures within the designed roof stratum, supporting the effectiveness of the proposed control approach. These results demonstrate that the fracture position of the main roof plays a key role in controlling GER stability, and its regulation provides an effective means for improving roadway performance under complex geological conditions. Full article

Natural fractures can significantly affect fluid seepage behavior and development performance in tight formations. However, the optimal configurations and performance of oriented horizontal wells under various natural fracture scenarios remain insufficiently understood. Numerical simulation models for a fractured horizontal well in a five-spot well pattern were established based on the embedded discrete fracture model (EDFM) to consider the coupled effects of hydraulic fractures and natural fractures. Optimization analyses were performed under different natural fracture conditions, with cumulative oil production used as the main evaluation criterion. The results indicate that natural fractures play a significant role in determining the optimal horizontal well orientation. For reservoirs without natural fractures and those with low- to medium-density single-set natural fractures, the optimal horizontal well orientation is perpendicular to the maximum horizontal stress direction. In contrast, for high-density single-set natural fracture systems, a slight rotation of the horizontal wellbore improves cumulative oil production, with an optimal orientation angle of approximately 15° identified in this work. For conjugate fracture networks, the influence of well orientation becomes more significant, and the optimal orientation angle varies with fracture density, ranging from 15° to 45°. This study indicates that the horizontal wellbore trajectory design may highly rely on the characteristics of natural fractures. Therefore, thorough and accurate characterization of natural fractures should be conducted before optimizing the orientation of fractured horizontal wells. The findings of this work provide theoretical guidance for the placement of fractured horizontal wellbores in naturally fractured tight formations. Full article