Search Results (204)

Overpressure prediction is critical for safe and efficient drilling, yet remains challenging in complex basins with multiple genetic mechanisms. This study systematically investigates the overpressure origins in the Xihu Sag, East China Sea, a prolific hydrocarbon-bearing sag with widespread overpressure and complex pressure regimes. By integrating well logging data and direct pore pressure measurements from nine wells across three major structural units, the Western Slope Belt, the Western Sub-sag and the Central Inversion Belt, a multi-method diagnostic framework is employed. This combines Bowers’ effective stress analysis with sonic-density cross-plots to discriminate between loading and unloading mechanisms. Results show obvious vertical zoning of pore pressure—normal-pressure zone, overpressure zone, and pressure reversal zone—with distinct horizontal heterogeneity. Results reveal a distinct spatial differentiation in dominant overpressure mechanisms. In the Western Slope Belt, overpressure in the deep Pinghu Formation primarily results from a composite of undercompaction (creating initial pressure seals) and subsequent hydrocarbon generation-induced fluid expansion. In contrast, in the Central Inversion Belt and Western Sub-sag, overpressure is predominantly driven by hydrocarbon charging along faults coupled with tectonic compression, with minimal undercompaction signatures. Previous studies on overpressure genesis in the Xihu Sag have largely focused on the Western Slope Belt. This study expands the analytical scope to the Western Sub-sag and Central Inversion Belt, and conducts a systematic comparative analysis of overpressure genesis across multiple tectonic units. The value of this work lies in the systematic application of classical diagnostic methods to fill the regional research gap regarding the overpressure characteristics of the Huagang Formation and the composite nature of overpressure. With accurately constrained genetic mechanisms, the findings can provide support for optimized drilling fluid design and wellbore stability management, and effectively mitigate deep hydrocarbon exploration risks in this sag and analogous overpressured basins. Full article

Child-well stimulation design has become a central challenge in mature unconventional reservoirs, where infill wells are commonly completed in reservoirs that have already been modified by parent-well production. Prior depletion changes pore pressure, stress distribution, and fracture-propagation pathways, causing child-well treatments to behave differently from parent-well completions. As a result, increasing fluid volume, proppant loading, stage density, or pump rate does not necessarily produce proportional gains in recovery. This review synthesizes the comprehensive literature on child-well stimulation intensity with emphasis on well performance, fracture-driven interactions, pad-scale economics, diagnostics, and resource-use considerations. The analysis shows that the production response is highly conditional: larger treatments can enhance reservoir contact when fractures access underdrained rock; however they may lose effectiveness when depletion-induced stress changes redirect fracture growth toward parent-well drainage areas or pre-existing fracture networks. In such cases, higher nominal intensity can increase interwell communication, reduce completion efficiency, impair parent-well performance, and weaken pad-level economic value. A key outcome of this review is the distinction between nominal stimulation intensity, represented by the treatment pumped, and effective stimulation intensity, represented by the fraction of that treatment that creates incremental productive fracture area. This distinction reframes child-well optimization from a treatment-size problem to a depletion-aware fracture-placement problem. Diagnostics, coupled modeling, production analysis, and mitigation strategies are therefore necessary to determine whether added stimulation intensity improves recovery or primarily redistributes production within the pad. From an economic perspective, the pad rather than the individual child well is the correct unit for evaluating stimulation-intensity decisions, since pad-level net present value integrates incremental child-well recovery, parent-well degradation, protection costs, spacing effects, and completion capital. Produced-water reuse and lifecycle emission benchmarking represent practical tools for reducing the environmental footprint of child-well development programs while simultaneously lowering freshwater demand and disposal volumes. These economic and environmental dimensions are inseparable from the technical optimization of stimulation intensity and are addressed explicitly in this review. This review concludes that child-well stimulation intensity should be optimized within a pad-scale framework that integrates depletion state, spacing, landing-zone selection, parent-well management, and long-term value rather than being uniformly maximized. Full article

The traditional geological view holds that overpressure resists mechanical compaction during sedimentary burial, keeping effective stress approximately constant and thereby passively preserving porosity and permeability in deep reservoirs. However, this assumption has relied predominantly on theoretical inference and struggles to explain a widespread geomechanical paradox: in many overpressured formations, the magnitude of permeability enhancement significantly outpaces the degree of porosity preservation. To decode this paradox, we conducted a fundamental proof-of-concept study utilizing high-pressure percolation coupled with in situ micro-CT and fractal analysis to dynamically simulate the effects of overpressure on porous sandstone under constant mean effective stress. The results reveal a counterintuitive phenomenon: while the global porosity remained fundamentally stable, the absolute permeability demonstrated a significant ~8% enhancement. Microstructural analysis indicates that this enhancement is driven by localized hydraulic wedging and the reactivation of sub-resolution throats acting as topological bridges. This active topological optimization physically and mathematically manifests as: (1) the massive reconnection of macroscopic isolated pores; (2) an enhanced space-filling capacity of the flow network, evidenced by an increased coordination number and 3D pore space fractal dimension (D_f); and (3) the structural straightening of fluid pathways, rigorously quantified by a reduction in flow tortuosity and tortuosity fractal dimension (D_T). These findings, derived from a single well-characterized sandstone sample, demonstrate that overpressure-driven permeability enhancement is a physically plausible mechanism in tight sandstones. This discovery offers a candidate physical explanation for the anomalously high permeability observed in certain deep overpressured reservoirs. However, the generalizability of these results to reservoirs with differing porosities, mineralogies, and diagenetic histories remains to be evaluated through multi-sample studies. Full article

Large-scale physical model experiments play a critical role in understanding the coupled thermo–hydro-mechanical responses during hydrate dissociation. In this study, a specially designed large-scale physical simulation apparatus (effective volume: 1178 L) was employed to investigate the depressurization-induced dissociation behavior of CO₂ hydrate, which was used as a model system to simulate the macroscopic response of hydrate-bearing sediments under controlled laboratory conditions. Key reservoir parameters—including temperature, pressure, electrical resistivity, gas production rate, and stratum displacement—were continuously monitored using an integrated array of temperature sensors, pressure transducers, electrical resistivity probes, and displacement meters. During depressurization, the system pressure decreased from 3 MPa to 1 MPa (matching the backpressure), while the internal temperature dropped from 3.5 °C to approximately 1 °C due to the endothermic dissociation of the hydrate. Gas production exhibited a three-stage evolution: an initial slow release, a rapid increase as the dissociation front propagated through the sediment, and a plateau upon completion of hydrate dissociation. Based on the measured gas production and CO₂ consumption, the hydrate saturation was estimated to be approximately 0.248. The dissociation process led to measurable sediment settlement, with a maximum vertical displacement of 88.3 mm (approximately 5.88% of the model height). Analysis of the evolution of effective stress indicates that depressurization reduced pore pressure and increased vertical effective stress by approximately 0.55 MPa, while hydrate dissociation weakened the sediment skeleton, jointly causing settlement. This study demonstrates the feasibility of using a large-scale apparatus to capture the coupled processes during hydrate dissociation. It provides benchmark experimental data for validating numerical models of hydrate-bearing sediment behavior. Further validation is required before these results can be extrapolated to CH₄ hydrate systems. Full article

Long-term agricultural irrigation in the loess platform region of Northwest China has raised the groundwater level and triggered numerous irrigation-induced loess landslides. The widely developed fissures in loess provide preferential pathways for irrigation water infiltration and serve as key factors that control the hydrological evolution and stability of landslides. The Jiaojiayatou landslide in the Heifangtai platform, Gansu Province, was selected as the study case. A coupled saturated-unsaturated seepage–stress numerical model incorporating fissure structures was established to systematically investigate the effects of fissure depth, location, and number on the seepage field evolution, stability, and deformation characteristics of loess landslides under long-term irrigation. The results show that fissures significantly accelerate the advance of the wetting front, enlarge the high-water-content zone, increase pore water pressure, and reduce the factor of safety. Among these parameters, the effect of fissure depth is the most significant: for fissure depths of 5 m and 10 m, the simulated average annual rise in groundwater level is 0.63 m/a and 1.21 m/a, respectively. When the fissure depth increases to 15 m, irrigation water directly recharges the groundwater, leading to landslide instability (factor of safety drops to 0.97). The displacement at the slope shoulder increases by 54% compared with that in the no-fissure case, and the displacement pattern shifts from predominantly horizontal sliding to vertical settlement. Furthermore, the closer the fissure is to the platform edge and the greater the number of fissures, the lower the stability becomes and the larger the soil displacement at the slope shoulder. Full article

Autonomous subterranean mobility remains one of the least unified domains in robotics because locomotion emerges from coupled interactions among deformable geomaterials, structural mechanics, energy dissipation, and environment-dependent sensing constraints. This foundational pioneer technical review synthesizes 35 years of research on burrowing and underground robotic systems through a terradynamic and multiphysics perspective. Following PRISMA guidelines, 143 peer-reviewed studies were analyzed across granular soils, cohesive sediments, saturated media, fractured geomaterials, and extraterrestrial regolith analogs. The review evaluates six dominant locomotion classes, including peristaltic, undulatory, fluidization-assisted, excavation-based, tip-extension, and hybrid architectures. Results demonstrate that locomotion performance is governed primarily by regulation of substrate response rather than propulsion generation alone. Across all architectures, mobility depends on the coupled evolution of confinement-dependent stress redistribution, yielding mechanics, pore-pressure dynamics, fracture propagation, structural stability, thermomechanical loading, and energy partitioning. The analysis further reveals a convergence toward stress-regulated locomotion, where successful systems minimize drag accumulation, control force-chain evolution, and adapt to changing terradynamic conditions. Major unresolved challenges include the absence of transferable scaling laws, standardized benchmarking methodologies, predictive terradynamic models, and multi-medium autonomy. The review concludes by proposing the foundations of a unified multiphysics terradynamic robotics paradigm capable of linking robot design, substrate mechanics, control, and deployment across terrestrial and planetary subsurface environments. Full article

During the service of solid rocket motors, propellant grains need to bear various loads such as curing cooling, gravity, and combustion internal pressure. The internal pore defects will seriously affect the structural integrity. In this paper, a three-dimensional finite element model of propellant–insulation layer–mold is established to study the structural responses of pore defects with different sizes (30–100 mm) under three typical working conditions: curing cooling, curing cooling coupled with gravity, and internal pressure loading. It is found that under the curing cooling condition, compared with the non-porous propellant grain structure, the structure with pores will raise the overall mechanical response of the propellant grain, and the maximum stress and strain are mainly concentrated in the front end of the core hole and the wing groove area. The pore size has a limited impact on the overall stress distribution, but will change the local stress concentration degree. Among them, the 80 mm pore reduces the stress in the wing groove area through stress field interference. Moreover, large-size pores will significantly weaken the structural bearing capacity and increase the contact pressure between the propellant and the core mold. Under the condition of curing cooling coupled with gravity, the stress and strain are mainly distributed at the edge of the pores, and the values increase with the increase of pore size. Under the action of internal pressure load, the stress and strain in the middle section of the propellant grain have no obvious change, but stress concentration occurs in the transition area between the core hole and the wing groove and at the end of the wing groove. The results of this study provide a reference for the integrity evaluation and structural optimization of propellant grains with pore defects. Full article

Intense rainfall and the resulting increase in ground saturation can significantly modify the mechanical performance of rock masses in natural slopes, particularly when fractured material is present. Extended infiltration reduces shear strength along discontinuities and increases pore-water pressures, raising the probability of large-scale landslides. When a tunnel is built within or near an unstable slope, the response of both structures becomes coupled, and this tunnel–slope interaction has proven to be an important aspect in the design and safety assessment of underground infrastructure in mountainous regions. This study evaluates the static and seismic performance of a tunnel–slope system in a fractured shale–limestone slope that failed after heavy rainfall. Since ground exploration was limited, the observed failure was reproduced through a back-analysis within a performance-based design (PBD) framework to calibrate representative geomechanical parameters. These parameters were then used in three-dimensional finite difference models to simulate the tunnel construction process and the seismic response of the system. During construction, the interaction between the tunnel and the slope was found to be minor. Under seismic loading, however, the simulations revealed notable interaction effects: slope displacements accumulate in the zone where the tunnel runs closest to the unstable critical section, and the stresses in the tunnel lining increase as a result of both the interaction with the slope and the curvature of the alignment. These results indicate that tunnel–slope interaction should be explicitly considered in the analysis and design of underground infrastructure whenever the tunnel lies within about four diameters of an unstable slope. Full article

Coal reservoirs commonly contain both adsorbed and free methane, but their depth-dependent partitioning and controlling mechanisms remain difficult to quantify. Taking the Daning–Jixian block as example, the coupled storage of adsorbed and free gas was evaluated considering coal rank, pore-size distribution, reservoir pressure, effective stress, and temperature. The results show that coal rank controls nanopore development and methane adsorption capacity. With increasing R_o, the CO₂-derived micropore specific surface area and micropore volume increase, whereas the N₂-derived mesopore–macropore parameters generally decrease. Langmuir volume increases with coal rank but decreases with increasing temperature. Porosity generally increases with R_o but decreases exponentially with increasing confining pressure. By integrating stress-dependent porosity, reservoir pressure, temperature, gas compressibility factor, and coal rank, depth-dependent predictive models for adsorbed and free gas contents were established. The calculated theoretical maximum adsorbed gas capacity increases from 7.61 m³/t at 500 m to 19.95 m³/t at 3000 m, with a gradually decreasing growth rate. Free gas content increases from 0.44 to 4.42 m³/t over the same depth interval, and its contribution to total gas content increases from 5.5% to 18.1%. Comparison with measured gas content indicates that coal seams shallower than 1500 m are commonly adsorption-undersaturated, whereas deeper intervals locally exceed the adsorbed gas storage capacity, implying the accumulation of free gas. These results provide a quantitative framework for evaluating adsorbed-free gas partitioning and deep coalbed methane enrichment. Full article

The exploitation of deep coalbed methane is of great significance for easing China’s energy supply pressure and realizing the “Dual Carbon” goals. However, local grid refinement simulation methods for coalbed methane cannot well capture the characteristics of deep coalbed methane reservoirs, including strong stress sensitivity and high brittleness. To tackle this issue, this paper develops a novel numerical simulation approach dedicated to deep coalbed methane development. Integrated with the fluid–solid coupling effect in rock mechanics, this approach considers the interporosity flow between matrix pores and cleat fractures as well as that between cleat fractures and hydraulic fractures, and establishes a multi-dimensional coupled simulation framework on the basis of the dual-porosity single-permeability model and embedded discrete fracture model. Simulation results show that compared with the local grid refinement model, the daily gas production curve simulated by the proposed method is more consistent with the actual field curve. The local grid refinement method fails to accurately characterize the specific morphology of hydraulic fractures. The average relative error of the local grid refinement model reaches 25.61%, while that of the model in this paper is only 7.54%, representing an accuracy improvement of 18.07%. Sensitivity analysis draws the following conclusions: reservoir gas content is the dominant geological factor governing deep coalbed methane output, and raising reservoir gas content can boost cumulative gas production by 45.77%; hydraulic fracture length mainly affects gas production performance in the middle and late production stages, while fracture conductivity dominates early-stage productivity. This method can fully characterize the coupled flow behaviors of three types of media (matrix pores, cleat fractures and hydraulic fractures), and offers solid technical support for productivity forecasting and development scheme optimization of deep coalbed methane reservoirs. Full article

Alkali-activated slag–loess cemented backfill materials are subjected to coupled confining pressure and curing temperature in open-pit end-slope backfill mining. This study investigates their mechanical properties and microscopic mechanisms under four confining pressures (0, 2, 4, and 6 MPa) and four curing temperatures (5, 20, 35, and 50 °C) at a curing age of 7 days. Uniaxial and triaxial compression tests were conducted to obtain stress–strain curves, peak strength, elastic modulus, cohesion, and internal friction angle. Scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and an improved simulated annealing algorithm for three-dimensional pore reconstruction were used to characterize pore diameter, porosity, connectivity, and hydration products (calcium-silicate-hydrate (C-S-H), calcium-aluminosilicate-hydrate (C-A-S-H), and sodium-aluminosilicate-hydrate (N-A-S-H)). The results show that increasing confining pressure flattens the post-peak softening curve and transitions failure from brittle to ductile, while rising curing temperature shortens the compaction stage and increases elastic modulus. Both factors increase peak strength synergistically. Cohesion increases nonlinearly with temperature (2.64 MPa at 5 °C to 6.27 MPa at 50 °C), whereas the internal friction angle (13°) is temperature-insensitive. Microscopically, confining pressure reduces pore diameter, porosity, and connectivity via physical compaction; curing temperature promotes gel production, decreasing porosity from 26.23% to 13.95% and connectivity from 64.87% to 34.89%. This study provides a theoretical basis for backfill design and ground pressure management in open-pit end-slope mining. Full article

In slurry shield tunneling, inefficient control of slurry permeability in sandy formations can cause excessive slurry loss, increased material consumption, reduced bentonite reuse, and compromised tunnel-face stability. To address these challenges and enhance material efficiency, this study investigates slurry infiltration and bentonite particle deposition mechanisms in saturated sandy soils. Based on deposited-particle mass conservation and slurry volume conservation coupled with excess pore-water pressure, a mathematical model is established to capture the evolution of slurry rheological properties and soil pore characteristics during infiltration. Through multilayer infiltration column experiments, a multiple regression formula for the filtration coefficient is established, considering the spatiotemporal variability of slurry and soil properties. Furthermore, a dynamic penetration criterion for slurry particles is proposed and verified through single-soil infiltration tests. Results demonstrate that most bentonite particles deposit on the soil surface, with only a minimal fraction migrating into deeper pores until reaching shear stress equilibrium. The maximum infiltration distance is positively correlated with soil particle size but negatively correlated with slurry mass concentration. Increasing the slurry mass concentration or shear strength promotes the development of a well-structured filter cake and infiltration zone. These findings provide a theoretical framework for precisely regulating slurry permeability, thereby minimizing material waste and supporting sustainable shield tunneling operations. Full article

This study investigates staged surcharge preloading at a coastal test section by integrating field monitoring (pore-water pressure, settlement/settlement rate, and layer-by-layer deformation) with laboratory consolidation tests and field vane shear measurements. Responses at the surcharge center and slope-toe margin are compared to quantify spatial non-uniformity and pore-pressure–deformation coupling. Pronounced heterogeneity is observed (this field response represents three-dimensional deformation behavior that cannot be reproduced by 1D consolidation tests), with an empirical transition depth of ~24 m for this Wenzhou coastal soft soil site: above this depth, strains concentrate near the margin, whereas below it, compression at the center becomes dominant. The pore-pressure–settlement relationship is stage-dependent: during loading, pore pressure fluctuates markedly and settlement lags; during maintained consolidation, pore pressure dissipates, effective stress develops, and settlement is governed mainly by consolidation compression. After surcharging, water content decreases, and soil sensitivity reduces from 4.0 to 3.0 and stabilizes, indicating post-disturbance structural re-stabilization. These findings inform surcharge scheme design, monitoring layouts, and subsequent model calibration. Full article

Fluid injection in deep reservoirs can induce fault reactivation and seismicity, posing challenges for geothermal and subsurface energy development. This study investigates the mechanical interaction between two adjacent non-intersecting faults under fluid injection using a pseudo-three-dimensional thermo-hydro-mechanical (THM)-coupled numerical model. The results show that injection first triggers slip on F2, which then redistributes stress onto F1. The response of F1 is strongly heterogeneous: some segments are stabilized due to a decrease in Coulomb failure stress, whereas other segments are destabilized due to an increase in Coulomb failure stress. Stress-path analysis indicates that the immediate response of F1 to F2 slip is mainly governed by changes in effective normal stress and shear stress, rather than abrupt pore pressure changes on F1. These findings demonstrate that fault slip can act as a mechanical stress source that either promotes or inhibits adjacent fault reactivation. Therefore, slip-induced stress transfer should be explicitly considered when assessing fault stability in reservoirs containing multiple closely spaced faults. Full article

Ensuring the long-term serviceability of nearshore breakwaters constructed on weak seabeds is important for sustainable port infrastructure. This study investigates the wave-induced dynamic response of a vibro-compaction sand-pile composite foundation used in the Jinpai Port breakwater project in Lingao, Hainan, China. A coupled wave–structure–seabed numerical model was established using FssiCAS. Four representative monitoring points were selected inside and outside the structural influence zone and at different burial depths. The displacement, effective stress, shear stress, and pore water pressure responses were analyzed by combining full-field contour distributions with local time-history results. The results show that the foundation response is strongly location-dependent. The maximum horizontal displacement follows the order D > C > A > B, with values of approximately 10.8, 7.6, 0.5, and 0.3 mm, respectively. The final settlement follows the order A > B > C > D, with values of approximately 84, 43, 31, and 19 mm, respectively. Residual pore pressure is more significant beneath the breakwater, especially at Point B. The breakwater toes, structural boundaries, shallow seabed, and improved–natural foundation transition zones are identified as stability-sensitive zones, providing guidance for targeted monitoring, local reinforcement, drainage improvement, and maintenance planning. Full article