Cavities may form above the tail of torpedo anchors after installation and can significantly influence their subsequent vertical pullout behavior. However, there is still a lack of deep understanding of the effect of a fully open cavity on the vertical pullout capacity of torpedo anchors. This study investigates the vertical pullout capacity of torpedo anchors with a fully open cavity using the coupled Eulerian–Lagrangian (CEL) method implemented in abaqus. A comprehensive parametric analysis was conducted considering the impacts of anchor aspect ratio, fin aspect ratio, embedment depth, and soil strength profile. The results indicate that cavities reduce markedly both the end bearing and shaft friction resistance by altering the soil flow near the anchor tail, and thus affecting the anchor vertical bearing capacity. A capacity reduction coefficient β, defined for quantifying the loss in capacity due to the cavity, is found depending primarily on the shaft and fin aspect ratios. Specifically, β increases logarithmically with the shaft aspect ratio and decreases exponentially with the fin aspect ratio, while it is insensitive to embedment depth and soil strength gradient. Based on these findings, a simplified design procedure is proposed to predict the vertical pullout capacity considering cavity effect for engineering design in practice. This work improves the understanding of soil flow behavior influenced by cavities and provides a practical method to enhance the reliability of torpedo anchor design under vertical loading.
Pre-existing faults play an important controlling role in the formation and later evolution of sedimentary basins. The oblique extension of strike-slip faults cause subsidence and lead to the formation of pull-apart basins, this foundational mechanism is verified by fault interpretations from seismic reflection data, laboratory experiments, and numerical models. While this mechanism is observed in contemporaneous faults, how pre-existing thrust faults affect the formation of an overlying rift basin is still being studied. Here we reveal a set of Indosinian thrust faults and their reactivation effects within the basement of the offshore Bohai Bay Basin (OBBB), eastern China. These faults provide insights into the influence of pre-existing faults on the overlying basin. Structural analysis of seismic profiles of the OBBB show that its basinal basement developed a series of NWW-trending pre-existing faults, most of which cease at the top of the basement, though some were profoundly reactivated, crossing the unconformity upward into Cenozoic strata, hence, influencing the morphology and structural evolution of the Cenozoic Bohai Bay Basin (BBB). The NWW-trending faults were in response to the collision between the South China and North China blocks. Zircon and apatite fission-tracks reveal the fault-controlled basement suffered three intervals of uplift-cooling processes during the Late Triassic−Early Jurassic, Late Cretaceous, and Oligocene, and three corresponding intervals of subsidence-heating stages during the Early Cretaceous, Paleogene, and Neogene. The reactivations during the Early Cretaceous and Paleogene induced NWW-trending half-grabens. The strike-slip activity of the NNE-trending Tanlu Fault zone formed a set of NNE-trending pull-apart sags in the eastern BBB and reactivated some NWW-trending pre-existing thrust faults reversely with slight rotation. The two orthogonal extensions on interconnected half-grabens and strike-slip faults drove the formation of the pull-apart BBB. Broadly, pre-existing faults could partially be reactivated by later orthogonal strike-slip faults and jointly form a pull-apart rhomboid basin characterized by multiple sags.
The complexity of pipeline systems and variable flow patterns in multiphase flow with hydrate particles (HPs) makes it challenging to understand and to mitigate erosion. Considering a sand- water flow erosion test in an elbow, a set of ice- sand- water erosion tests were conducted in a spinning slurry. The mass loss rate (LR) and erosion rate (ER) were compared with different test parameters. Furthermore, the ER of oil- gas- hydrate- sand slurry in four types of elbow (90 degrees bend, 120 degrees bend, V- bend, and N- bend) was predicted using a computational fluid dynamics (CFD) simulation software. The addition of 5% mass fraction of sand particles (SPs) significantly increased LR of ice slurry with a mixed flow speed of 500 rev/ min. In the investigated cases, LR of Group B (ice- sand slurry) was 100 times higher than LR of Group A (ice slurry) on average. When the ice content was increased from 12.5% to 25%, the LR increased by 100%. A certain mass fraction of ice cubes also increased the LR of the sample in the sand- containing water flow. In general, as the ice concentration increased, the LR also increased. With an initial gas volume fraction phi of 10% to 25%, the flow patterns of slurry were simulated from layered flow in an elbow. Given the alternating transformation of bubbles and liquid columns, particles obtained more energy from the flow, resulting in higher ER and more eroded areas, especially in the N- bend. The 120 degrees bend layout was the most erosion resistant among the tested bends, having the lowest ER average and an ER maximum at phi = 15-20%.
Mesozoic volcanic buried hills constitute the most extensively exposed basement-type paleo-buried hills in the Bohai Sea area. Recently, CNOOC Bohai Oilfield achieved a major breakthrough by discovering a super-thick hydrocarbon pay zone within Mesozoic volcanic rocks at depths exceeding 5000 m in the Bozhong Uplift, setting a new record for natural productivity from deep offshore exploratory wells in China. This discovery confirms the substantial exploration potential of deep to ultra-deep Mesozoic volcanic buried hills in the Bohai Sea area. Focusing on the Mesozoic volcanic edifice assemblages in the Bozhong Uplift of the Bohai Bay Basin, this study integrates three-dimensional seismic data, mud logging, thin-section petrography, well logging, porosity–permeability measurements, mercury intrusion capillary pressure analyses, and zircon isotopic geochronology to comprehensively characterize the geometry, genesis, and evolution of the conjugate strike-slip fault system in the Bozhong Uplift. On this basis, the controlling effects of the fault system on volcanic eruption and emplacement, fracture development, and the formation of high-quality reservoirs are systematically elucidated. The results indicate that: (1) Three major fault sets are developed in the Bozhong Uplift, striking N-S, NW-SE, and NE-SW. Among them, the N-S- and NW-trending faults constitute a conjugate left-lateral strike-slip fault system consistent with the Andersonian faulting model. In map view, these faults mutually intersect, while in cross-section they display a series of characteristic strike-slip structural styles, including positive flower structures, negative flower structures, vertical structures, and the “dolphin effect.” (2) During the Yanshanian, the inherited activity of the N-S- and NW-trending conjugate strike-slip faults provided preferential pathways for the ascent of deep-seated intermediate to acidic magmas, thereby controlling typical central-type volcanic eruptions. Under this structural control, medium- to high-relief volcanic edifices are distributed in a pronounced checkerboard pattern along the conjugate fault zones. Concurrently, widespread intrusion and diapirism of acidic magmas during this period ultimately shaped the transverse anticline structure of the Bozhong Uplift. (3) Segmental overlap of the major strike-slip faults generated broad overlap zones that served as loci for concentrated tectonic stress release. Intense stress release strongly modified the volcanic rocks and produced abundant secondary fractures. The coupling of faults and fractures ultimately formed a highly connected fracture network, providing efficient pathways for the dissolution by various acidic fluids and leading to significant enhancement of porosity and permeability. Consequently, even in medial to distal volcanic facies away from eruption centers, favorable conditions for the development of high-quality reservoirs persist. Based on these insights, a ternary coupled reservoir-forming model for volcanic rocks in the Bozhong Uplift is proposed, characterized by “dominance of intermediate-acidic lithologies, superimposed modification by strike-slip faults, and multi-fluid dissolution-induced porosity enhancement.” This model elucidates the formation mechanism of laterally extensive, high-porosity and high-permeability fractured-vuggy reservoirs and large-scale volcanic hydrocarbon accumulations.
Ultra-low-permeability clastic reservoirs, characterized by poorly developed pore structures and extremely low permeability, pose significant challenges for accurately estimating oil saturation using the Archie equation and its derivative models. The primary limitation arises from the difficulty of oil–water displacement at the pore scale, which prevents reliable resistivity measurements under low water saturation conditions in laboratory experiments. These constraints ultimately hinder both sweet-spot identification and reserves evaluation. In this work, digital rock physics is applied to construct digital rocks with varying water saturations, enabling investigation of low-saturation resistivity through pore-scale numerical simulations. By integrating core-scale experimental results with pore-scale simulation data, we propose a two-scale integrated saturation model. The Wenchang Formation in the Huizhou region is selected as the case study. Digital rocks are reconstructed from X-CT images at two resolutions. The resistivity of fully saturated rock is simulated using the finite element method, and the formation factor–porosity relationship (F–ϕ) is established through cross-plots. The resistivity of cores at different water saturations is further simulated to derive the resistivity index, and the resistivity index–water saturation relationship (RI–Sw), which follows an exponential trend in double-logarithmic coordinates. Based on these results, a new saturation equation tailored for ultra-low-permeability reservoirs is developed by integrating pore-scale simulations with core-scale experiments. The calculated oil saturation values using this model are in good agreement with sealed-core measurements, whereas Archie-based results are underestimated. The proposed model substantially improves oil saturation prediction in ultra-low-permeability reservoirs, enabling more accurate hydrocarbon reserve assessment and enhancing exploration potential in the study area.
