Differential Stress Levels Research Articles

Experimental Investigation into the Process of Hydraulic Fracture Propagation and the Response of Acoustic Emissions in Fracture–Cavity Carbonate Reservoirs

Fracture–cavity carbonate reservoirs account for a considerable proportion of oil and gas resources. Because of the complicated relationships between cavities, fractures and pores in these reservoirs, which are defined as cavity clusters, fracturing technology is employed to enhance their hydrocarbon productivity. However, almost all previous studies have just considered the effect of a single natural cavity or fracture on the propagation of a hydraulic fracture; therefore, the mechanism by which a hydraulic fracture interacts with a cavity cluster needs to be clarified. In this study, cavity clusters with different distributions were accurately prefabricated in synthetically made samples, and large-scale simulation equipment was employed to systematically perform fracturing experiments considering different horizontal differential stress levels. Meanwhile, the hydraulic fracture propagation behaviors were comprehensively analyzed through fracture morphology, fracturing curves, the complexity of the fracture network and acoustic emission monitoring. It was found that a natural fracture with a smaller approach angle is favorable in guiding a hydraulic fracture to a cavity. The fracturing curves were divided into the following four types: frequent fluctuations with “step-like” shapes, great fluctuations with slightly lower closure pressure, fluctuations with obviously lower closure pressure, and little fluctuations with obviously lower closure pressure. And different cavity cluster distributions play a dominant role in the complexity of generated hydraulic fracture networks. In addition, AE energy was used to judge the ease of crossing the cavity. The above findings indicated that for the actual exploration and exploitation of carbonate reservoirs, the geological exploration of different fracture–cavity structures in reservoirs would be required, and targeted fracturing engineering designs need to be carried out for different fracture–cavity carbonate reservoirs.

Earth's Free Surface Complicates Inference of Absolute Stress From Earthquake‐Induced Stress Rotations

AbstractThe stress redistribution from an earthquake can produce localized measurable rotations of the principal stress axes if the absolute level of differential stress in the crust is on the order of the earthquake stress drop. Two simple analytic solutions have been developed to estimate the differential stress from an observed stress rotation. However, each has assumptions that may not be accurate near Earth's free surface. I model synthetic earthquakes in an elastic half‐space, and show that the assumptions of the methods are accurate for strike‐slip earthquakes, and for deep dip‐slip earthquakes. However, they are incorrect for shallow dip‐slip earthquakes. I introduce a free surface correction for one of the methods for dip‐slip earthquakes. I revise an analysis of stress rotations due to great subduction zone earthquakes, including this correction. The results support the original conclusion of near complete stress drop for many shallow subduction zone earthquakes.

Fluid-mineral Equilibrium Under Nonhydrostatic Stress: Insight From Molecular Dynamics

The interpretation of phase equilibria and reactions in geological materials is based on standard thermodynamics that assumes hydrostatic and homogeneous stress conditions. However, rocks and minerals in the lithosphere can support stress gradients and nonhydrostatic stresses. Currently, there is still not an accepted macroscopic thermodynamic theory to include the effect of nonhydrostatic stress on mineral reactions, and the use of several thermodynamic potentials in stressed geological system remains under debate. In experiments under nonhydrostatic stress, it is often difficult to resolve the direct effect of differential stress on phase equilibria because pressure gradients may be developed. Such gradients can affect the metamorphic equilibria at the local scale. Here, we investigate the direct effect of a homogeneous, nonhydrostatic stress field on the solid-fluid equilibrium using molecular dynamics simulations at non-zero pressure and elevated temperature conditions. Our results show that, for simple single-component systems at constant temperature, the equilibrium fluid pressure of a stressed system is always larger than the value of fluid pressure at hydrostatic stress conditions. The displacement of the equilibrium value of the fluid pressure is about an order of magnitude smaller compared to the level of differential stress in the solid crystal. Thus, phase equilibria can be accurately predicted by taking the fluid pressure as a proxy of the equilibration pressure. On the contrary, the mean stress of the solid can deviate substantially from the pressure of the fluid in stressed systems at thermodynamic equilibrium. This has implications on the use of thermodynamic pressure in geodynamic models since the fluid pressure is a more accurate proxy for predicting the location of metamorphic reactions, while the equilibrium density of the solid has to be determined from its mean stress.

Experimental Study on the Compressive Strength and Fatigue Life of Cement Concrete under Temperature Differential Cycling

Concrete, as an engineering material with extremely wide applications, is widely used in various infrastructure projects such as bridges, highways, and large buildings. However, structures such as highways and bridges often need to be situated in variable and harsh service environments for long periods. They not only face cyclic reciprocating vehicle loads but also have to contend with the effects of temperature cycling. Therefore, studying the impact and mechanism of temperature differential cycling on the compressive strength and fatigue life of cement concrete has certain theoretical significance and practical value. This study employed a comprehensive experimental design to investigate cement concrete specimens subjected to typical temperature variations (20–60 °C) and different numbers of temperature differential cycling (0, 60, 120, 180, 240, 300). Axial compressive strength tests, ultrasonic tests, and compressive fatigue tests were conducted. The axial compressive strength test measured the compressive strength of the cement concrete. It was found that with an increase in the number of temperature differential cycling, the compressive strength exhibited a trend of an initial increase followed by a decrease: at 60 cycles, the strength increased by 10.8%, gradually declined; returned to near-initial strength at 120 cycles, and continued decreasing, reaching a decline of 19.4% at 300 cycles. The ultrasonic test measured the ultrasound velocity of the concrete specimens after different temperature differential cycling. It revealed a decreasing trend in ultrasound velocity with an increase in times of temperature differential cycling, showing a strong linear relationship between the ultrasound velocity loss and strength loss, confirming the correlation between the degree of concrete strength degradation and internal damage. The compressive fatigue test analyzed the fatigue life variation in cement concrete under different times of temperature differential cycling and stress levels, showing good adherence to the Weibull distribution pattern. Based on the approximation assumptions of log-normal distribution and the Weibull distribution, the Weibull distribution parameters for the compressive fatigue life of cement concrete under temperature differential cycling were obtained.

Winter locations of red-throated divers from geolocation and feather isotope signatures.

Migratory species have geographically separate distributions during their annual cycle, and these areas can vary between populations and individuals. This can lead to differential stress levels being experienced across a species range. Gathering information on the areas used during the annual cycle of red‐throated divers (RTDs; Gavia stellata) has become an increasingly pressing issue, as they are a species of concern when considering the effects of disturbance from offshore wind farms and the associated ship traffic. Here, we use light‐based geolocator tags, deployed during the summer breeding season, to determine the non‐breeding winter location of RTDs from breeding locations in Scotland, Finland, and Iceland. We also use δ15N and δ13C isotope signatures, from feather samples, to link population‐level differences in areas used in the molt period to population‐level differences in isotope signatures. We found from geolocator data that RTDs from the three different breeding locations did not overlap in their winter distributions. Differences in isotope signatures suggested this spatial separation was also evident in the molting period, when geolocation data were unavailable. We also found that of the three populations, RTDs breeding in Iceland moved the shortest distance from their breeding grounds to their wintering grounds. In contrast, RTDs breeding in Finland moved the furthest, with a westward migration from the Baltic into the southern North Sea. Overall, these results suggest that RTDs breeding in Finland are likely to encounter anthropogenic activity during the winter period, where they currently overlap with areas of future planned developments. Icelandic and Scottish birds are less likely to be affected, due to less ship activity and few or no offshore wind farms in their wintering distributions. We also demonstrate that separating the three populations isotopically is possible and suggest further work to allocate breeding individuals to wintering areas based solely on feather samples.

Enhancement of polarization and magnetization in polycrystalline magnetoelectric composite

Electrical control of magnetization or magnetic control of polarization offers an extra degree of freedom in materials possessing both electric and magnetic dipole moments, viz., magnetoelectric (ME) multiferroics. A microstructure with polycrystalline configurations that enhances the overall polarization/magnetization and that outperforms single crystalline configurations is identified in a 1–3 CoFe2O4–BaTiO3 (or CFO–BTO) composite. The characterization of local fields corresponding to the polycrystal configuration underlines a nontrivial role played by randomness in better cross coupling mediated by anisotropic and asymmetric strains. The microscopic field (local field) profile of the composite provides rich information regarding the distribution of key parameters central to the magnetoelectric effect. The differential contractual stress level observed in the local stress profile of CFO–BTO composite upon applying an external magnetic field conforms with the previous experimental magnetostriction observed in CFO. The role played by residual stresses stemming from misalignment of the polarization in the neighboring grains in enhancing the ME coupling is briefly discussed.

Brittle Creep and Viscoelastic Creep in Lower Palaeozoic Shales from the Baltic Basin, Poland

The paper analyses the mechanical properties of shales from the Baltic Basin, focusing on creep strain in conditions of variable stress and elevated temperature (85 °C). Rock samples were collected from drill cores from various depths between 3600–4000 m. A series of creep tests was performed using a triaxial apparatus in simulated pressure and temperature conditions in the reservoir. The creep tests were conducted at variable levels of differential stress in variable time intervals. The laboratory experiments were performed in order to study brittle and viscoelastic creep proceeding in time in shales rich in organic matter and clay minerals. Creep compliance of shale formations rich in organic matter influences the success of hydraulic fracturing procedures, as well as migration of natural gas during exploitation. Laboratory characteristics of geomechanical properties (compressive strength, strain and elastic moduli) is crucial for planning natural gas exploitation from unconventional resources. The results indicate that the level of constant differential stress and creep time significantly influence the mechanical properties of shales. The paper presents the differences between brittle and viscoelastic strain registered during creep tests at variable stress conditions and time intervals. In viscoelastic creep tests, creep strain is over two times larger in the second stage of creep in comparison to the magnitude of strain registered in the first stage. In brittle creep tests, axial strain in the first creep stage is two times larger than in viscoelastic creep tests in the second stage. Based on the experiments, elastic parameters, i.e., Young’s modulus and Poisson’s ratio, have been determined for each of the analysed samples. In brittle creep tests, Young’s modulus is smaller than in viscoelastic creep tests. In viscoelastic creep tests Young’s modulus increases in successive stages. Whereas Poisson’s ratio is larger for samples from brittle creep tests than for samples from viscoelastic creep tests and does not change with subsequent creep stages in viscoelastic creep tests.

Role of lycopene in mitochondrial protection during differential levels of oxidative stress in primary cortical neurons

Reactive oxygen species (ROS) are a major contributor to intracellular organelle damage in neurons. ROS-induced mitochondrial dysfunction is highly associated with impaired energy metabolism that occurs during neurodegeneration. Therefore, the use of antioxidants may be beneficial in protecting the brain from injury caused by ROS. Lycopene is a carotenoid that exhibits neuroprotective properties via its antioxidant capacity. In this study, we hypothesize that treatment with lycopene attenuates hydrogen peroxide-induced oxidative stress in primary cortical neurons. We further tested if lycopene reverses mitochondrial dysfunction caused by ROS. Primary rat cortical neurons were treated with lycopene, hydrogen peroxide, or a combination of both. We used two different concentrations of hydrogen peroxide (25 and 100 μM) to examine a moderate or a high ROS challenge. Treatment with hydrogen peroxide significantly increased intracellular ROS, decreased mitochondrial membrane potential, and depleted neuronal ATP. When neurons were co-treated with lycopene and 25 μM hydrogen peroxide, lycopene attenuated hydrogen peroxide-mediated mitochondrial dysfunction. Interestingly, neuroprotective properties of lycopene were only found during exposure to 25 μM hydrogen peroxide, but not in a 100 μM treated group. We further found that lycopene prevents accumulation of the cleaved form of B-cell lymphoma-extra large, ΔN-Bcl-xL. Since ΔN-Bcl-xL causes loss of mitochondrial inner membrane potential which impairs energy metabolism and leads to neuronal death, prevention of ΔN-Bcl-xL accumulation may be an important mechanism explaining lycopene-mediated neuroprotection. Our study suggests that mitochondria are a key target during lycopene-mediated neuroprotection but that the efficacy of lycopene as a neuroprotectant may vary under different levels of ROS.

Dependence of earthquake faulting type on fault strike across the Korean Peninsula: Evidence for weak faults and comparison with the Japanese Archipelago

We re-evaluate the stress state across the Korean Peninsula by analyzing the stress tensor inversion results, strike–rake relationships, and faulting-type indicator–magnitude relationships of published earthquake focal mechanism data. Previous studies have shown that the stress state is characterized by a strike-slip faulting stress regime with ENE–WSW maximum compression, NNW–SSE minimum compression, and vertical intermediate stress. The differential stress magnitudes are estimated to be on the order of 100 MPa under the assumption of the strong-fault earthquake with a high Coulomb friction coefficient of 0.85. Conversely, other studies have shown that the weak-fault model is applicable across the Japanese Archipelago, where the differential stress level is only a few tens of MPa. We, therefore, review previous studies, evaluate the focal mechanism data across the Korean Peninsula, and infer the absolute stress level. Our comparison of the geological and tectonic histories of the Korean Peninsula and Japanese Archipelago suggests that the NNW–SSE tensional stresses in the Korean Peninsula are caused by slab rollback of the Philippine Sea Plate at around 5 Ma, west of Kyushu Island, southwestern Japan. We also find that the earthquake faulting types across the Korean Peninsula are closely related to their fault strikes, confirming a spatially uniform distribution of the strike-slip stress regime with nearly equal tensional and compressional tectonic stresses. However, this situation leads to the apparent paradox of strike-dependent fault strength. We eliminate this paradox by proposing a model whereby reverse- and normal-faulting events are caused by local stress heterogeneities around the edges of strike-slip faults under a uniform low-strength strike-slip stress state for all faulting types. We also highlight the usefulness of strike–rake and faulting-type indicator–magnitude diagrams in considering stress states.

Influence of stress-induced microcracks on viscoplastic creep deformation in Marcellus shale

Time is one of the often-neglected factors in the assessment of the erratic failure of shale rock. Laboratory-creep experiments showed that constant stress induces time-dependent failure in brittle shale. However, the microscopic reason for time-dependent deformation in shale is still unknown. In the current study, triaxial creep and recovery experiments showed that the brittle shale specimens exhibit viscoelastic and viscoplastic creep deformation at constant stress state. In addition, the X-ray computed tomography scan of Marcellus shale showed that the shale specimens contained significant volume of preexisting microcracks. The statistical correlation among permanent strain during the triaxial experiment and stress-induced change in the three-dimensional geometry of microcracks showed that the microcracking is the microscopic reason for viscoplastic creep deformation in shale. In addition to the time and level of constant differential stress, factors such as orientation of bedding planes and specimen heterogeneity also influence the nature of creep deformation.

Differential Levels of Endoplasmic Reticulum Stress in Peripheral Blood Mononuclear Cells from Patients with Sudden Sensorineural Hearing Loss.

BackgroundSudden sensorineural hearing loss (SSNHL) is currently treated with a combination of drugs, predominantly with glucocorticoids (GCs). However, the mechanisms of action of GCs in SSNHL are unknown. This study aimed to analyze the role of endoplasmic reticulum stress (ERS) in SSNHL pathogenesis and prognosis.Material/MethodsIn this study, we evaluated the expression and activation status of the protein kinase RNA-like endoplasmic reticulum kinase (PERK)-C/EBP homologous protein (CHOP) pathway in peripheral blood mononuclear cells (PBMCs) from patients with SSNHL and compared them with those in healthy controls. We also compared differences in expression of activating transcription factor 4 (ATF4) and CHOP before and after glucocorticoid treatment in patients with improved and unimproved SSNHL.ResultsTreatment with GCs significantly improved hearing in 55% of patients with SSNHL. Levels of phosphorylated PERK (p-PERK) and phosphorylated eukaryotic initiation factor 2α were increased in PBMCs from patients with SSNHL compared with healthy controls. ATF4 and CHOP expression were also significantly elevated. After treatment, the amount of ATF4 and CHOP proteins in PBMCs in the patients whose SSNHL improved was significantly reduced compared with the levels measured before treatment in all patients with SSNHL. The expression of the ATF4 and CHOP proteins in PBMCs in the unimproved group, however, was not significantly changed relative to pretreatment levels.ConclusionsERS may play a significant role in the pathogenesis of SSNHL, and the responsiveness of the condition to GC-mediated mitigation of ERS may be one of the key factors that affect patient prognosis.

Density-dependence tips the change of plant\u2013plant interactions under environmental stress

Facilitation studies typically compare plants under differential stress levels with and without neighbors, while the density of neighbors has rarely been addressed. However, recent empirical studies indicate that facilitation may be density-dependent too and peak at intermediate neighbor densities. Here, we propose a conceptual model to incorporate density-dependence into theory about changes of plant–plant interactions under stress. To test our predictions, we combine an individual-based model incorporating both facilitative response and effect, with an experiment using salt stress and Arabidopsis thaliana. Theoretical and experimental results are strikingly consistent: (1) the intensity of facilitation peaks at intermediate density, and this peak shifts to higher densities with increasing stress; (2) this shift further modifies the balance between facilitation and competition such that the stress-gradient hypothesis applies only at high densities. Our model suggests that density-dependence must be considered for predicting plant–plant interactions under environmental change.

Chronic Stress and Cardiovascular Disease among Individuals Exposed to Lead: A Pilot Study.

Chronic stress and cardiovascular disease risk were explored in a predominately middle-aged adult population exposed to elevated lead levels in this cross-sectional study using data from the National Health and Nutrition Examination Survey (NHANES) from the period 2007-2010. Elevated lead exposure was defined using the epidemiological threshold of a blood lead level (BLL) > 5 μg/dL as defined by the U.S. Centers for Disease Control and Prevention (CDC). Allostatic load (AL), a measure of chronic stress, was operationalized using 10 clinical markers. The geometric mean values for clinical cardiovascular disease risk markers of interest (a) Gamma glutamyl-transferase (GGT) (a marker of oxidative stress), and (b) non-HDL cholesterol (non-HDL-c) (a marker of cardiovascular disease risk) were explored among lead-exposed and less lead-exposed individuals with differential chronic stress (AL) levels. Associations between AL and GGT/non-HDL-C were analyzed using linear regression models. The likelihood of increased clinical markers in lead-exposed individuals with high compared to low AL was explored using binary logistic regression models. In analyzing lead-exposed as compared to less lead-exposed populations, the geometric mean of the variables of interest showed significant elevations among lead-exposed individuals as compared to less lead-exposed individuals. Simple linear regression revealed that AL was positively associated with the variables of interest among the lead-exposed. In binary logistic regression among the lead-exposed, those with high AL, as compared to those with low AL, had significantly higher odds of having elevated non-HDL-C. This study submits that those exposed to lead with increasing AL may experience adverse cardiovascular health outcomes.

Effects of larval density on dispersal and fecundity of western corn rootworm, Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae).

The western corn rootworm, Diabrotica virgifera virgifera (LeConte) (Coleoptera: Chrysomelidae), is an economically important pest of corn in the northern United States. Some populations have developed resistance to management strategies including transgenic corn that produces insecticidal toxins derived from the bacterium Bacillus thuringiensis (Bt). Knowledge of insect dispersal is of critical importance for models of resistance evolution. Larval density affects survival in the field, and stress from crowding often affects facultative long-distance dispersal of adult insects. In this study, we used laboratory flight mills to characterize western corn rootworm flight performance as a function of larval rearing density. Larvae were reared under three densities and the resulting adult females were either allowed to fly voluntarily for 22 h or forced to fly specified durations. For both experiments we also measured lifetime fecundity following flight. The three rearing densities placed differential levels of stress on individuals, as evidenced by decreased survival to adulthood and decreased size of adults at greater rearing density. When larvae were reared under crowded conditions the resulting females were more likely to engage in flight activity, including long uninterrupted flights lasting >10 min, than those reared under low density conditions. Flight and egg production are both energy intensive processes. However, we found no evidence in either voluntary or forced flight experiments of a tradeoff between flight activity and female fecundity. The results suggest that females emerging from high density populations in cornfields are more likely to disperse and disperse farther than those emerging from low density populations. These results are important because they imply that variation in population density in the landscape will affect dispersal, which may in turn require computer models of resistance evolution to incorporate multiple dispersal rates arising from varying larval densities among fields.

Investigation of static/dynamic moduli and plastic response of shale specimens

High clay and organic content, irregular voids, presence of micro-cracks, and obvious bedding planes are among features of shale gas rocks that affect their mechanical response. These sedimentary rocks exhibit substantial degree of anisotropy and inhomogeneity, with non-linear response when subjected to axial loading. The laboratory characterization of these rocks is necessary to design an optimized hydraulic fracturing programme, and for reliable constitutive and numerical modeling of these formations. In this study, a series of triaxial multi-stage elastic and cyclic tests were performed on Marcellus Shale specimens, retrieved from a deep well (~2270 m deep) located in West Virginia, to characterize their elastic-plastic and hysteresis response subjected to loading/unloading cycles. The experimental results indicated a nonlinear response, particularly at low differential stress levels. In addition, the specimens with higher clay content exhibited a softer response with lower estimated static and dynamic moduli at different stress levels compared to those of specimens with higher calcite/quartz content. The ultrasonic P- and S-wave velocity measurements were found to be sensitive to the changes in the micro-structure of the rock caused by variation in stress condition. In general, the estimated Young's modulus during unloading was found to be higher than loading. The plastic deformations were pronounced in the first cycle of loading, followed by a decreasing trend in the subsequent cycles.

Development and Recovery of Stress‐Induced Elastic Anisotropy During Cyclic Loading Experiment on Westerly Granite

AbstractIn the upper crust, where brittle deformation mechanisms dominate, the development of crack networks subject to anisotropic stress fields generates stress‐induced elastic anisotropy. Here a rock specimen of Westerly granite was submitted to differential stress cycles (i.e., loading and unloading) of increasing amplitudes, up to failure and under upper crustal conditions. Combined records of strains, acoustic emissions, and P and S elastic wave anisotropies demonstrate that increasing differential stress promotes crack opening, sliding, and propagation subparallel to the main compressive stress orientation. However, the significant elastic anisotropies observed during loading (≥20%) almost vanish upon stress removal, demonstrating that in the absence of stress, crack‐related elastic anisotropy remains limited (≤10%). As a consequence, (i) crack‐related elastic anisotropies measured in the crust will likely be a strong function of the level of differential stress, and consequently (ii) continuous monitoring of elastic wave velocity anisotropy along faults could shed light on the mechanism of stress accumulation during interseismic loading.

Time-Dependent Behaviour of Brittle Rocks Based on Static Load Laboratory Tests

Cumulative elastic and inelastic strain and associated internal stress changes as well as damage evolution over time in brittle rocks control the long-term evolution of the rockmass around underground openings or the land surface settlement. This long-term behaviour is associated with time-dependent deformation and is commonly investigated under static load (creep) conditions in laboratory scale. In this study, low Jurassic and Cobourg limestone samples were tested at different static load levels in unconfined conditions to examine the time to failure. Comparisons are made with longterm testing data in granites and limestones associated with the Canadian nuclear waste program and other data from the literature. Failure typically occurred within the time limits of the test program (4 months) with axial (differential) stress levels near or above the crack damage threshold (CD) estimated from baseline testing. The results also suggest that the time to failure of limestone is longer than that of granite at a given driving stress. Further insight into samples that did not reach failure was investigated and it was found that there was a clear division between failure and no failure samples based on the Maxwell viscosity of the samples tested (indicating that viscosity changes near the yield threshold of these rocks. Furthermore, samples showed a clear tendency towards failure within minutes to hours when loaded above CD and no failure was shown for samples loaded below CI (crack initiation threshold). Samples loaded between CD and CI show a region of uncertainty, with some failing and other not at similar driving stress-ratios. Although such testing is demanding in terms of setup, control of conditions, continuous utilization of test and data acquisition equipment and data processing, it yields important information about the long-term behaviour of brittle rocks, such as the expect time to failure and the visco-elastic behaviour. The information presented in this paper can be utilized for preliminary numerical studies to gain an understanding of potential impact of long-term deformations.

Persistent annual migration patterns of a specialist seabird

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 593:231-245 (2018) - DOI: https://doi.org/10.3354/meps12459 Persistent annual migration patterns of a specialist seabird Rachael A. Orben1,*, Nobuo Kokubun2, Abram B. Fleishman3,4, Alexis P. Will2,5, Takashi Yamamoto2,6, Scott A. Shaffer3, Rosana Paredes7, Akinori Takahashi2, Alexander S. Kitaysky5 1Department of Fisheries and Wildlife, Oregon State University, Hatfield Marine Science Center, 2030 SE Marine Science Dr., Newport, OR 97365, USA 2National Institute of Polar Research, Tachikawa, Tokyo 190-8518, Japan 3Department of Biological Sciences, San Jose State University, One Washington Square, San Jose, CA 95192-0100, USA 4Conservation Metrics, Inc., 145 McAlister Way, Santa Cruz, CA 95060, USA 5Department of Biology and Wildlife, Institute of Arctic Biology, University of Alaska Fairbanks, Fairbanks, AK 99775-7000, USA 6Graduate School of Environmental Studies, Nagoya University, Nagoya, Aichi 464-8601, Japan 7Department of Fisheries and Wildlife, 104 Nash Hall, Oregon State University, Corvallis, OR 97331-3803, USA *Corresponding author: raorben@gmail.com ABSTRACT: Specialization can make animals vulnerable to rapid environmental changes. For long-lived seabirds, foraging specialization may make individuals especially sensitive, as climatic changes are currently occurring over the course of one lifetime. The Bering Sea is a dynamic subarctic and arctic ecosystem where windblown sea ice mediates annual productivity and subsequent pathways to upper trophic levels. Red-legged kittiwakes Rissa brevirostris are endemic surface foraging seabirds specializing on myctophid fishes during reproduction. Their degree of specialization outside the breeding season is less understood. We examined their non-breeding ecology (migration, distribution, isotopic niche) during 4 winters with varying sea ice extent. Although we found annual variation in core distributions, diets (as reflected in feather stable isotope signatures), and outbound migratory timing, the winter range of red-legged kittiwakes was restricted to the western regions of the Bering Sea and North Pacific. Contrary to expectations, sea ice did not limit distributions in the Bering Sea in 3 yr: e.g. sea ice associations (<100 km) were infrequent (8.7% mo-1). Yet, their wintering range often overlapped with areas of seasonal ice cover, suggesting range-wide use of sea ice ecosystems. Stress levels measured by corticosterone in feathers were generally low. However, birds that concentrated in the Bering Sea in February had higher stress levels and fed at a lower trophic level than those in the western Aleutians and western subarctic. As conditions change, this persistence in wintering locations, while incurring differential stress levels, may contribute to rapid population fluctuations as has been observed in the recent past. KEY WORDS: Feather corticosterone · Life-history trade-off · Geolocation · Migration · Net‑squared displacement · Stable isotopes · Red-legged kittiwake · Rissa brevirostris Full text in pdf format Supplementary material PreviousNextCite this article as: Orben RA, Kokubun N, Fleishman AB, Will AP and others (2018) Persistent annual migration patterns of a specialist seabird. Mar Ecol Prog Ser 593:231-245. https://doi.org/10.3354/meps12459 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 593. Online publication date: April 12, 2018 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2018 Inter-Research.

Dynamic conductivity of proppant-filled fractures

A common challenge in matching the production from hydraulically fractured wells is related to changes in conductivity of propped fractures due to changes in the stress configuration. In this paper, we investigate the dynamic nature of fracture conductivity, by evaluating the permeability and width of a propped fracture segment at variable differential stress conditions. We combine the Discrete Element Method (DEM) and the Lattice Boltzmann Model (LBM) to simulate the evolution of a fracture segment filled with proppant and to evaluate the related fracture conductivity at various stress states. An ensemble of spherical proppant particles is generated and compacted under a specified confining stress via DEM to generate an initial propped fracture segment. A representative elementary volume (REV) of the proppant pack is then extracted from the DEM representation and used for fluid flow simulations. LBM is used to calculate the detailed single-phase flow field, at the pore-scale, and allows for calculation of permeability. In-situ conditions are then simulated by applying confining and differential stress on the REV via DEM. The differential stress is gradually increased on the REV, to simulate a reduction in the pore pressure due to production, until the formation of shear band(s) is observed and fracture failure begins. Permeability of the proppant pack, from LBM simulations, combined with the fracture aperture, from DEM simulations at different differential stress levels, allow us to determine the dynamic conductivity of the propped fracture segment. From a range of proppant-size distributions, we demonstrate that a well-graded proppant pack outperforms a poorly graded pack in maintaining the fracture conductivity over a broader range of differential stress conditions. Furthermore, we demonstrate that a well-graded proppant pack keeps the fracture segment open over a larger range of differential stress states.

The Influence Between Natural Fracture and Hydraulic Fracture in Shale Reservoir

Based on linear elastic fracture mechanics, the forming mechanism of complex fracture system in shale reservoir are studied. The interference of the natural fracture and hydraulic fracture model and a new crack initiation angle calculation model is established, and the influence factors of critical differential stress level and new crack initiation angle are analyzed. The impact of natural fracture dip and natural fracture density on complex fracture network are researched. Calculation results show that the lower approximation Angle and high pressure in the fracture is the necessary conditions of forming fracture network; Under the condition of lower stress difference and high internal pressure, the new crack can be faster to the maximum horizontal in-situ stress direction, connect more natural fracture; in order to get a complex fracture network in shale reservoir in the need to constantly improve the pressure to activate and connect higher approaching Angle and distance of natural fracture. The research results have important reference value in understanding the formation mechanism of complex fracture network in shale reservoir, can also be used for shale fracturing design and fracturing interval optimization.