Hydraulic Stress Research Articles

Evaluating key climatic and ecophysiological parameters of worldwide tree mortality with a process-based BGC model and machine learning algorithms

Drought-induced tree mortality has been increasing worldwide under climate change; therefore, forests will become more vulnerable as warming continues. Meanwhile, carbon starvation and hydraulic failure have been proposed as main drought-induced mortality mechanisms, mostly validated through individual tree-level experiments. However, there lack of a unified way to monitor and assess tree mortality across the different biomes and climate regions. In this sense, process-based biogeochemical (BGC) modeling may be an effective tool for simulating and understanding ecophysiological processes for tree mortality at large spatial scales. In this study, a hydraulic vulnerability curve for percentage loss of conductivity (PLC) was added to the BGC-NSCs model, the modified version of the BIOME-BGC with two additional non-structural carbohydrates (NSCs) pools. And then, we simulate the model at the sites around the world where tree mortality were reported. Using sensitivity analysis and machine learning algorithms for hydraulic stress, PLC and NSCs showed a high sensitivity and significance to tree mortality within the modeling framework. The model simulations also reveal the relationship between PLC and NSCs based on mortality stress intensity, plant functional types, and climate conditions, further validated with the results of previous experiment studies at the plot scale. This study proposes a potential to estimate eocphysiological variables at the regional scale using the BGC model, and to use high sensitivity variable, such as PLC and NSCs, as effective diagnostics for hydraulic stress across different biomes and climate regions.

Global patterns of tree wood density.

Wood density is a fundamental property related to tree biomechanics and hydraulic function while playing a crucial role in assessing vegetation carbon stocks by linking volumetric retrieval and a mass estimate. This study provides a high-resolution map of the global distribution of tree wood density at the 0.01° (~1 km) spatial resolution, derived from four decision trees machine learning models using a global database of 28,822 tree-level wood density measurements. An ensemble of four top-performing models combined with eight cross-validation strategies shows great consistency, providing wood density patterns with pronounced spatial heterogeneity. The global pattern shows lower wood density values in northern and northwestern Europe, Canadian forest regions and slightly higher values in Siberia forests, western United States, and southern China. In contrast, tropical regions, especially wet tropical areas, exhibit high wood density. Climatic predictors explain 49%-63% of spatial variations, followed by vegetation characteristics (25%-31%) and edaphic properties (11%-16%). Notably, leaf type (evergreen vs. deciduous) and leaf habit type (broadleaved vs. needleleaved) are the most dominant individual features among all selected predictive covariates. Wood density tends to be higher for angiosperm broadleaf trees compared to gymnosperm needleleaf trees, particularly for evergreen species. The distributions of wood density categorized by leaf types and leaf habit types have good agreement with the features observed in wood density measurements. This global map quantifying wood density distribution can help improve accurate predictions of forest carbon stocks, providing deeper insights into ecosystem functioning and carbon cycling such as forest vulnerability to hydraulic and thermal stresses in the context of future climate change.

Study of stress field induced by natural fracture and its influence on hydraulic fracture propagation

The change of fracture propagation direction caused by stress interference between fractures is one of the main reasons that affect shale gas productivity. Natural fractures will be damaged by the induced stress, and the induced stress field produced by natural fracture damage will in turn affect the propagation of hydraulic fractures. Previous studies usually ignored the impact of stress field variation caused by natural fracture damage, leading to inaccurate fracture propagation simulation results. A new model for simulating hydraulic fracturing-induced stress field is established with consideration of the influence of natural fracture damage. Then, the natural fracture-induced stress is analyzed in open and closed fracture states. Through superposition of stress fields of natural fractures and hydraulic fractures, the interaction among open and closed natural fractures, reservoir and hydraulic fracture, and the corresponding fracture property evolution are studied. The results demonstrate that induced stress is not affected by elastic modulus and Poisson's ratio of the rock. The induced stress difference at the fracture tip is proportional to the net pressure and the length of hydraulic fractures. The results provide guidance for the optimal design of fracturing. When the deflection angle of hydraulic fracture is less than 90°, the maximum induced stress difference at the fracture tip decreases with the reduction in the deflection angle. When the deflection angle of hydraulic fracture is smaller than 90° or the deflection angle of natural fracture is smaller than 45°, the steering of hydraulic fracture is less hindered, which is beneficial to the formation of complex fracture network.

Heat Tolerance of Gammarus fossarum (Crustacea, Amphipoda) Is Influenced by the Level of Stress Associated with Reproduction and the Water Quality of Their Habitat

To elucidate how rising temperatures influence native amphipods, and how the expansion of wastewater treatment plants (WWTPs) with a fourth purification stage can alter temperature-related effects, studies were conducted in anthropogenically influenced rivers in Southwest Germany. Gammarids were sampled up- and downstream of two WWTPs, and exposed to elevated temperatures in the laboratory. Gammarids separated from precopula pairs were more sensitive to exposure-related stress and to a temperature increase than animals that were not in the reproductive phase. Furthermore, females were significantly more sensitive than males. Adult gammarids sampled individually at the WWTP with three purification stages showed a similar tolerance to elevated temperature at the up- and downstream site. In contrast, mortality following heat stress was substantially lower in animals sampled downstream of the WWTP with four treatment stages than in those sampled at the upstream site. We assume that the increased sensitivity of artificially separated precopula pairings reflect the situation under high hydraulic stress after heavy rainfall. Our results further illustrate the importance of WWTP upgrades in the context of climate change: gammarids sampled downstream the upgraded fourth-stage WWTP were likely exposed to lower levels of micropollutants, and thus may have allocated energy from detoxification to heat stress responses.

Exact analytical solution for deep tunnels in viscoelastic–plastic rock considering the actual loading path

The rock rheology and time process of tunnel construction make the mechanical responses of rock a function of time and are closely related to the loading path, however, these factors are not properly or correctly considered in analytical studies. The analytical study for deep tunnels in rheological rock is performed in this study, strictly taking into account the viscoelastic‒plastic characteristic of surrounding rocks and the actual loading path during excavation and supporting stages.The plane-strain problem of a circular tunnel in infinite plane under hydraulic far-field stress is simplified, with the time-dependent supporting pressure exerted at a specific time to consider the effect of delay installation of yielding support. The solving procedure as well as the time-dependent analytical solution of displacement, stress, and plastic radius in the excavation and supporting stages are presented in detail when the rocks satisfy the Unified strength theory and perfect plasticity. The analytical solutions agree very well with the numerical results, where the time procedure of tunnel excavation and support is simulated in the numerical model. Furthermore, the proposed solution can predict well the long-term deformation of the Shangxinzhai tunnel, which validates its applicability in real engineering. It is found that the tunnel deformation is underestimated if the unloading stage is not correctly considered. Based on the analytical solution, the influence of the supporting time, start time of the yielding stage and viscosity coefficient on the time dependence of tunnel convergence and stresses is investigated.

Exploring the mechanisms of diverging mechanical and water stability in macro‐ and microaggregates

AbstractBackgroundSoil stability is often evaluated using either mechanical or hydraulic stress. The few studies that use both approaches suggest that these two types of stability behave differently.AimsOur aim was to explore the mechanisms of aggregate stability regarding mechanical and water stability at the macro‐ and microscale, among other things, the effect of differing pore structure and soil organic matter content.MethodsSamples were taken from two adjacent plots that were expected to differ in stability due to land use, that is, cropped versus bare fallow (BF). The stability of dry‐separated macroaggregates (8–16 mm) and microaggregates (53–250 µm) was determined via wet sieving and unconfined uniaxial compression tests. To explore the mechanisms of stability, 3D pore characteristics were analyzed with microtomography scans. Furthermore, the contents of carbon and exchangeable polyvalent cations as well as contact angles were determined.ResultsWater stability of macroaggregates was much higher in the cropped plot (geometric mean diameter 0.65–2.37 mm [cropped] vs. 0.31–0.56 mm [BF]), while mechanical stability was very similar (median work 17.3 [cropped] and 17.5 N mm [BF]). The two size fractions behaved similarly regarding both types of stability, with more pronounced differences in macroaggregates. Several soil characteristics, like carbon, exchangeable calcium, and higher connectivity of pores to the aggregate exterior, contributed to water stability. Regarding mechanical stability, the destabilizing effect of lower carbon content and exchangeable calcium in the BF plot was counterbalanced by a lower porosity.ConclusionsMechanical and water stability behaved differently in the two plots due to the different deformation mechanisms.

Light and temperature controls of aquatic plant photosynthesis downstream of a hydropower plant and the effect of plant removal

Many rivers worldwide are regulated, and the altered hydrology can lead to mass development of aquatic plants. Plant invasions are often seen as a nuisance for human activities leading to costly remedial actions with uncertain implications for aquatic biodiversity and ecosystem functioning. Mechanical harvesting is often used to remove aquatic plants and knowledge of plant growth rate could improve management decisions. Here, we used a simple light-temperature theoretical model to make a priori prediction of aquatic plant photosynthesis. These predictions were assessed through an open-channel diel change in O2 mass balance approach. A Michaelis-Menten type model was fitted to observed gross primary production (GPP) standardised at 10 °C using a temperature dependence from thermodynamic theory of enzyme kinetics. The model explained 87 % of the variability in GPP of a submerged aquatic plant (Juncus bulbosus L.) throughout an annual cycle in the River Otra, Norway. The annual net plant production was about 2.4 (1.0–3.8) times the standing biomass of J. bulbosus. This suggests a high continuous mass loss due to hydraulic stress and natural mechanical breakage of stems, as the biomass of J. bulbosus remained relatively constant throughout the year. J. bulbosus was predicted to be resilient to mechanical harvesting with photosynthetic capacity recovered within two years following 50–85 % plant removal. The predicted recovery was confirmed through a field experiment where 72 % of J. bulbosus biomass was mechanically removed. We emphasise the value of using a theoretical approach, like metabolic theory, over statistical models where a posteriori results are not always easy to interpret. Finally, the ability to predict ecosystem resilience of aquatic photosynthesis in response to varying management scenarios offers a valuable tool for estimating aquatic ecosystem services, such as carbon regulation. This tool can benefit the EU Biodiversity Strategy and UN Sustainable Development Goals.

Enhancement of sewer sediment control and disruption of adhesive gelatinous sediment structure using low-dose calcium peroxide

Large quantities of sediments in urban sewer systems pose significant risk of pipe clogging and corrosion. Owing to their gel-like structure, sewer sediments have strong resistance to hydraulic shear stress. This study proposed a novel approach to weaken the erosion resistance of sewer sediments by destroying viscous gel-like biopolymers in sediments with low doses of calcium peroxide (CaO2). After treatment with 10–50 mg g−1 TS of CaO2, the critical erosion shear stress was significantly reduced by 25.7%–59.9%. The sediment aggregates gradually disintegrated into small diameter particles with increasing CaO2 dosage. Further analysis showed that the strong oxidizing and alkaline environment induced by CaO2 treatment led to cell lysis and changes in the composition and property of extracellular polymeric substances (EPS). After CaO2 treatment, aromatic proteins and humic acid-like substances associated with adhesion translocated from the inner EPS layers to outer layers while being disintegrated into small organic molecules. Concomitantly, CaO2 treatment disrupted the main functional groups (–OH, COO−, C–N, CO, and CN) in inner EPS layers, thus weakening EPS adhesion. Analysis of protein secondary structure and zeta potential reflected the reduced aggregation capacity of sediment microorganisms and loosening of sediment structure after CaO2 treatment. Thus, CaO2 treatment facilitated fragmentation and disaggregation of the gelatinous structure of sewer sediments. Such green strategy decreased the cost of sewer sediment disposal by 42.10–68.95% when compared to water flushing, and it would improve the self-cleaning capacity of sewer system and efficiency of dredging equipment.

Evaluation of Metal Ageing of Reactor Pressure Vessels in a Nuclear Power Plant under the Influence of Ageing Mechanisms and Environmental Conditions

The reactor metal in NPP is subjected to ageing mechanisms – corrosion, erosion, fatigue and neutron fluence embrittlement. These mechanisms cause degradation of the mechanical properties and may lead to the occurrence of defects. Environmental conditions are high pressure and fluid temperature values along the primary circuit. Where defects occur, it is possible to observe fracture of the metal under the environmental conditions. The RPV metal was studied by non-destructive testing methods. The article contains the results of the methods applied. In the defects site the values of fluence, circular stresses and temperatures were defined for a reactor operating under accident conditions. The impact of these factors in creating zones in the metal with risk of destruction has been considered. The results show, that corrosion and erosion have not caused defects in the metal. Neutron fluence and its thermal impact on the metal have caused embrittlement which is manifested by indications of defects. Two main factors were observed that lead to the creation of sites with a potential risk of fracture: 1) the neutron and thermal embrittlement of the metal, and 2) the hydraulic stresses from the fluid. The stress intensity factors of some defects, as well as the critical values of these factors have been calculated. The comparison of these values demonstrates that the metal in the danger zones will not be damaged.

Suffusion in densely compacted Satozuka pumice sand and its impact on static loading undrained shear strength and dilation behaviour

Pumice sand of volcanic origin contains a high fraction of non-plastic fines (>40 % for Satozuka pumice sand in Sapporo, Japan). Suffusion in such soil can wash away a portion of the fine particles and alter the soil microstructure. The moisture content and degree of compaction can affect the suffusion characteristics of soil deposits, however their effect has not yet been evaluated. Future construction sites in growing Sapporo City, consisting of pumice sand, will require a high degree of compaction (over 90 % and preferably over 95 %) as this sand is prone to suffusion in spite of its dense state. The aim of this study is to assess the impact of suffusion on densely compacted pumice sand with a high proportion of fines, based on its mechanical properties, with an emphasis on shear strength and dilatancy. Firstly, the suffusion characteristics of Satozuka pumice sand were evaluated. Subsequently, undrained triaxial tests (CU¯ tests) under monotonic loading were conducted on high-density specimens, with suffusion and without suffusion, to study the impact of suffusion. It is seen in the results that the hydraulic conductivity, shear strength, stress paths, and dilatancy are all noticeably affected by suffusion. The specimens with suffusion exhibit an increase in residual shear strength and maximum deviator stress under shearing and experience an earlier occurrence of phase transformation from contraction to dilation during shearing. This tendency implies that suffusion has no significant negative impact on the deterioration of earth fill made from pumice sand and non-plastic fines, and that it persists at degrees of compaction between 80 % and 100 %.

Patterning and dynamics of membrane adhesion under hydraulic stress

Hydraulic fracturing plays a major role in cavity formation during embryonic development, when pressurized fluid opens microlumens at cell-cell contacts, which evolve to form a single large lumen. However, the fundamental physical mechanisms behind these processes remain masked by the complexity and specificity of biological systems. Here, we show that adhered lipid vesicles subjected to osmotic stress form hydraulic microlumens similar to those in cells. Combining vesicle experiments with theoretical modelling and numerical simulations, we provide a physical framework for the hydraulic reconfiguration of cell-cell adhesions. We map the conditions for microlumen formation from a pristine adhesion, the emerging dynamical patterns and their subsequent maturation. We demonstrate control of the fracturing process depending on the applied pressure gradients and the type and density of membrane bonds. Our experiments further reveal an unexpected, passive transition of microlumens to closed buds that suggests a physical route to adhesion remodeling by endocytosis.

Enhanced Bacterium-Phage Symbiosis in Attached Microbial Aggregates on a Membrane Surface Facing Elevated Hydraulic Stress.

Phages are increasingly recognized for their importance in microbial aggregates, including their influence on microbial ecosystem services and biotechnology applications. However, the adaptive strategies and ecological functions of phages in different aggregates remain largely unexplored. Herein, we used membrane bioreactors to investigate bacterium-phage interactions and related microbial functions within suspended and attached microbial aggregates (SMA vs AMA). SMA and AMA represent distinct microbial habitats where bacterial communities display distinct patterns in terms of dominant species, keystone species, and bacterial networks. However, bacteria and phages in both aggregates exhibited high lysogenicity, with 60% lysogenic phages in the virome and 70% lysogenic metagenome-assembled genomes of bacteria. Moreover, substantial phages exhibited broad host ranges (34% in SMA and 42% in AMA) and closely interacted with habitat generalist species (43% in SMA and 49% in AMA) as adaptive strategies in stressful operation environments. Following a mutualistic pattern, phage-carried auxiliary metabolic genes (pAMGs; 238 types in total) presumably contributed to the bacterial survival and aggregate stability. The SMA-pAMGs were mainly associated with energy metabolism, while the AMA-pAMGs were mainly associated with antioxidant biosynthesis and the synthesis of extracellular polymeric substances, representing habitat-dependent patterns. Overall, this study advanced our understanding of phage adaptive strategies in microbial aggregate habitats and emphasized the importance of bacterium-phage symbiosis in the stability of microbial aggregates.

Study on the Influence of Gas Desorption Characteristics of Different Coal Bodies under Hydraulic Permeability Enhancement

To investigate the influence of hydraulic permeability enhancement on the gas desorption and accumulation characteristics of water-bearing coal bodies and deeply implement hydraulic fracturing measures to prevent and control gas disasters in coal seams, isothermal adsorption/desorption tests, low-pressure CO2 adsorption tests, and X-ray diffraction (XRD) tests were performed on anthracite from Anbao Coal Mine in Guizhou and bituminous coal from Huainan Pan’er Coal Mine. The study results showed that the gas desorption by kinds of water-bearing coal samples (anthracite) and bituminous coal (PE) was evidently promoted by stress. After each stress path, differences were observed between coal samples in gas desorption, which, however, presented similar variation trends. The gas desorption of the AB coal samples was greater than that of PE coal samples, and the increment of the limiting gas desorption by the AB coal samples was greater than that by the PE coal samples. The specific surface area of the AB coal samples was larger than that of the PE coal samples, and they both contained hydrophilic minerals and moisture. It was found through field observation that after hydraulic permeability enhancement acted upon water-bearing coal, the desorbed gas passively migrated under the stress action. When the coal body was free from the hydraulic stress action, gas started flowing back. The study results reveal the influence of hydraulic permeability enhancement on the accumulated gas desorption characteristics of water-bearing coal, and will be of theoretical and practical engineering significance for the prevention and control of gas disasters in coal seams using hydraulic measures.

Deficit irrigation limits almond trees’ photosynthetic productivity and compromises yields

Almond yields vary between rainfed and intensively irrigated systems, but how to match irrigation to potential productivity is unclear. Hence, we compared almond physiology under deficit (600 mm) and full (1300 mm) irrigation to identify stress indices and determine the production overheads of mismanaged watering. We hypothesized that trees alter their growth to conserve resources during drought and mitigate their hydraulic stress responses. Thus, we monitored stem water potential and stomatal conductance to characterize the hydraulic responses of trees to deficient and hydrated water conditions. Adapting the seasonal relationship between irrigation coefficients and tree water potential was also tested. Finally, soil water status and trunk development were considered physical stress indices for field conditions. Soil water depletion in deficit irrigation reduced stem water potential below − 2 MPa and checked stomatal conductance at 0.15 mol m-2 s-1 for most of the growing season. An empirical productivity model determined that, under deficit irrigation, almond trees suffer from chronic stress that limits their photosynthetic capacity to ∼14 µmol m-2 s-1. Consequently, nominal assimilation limitations (10%) in early summer manifested to 4 kg C tree-1 metabolic losses by autumn. The inter-annual vegetative limitations in deficit irrigation resulted in significant yield reductions (35%) by the second experimental season. Temporal changes in the correlations between stem water potential, stomatal conductance, and trunk contractions made it difficult to use water stress indices to make irrigation decisions. However, normalizing tree performance by phenology indicated a 960 mm irrigation that supported high yields. Further, integrating the variability in soil water with trunk dendrometry illustrated that trees could maintain constant growth between irrigation days under well-watered conditions. Hence, in commercial operations, variable growth rates and trunk contraction measures signal insufficient irrigation and could guide practical irrigation adaptations.

An XFEM-based CZM Numerical Strategy for Modeling Hydraulic Fracture Propagation under Different Fracturing Schemes

Segmented and cluster fracturing can improve the efficiency of volume stimulation in shale reservoirs and reduce construction costs. It is important to clarify the propagation characteristics of hydraulic fractures and stress interference under different fracturing techniques to optimize the process of clustered and staged fracturing. For this purpose, we have developed a 2D XFEM-based CZM hydraulic fracturing model. The capability of this model was validated by analytical solutions and then used to study the propagation paths of hydraulic fractures and the characteristics of stress interference under simultaneous fracturing, sequential fracturing, zipper fracturing and multi-cluster fracturing. The results show that in simultaneous fracturing, the middle cluster is compressed by the external position clusters, and the opening width of hydraulic fractures is reduced. In sequential fracturing, the fracture that first initiates create an additional stress field that inhibits the propagation distance of subsequent fractures and the propagation path of hydraulic fractures is also affected in the stress shadow region. Zipper fracturing can effectively alleviate stress interference between multiple fractures, and internal fractures can also propagate a certain distance. In multi-cluster fracturing, the fluid rate into the internal fractures may be limited, and fracture propagation is also limited by stress interference. Therefore, it is necessary to optimize the parameters of clusters to ensure that all clusters can initiate fractures normally. The research results are important for the parameter optimization of clustered and staged fracturing, especially for well factory fracturing mode.

Weakening amplification of grassland greening to transpiration fraction of evapotranspiration over the Tibetan Plateau during 2001-2020

Evapotranspiration plays a key role in regulating the water cycle in terrestrial ecosystems. Transpiration fraction to evapotranspiration (TF) characterizes the role of vegetation function in evapotranspiration and is a pivot feature for the interactions between soil, ecosystem, and the atmosphere. Generally, the amount of vegetation has the most direct impact to TF, but the sensitivity of TF to vegetation change is not known, which could be a metric to evaluate the role of greening in land-atmosphere water transportation. To address this key knowledge gap, we used remote sensing observations of leaf area index (LAI) as a proxy for vegetation amount and derived the sensitivity of TF to LAI (θ =∂TF∂LAI) for the grasslands over the Tibetan Plateau (TP). Results showed there is a substantial decline of θ over the TP during 2001–2020, decreasing from over 4% m−2 m2 to below 1.5% m−2 m2. To analyze the drivers of θ, we introduced a diagnosis framework for soil-atmosphere hydraulic stress shifts based on trends in soil moisture and atmospheric vapor pressure deficit. Results implicated the θ decline over the TP is likely not induced by the dry-wet balance shifts by soil and atmosphere, nor by the vegetation's stomal conductance. Instead, we speculate that the θ decline is induced by a marginal damping effect in case the TF rise is approaching to grassland's limit over the TP. Our finding appeals future studies on terrestrial evapotranspiration to consider the role of vegetation in controlling transpiration and emphasize that greening may not lead to higher TF.

Observed forest trait velocities have not kept pace with hydraulic stress from climate change.

The extent to which future climate change will increase forest stress and the amount to which species and forest ecosystems can acclimate or adapt to increased stress is a major unknown. We used high-resolution maps of hydraulic traits representing the diversity in tree drought tolerance across the United States, a hydraulically enabled tree model, and forest inventory observations of demographic shifts to quantify the ability for within-species acclimation and between-species range shifts to mediate climate stress. We found that forests are likely to experience increases in both acute and chronic hydraulic stress with climate change. Based on current species distributions, regional hydraulic trait diversity was sufficient to buffer against increased stress in 88% of forested areas. However, observed trait velocities in 81% of forested areas are not keeping up with the rate required to ameliorate projected future stress without leaf area acclimation.

The design and evaluation of the outflow structures of an interventional microaxial blood pump.

Blood pump design efforts are focused on enhancing hydraulic effectiveness and minimizing shear stress. Unlike conventional blood pumps, interventional microaxial blood pumps have a unique outflow structure due to minimally invasive technology. The outflow structure, composed of the diffuser and cage bridges, is crucial in minimizing the pump size to provide adequate hemodynamic support. This study proposed four outflow structures of an interventional microaxial blood pump depending on whether the diffuser with or without blades and cage bridges were straight or curved. The outflow flow structure's effect on the blood pump's hydraulic performance and shear stress distribution was evaluated by computational fluid dynamics and hydraulic experiments. The results showed that all four outflow structures could achieve the pressure and flow requirements specified at the design point but with significant differences in shear stress distribution. Among them, the outflow structure with curved bridges would make the blood dispersed more evenly when flowing out of the pump, which could effectively reduce the shear stress at the cage bridges. The outflow structure with blades would aggravate the secondary flow at the leading edge of the impeller, increasing the risk of flow stagnation. The combination of curved bridges and the bladeless diffuser had a relatively better shear stress distribution, with the proportion of fluid exposed to low scalar shear stress (<50Pa) and high scalar shear stress (>150Pa) in the blood pump being 97.92% and 0.26%, respectively. It could be concluded that the outflow structure with curved bridges and bladeless diffuser exhibited relatively better shear stress distribution and a lower hemolysis index of 0.00648%, which could support continued research on optimizing the microaxial blood pumps.

Effect of hydraulic fracturing induced stress field on weak surface activation during unconventional reservoir development

Unconventional reservoirs usually contain many weak surfaces such as faults, laminae and natural fractures, and effective activation and utilization of these weak surfaces in reservoirs can significantly improve the extraction effect. In hydraulic fracturing, when the artificial fracture approaches the natural fracture, the natural fracture would be influenced by both the original in-situ stress field and the hydraulic fracturing-induced stress field. In this paper, the hydraulic fracturing-induced stress field is calculated based on the relative position of hydraulic fracture and natural fracture, the original in-situ stress, the net pressure inside the hydraulic fracture and the pore pressure of the formation. Furthermore, the stability model of the natural fracture is established by combining the Mohr-Coulomb rupture criterion, and extensive parametric studies are conducted to explore the impact of each parameter on the stability of the natural fracture. The validity of the proposed model is verified by comparing with the reservoir characteristics and fracturing process of the X-well 150–155 formation in the Songliao Basin. It is found that the stress field induced by the hydraulic fracture inhibits the activation of the natural fracture after the artificial fracture crossed the natural fracture. Therefore, for similar reservoirs as X-well 150–155, it is suggested to connect natural fractures with hydraulic fractures first and then activate natural fractures which can effectively utilize the natural fractures and form a complex fracture network.

Characteristics and quantification of fine particle loss in internally unstable sandy gravels induced by seepage flow

For internally unstable sandy gravels, fine sand particles might pass through the constrictions of the coarse gravel particles and flow out of the soil with seepage flow. This phenomenon is referred to as internal erosion, which induces fine particle loss and changes particle size distribution, thereby significantly affecting the soil's hydraulic and mechanical properties. To investigate the characteristics of the fine particle loss process in sandy gravels induced by seepage flow, as well as the corresponding permeability change and skeleton deformation, systematic large-scale internal erosion tests were conducted on two kinds of internally unstable sandy gravels under two hydraulic loading paths (constant and multi-stage hydraulic gradients) and three surcharge pressures (0.5, 1.0 and 2.0 MPa). The initiation and progression of internal erosion under different hydraulic loading paths and stress conditions were summarized. An incremental erosion model, which can describe the fine particle loss process under different hydraulic loading paths and incorporate the effect of surcharge pressure, was established based on the volumetric exchange rate of the deposited fines to the fluidized fines. The model was validated not only with the test results of sandy gravels in this study but also with other test results under various hydraulic and stress conditions to demonstrate its versatility.