Plugging Properties Research Articles

Host-Guest recognition strengthened supramolecular preformed particle gel for conformance control in low-permeability fractured reservoirs

The conventional preformed particle gels (PPG) have problems of irreversibly shear-breakage after shearing actions during the application for conformance control in low-permeability fractured reservoirs. Herein, a novel host–guest recognition strengthened supramolecular PPG (S-PPG) was synthesized with synergistically non-covalent and covalent crosslinking structures, using host (allyl β-CD) and guest (cetyldimethylallyl ammonium chloride) monomers. Firstly, the chemical structures and micro-morphologies of S-PPG were systematically characterized by FTIR, Cryo-SEM, EDS and AFM methods. Then, the dispersion and thermal stability, swelling properties and rheological properties, including viscoelasticity, creep-recovery and shear self-healing abilities, of S-PPG were investigated, respectively. Finally, the low-permeability cores with fractures were used to identify the injectivity and plugging properties of S-PPG and conventional PPG. The results showed that the S-PPG exhibited strong crosslinking network, high dispersion and thermal stability. In addition, the S-PPG also displayed better swelling, viscoelasticity, creep-recovery and shear self-healing ability after equilibrated swelling process compared to those of conventional PPG. Finally, the S-PPG exhibited considerable compatibility with low-permeability cores with fracture width of 0.05 mm, and produced higher plugging rate (93.53 %) than conventional PPG (86.35 %). Therefore, the newly formulated supramolecular preformed particle gel (S-PPG) containing host–guest recognition can provide a new approach for solving the problems of irreversibly shear-breakage and improving the conformance control performance of PPG in low-permeability fractured reservoirs.

The effect of well temperature on the microstructure and the mechanical performance of bismuth-based plugs in well plugging and abandonment operations

Bismuth-tin (BiSn) alloys are considered promising candidates as potential alternatives to cement for well plugging and abandonment (P&A) applications to prevent any uncontrolled flow of deep reservoir fluids to the surface. As the temperature in the well varies with depth, the final microstructure of the BiSn alloy after its solidification might be influenced, and consequently the mechanical performance of the alloy will also be affected. The aim of this study is to evaluate the microstructural evolution of the BiSn alloy after solidification, at temperatures between 20 °C and 90 °C, to reflect in situ conditions from approximately 800m to 3 km depth. Microstructural characterization was carried out by using scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). The microstructural analysis revealed that the plug subjected to the lower temperature resulted in finer microstructure, whereas the grain size of the material was increased at higher temperatures. Additionally, the theoretical lever rule was employed to validate the microstructural findings. The observed microstructures were also correlated with tensile tests, which showed that the plug at 20 oC had a yield strength of 67,25 MPa, whereas the yield strength of the plug at 90 oC was only 41,64 MPa. In addition, hydraulic push-out tests further confirmed that lower testing temperatures correlate with higher pressure tolerance of the plug. The plug at 20 oC could withstand 113,5 bar, compared to the plugs at 60 oC and 90 oC which could withstand 90,4 and 50,9 bar, respectively. From all the experimental tests conducted, the outcome was that the lower temperature of 20 oC resulted in a material with a finer microstructure, which plays an important role in enhancing the mechanical properties of the bismuth-based plug. This study aims to advance the understanding of BiSn alloy performance in plug and abandonment (P&A) operations, thereby supporting the industry's efforts to qualify BiSn alloys as effective well-barrier materials.

Characterization of biological and mechanical properties of 3D-bioprinted osteochondral plugs

Three-dimensional (3D) bioprinting offers significant potential for the repair of articular cartilage by engineering functional osteochondral tissue. However, progress has been hindered by a lack of printable bioinks that promote the development of bone and chondral tissue while also maintaining sufficient cytocompatibility and mechanical strength. Herein, we designed a biphasic osteochondral plug with distinct chondral and bone regions and developed suitable bioinks for each tissue using photorheology and compression testing. The chondral region consisted of human bone marrow-derived mesenchymal stem cells (hbMSCs) encapsulated in a chondral bioink composed of methacrylated hyaluronic acid and high molecular weight hyaluronic acid. The bone region was 3D bioprinted from an hbMSC-laden methacrylated gelatin (GelMA) bioink and a biodegradable thermoplastic and ceramic lattice that provided mechanical strength. The viability and functionality of hbMSC encapsulated in the bioinks were confirmed through live/dead assays, histology, biochemical assays, and fluorescence microscopy. Over 56 days of culture in a chondrogenic medium, hbMSCs encapsulated in chondral bioink deposited cartilage-like extracellular matrix components, such as type II collagen and glycosaminoglycans. Similarly, cells encapsulated in the bone bioink and cultured in osteogenic medium deposited hydroxyapatite, a key component of bone. These findings provide promising initial results for using 3D-bioprinted plugs to repair osteochondral defects in articular cartilage.  

Investigation of petrophysical properties of synthetic carbonate plugs: Adding a novel 3D printing approach to control pore networks

The physical and chemical properties of rocks play a crucial role in understanding fluid-solid flow behavior at the pore level. Thus, studying pore space characteristics is important in evaluating and determining petrophysical properties of various rock types, including synthetic rocks, which can be designed to mimic natural rocks. This study investigates the petrophysical properties of synthetic carbonate plugs by using a new approach that correlates the base material and a 3D printing solution with porosity and permeability. The research shows that with precise particle size and morphology, pure mineral materials such as calcite, dolomite, quartz and a non-structural Portland cement may produce a controlled rock matrix. The synthetic plugs developed in this study exhibit controlled macro, meso, and micro porosities, including fractures and vuggys, by the solubilization of materials with controlled morphology by 3D printing, which provides valuable information on regulating pore space in synthetic carbonate rocks.

Migration characteristics and plugging rules of self-aggregating nanoparticles in the porous media

Nanoparticle profile control agents have been widely used for water shut-off treatment due to their excellent properties. However, the contradiction between the injectability and the plugging characteristics of profile control agents seriously affects their sealing effect. Self-aggregating nanoparticles can effectively alleviate this contradiction by forming particle clusters in the formation to block water channels. In this study, the injection and plugging properties of self-aggregating nanoparticle profile control agents in porous media were investigated using visual porous media etching model and core model. Injectivity experiments showed that the flow of nanoparticles in porous media was divided into three phases: flow dispersion phase, self-aggregation migration phase, and aggregation plugging state. They could aggregate to form clusters similar to the pore throat size after 60 min. Core displacement experiments showed that the plugging performance of nanoparticles was influenced by solution concentration, injection volume, injection rate, and core permeability. When injecting a 3000 mg/L nanoparticle solution into the core at 60 °C, the residual resistance coefficient was larger than 1.83. The retention rate in pore throats was superior to 60%.

Development and Characterization of Thermoresponsive Smart Self-Adaptive Chitosan-Based Polymer for Wellbore Plugging.

To meet the escalating demand for oil and gas exploration in microporous reservoirs, it has become increasingly crucial to develop high-performance plugging materials. Through free radical grafting polymerization technology, a carboxymethyl chitosan grafted poly (oligoethylene glycol) methyl ether methyl methacrylate acrylic acid copolymer (CCMMA) was successfully synthesized. The resulting CCMMA exhibited thermoresponsive self-assembling behavior. When the temperature was above its lower critical solution temperature (LCST), the nanomicelles began to aggregate, forming mesoporous aggregated structures. Additionally, the electrostatic repulsion of AA chains increased the value of LCST. By precisely adjusting the content of AA, the LCST of CCMMA could be raised from 84.7 to 122.9 °C. The rheology and filtration experiments revealed that when the temperature surpassed the switching point, CCMMA exhibited a noteworthy plugging effect on low-permeability cores. Furthermore, it could be partially released as the temperature decreased, exhibiting temperature-switchable and self-adaptive plugging properties. Meanwhile, CCMMA aggregates retained their reversibility, along with thermal thickening behavior in the pores. However, more detailed experiments and analysis are needed to validate these claims, such as a comprehensive study of the CCMMA copolymer's physical properties, its interaction with the reservoir environment, and its performance under various conditions. Additionally, further studies are required to optimize its synthesis process and improve its efficiency as a plugging material for oil and gas recovery in microporous reservoirs.

Evaluation of Supramolecular Gel Properties and Its Application in Drilling Fluid Plugging

Supramolecular gels are physically cross-linked hydrogels formed by non-covalent interactions. The synthesis, structure optimization, property regulation, and application expansion of supramolecular gels has gradually become the research hotspot in the field of gel materials. According to the non-covalent interactions such as hydrophobic association and hydrogen bonding, the supramolecular gel prepared in this study has excellent rheological properties and adaptive filling and plugging properties, and can be used in the field of drilling fluid plugging. In this paper, the microstructure, rheological properties, temperature resistance, and plugging properties of supramolecular gels were studied and characterized in detail. The experimental findings demonstrated that when the strain was less than 10%, the supramolecular gel displayed an excellent linear viscoelastic region. The increase in strain weakens the rheological properties of supramolecular gel and reduces the elastic modulus of supramolecular gel to a certain extent. The supramolecular gel still had a neat three-dimensional reticular structure after curing at high temperatures, and the network of each layer was closely connected. Its extensibility and tensile properties were good, and it had excellent temperature resistance and mechanical strength. The supramolecular gel had excellent tensile and compressive properties and good deformation recovery properties. When the elongation of the supramolecular gel reached 300%, the tensile stress was 2.33 MPa. When the compression ratio of supramolecular gel was 91.2%, the compressive stress could reach 4.78 MPa. The supramolecular gel could show an excellent plugging effect on complex loss layers with different fracture pore sizes, the plugging success rate could reach more than 90%, and the plugging layer could withstand 6.3 MPa external pressure. The smart plugging fluid prepared with supramolecular gel material could quickly form a fine barrier layer on the rock surface of the reservoir. It could effectively isolate drilling fluid from entering the reservoir and reduce the adverse effects, such as permeability reduction caused by drilling fluid entering the reservoir, so as to achieve the purpose of reservoir protection.

Digital workflow of labial guides for alveolar ridge reduction during implant-supported full-arch rehabilitation.

This article presents a technique for fabricating two-piece labial guides using resin and cobalt-chromium for guided alveoloplasty, implant placement, and immediate full-arch implant-supported fixed complete denture placement. This technique reduces tissue damage and overcomes the negative effect of the anatomical position of the palatal neuropore and mobility of the tissue flap on the positioning and stability of the guide. The use of labial retention metal guides with improved mechanical properties of cobalt-chromium and fixation plugs allows the unilateral placement of the guide. Thus, minimally invasive implant surgery can be performed owing to the absence of large palatal flap elevation and obstruction. Such a design provides better stability of the guides and clear visual access during surgery.

Influence of base material particle features on petrophysical properties of synthetic carbonate plugs

Rocks with representative physical and chemical properties are essential to understanding fluid-solid flow behaviors at the pore scale. In this way, studying the pore space characteristics is a key point for evaluating and providing petrophysical properties for distinct rock types, such as synthetic rocks, with controlled and representative properties like natural ones. This work studies the petrophysical properties of synthetic carbonate plugs with a novel approach by correlating particle size, particle size fraction, and the morphology of particles with porosity and permeability, which could guide the scientific community to further forming of carbonate rocks with a controlled pore network. Results indicated that particle shape influenced the accommodation of particles in the porous space and, therefore, in the petrophysical properties, where an increase in particle size decreases porosity and increases permeability. Also, the obtained plugs showed the following petrophysical features: gas porosity from 10% to 17%, mercury porosity from 11% to 19%, gas permeability from 0.07 mD to 0.70 mD, and mercury permeability from 0.02 mD to 0.35 mD, providing important insight on controlling pore space in synthetic carbonate rocks.

Performance of Self-healing microgel incorporating Nano-Silica as plugging material for drilling fluid

Lost circulation severely threatens the safety and efficiency of oil drilling operations, especially at high temperatures. Accordingly, many plugging materials have also been studied and applied, such as inert bridge materials, inorganic cement, and organic gel. However, these materials have certain limitations. For example, inert materials are difficult to form dense plugging slugs and have weak cementing strength with the formation; cement slurry is easily diluted and dispersed in the formation, and the waiting process required for cement greatly increases the non-productive time. Comparatively, polymer gel is widely used in lost circulation control because of its good deformability and water absorption capacity. However, conventional polymer gel usually has low mechanical strength and weak retention capacity in the leakage layer. In addition, there is a lack of sufficient interaction between plugging materials and the formation, which makes it difficult to form a highly dense plugging layer. Under high pressure conditions, a large amount of leakage still occurs, and even repeated leakage occurs under frequent pressure fluctuations, resulting in plugging failure. Herein, we synthesized zwitterionic polymer/nano-silica microgels (P(MPTC-co-NaSS)@SiO2) with dynamic crosslinking ability through a facile one-step reversed phase emulsion polymerization. They exhibited spherical particles of narrow size distribution around 40 μm in aqueous dispersion. The strong hydrogen bonds, electrostatic interaction and cation-π endowed microgels excellent self-healing ability, strong mechanical strength and sufficient high-temperature stability. Although high temperature and salinity decreased the particle size and rheological property as well as the particle quantity, P(MPTC-co-NaSS)@SiO2 microgels maintained fairly constant rheological properties and achieved significant plugging performance in the temperature range of 70–––150 °C and simultaneously with a salinity tolerance up to 7.5 wt% NaCl contamination. P(MPTC-co-NaSS)@SiO2 microgels were well dispersed in water-based drilling fluid (WBDF) and generated a bulk gel when concentrated in pores and cracks, which was applicable to plugging drilling fluids. A conventional WBDF with remarkable plugging property was formulated by facilely supplementing with P(MPTC-co-NaSS)@SiO2, which has a great prospect for applications in oil field. This study builds a bridge between fundamental research on microgels and the applications of soft particles as plugging materials in oil industry.

Characteristics and Model of Heterogeneous Wettability in a Marine-Continental Transitional Shale Reservoir: Insights from Organic Matter, Minerals, and Microstructure

Heterogeneous wettability is characterized by a spotted-wet or mixed-wet state. It has a great influence on reservoir evaluation and the efficient development of shale gas reservoirs. In this study, reservoir properties of several core plugs from the Permian Shanxi–Taiyuan formations were studied. These formations are typical marine-continental transitional shale located in the Southern North China Basin. To investigate the effects of reservoir properties on heterogeneous wettability, we measured the water–air contact angle and compared it with other properties such as organic petrology, organic geochemistry, mineralogy, and microstructure features, including pore-fractures and surface roughness. The results reveal a negative correlation of vitrinite content with contact angle, in addition to the high clay content and residual polar groups within type III kerogen, indicating that the Shanxi–Taiyuan shales with a high maturity level still have a higher affinity to water. The contact angle of the core samples decreases with increasing surface roughness, partially due to the influence of pore-fracture development. The crossplots indicate that the majority of pore-fractures that exist in the shale preferentially tend to be water-wet. Therefore, the heterogeneous wettability of shale is dominated by the random mixture and arrangement of hydrophilic and hydrophobic components as well as the complexity of the microstructure, such as the rough pore wall. Furthermore, on the basis of improving and examining the Cassie model, a triangle method and a detailed workflow for evaluating the heterogeneous wettability of shale are proposed by comprehensively analyzing “three factors”, including pore structure, mineral components, and organic matter. The test results demonstrate that the Shanxi–Taiyuan shales are mainly water-wet (controlled by organic factor) and neutral-wet, which evidently differ from the marine Longmaxi shale with a wide distribution covering the zones of strongly water-wet, weakly water-wet (controlled by pore factor), and weakly water-wet (neutral-wet). The proposed approach can be applied to promptly and comprehensively evaluate and predict the heterogeneous wettability of shales during shale oil and gas exploration.

Improved oil recovery in sandstone reservoirs with deep profile control technology: A comparative study between modified starch gel and polymer gel

Deep profile control technology was implemented to alleviate the issue of oil recovery in sandstone reservoirs caused by varying permeability. Rheological tests, injection and plugging capacity experiments, core flooding experiments combined with Computed Tomography (CT) analysis were performed to study the mechanisms of profile control, oil displacement, rheology, and plugging properties of modified starch gel and polymer gel. Moreover, a comparison was made regarding the oil recovery, sweep efficiency, and displacement efficiency for both the starch gel and polymer gel. The results show that the ease of injection and plugging properties of the modified starch gel are superior to those of the viscoelastic polymer gel. The visual and quantitative distribution profiles and migration characteristics of water, oil, and gels are characterized. During the gel flooding stage, the modified starch gel maintained a rigid shape compared to the lax behavior observed with polymer gel. During the subsequent water flooding stage, the modified starch gel became immobile. It plugged the high permeability layer of the core plug, increasing the sweep efficiency of the low permeability layer by 60.00%. The sweep efficiency of the low permeability layer increased by 37.26% as the polymer gel continued to flow through the core plug. In addition, the high permeability layer is easily swept clean, so enhancing the displacement efficiency is the key to maximizing recovery. The displacement and relatively easy sweep efficiencies must be improved simultaneously to maximize the recovery in the low permeability layer. The study's findings shed light on the significance of modified starch gel in enhancing oil recovery in sandstone reservoirs with varying permeability.

The spark plug’s thermal state simulation

BACKGROUND: Spark plugs are among the most important components of the ignition system of an internal combustion engine (ICE). The spark-over produced by the ignition system should have sufficient energy for combustion initiation at any engine operation mode in all the service conditions. Start properties, reliability, power, fuel efficiency and exhaust toxicity of an ICE essentially depend on excellence of design and manufacturing quality of a spark plug. On the other hand, functional properties of spark plugs depend on concordance to ICE with main dimensions, design, heat properties and value of a spark gap. AIMS: Use of the methods of the spark plugs temperature field simulation in order to reduce the amount of experimental studies and to reduce the number of specimen options subjected to laboratory and on-road engine tests.
 METHODS: The simulation model of the temperature field of a spark plug of an internal combustion engine (ICE) was developed in this study.
 RESULTS: The representation of temperature distribution in the insulators thermal cone and other ceramic elements of a spark plug was obtained. The dependence of thermal conductivity and specific thermal capacity of corundum ceramics on temperature was derived. The dependencies of thermal conductivity coefficient of the plugs ceramic insulator are presented. The analysis of thermophysical properties of the substances made of various materials was carried out. The equations of the temperature field of all parts of a spark plug (a contact head, glass sealant, an insulator, a central electrode, a spark plugs body, a heat-sinking washer, a sealing ring, a cylinder head) were developed. The schematic representation of the spark plug geometry, used in the temperature field calculation, was obtained. The requirements for boundary conditions for calculation of the temperature field of a spark plug were determined. The calculation scheme of the spark plug is presented. The condition of heat exchange at the boundary between the selected part of a cylinder head and the cooling system of an internal combustion engine was considered. The study of heat transfer between the structural elements of a spark plug and the air inside the ICE compartment was carried out. The dependencies describing the heat exchange between the structural elements of a spark plug being in thermal contact with each other were determined and the internal boundary conditions were established.
 CONCLUSIONS: The presented methods and algorithms of spark plugs thermal state simulation helped to perform the calculation of dependencies of thermal conductivity and specific thermal capacity of the insulators ceramics as well as the thermal conductivity coefficient of the plugs ceramic elements on temperature.

Styrene-Lauryl Acrylate Rubber Nanogels as a Plugging Agent for Oil-Based Drilling Fluids with the Function of Improving Emulsion Stability.

With the exploration and development of unconventional oil and gas, the use frequency of oil-based drilling fluid (ODF) is increasing gradually. During the use of ODFs, wellbore instability caused by invasion of drilling fluid into formation is a major challenge. To improve the plugging property of ODFs, nano-sized poly(styrene-lauryl acrylate) (PSL) rubber nanogels were synthesized using styrene and lauryl acrylate through soap-free emulsion polymerization method and were characterized using FTIR, NMR, SEM, TEM, particle size analysis and TGA. The results show that, due to good dispersion stability and oil-absorbing expansion ability, the PSL rubber nanogels have a wide range of adaptations for nano-scale pores to deposit a layer of dense filter cake on the surface of filter paper with various pore diameters, reducing the filtration of mineral oil and W/O emulsion significantly. Due to the unique wettability, the PSL rubber nanogels can be adsorbed stably at the oil-water interface and form a dense granular film to prevent droplets coalescing, which improves the emulsification stability of W/O emulsion. Furthermore, the PSL rubber nanogels are soap-free and compatible with ODFs without foaming problems. The PSL rubber nanogels can increase the hole-cleaning performance of ODFs by raising viscosity and yield point. The PSL rubber nanogels outperformed hydrophobic modified nano silica and polystyrene nanospheres in plugging and filtration reduction. Therefore, the PSL rubber nanogels are expected to be used as a new plugging agent in oil-based drilling fluid. This research provide important insights for the use of organic nanogels in ODFs and the optimization of plugging conditions.

Synthesis and properties of PAM/PLA composite degradable particle temporary plugging agent

AbstractUsing acrylamide (AM) and polylactic acid (PLA) as raw materials, polyacrylamide (PAM)/PLA composite temporary plugging agent TZDJ (TZDJ is a synthetic self‐degrading stopper) was synthesized by inverse suspension polymerization. The chemical structure, chemical composition, microscopic morphology, and thermal stability of TZDJ were characterized by Fourier infrared spectral analysis (FT‐IR), carbon nuclear magnetic resonance spectroscopy (13C NMR), scanning electron microscope (SEM), and thermogravimetric analysis (TGA). The degradation and plugging properties of TZDJ were studied by laboratory experiments, and the self‐degradation mechanism of TZDJ was proposed. The results show that TZDJ has good temperature and salt resistance, and it is completely degraded by hydrolysis at 180°C for 120 h. The fracture core displacement experiment shows that TZDJ has good plugging and core permeability recovery performance, and its temporary plugging rate and core permeability recovery rate are all above 90%. The FT‐IR and SEM analysis showed that the degradation process of TZDJ was divided into three stages: degradation of shell PAM, core PLA, and degradation product small molecules.

Preparation and migration study of multi-walled carbon nanotube hybrid polymer gel particles: EOR implication

Multi-walled carbon nanotube hybrid polymer gel particles (MWCNTs-HPGP) were synthesized using the free radical polymerization approach to reduce water cut at first. The microstructure, particle size distribution, swelling characteristics, strength, and rheology of MWCNTs-HPGP were then studied. Subsequently, the migration and plugging properties of MWCNTs-HPGP were assessed at the core scale. The morphology of MWCNTs-HPGP was irregular with interpenetration networks, and MWCNTs-HPGP could expand by 48.2–94 times in brine with varying salinities (0 ∼ 20 × 104 mg/L NaCl), according to experimental results. After 60 d of age, MWCNTs-HPGP retained 95.2% of strength. Furthermore, the MWCNTs-HPGP dispersion solution’s shearing thinning feature and good viscoelasticity supplied it with favorable injectivity and deep migrating capacity. In addition, when the matching coefficient (β, the ratio of MWCNTs-HPGP median particle size to the average pore diameter) was 1.06 and the flooding rate was 0.5 mL/min, MWCNTs-HPGP could efficiently plug large pores with a plugging efficiency ranging from 82.7% to 98.5%. All the experimental results demonstrated that MWCNTs-HPGP could be a potential candidate for deep profile control and displacement in high temperature and high salinity reservoirs.

Injectivity and plugging characteristics of CO2-responsive gel particles for enhanced oil recovery in fractured ultra-low permeability reservoirs

Although CO2-responsive gel particles (CRGP) injection appears to be a viable strategy for delaying CO2 breakthrough in fractured ultra-low permeability reservoirs, few studies have been conducted to thoroughly evaluate the injectivity and plugging characteristics of CRGP. In the present work, the swelling kinetics of the CRGP at various temperatures and salinities were first characterized through the change in CRGP volume. The rheology properties of CRGP dispersion solution were then evaluated. And then, taking the resistance coefficient, residual resistance coefficient and plugging rate as indexes, the effect of matching coefficient, injection rate, and injection concentration on CRGP injectivity and plugging characteristics was investigated using simulated artificial fractures. Finally, the empirical correlations of injectivity and plugging characteristics parameters were established. According to experimental result, the equilibrium swelling ratio of CRGP was greater than 40-fold at varied salinity conditions. The shear-thinning capabilities of the CRGP dispersion solution, as well as its favorable storage modulus, endowed CRGP with favorable in-depth migration and plugging properties. In addition, in terms of fracture plugging by CRGP, the plugging ratio of CRGP in fractures was higher than 90% when the matching coefficient ranged from 1.0 to 2.0, the injection rate was less than 0.5 mL/min, and CRGP concentration was larger than 3000 mg/L. The acquired empirical relationships were expected to guide the selection of CRGP size, injection concentration, and injection rate for fracture plugging.

A review of decentralized and distributed control approaches for islanded microgrids: Novel designs, current trends, and emerging challenges

Distributed energy resources (DER) on the demand side have been fast growing, which could boost energy resilience by uninterruptedly supplying the commercial and residential sectors in the form of islanded microgrids when the utility electricity grid is out of service. Nevertheless, simply applying the centralized hierarchical control strategies, traditionally used for utility electricity grids, onto the islanded microgrids would encounter several critical issues. For instance, the control goals in secondary and tertiary control could be activated tardily, the single-point fault could cause critical system failure, and the properties of dynamic plug and play would be hard to achieve. To this end, decentralized and distributed control approaches have been explored to cope with the issues. Specifically, compared to the centralized hierarchical control, decentralized and distributed control strategies can (i) respond to disturbances more promptly, enhancing the performance of islanded microgrids with limited resources; (ii) guarantee system stability especially when a fault occurs and certain DERs are disconnected from the network; and (iii) facilitate deeper penetration of DERs in the microgrid, owning to the low computational complexity and sparse communication network. In this article, the common approaches for decentralized and distributed control are reviewed, and the current design trends and critical technical challenges are discussed to offer a comprehensive understanding of decentralized and distributed controlled microgrids.

Texture, composition and properties of plugs formed by carbon dioxide hydrate and wax

Gas hydrates and wax are the major flow assurance problems for the transportation of produced hydrocarbons through pipelines. However, in most research works both these two problems are studied separately. Although simultaneous precipitation or deposition of these compounds in pipelines can lead to different mitigation/prevention strategies, the investigations in which both these problems are considered simultaneously appeared only recently. There is no information in the literature on the texture/composition and features of decomposition process of mixed wax/hydrate plugs. At the same time, this information could be useful to understand how to treat the problem of formation of these plugs. In this work, three wax/gas hydrate plugs were collected at quasi-static conditions from a water-in-oil emulsion to study their texture, composition and the features of decomposition process. Powder X-ray diffraction and IR (infrared spectroscopy) analyses showed that the plugs consisted of wax and gas hydrate. Thermovolumetric and DSC (Differential Scanning Calorimetry) experiments showed that the main part of gas hydrate in the plugs at the ambient pressure started to decompose at about 268 K. This temperature was higher than the equilibrium temperature of carbon dioxide hydrate at this pressure, indicating that the gas hydrate in the plugs could be effectively preserved at temperatures below the ice melting point (273.2 K). It was found through observation of the hydrate decomposition process in the plugs under the microscope that the gas in the samples released in small bubbles, while the hydrate particles were not visible at this magnification, indicating that the hydrate was indeed highly dispersed in the samples. A residual wax was jelly-like after decomposition of hydrate in all the cases. Rheological experiments showed that the plugs residues after decomposition of the hydrates had higher yield points and viscosities than the initial waxy crude oil originally used for the experiments.

Source models of long-period seismic events at Galeras volcano, Colombia

SUMMARY Long-period (LP) seismic events have occurred repeatedly at Galeras volcano, Colombia, during the transition from effusive dome formation to explosive Vulcanian eruptions. Since 1989, two types of LP events have been observed there: one characterized by long-lasting, decaying harmonic oscillations (NLP events) and the other by non-harmonic oscillatory features (BLP events). NLP events are attributed to resonances of a dusty gas-filled crack in the magma plugging the eruptive conduit. Sixteen episodes of NLP events occurred at Galeras during 1992–2010, each characterized by systematic temporal variations in the frequencies and quality factors of NLP events. Our and previous estimates of crack model parameters during three of those NLP episodes indicate that the similar temporal variations in crack geometry and fluid properties can be explained by an increase in the ash content within the crack and a decrease in crack volume. We found that NLP events, associated with low SO2 fluxes, are anticorrelated with BLP events, which are accompanied by high SO2 emissions. From our observations and analytical results, we inferred that BLP events are generated by resonances of open cracks in the uppermost magma plug, corresponding to tuffisite veins, that efficiently transfer volcanic gases. After sufficient degassing and densification, the magma plug effectively seals the conduit. The growing overpressure in the deeper magma is then released through a shear fracture along the conduit margin. The intrusion of deeper, vesiculated magma into the shear fracture depressurizes and fragments the magma, producing a dusty gas and triggering the crack resonances that generate NLP events. Our results thus indicate that the evolution of the properties of the magma plug controls the occurrences of BLP and NLP events at Galeras. Although NLP events do not always precede explosive eruptions, they indicate that an important overpressure is building in the shallow conduit.