Study on mechanical properties and microscopic damage mechanism of tight sandstone reservoir under uniaxial compression

Due to the characteristics of low porosity, low permeability and serious anisotropy in tight reservoirs, it is difficult for conventional hydraulic fracturing theory to accurately guide the efficient exploitation of tight reservoirs. It has been shown that the reservoir rock mechanical properties are the key factor impacting the fracturing effect, but the current research on the damage properties of tight reservoir rocks is not comprehensive enough. Therefore, in order to improve the fracturing theory of tight reservoirs, this paper first explores the evolution mechanism of rock fractures through uniaxial compression experiments. Secondly, based on the particle discrete element method, the damage and failure process of tight sandstone under uniaxial compression is simulated from the microscopic scale. The test results show that the rock failure mainly includes tensile failure, shear failure, and tensile-shear failure; Internal micro-fractures will interconnect during rock destruction to form primary fractures through the rock mass, while secondary micro-fractures will also be generated. The numerical simulation results show that when the rock is subjected to tensile-shear failure, with the increase of load, tensile micro-fractures are mainly produced in the specimen, accompanied by a few shear fractures. Under the joint action of shear failure and tensile failure, V-shaped cracks are easily formed in rock. The tensile strength of rock is mainly affected by the microscopic tensile strength, and the cohesive force, modulus, stiffness ratio, friction coefficient and friction angle have significant effects on the compressive strength of rock. Therefore, a reasonable choice of microscopic parameters can realistically simulate the compression-tensile strength ratio of the rock. The research results of this paper can provide the theoretical basis of rock mechanics for the efficient exploitation of tight reservoirs

Investigating the simultaneous fracture propagation from multiple perforation clusters in horizontal wells using 3D block discrete element method

Multi-cluster horizontal well fracturing is one of the key technologies to develop the unconventional reservoirs such as shales. However, the field data shows that some perforation clusters have little production contribution. In this study, a three-dimensional (3D) numerical model for simulating the multiple fracture propagation based on 3D block discrete element method was established, and this model considers the stress interference, perforation friction and fluid-mechanical coupling effect. In order to determine the most appropriate measures to improve the uniformity of multiple fracture propagation, the effect of the geologic and engineering parameters on the multiple fracture propagation in shale reservoirs is investigated. The modeling results show that the geometry of each fracture within a stage is different, and the outer fractures generally receive more fracturing fluid than the interior fractures. The vertical stress almost has no effect on the geometries of multiple fractures. However, higher horizontal stress difference, larger cluster spacing, smaller perforation number, higher injection rate, and smaller fracturing fluid viscosity are conducive to promote the uniform propagation of multiple fractures. The existence of bedding planes will increase the fluid filtration, resulting in a reduction in fracture length. The middle two fractures receive less fluid and the width of them is smaller. Through analyzing the numerical results, a large amount of fracturing fluid should be injected and the proppant with smaller size is suggested to be used to effectively prop the bedding planes. Cluster spacing and perforation number should be controlled in an appropriate range according to reservoir properties. Increasing the injection rate and reducing the viscosity of fracturing fluid are important means to improve the geometry of each fracture

Capacitor voltage balancing control with reducing the average switching frequency in MMC

Modular multilevel converter (MMC) has a very good application prospect in the flexible high-voltage direct current (HVDC) transmission, but higher self-loss will affect the efficiency and restrict the development. Here, the MMC in voltage source converter-based HVDC as the research object, the control and modulation algorithms are designed on the basis of analysing its working principle. The capacitor voltage imbalance problem is existed in the converter because that the capacitor of each module is independent. The factors of switching actions in balancing control technology based on the sorting method of sub-module capacitor voltages are analysed in detail. Then, an optimised balancing control algorithm by setting two balancing control thresholds is proposed to reduce the average switching frequency. Finally, a two ends HVDC transmission system was built in MATLAB/SIMULINK to verify the optimisation. The simulation research results show that the optimised balancing control can ensure sub-module capacitor voltage is in balanced and does not increase the AC output voltage harmonic content, and obviously reduce the average switching frequency of switching devices, and with the increase of number of level, the more obvious optimisation effect

Characteristics of steady burning over inclined polymethyl methacrylate surface in different pressure environments

This paper aims to examine the characteristics of steady burning of inclined polymethyl methacrylate (PMMA) slabs under different ambient pressures. A sequence of experiments concerning steady burning intensity and combustion efficiency of the sample were conducted under both normal pressure (Hefei: altitude 30 m, 100 kPa) and reduced pressure (Lhasa: altitude 3650 m, 64 kPa). An in situ calorimeter based on oxygen consumption method was employed to measure the heat release rate of materials. A semi-theoretical model based on convective heat feedback from the flame was proposed and fitted well with laminar-dominated flame over the inclined PMMA surface. The critical maximum side length of sample for laminar-dominated flame in Hefei is lower than Lhasa. The trend of combustion efficiency with increasing inclination angle shows a great difference under different pressure environments

Effect of Sn Addition on Microstructure and Corrosion Behavior of As-Extruded Mg–5Zn–4Al Alloy

The effect of Sn addition on the microstructure and corrosion behavior of extruded Mg–5Zn–4Al–xSn (0, 0.5, 1, 2, and 3 wt %) alloys was investigated by optical microscopy (OM), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), electrochemical measurements, and immersion tests. Microstructural results showed that the average grain size decreased to some degree and the amount of precipitates increased with the increasing amount of Sn. The extruded Mg–5Zn–4Al–xSn alloy mainly consisted of α-Mg, Mg32(Al,Zn)49, and Mg2Sn phases as the content of Sn was above 1 wt %. Electrochemical measurements indicated that the extruded Mg–5Zn–4Al–1Sn (ZAT541) alloy presented the best corrosion performances, with corrosion potential (Ecorr) and corrosion current density (Icorr) values of −1.3309 V and 6.707 × 10−6 A·cm−2, respectively. Furthermore, the corrosion mechanism of Sn is discussed in detail

Human coronavirus HKU1 spike protein uses O-acetylated sialic acid as an attachment receptor determinant and employs hemagglutinin-esterase protein as a receptor-destroying enzyme

Human coronavirus (hCoV) HKU1 is one of six hCoVs identified to date and the only one with an unidentified cellular receptor. hCoV-HKU1 encodes a hemagglutinin-esterase (HE) protein that is unique to the group a betacoronaviruses (group 2a). The function of HKU1-HE remains largely undetermined. In this study, we examined binding of the S1 domain of hCoV-HKU1 spike to a panel of cells and found that the S1 could specifically bind on the cell surface of a human rhabdomyosarcoma cell line, RD. Pretreatment of RD cells with neuraminidase (NA) and trypsin greatly reduced the binding, suggesting that the binding was mediated by sialic acids on glycoproteins. However, unlike other group 2a CoVs, e.g., hCoV-OC43, for which 9-O-acetylated sialic acid (9-O-Ac-Sia) serves as a receptor determinant, HKU1-S1 bound with neither 9-O-Ac-Sia-containing glycoprotein(s) nor rat and mouse erythrocytes. Nonetheless, the HKU1-HE was similar to OC43-HE, also possessed sialate-O-acetylesterase activity, and acted as a receptor-destroying enzyme (RDE) capable of eliminating the binding of HKU1-S1 to RD cells, whereas the O-acetylesterase-inactive HKU1-HE mutant lost this capacity. Using primary human ciliated airway epithelial (HAE) cell cultures, the only in vitro replication model for hCoV-HKU1 infection, we confirmed that pretreatment of HAE cells with HE but not the enzymatically inactive mutant blocked hCoV-HKU1 infection. These results demonstrate that hCoV-HKU1 exploits O-Ac-Sia as a cellular attachment receptor determinant to initiate the infection of host cells and that its HE protein possesses the corresponding sialate-O-acetylesterase RDE activity. IMPORTANCE Human coronaviruses (hCoV) are important human respiratory pathogens. Among the six hCoVs identified to date, only hCoV-HKU1 has no defined cellular receptor. It is also unclear whether hemagglutinin-esterase (HE) protein plays a role in viral entry. In this study, we found that, similarly to other members of the group 2a CoVs, sialic acid moieties on glycoproteins are critical receptor determinants for the hCoV-HKU1 infection. Interestingly, the virus seems to employ a type of sialic acid different from those employed by other group 2a CoVs. In addition, we determined that the HKU1-HE protein is an O-acetylesterase and acts as a receptor-destroying enzyme (RDE) for hCoV-HKU1. This is the first study to demonstrate that hCoV-HKU1 uses certain types of O-acetylated sialic acid residues on glycoproteins to initiate the infection of host cells and that the HKU1-HE protein possesses sialate-O-acetylesterase RDE activity