28 September 2014, Volume 32 Issue 27

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  • SONG Bo, TIAN Yuansi, WANG Xiaopo, LIU Zhigang
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    The CO2/HCs fluids are widely discussed in the engineering and scientific studies. The viscosity, as one of the most important transport properties, plays a key role in the applications of CO2/HCs mixtures in different fields. The theoretical approaches are effective to supplement the experimental viscosity data in wide thermodynamic ranges. In this paper, viscosity models are built for CO2/HCs binary mixtures based on the Vesovic-Wakeham theory. The viscosity correlations and the potential parameters of pure species are selected from literature and utilized in the constructions of models. The viscosities of five industrially important CO2/HCs binary systems are predicted in the temperature range from 273.15 K to 973.15 K and at the pressure up to 200 MPa. The studied systems are CO2/CH4,CO2/C2H6,CO2/C3H8,CO2/n-C4H10 and CO2/iso-C4H10. The calculated values are compared with a large amount of experimental viscosity data over a wide temperature-pressure range. The extensive analysis shows that the calculated viscosity values of the present work could be used with confidence in different industrial applications.
  • ZHU Weiyao, TIAN Ying'ai, YU Mingxu, HAN Hongyan, ZHANG Xueling
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    Low permeability reservoir usually has small pore throat.Based on this characteristic, the mechanism of microscopic fluid flowin microtubes was studied using one-dimensional microtubeswith inner diameters of 5, 10, 15, 20 μm with deionized water and kerosene as the flow media.The relationshipsamong flow rate, boundary layer thickness and pressure gradient are revealed.Microtubule surface wettabilitywas changed from hydrophilic to hydrophobic with mixtures of dimethyldichlorosilane, kerosene, and siliconearein different proportions to form four different simulated oil viscosities to study the law governing the fluid flow. The results show that the fluid flow rate hada linearrelationship with pressure gradient, but nonlinearity of fluid flow gradually increased with decrease of the microtubule diameter. Also, the higher the driving pressure, the smaller the effective boundary layer thickness, and the ratio of effective fluid boundary layer thickness to microtubulediameter decreased with pressure gradient. The fluid velocity was higher than that before modification of the microtubule wall from hydrophilic to hydrophobic,and the larger the size of microtubules, the more significant the effect. The pressure gradientincreased significantly from 1.26 MPa/m to 6.83 MPa/m when the viscosity of the flow media was changedfrom 2.40 mPa·s to 10.20 mPa·s.
  • HUANG Si, YANG Fuxiang, SU Xianghui
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    In a numerical simulation of the solid-liquid two-phase flow in pumps, the conventional approach is to treat solid particles as a kind of quasi-fluid, using the CFD in the multiphase flow case, but in this way, the characteristics of the solid particles, such as the shape and size, the collision, the agglomeration and the separation, can not be reasonably represented. In this paper, the unsteady solid-liquid two-phase flow in an IS type centrifugal pump is simulated numerically, using the discrete element method (DEM) combined with the CFD method. With the coupling of the codes of the EDEM and the Fluent, the motions of the solid particles and their effects on the pump performance are investigated. In the simulation, the solid particles are 1500 kg/m3 in density, 15% in the volume rate at the pump inlet, 1.0-3.0 mm in sizes in a random distribution. Some valuable conclusions are reached, including the time-dependent head, the particle trajectories and the solid volume rate distributions in the centrifugal pump.
  • CHEN Xue
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    The FRC fractal antenna is analyzed using the MOM (method of moment), and verified with Designer, and a good agreement is obtained between their results. The FRC fractal is smaller than the theoretical size of the original rectangular patch with the similar VSWR (voltage standing wave ratio) and pattern. A 32-element microstrip array antenna is designed, with high gain, low sidelobe and fan beam with Taylor distribution, pointing at 47° from the normal. The experimental results of the fabricated antenna reach a bandwidth of 5°, the SLL of-20 dB and the cross polarization level of-20 dB, which verifies the result of the MOM.
  • LIU Qiang, CHEN Xihong, HU Denghua, XUE Lunsheng, HAN Beibei, ZHANG Qun
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    Accurate satellite clock error prediction is of vital importance for satellite stable operation when satellites' clocks are not able to compare with those on the ground. Aiming at the problem of intermediate and short-term clock error prediction, a polynomial model is chosen to predict the clock error, and an orthogonal iterative functional networks algorithm based on a sliding window model is designed, which takes advantage of the non-linear learning ability of functional networks to fit and analyze the clock error model. The analysis shows that when the prediction time is less than 12 h, the predicted errors are between 0.2 ns and 0.5 ns, which is equivalent to IGU P. When the prediction time is 24 h, the overall errors are around 1ns, which is slightly less than IGU P. When the prediction time is a satellite week, the maximum error may reach 130 ns, which does not meet the reqirement of the satellites. It is concluded that the algorithm of the paper is suited for the short-term clock error prediction but not the intermediate and longtime prediction.
  • YIN Qixiang, ZHOU Guoqing, WANG Tao, XIA Lijiang, LIU Yuyi
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    A 3-D numerical simulation model of the permafrost roadbed-bridge transition section thermal field is built and the finite element method is adopted to predict and compare the thermal field for different transition section heights and different types of permafrost by raising temperature by 2.6℃ in the coming 50 years. The calculated results show that with the time, the maximum thawing depth profile parts move gradually from the transition section to the place behind the embankment,and the positions of the largest cross-sectional thawing depth and the maximum thawing rate move from the northern slope foot between the slope shoulders to the subgrade center. The cross sectional northern slope foot thawing rate becomes greater than the natural thawing permafrost foundation rate. For the rest parts of the transect before the operational 25 years, the artificial permafrost table degradation rate is lower than the natural permafrost table, and 25 years later, the artificial permafrost table degradation rate is higher than that of the natural permafrost table gradually.
  • ZHAO Guoyan, DENG Qinglin
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    A new method for non-stationary signal analysis, local characteristic-scale decomposition (LCD), was applied to analysis of micro-seismic signals to overcome the difficulties in differentiation of micro-seismic signals in mines and improve differentiation efficiency. With LCD, a complicated signal self-adaptively decomposes into a number of intrinsic scale components (ISC), whose instantaneous frequencies have physical significance. Compared with the conventional empirical mode decomposition method, LCD method has higher computational efficiency and better restriction of end effects. The results show that it was effective to use LCD to decompose typical micro-seismic signals of rock fracture and blast vibration signals in mines. Frequency spectrum analysis of each ISC shows that the two kinds of signals had significant difference in frequency distribution, and Hilbert transformation of each ISC shows that they also had significant difference in energy distribution. This method provides a new approach to rapid and accurate differentiation of micro-seismic signals.
  • XUE Pei, WANG Yanbin, WANG Jin, ZHANG Xin, FAN Jingjing
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    Based on the stress-strain constitutive equation of porous media, a permeability model of the undersaturated coal reservoir is presented considering the effective stress, matrix shrinkage and slippage effect. Using the model and parameters from the coal seam of Qinshui basin, the permeability variations of this block in the process of reservoir pressure changing from the initial value to the depletion pressure are predicted. The model parameter sensitivity is analyzed. The results indicate that at the initial stage of the undersaturated coal reservoir, the coal reservoir permeability declines until the reservoir pressure drops to the critical desorption pressure, after that the permeability rises. The simulated results show that the coal reservoir permeability in the Qinshui basin reduces to 0.186×10-3μm2 when the pressure drops to the critical desorption pressure (3.80 MPa), then the permeability begins to rise with the pressure decline and rebounds to the initial value at 2.77 MPa, and finally at the depletion pressure 0.92 MPa, the permeability reaches to 3.182 times the initial value. Sensitivity analysis points out that the larger the value of Langmuir volume strain, Young's modulus and slippage coefficient, the better the improvement of the final permeability, while the Poisson ratio is just the reverse. The Langmuir volume strain is the most important influence parameter of the final permeability.
  • WANG Yanchun, WANG Yongyan
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    Due to the complexity of the deep environment, a creep model is built and the experimental study is conducted of the shale under the thermal-mechanical-chemical joint actions, and the corresponding mechanisms are investigated. The experimental study reveals that both the temperature and the chemical pH value play a promoting role in the process of creep under this complex environment, and the higher the temperature or the chemical pH value, the larger the instantaneous strain, the creep strain and the creep rate will be, and the longer it will take to reach the steady-state creep stage; the impact of the chemistry pH value on the creep properties of the shale is more significant than that of the temperature. With the method of replacing the conventional linear component with a nonlinear one to describe the nonlinear rheological part, a nonlinear viscoelasto-plastic creep model of the shale under the deep complex environment is built, which agrees well with the actual deep situation and can better simulate the creep properties under the deep complex environment. The model parameters are obtained, and the correctness of the model is verified by comparing the experimental curves with the theoretical ones. This model can be used for the shale creep behavior prediction at various temperatures and pH values within the range discussed in this paper.
  • ZHAO Mingguo, CHA Xingchen, JIA Huimin, YANG Hongyu
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    The reservoir development in Daqing Sabei transitional zone is now in the high water cut period, and the production declines rapidly. Under such situations, the composite heat carrier huff and puff in-house experiments are carried out to achieve a better reservoir performance. Through the evaluation of the recovery rate, the water cut and the production gas-oil ratio, the feasibility of the composite heat carrier huff and puff in Sabei transitional zone is explored. It is shown that the composite heat carrier huff and puff enjoys a very good performance, which depends highly on the soak time and the huff and puff cycles. The over-time or too-shorttime well soak will both result in a poor performance. Under the same conditions, the water cut and the production gas-oil ratio decline with the increase of the soak time. When the soak time goes over 150 s, the water cut and the production gas-oil ratio will increase. With the increase of the huff and puff cycle, the oil recovery, the water cut and the gas-oil ratio increase. The optimal soak time for the Daqing Sabei transitional zone reservoir is 150 s, with the highest recovery and the lowest production gas-oil ratio and the huff and puff cycles should be three or four periods and three cycles for the best result.
  • YANG Peng
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    With the increasing maturity of oil and gas exploration in Jiangsu oilfield, mining reserves that are difficult to exploit increase year by year, which are characterized by low permeability or extremely low permeability, deep layer, thin layer with low abundance, low natural capacity, dramatic decrease of production capacity, and difficulties in effective exploitation by conventional mining methods. Considering the geological features of these reserves, this paper studies five evaluation parameters for these reserves including the mainstream throat radius, movable fluid saturation, quasi start-up pressure gradient, crude oil viscosity, and clay minerals content, on the basis of core analysis of 26 blocks in Gaoyou sag, Jinhu sag and Hai'an sag. The effective development potential of these reserves was evaluated with their characteristics and the technical and economic risks taken into consideration. The results show that more than 50% of blocks in the unexploited difficult reserves of Jiangsu oilfield remain difficult to exploit with no reservoir transformation under the oil price of 730 $/t. A higher oil price and transformation of the reservoir are necessary for effective development of these reserves.
  • Reviews
  • WAN Xiaofeng, KANG Liping, YU Yunjun, ZHANG Yanfei
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    The photovoltaic pump system is promising in the arid and non power grid areas, because it can operate without fuel and without being connected to a power grid. However, there are a lot of problems in the actual application of the photovoltaic pump system, such as the cost of the system is much too expensive, the maximum power point tracking (MPPT) involves various methods with poor universality, and the matching of the motor and the pump capacity is complex. These problems need to be addressed for the application of the photovoltaic pump system. This paper reviews the related domestic and overseas studies, including the MPPT technology, the selection schemes and their capacity matching of motor and water pump. The advantages and disadvantages of all kinds of MPPT technologies are summarized with respect to the scope of application, and the application scope of all kinds of drive motor and the capacity matching principle of pump as an adequate theoretical basis for the reasonable selection of a photovoltaic pump system under different application conditions. Finally we put forward five suggestions for the optimum design of the photovoltaic pump system, and point out the direction for further research and development of the photovoltaic pump system.