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With the development of gas well exploitation, the calculation of wellbore with single-phase state affected by single factor cannot meet the actual needs of engineering. We need to consider the simulation calculation of complex wellbore environment under the coupling of multiphase and multiple factors, so as to better serve the petroleum industry. In view of the problem that the commonly used temperature and pressure model can only be used for single-phase state under complex well conditions, and the error is large. Combined with the wellbore heat transfer mechanism and the calculation method of pipe flow pressure drop gradient, this study analyzes the shortcomings of Ramey model and Hassan & Kabir model through transient analysis. Based on the equations of mass conservation, momentum conservation and energy conservation, and considering the interaction between fluid physical parameters and temperature and pressure, the wellbore pressure coupling model of water-bearing gas well is established, and the Newton Raphael iterative method is used for MATLAB programming. On this basis, the relationship between tubing diameter, gas production, gas–water ratio, and wellbore temperature field and pressure field in high water-bearing gas wells is discussed. The results show that the wellbore temperature pressure coupling model of high water-bearing gas well considering the coupling of gas–liquid two-phase flow wellbore temperature pressure field has higher accuracy than Ramey model and Hassan & Kabir model, and the minimum coefficients of variation of each model are 0.022, 0.037 and 0.042, respectively. Therefore, the model in this study is highly consistent with the field measured data. Therefore, the findings of this study are helpful to better calculate the wellbore temperature and pressure parameters under complex well conditions.

Purpose: To maintain the well integrity, the strength and sealing ability of premium connection should be in the safe scope. ISO 13679 is widely used for evaluating the ability of tubing and casing connection all over the world. FE is adopted to simulate the ISO 13679 tests. Design/methodology/approach: Because of the disadvantage of experiment such as long period, high cost and high requirement on the facility, considering the convenience and universality of finite element method, as well as the contacting nonlinearity and material nonlinearity, three-dimensional finite element model of a certain type of premium connection is established with the consideration of helix angle. The loads exerted on the premium connection are the loads in series B test and thermal cycle test of ISO 13679. The distributions of Von Mises stress and contact pressure in various cases were studied. Findings: The results showed that the bending load has a great influence on the distribution of Von Mises stress and contact pressure for premium connection. The Von Mises stress and contact pressures on the sealing surface are smaller on the tension side and greater on the compression side. With increasing axial compression load, the contact pressures on the tension side are too small, which may lead to sealing failure. The influence of temperature on the performance of premium connection cannot be ignored when choosing or designing premium connections. Both the Von Mises stress and contact pressure decrease slightly during a period of thermal cycle. Although the performance of the premium connection is good in a period of thermal cycle, its performance in a long period should be evaluated. Finite element simulation can effectively simulate the ISO 13679 test procedure and obtain the stress and contact pressure distribution. It can be used as a reference for evaluating the performance of premium connections. Originality/value: Considering the convenience and universality of finite element method, as well as the contacting nonlinearity and material nonlinearity, three-dimensional finite element model of a certain type of premium connection is established with the consideration of helix angle.

Variations in the rates of hydraulic perforation among different jetting nozzles are common in gas well development, especially the multi-stage perforation of horizontal wells. A laboratory experiment was conducted to analyse the effects of flow rate, liquid viscosity and particle size on the perforation rate of a multi-nozzle structure under liquid-solid flow. The spatial distributions of particles and sample erosion rates were documented for multi-stage multi-angle nozzles with a variable difference in liquid properties and flow parameters using three particle sizes. Results showed that the difference in perforation rates among the nozzles was affected by the particle distribution inside the jetting tool and the energy of the particles ejected by each nozzle. Furthermore, the difference in perforation rates of nozzles with different jet angles was markedly affected by particle size, then by the flow rate and finally the liquid viscosity. [Received: April 15, 2018; Accepted: May 28, 2018]

Owing to the influence of fluctuating wellbore pressure, the fracture width in formations with induced fractures changes dynamically. However, the traditional design method for fracture plugging materials is based on a static fracture width, which leads to the easy destruction of the plugging layer established underground and a high risk of secondary loss. Until now, the treatment of lost circulation in formations with induced fractures still faces significant challenges. Therefore, a dynamic fracture plugging experimental device was developed in this study to investigate the bridging–plugging process of lost circulation materials (LCMs) within induced fractures, as well as the design criteria for LCMs for induced fractures. Experimental results show that through adjusting the mass ratio and total concentration of the three-level bridging material, the plugging position of the bridging material can be changed, and the fracture reopen pressure can be increased. When the mass ratio of the three-level bridging material satisfies primary bridge particle: secondary bridge particle: tertiary bridge particle = 1:1:2 and the total concentration is greater than 5%, the plugging material compounded with the three-level bridging material is prone to retention in the induced fractures, exhibiting short bridging time, low permeability of the formed plugging layer, and high fracture initiation pressure. It can effectively prevent the opening of fractures or the generation of new induced fractures, thereby reducing the risk of secondary loss. The research results can provide a scientific theoretical basis for the loss control of induced fractures.

The energy absorption density of the nanofluidic energy absorption system (NEAS) is much higher than that of traditional energy-absorbing materials. An NEAS based on different pore size combinations may exhibit cascade characteristics, which can achieve energy absorption of different grades. In this work, two NEAS models based on carbon nanotubes are constructed, which are DNEAS and SNEAS. In DNEAS, two tubes with both ends immersed in a water reservoir are used. In SNEAS, two tubes are connected end to end, with the end of the bigger tube immersed in the water reservoir. The effects of loading rate coupled to pore size on the infiltration processes of water molecules into two models were investigated. The fitting correlations between the critical pore size difference and temperature were established. It has been observed that an increase in loading rate will transform a single-stage system into a multistage system that displays cascade characteristics. The critical pore size difference for the system to display cascade characteristics decreases with an increase in the loading rate. In DNEAS, the infiltration processes of the large and small tubes are independent of each other, and the effect of the loading rate on energy absorption characteristics is the superposition of that of the corresponding single tube NEAS. In SNEAS, the water molecules fill up the bottom tube before infiltrating the top tube. The influence of the effective viscosity change caused by loading rate on infiltration pressure is greater than that of hydrogen bond loss during the entry of water molecules into the bottom tube. The critical infiltration pressure as well as the total energy absorption of the top tube in SNEAS were higher than that of the corresponding single tube NEAS. The research findings have expanded the fundamental database of cascade nanofluidic systems and offer valuable insights for the application design of such systems.

Oil country tubular goods (OCTG) may develop plastic bending, fracture and fatigue failure during the service due to dynamic loads, such as perforation impacts and fluid-induced vibration. This paper was to evaluate strength safety of OCTG under dynamic loading. To gain key parameters of the dynamic constitutive model of materials represented by Johnson-cook. An axial tensile test of the common P110S and an experiment on its mechanical properties under high strain rate were carried out for the first time. The yield limit of P110S under static loads was 775MPa, which was 2.2% higher than the nominal value (758MPa). The ultimate strength was 835MPa and the yield ratio was 1.08. Since it often requires the yield ratio higher than 1.2 5, the P110S had slightly poor tenacity. Under the strain rates of 500s-1 and 1000s-1, the yield limits of P110S were 15.5% and 41.4% higher than the static measured values. The dynamic loading experiments of P110S under the strain rates of 500s[?]1 and 1000s[?]1 were corresponding to the fluid-induced vibration of columns and perforation i mpacts-induced vibration of columns, respectively. Dynamic factor of P110S was determined according to engineering habits. According to conversion and contrast analysis on increment of yield limit under dynamic loading, the final strengths were decreased by 4.5% and 8.6%, respe ctively. Moreover, key parameters of the Johnson-cook dynamic constitutive model of P110S material were determined through experiments for the first time. They could provide key para meters of constitutive model for accurate finite element simulation analysis under dynamic loa ds.