Research On Slug Flow Mechanism And Stability Control Methods In The Curved Section Of Undulating Horizontal Well

In the process of oil and gas production,gas-liquid two-phase flow exists in many oil and gas wells.Due to the complex structure of different terrains,almost all horizontal wells have curved sections,and the special structure of the curved section will cause flow pattern changes and flow parameters change compared with straight pipes.In oil and gas wells,gas-liquid two-phase flow often occurs in various flow patterns,the most unstable one is slug flow,which has the most complex migration law.Its existence will affect the working efficiency of the pump,thus affecting the oil and gas production and reducing the service life of the tubes.Therefore,in order to improve the efficiency of pump and the production of oil and gas wells,it is of great significance to accurately predict the flow mechanism of slug flow in horizontal wells and explore the influence of different factors on the flow instability.In order to study the flow characteristics of the slug flow in the curve section more intuitively,a visualization experimental platform of gas-liquid two-phase flow in the curve section was designed and established,and the accuracy and repeatability of the experimental platform were verified by single-phase fluid experiments.Then,the experiments of gas-liquid two-phase flow were carried out under three kinds of curvature radius,five kinds of inflow angles,six kinds of liquid velocities and 26 kinds of gas velocities.The pressure data and liquid holdup data at different positions were measured,the transformation of flow patterns was observed and recorded by camera,and the transitional boundary charts of plug flow to slug flow and slug flow to churn flow in different conditions were given The slug frequency is obtained by analyzing liquid holdup curves,and the dimensionless slug lengthφ_D=Ls/D is defined,then the distribution ofφ_Dis obtained.It is found that with the increase of the curvature radius,the fluctuation of pressure and liquid holdup becomes smaller,the slug frequency decreases,the transitional boundary of slug flow and churn flow also needs larger gas velocity,and the distribution ofφ_D is gradually concentrated;with the increase of the inflow angle,the slug frequency decreases and the transitional boundary of slug flow and churn flow needs a larger gas velocity,and the dimensionless slug lengthφ_D decreases firstly and then increases;with the increase of the gas-liquid ratio,the slug frequency increases gradually,and the fluctuation of the pressure and liquid holdup increases obviously;under the condition of low gas-liquid ratio,the slope of the dimensionless slug lengthφ_D curve is larger,indicating that the increase rate ofφ_D is larger;while under the condition of high gas-liquid ratio,the slope ofφ_D curve begins to decrease.In this research,a two-dimensional dynamic model of gas-liquid two-phase flow in curved pipe is established.The model is based on N-S equation,which ignores the heat transfer between wellbore wall and fluid as well as the heat transfer between gas and liquid,including the mass and momentum conservations of gas phase and liquid phase,and supplements the closure relations.According to the structural characteristics of the curve section,the two-dimensional rectangular coordinate is converted into polar coordinate,and the most used finite difference numerical method is used to solve the equations,the conversion reduces the calculation time and complexity.Then,calculated the pressure profile of the whole pipe,the pressure and liquid holdup curve of the curve section,which are compared with the experimental results,and the pressure and liquid holdup amplitude error analysis,the fluctuation time error analysis of curve are carried out.The slug frequency f,the maximum and minimum pressures,the maximum and minimum liquid holdups are selected as the characteristic parameters in weight error analysis,it is found that the error between the measured value and the calculated value is within 20%,and the accuracy of model calculation is good.At last,the dynamic model is used to simulate the slug flow of large-scale horizontal wells.According to the calculation results of different cases,we analyzed the influence of different curvature radius,inflow angle,gas-liquid velocity ratio and completion methods on the slug flow instability in the actual horizontal wells,and found that the small-staged segregated completion and central pipe completion cannot reduce the flow instability,the large-staged segregated completion can reduce the flow instability of slug flow.At the same time,the effect of throttle valve on slug flow instability is studied when the wellhead and downhole throttle valves are used,and found that with the decrease the valve opening,the liquid holdup and pressure fluctuation in the wellbore decrease,and the slug flow instability weakens,but the back pressure in the wellbore increases which has disadvantage of reducing production.