Study On The Law Of Gas-liquid Two-phase Flow In Horizontal Wells With Gas Pocket

During horizontal well drilling process,the irregular shape of the horizontal well such as undulations and expansions is affected by the difficulty of controlling the trajectory of the hole,the collapse of the shaft lining and the water swelling in the formation.If the gas in the reservoir invades the wellbore,it will accumulate in the "pocket" of the flexural and variable diameter wells under the effect of density difference,which leads to the complex multiphase flow of the wellbore and the control of the wellbore pressure.The experiment system of multiphase flow simulation of unconventional horizontal wellbore is designed.The flow pattern transformation law of gas-liquid two phase flow and the law of gas migration in horizontal wellbore with "pocket" are obtained according to the experimental datum.Through the gas-liquid two phase flow pattern experiment,the flow pattern conversion rules of the unconventional horizontal tube is revealed.On the basis of the experimental data,the flow pattern map of each tube section is drawn,the special flow pattern converting model is established and the conversion boundary is judged.The empirical model of the conversion boundary of gas "pocket" to intermittent flow,impact flow and annular flow is verified by flow pattern map.The flow pattern in the undulating tube section is more complex,and the bubbly flow,batch flow,impact flow and annular flow are observed,Because of the gas "pocket" in the lower liquid flow,the stratified flow is not observed in the upper tube section.The gas bubble in the shape change of the undulating section is easily broken or merged,and it is easy to form a special flow pattern,namely the impact flow.There are bubbly flow,stratified flow,wavy flow,intermittent flow and annular flow in the variable diameter tube section.It is easier to produce gas aggregation and form "pocket" relative to the curved section.In the tube section after expansion,the small bubble distribution is more dispersed and slower,and it is easier to produce wave flow than the tube section before the expansion.The effects of liquid flow velocity,gas flow velocity,liquid viscosity,tube curvature and tube diameter on the gas movement in the "pocket" are investigated by gas migration experiments.The analysis shows that slug displacement and liquid shear are two important factors to control gas migration.According to the experimental results,the factors affecting the gas migration velocity are liquid velocity,liquid viscosity,tube curvature and variable diameter in order of sensitivity from big to small.The experimental and calculated results show that there is a critical liquid velocity in the gas discharge in the horizontal well "pocket".When the velocity of liquid phase is less than the critical liquid velocity,the gas will continue to gather at the top of the undulating section and the change diameters of the tube,and the gas will not be discharged.Based on this,the critical liquid velocity model for gas discharge "pocket" and the velocity model of gas migration are proposed and verified by the experiment results.The experiment results and the established models can provide references for the design of gas discharge operation parameters in the "pocket" horizontal wellbore and the determination of the wellbore pressure control technology.