Study On Mechanism Of Gas Carrying Liquid Of Annular Flow In Vertical Wellbore

In vertical annular flow,the gas phase forms a continuous gas core,and the liquid phase distributes in two forms,which are continuous liquid film flowing along the tube wall and dispersed droplets entrapped in the gas core.The interaction between the liquid film and the gas core leads to the continuous mass and momentum transfer between these two continuous fluids.The vertical annular flow widely exists in the exploitation and transportation of petroleum and natural gas.For example,the flow is under annular flow in the events the formation water is encountered in air and gas drilling,blowout due to severe gas invasion in drilling,well killing of relief wells,gas well testing,and gas wells producing liquids,et al.At present,it is difficult to accurately determine the state of gas carrying liquid under the aforementioned events restricted by the limitation of understanding the mechanism of gas carrying liquid in annular flow.This leads to the formation water or test liquid in the wellbore cannot be fully carried out of the wellbore,resulting in liquid loading,or injected killing fluid cannot continue to accumulate in the wellbore,thus killing failure.Therefore,it is necessary to clarify the liquid distribution characteristics under the action of high-speed gas in the annular flow,reveal the ways of gas carrying various forms of liquid,explain the mechanism of gas carrying liquid,and construct corresponding theoretical models,so as to provide a powerful means for accurately determining the state of gas carrying liquidUsing the self-developed vertical annular flow liquid film-droplet synchronous observation experimental system,the experiments of gas carrying liquid characteristics are carried out,and the entrainment ways of bubble and droplet which determine the gas-liquid distribution characteristics are revealed.Results show that the mass transfer between the liquid film and the gas core is carried out by three kinds of bubble entrainment way,which are large wave roll and overturn,synchronized bubble entrainment along with droplet entrainment,and droplets from gas core impacting liquid film,as well as five kinds of droplet entrainment way,which are the destruction of waves containing bubbles,bag break-up of waves,ligament break-up of waves,droplet impacting liquid film,and burst of bubbles.With the increase of the gas velocity,the dominate bubble entrainment way changes from the large wave roll and overturn to the synchronized bubble entrainment along with droplet entrainment and the dominate droplet entrainment way changes from destruction of waves containing bubbles to ligament break-up of waves.There is a close relationship between bubble entrainment and droplet entrainment.The entrained bubbles reduce the strength of large-scale waves and facilitate the shearing-off of waves.A small number of droplets are entrained due to the burst of residual bubbles in the liquid film.Droplets generated by bag and ligament break-up of waves can directly impact the nearby liquid film,leading to synchronous bubble entrainment.A small number of bubbles can be entrapped by droplets from gas core impacting liquid filmBased on the variation of gas core-liquid film interface roughness with geometry and motion parameters of interface roughness element in vertical annular flow,the correlation for the interface roughness is proposed.The interfacial resistance regime is identified and a prediction method of the interfacial friction factor suitable for the whole resistance regime is established.Results show that the gas core is under turbulent flow.The resistance regime transits from the fully rough regime to the transitional regime and then to the smooth regime with an increase in the gas core Reynolds number.Compared with the experimental data,the predictive accuracy of the proposed correlation is higher than that of the ten existing ones in the whole resistance regime.The proposed correlation ensures the accurate calculation of the interfacial shear stress which characterizes the degree of the gas care carrying the liquid filmBased on the droplet size and velocity measured by the three-dimensional phase Doppler anemometry at various radial positions in vertical annular flow,the influence of droplet entrainment way on the characteristics of gas carrying droplets is analyzed.Besides,the radial evolution characteristics of entrained droplets and the momentum exchange between the gas and the droplets are figured out.Results show that,at high gas velocities,the ligament break-up of waves produces a large number of entrained droplets with high sphericity and initial velocity.Droplet size presents continuous distribution.At low gas velocities,the destruction of waves produces a small number of entrained droplets with low sphericity and initial velocity The size distribution of droplets is dispersed.The distribution characteristics of droplet size and velocity remain unchanged during the movement of droplets from the gas core-liquid film to the axis.The proportion of large droplets is low,but the corresponding momentum ratio is highAir-water and SDBS solution-air vertical annular flow experiments are carried out to reveal the way of gas carrying liquid with the introduction of surfactants.The foaming mechanism is explained and the foaming model is constructed.The foaming ability of the annular flow is evaluated quantitatively.Results show that with the introduction of surfactants,the bubble entrainment degree is enhanced while the droplet entrainment degree is inhibited,making the liquid film along the wall pipe change from single-phase liquid to foam.The way of gas carrying liquid is the shearing effect of gas on the small-scale waves covering on the liquid film with uniform distribution in both the axial and circumferential directions.The momentum exchange degree between the gas and liquid is increased,and the liquid carrying efficiency of gas is improved.Considering the effect of surface tension on gas entrainment,the foaming model for annular churn flow is established according to the energy conservation law assuming that the turbulent kinetic energy of liquid film jet used for inducing gas entrainment is equal to the increase rate of surface energy of the entrained bubbles.The model is verified by the experimental data.The foaming ability depends on the gas entrainment rate,the area of foam generation zone,and the gas flow rate.The foaming ability of annular flow is stronger than that of other flow patterns,which is suitable for foam drainage.A correlation is proposed for the prediction of required surface tension for the foam drainage of annular flowThe flow region of the annular flow in the wellbore is divided according to the way of droplet entrainment.The prediction correlation for the size of the maximum initial entrained droplet is given for every flow region.Based on the force balance condition of the largest droplet in the gas core,the dimensionless critical gas mass flow rate is defined as the critical criterion for gas carrying liquid.The whole wellbore critical flow rate of gas carrying liquid method is constructed.The influence of different factors on gas carrying liquid is analyzed by an example.Results show that there are three kinds of flow region with the change of gas velocity along the axis of the wellbore,which are co-current annular flow with ligament breakup,co-current annular flow with bag breakup and churn-annular flow with wave under-cut.The maximum size of the droplet in the gas core depends on the droplet breakup process under co-current annular flow and droplet entrainment process under churn-annular flow.The dimensionless critical gas mass flow rate decreases with the increase of gas flow rate,the decrease of tube diameter and the decrease of the surface tension,which is more conducive to gas carrying liquid.The gas and liquid viscosities have a slight influence on the ability of gas carrying liquid.