Research Of The Gas Migration Along Wellbore And The Phase Transition In Annulus Fluids

Gas overflow is a common and serious drilling problem.It is necessary to study the gas overflow to meet the requirements of safe drilling and delicate controlled pressure drilling technology.The Overflow gas has two existing forms in the well.One is to enter the wellbore in the form of bubbles,and the other is that the gas mixture exists in the wellbore in the form of supercritical fluid when the wellbore temperature and pressure are higher than the critical temperature and pressure of the gas mixture.With the decrease of temperature and pressure in the drilling cycle,the transition of supercritical fluid occurs and a large amount of gas is produced,which affects the temperature,pressure and flow law of the wellbore.Therefore,the study of gas migration characteristics along the wellbore and the characteristics of the fluid phase transition in the annulus are of great significance for further understanding the wellbore flow pattern and for fine air pressure drilling.(1)The drilling strata gas flow calculation model is established based on previous horizontal well reservoir percolation model,using the pseudo pressure function different and taking into account the change of bottom hole pressure,drilling speed and flow conditions.The results of model calculation show that the gas overflow velocity is positively related to formation permeability,ROP and bottom hole pressure difference.(2)Simulated bubble formation process in the wellbore by FLUENT.The bubbles are asymmetric in the process of formation.The simulation results show that under certain conditions,the bubble detachment volume is positively correlated to diameter,the gas mass flow rate,fluid density;the bubble detachment time is negatively correlated to aperture,mass flow,gas fluid density.Considering the effect of fluid drag,a model of wellbore primary bubble is established.The simulation results show that the bubble size increases with the increase of permeability under certain conditions,and the bubble departure time decreases as the fluid flow rate increasing.(3)The rising velocity model of small bubbles at low Reynolds number is established through force analysis.It is found that the final velocity of small bubbles decreases with the increase of fluid density.The rising velocity of large bubbles in different models is calculated.The rising velocity of large bubbles and small bubbles is closely related to the bubble radius,and the general law of the bubble rising velocity and its radius is analyzed.(4)Based on the LBM method,the gas-liquid phase transition process in the wellbore is simulated and analyzed.It is found that the initial density has a significant influence on the gasliquid distribution after phase change.When the mixed fluid of initial density is less than the critical density,Continuous gas is formed in the position of phase transition;when the mixed fluid density is greater than the critical density,gas migration along wellbore in the form of bubble after phase transition.