Study On Multiphase Flow Law Of Supercritical Fluid Intrude Into Wellbore Annular

The study of multiphase flow law in wellbore is the premise of achieving accurate control of wellbore pressure,and it is the core scientific problem of achieving safe and efficient drilling.After the supercritical fluid intrudes into the annulus of the wellbore,the phase of the supercritical fluid changes with the decrease of the temperature and pressure which causes the thermal physical properties to change drastically,and the hydraulic column pressure of the wellbore will be seriously unbalanced,so that the wellbore and blowout accidents are prone to occur.Such as causing damage to drilling equipment,casualties and environmental pollution.Therefore,it is urgent to study the multiphase flow law of supercritical fluid intrude into wellbore annular to accurately obtain the distribution of the annulus pressure after the invasion of the supercritical fluid,so as to avoid the recurrence of the safety accident.However,there are few researchers study on the multiphase flow law of supercritical fluid intrude into wellbore annular,and the main medium of some studies is a single supercritical carbon dioxide.The composite component of the actual natural gas cannot be fully considered so that the multiphase flow law cannot be truly reflected.The law of multiphase flow in the annulus of the wellbore has certain limitations.The theoretical research,numerical simulation and laboratory experiments are combined to study the multiphase flow law of supercritical fluid intrude into wellbore base on the special physical properties of supercritical fluids.The specific research work of this thesis is as follows:(1)Based on the analysis of the phase characteristics and thermophysical properties of supercritical carbon dioxide,the phase and physical properties of natural gas containing carbon dioxide were studied.And based on the analysis and summary of the advantages and disadvantages of the existing physical property parameters and the applicable conditions,the calculation model for the physical parameters of carbon dioxide-containing natural gas ares selected.Finally,the selected models are used to calculate the thermal properties of the carbon dioxide such as natural gas deviation coefficient,density and viscosity and the law of change with temperature and pressure will be obtained.(2)Based on the classical fluid mechanics theory,the formula for determining the flow pattern transition and the calculation formula of fluid characteristic parameters after the supercritical fluid invades the annulus of the wellbore is established.The experiment of supercritical fluid intrusion into the wellbore flow chamber was carried out.The variation law of frictional pressure drop under different flow patterns after supercritical fluid intrusion was simulated and the model was also verified.The results show that the increase of gas or liquid flow rate under the same flow pattern will increase the frictional pressure drop in the wellbore.The calculation results of the model in this thesis are in good agreement with the experimental results,which can meet the needs of actual drilling construction.(3)The experiment of supercritical fluid intrusion into the wellbore annulus multiphase flow convection heat transfer was carried out,and the heat transfer law under different flow patterns was simulated.The results show that under the same flow pattern,the increase of gas or liquid flow rate will increase the convective heat transfer coefficient,but the increase of the convective heat transfer coefficient under different flow patterns has obvious difference.Based on the experimental data,a new calculation model of convective heat transfer coefficient is established.The calculation results show that the convective heat transfer coefficient calculation model established in this thesis can truly reflect the convective heat transfer characteristics under different flow patterns.(4)Based on thermodynamics,the basic principles of heat transfer and fluid mechanics,the heat exchanger system between wellbore and formation is studied,the calculation model of supercritical fluid intrusion into the annulus temperature model is established.Combined with the selected supercritical fluid physical property parameter equation and the established convective heat transfer coefficient model,the annulus temperature model is solved to analyze the temperature field distribution law,and its influencing factors after the supercritical fluid invades the annulus are concluded.The results show that the main factors affecting the annulus temperature of the wellbore are gas intrusion velocity,geothermal gradient and cycle time.The influence of geothermal gradient on the wellbore temperature is greater than other factors.The gas intrusion velocity and cycle time mainly affect the annulus temperature distribution of the lower section.(5)Based on the principle of mass conservation and conservation of momentum,and considering the influence of phase change factors,the continuity equation and momentum equation of supercritical fluid intrusion into the annulus of the wellbore are derived,then the supercritical fluid established in the previous article invades the annulus temperature of the wellbore,the field prediction model,the supercritical fluid physical property model,the drilling fluid physical property parameter calculation equation,the two-phase tube flow pressure drop model,the corresponding initial conditions,and the boundary conditions are combined to solve this equation by the finite difference method.The multiphase flow law of supercritical fluid intrusion into the annulus of the wellbore is obtained by calculation.The calculation results show that the higher the carbon dioxide content,the slower the downhole pressure decreases,the smaller the mud pool increment and the larger the gas volume fraction near the wellhead.The greater the drilling fluid density,the slower the downhole pressure drops,the smaller the mud pool increment,and the smaller the gas volume fraction near the wellhead.The greater the gas intrusion rate,the faster the downhole pressure drops,the greater the mud pool increment,and the greater the gas volume fraction near the wellhead.This thesis takes the carbon dioxide-containing natural gas as the research object,and starts with the special phase characteristics of supercritical carbon dioxide to analyzes the physical properties and phase transition behavior of carbon dioxide-containing natural gas,and innovatively considers the supercritical fluid phase transition factor into the wellbore annulus.A new calculation model that can reflect the multiphase flow law of the supercritical fluid intrusion into the annulus of the wellbore is derived to achieve a clearer understanding of the cause of the blowout caused by the intrusion of supercritical fluid into the annulus.The research results of this thesis provide an important theoretical basis for the development of acid gas fields.At the same time,it has important theoretical value for enriching the research content of multiphase flow and perfecting multiphase flow theory.