Numerical Simulation Study On Cuttings Migration Characteristics Of Dual Gradient Drilling Technology

Conventional deep-sea reservoir drilling is faced with problems such as poor stability of sea-bed shale,large amount of drilling fluid and narrow safe pressure range.In view of these difficulties,double gradient drilling technology came into being.Hollow spheres drilling technology is to pump the light material spheres into the annulus wellbore through the injection pipeline,so that the density of the mixed fluid is close to seawater,and then produce two pressure gradients in the wellbore.This technology avoids the problems of submarine pump blockage and blowout accident.In this paper,the Eulerian-Eulerian multi-fluid model based on Kinetic theory of granular flow is used to simulate the migration process of cuttings and hollow spheres.The influence of hollow spheres injection on the migration law of cuttings and the distribution of fluid flow field is summarized.The operating parameters affecting the flow characteristics are numerically simulated.The results show that the hollow spheres will form a mixed transition region with cuttings at the end of the injection pipeline.Appropriately increasing the concentration of the hollow spheres is conducive to expand the range of the transition region and improve the concentration distribution of cuttings at the same time;when the injection amount of cuttings increases,the hollow spheres will accumulate at the end of the injection pipeline.It needs a longer carrying distance of drilling fluid to fully mix the two particles.At the same time,the pressure gradient in the wellbore under the injection pipeline will also increase significantly;Properly increasing the rotating speed of drill pipe and the flow rate of drilling fluid is conducive to increasing the suspension effect of drilling fluid on particle phase,avoiding particle accumulation at the end of injection pipeline and the bottom of wellbore,and the lifting force of drilling fluid on particle phase increases accordingly;The increase of drilling fluid flow rate can expand the range of the flat flow rate area of particle phase,and the cuttings migration efficiency in the vertical pipe section has been significantly improved.The different physical parameters of hollow spheres and drilling fluid are numerically simulated.The results show that the diameter of small-size hollow spheres is closer to cuttings,and the appropriately reducing the diameter of hollow spheres can speed up the mixing process of two particles and improve the concentration distribution of mixed fluid at the same time;When the diameter of the hollow spheres decrease from 10 mm to 5 mm,the velocity difference between the two particles decreases obviously,and the velocity pulsation of the particle phase decreases at the same time;The use of low-density hollow spheres can quickly mix with cuttings at the end of the pipeline,and the double pressure gradient phenomenon of fluid in the vertical wellbore is significantly enhanced;Increasing the values of n and K can improve the carrying and suspension ability of drilling fluid to particles,and make cuttings migrate stably in the middle area of wellbore,so as to avoid deposition at drill pipe or wellbore wall.At the same time,the velocity distribution and dynamic pressure distribution of the two particles have been significantly improved.Based on CFD method,this paper simulates the flow process under different operating conditions and liquid-solid two-phase physical parameters,in order to provide theoretical guidance for the practical application of dual-gradient drilling.