Study On Cuttings Transport During Drilling In Inclined Section Of Complex Structure Wells

Cuttings transport has always been a key problem related to borehole cleaning and downhole safety in the drilling process of extended reach and branch wells with complex structure.If the cuttings could not be delivered to the ground,there may lead to many downhole accidents,such as excessive torque resistance,sticking pipe,well control failure,etc.Therefore,the study of related problems is very necessary to solve and guide production practice.Based on the theory and method of Computational Fluid Dynamics(CFD),this study takes the solid-liquid two-phase flow in the annulus of the inclined section of the complex structure well as the research object,establishes the downhole annulus structure and drillstring oscillation models.The influence mechanism of different drilling operating parameters on cuttings transport laws and patterns,as well as the influence of drill string oscillating motion on annular cuttings transport were obtained.At the same time,the change law of cuttings transport in the annulus of the small hole was obtained by visual experiments,which provides a reference for the numerical simulation results and inferences.The main conclusions of this study are as follows:(1)The influence mechanism of each parameter on annular pressure drop,cuttings concentration and cuttings motion patterns during drilling was analyzed by numerical simulation.Among them,wellbore characteristics and drilling fluid rheology have obvious effects on the law of cuttings transport.With the increase of specularity coefficient,the momentum loss of cuttings also increases.Under the condition of lower critical shear rate and higher circulating flow rate,it is more conducive to the cuttings transport and the cleaning of the wellbore.(2)The numerical model of drill string oscillation was established,and the influence of drill string oscillation motion on the flow field,pressure distribution characteristics and cuttings transport characteristics of the wellbore annulus was obtained through calculation.The research shows that the variation of drill string oscillation amplitude and frequency under different flow velocity conditions has significantly different effects on the annular flow field.Drill string oscillation has a beneficial effect on improving the annular flow field under eccentric conditions,helping to remove the cuttings bed,shortening the period for annular cuttings to reach equilibrium,and reducing the concentration of annular cuttings.Under the eccentric condition,the average improvement rate of drill string oscillation on the transport efficiency of annular cuttings can reach 7.55%,which can effectively improve the transport efficiency of cuttings downhole.(3)The effect of different drilling operating variables on the cuttings transport in the small-sized wellbore annulus and the changing mechanism of cuttings motion patterns are obtained through experiments.The transformation mechanism of cuttings motion patterns between suspension,fixed bed and moving bed pattern under different drilling parameters is revealed,which provides a basis for numerical simulation laws,results and inferences.