| In order to improve the drilling fluid to the drilling bit cleaning and cooling effect,improve the cuttings migration efficiency.Based on the study of the distribution and change of the bottom hole flow field under the action of multiple factors,this paper further analyzes the migration process and change law of cuttings,and proposes a parameter design model based on grey correlation degree,so as to achieve the purpose of protecting the performance of the bit and improving the efficiency of cuttings migration.The main contents are as follows:The numerical simulation model of PDC bit bottom hole flow field was established,and the distribution and variation of bit bottom hole flow field were analyzed from the aspects of the number of external nozzles under the condition of the same bit speed,jet velocity and total flow channel area.The results show that when the rotational speed is in the range of 90～110 r/min,the increase of rotational speed has a significant effect on the improvement of bottom hole turbulent kinetic energy and velocity.However,when the rotational speed reaches 120 r/min,the turbulent kinetic energy and velocity at bottom hole slightly decrease,and the speed change has a significant effect on the velocity distribution on the blade surface.When the jet injection velocity is within the range of5～9 m/s,the turbulent kinetic energy and velocity of bottom hole increase with the increase of jet injection velocity,and the low-speed area of bit body surface decreases greatly.When the total nozzle passage area is the same,if the diameter of the outer nozzle is reduced and one outer nozzle is added,the turbulent kinetic energy and velocity of the bottom hole will be increased,but the area proportion of the low speed area on the bit surface will be increased.Under the condition that the total flow area is the same,the nozzle arrangement scheme with relatively fewer nozzles or smaller difference between the diameter of the outer nozzle and the diameter of the central nozzle should be considered.The coupling simulation model of FLUENT and EDEM was established to reveal the migration process of cuttings from the aspects of bit speed,jet velocity and cuttings particle diameter,and to analyze the migration law of cuttings at the bottom hole.The results show that the trend of cuttings retention and the minimum cuttings velocity can be divided into three stages.In the cuttings generation stage,the fluid fluidity is low,the cuttings retention is on the rise,and the minimum cuttings velocity in the bottom hole decreases rapidly after reaching the maximum.In the transition stage of cuttings migration,the fluidity of drilling fluid increases,effective cuttings migration zone is formed at the bottom of the hole,the growth rate of cuttings retention decreases gradually,and the deceleration effect of drilling fluid on cuttings at the bottom of the hole is weakened.In the stage of smooth migration of cuttings,the flow field at bottom hole is stable,the cuttings retention and the minimum velocity of cuttings at bottom hole fluctuate in a small range.When the bit speed increased from 90 r/min to 110 r/min,the cuttings migration showed an upward trend and the cuttings retention decreased greatly,but when the bit speed reached 120 r/min,the cuttings migration decreased slightly and the cuttings retention showed an upward trend.When the jet velocity increases from 5m/s to 9 m/s,the cuttings retention decreases gradually,and the cuttings migration fluctuates somewhat,but the overall trend is upward.When the diameter of cuttings is in the range of 1～3 mm,the stability of cuttings migration effect decreases with the increase of cuttings diameter.The larger the diameter of cuttings,the higher the migration difficulty.Bit speed,jet velocity and drilling fluid density were selected as cuttings migration parameters,and cuttings retention,cuttings migration and minimum cuttings particle velocity at bottom hole were selected as optimization objectives.According to the parameters range given in the drilling design process under specific working conditions,orthogonal test is designed and numerical simulation is carried out.On the basis of principal component analysis,grey correlation theory is introduced to process the simulation data.The second-order parameter design model of cuttings migration parameters was established,the multi-objective optimization was transformed into the optimization of grey correlation degree,and the model was solved to determine the best parameters in the range,and the results were verified.The results show that the migration rate of cuttings is as high as 68.2%,and the minimum velocity of cuttings at bottom hole is increased by 32.8%. |
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