Research On Bottom Hole Flow Field Of Horizontal Well PDC Bit Based On CFD-DEM Coupling

Horizontal well PDC drill bit,as an important tool in drilling engineering,is widely used in the development of deep shale gas horizontal wells.It is required to design a hydraulic structure of horizontal well PDC drill bit for achieving the best effective bottomhole clearing while efficiently drilling in shale formations.The design of PDC drill bit hydraulic structure can be assisted and guided for by using numerical simulation technique to study the migration of cuttings in the bottomhole flow field of the horizontal well.The existing numerical simulation methods mostly use single-phase flow method and Lagrange particle sample tracking method.Both methods do not consider the two-way coupling effect of cuttings and drilling fluid and the size and mechanical behavior of cuttings when PDC drill bits have high drilling rate with a lot of cuttings generated.So,there are certain limitations and deficiencies.Based on the above considerations,this paper applies a coupled Computational Fluid Dynamics and Discrete Element Method(CFD-DEM)approach to simulate of cuttings migration in the bottomhole flow field of PDC drill bit in horizontal wells under the condition of a large number of cuttings volume.The main research works in this paper are as follows:(1)Based on the CFD-DEM coupling theory,considering gravity and rotation,the basic theory about PDC drill bit bottom hole multiphase flow field and mathematic model of numerical simulation are established.(2)Using CAD software to establish the three-dimensional flow field model at the bottomhole,and to extract the initial amount of cuttings generated by each cutting tooth and the initial velocity and direction of the cuttings according to the rock cutting capacity simulation theory.(3)A study method is implemented about the numerical simulation of the CFD-DEM coupling horizontal well PDC drill bit bottomhole flow field.The polyhedral mesh division technology is used to verify the grid independence of the calculation model.Under different conditions,trajectory tracking method is applied to analyze the drilling fluid movement and cuttings migration.The cuttings-retention and cuttings-migration-ratios are proposed as the evaluation indicators for the cuttings-carrying performance in the bottomhole flow field of horizontal well PDC drill bit.(4)For the horizontal well PDC drill bit,based on the two evaluation indicators,analysis and comparison are implemented to study the effect of the waterway profile,the number of nozzles,the nozzle size and the nozzle arrangement on the cutting-carrying performance.According to the recommendations of hydraulic structure after the study,the horizontal well PDC drill bits were used in the field where their drilling performance were better.The above research results enrich and develop simulation methods of PDC drill bit in bottom hole flow field.Furthermore,it displays intuitively the cuttings migration and guides PDC bit hydraulic structure optimization design to improves PDC bits performance,which will have important academic value and engineering significance.