Research On Adhesion Mechanism Of Shaly Cuttings And Hydraulic Optimization Design Of PDC Bits

Bit balling is one of the main reasons for the decrease of ROP of PDC bits in shaly formation drilling.The adherence of shaly cuttings and poor hydraulic cleaning are the two main reasons for bit balling.In this paper,the adhesion mechanism of shaly cuttings and the hydraulic optimization design of PDC bits were studied mainly.The adhesion mechanism of shaly cuttings was revealed from the microcosmic and mechanical perspective.Combining the friction theory and adhesion theory,the model for calculating the interfacial adhesion resistance between PDC bit and shaly cuttings was established,drawing lessons from the calculation form of sliding friction.Adhesion coefficient was used to eva luate the adhesion property of shaly cuttings.The method of obtaining adhesion coefficient was developed based on direct shear test.The effects of water content of cuttings,clay mineral type and content,cutting size,PDC bit material type and roughness,liquid medium type and water based addictive on adhesion coefficient were studied quantitatively.Taking clay mineral content and cutting size into consideration,the computational model of adhesion coefficient was establis hed for common shaly cuttings,the clay mineral type of which was illite-smectite mixed layer and illite,when the water-based drilling fluid was used.Assuming shaly cuttings as identical spheres,the adhesion contact area of cutting particles on the surface of bit body was obtained by numerical simulation based on ANSYS.The computational model of adhesion contact area was established,taking well bore hydrostatic pressure and cutting size into consideration.Based on computational fluid dynamics theory,the flow control equations of PDC bit bottom hole flow field were established based on discrete particle model(DPM).Taking the depth of cutting into consideration,the realistic bottomhole fluid volume model was established based on a field's commonly used PDC bit.Cuttings were modeled as spherical particles which were injected into the flow field from the cutters surface,and the mass flow rate and initial velocity of cuttings were considered.Then the cuttings transport track can be obtained through numerical simulation.By analyzing the force and starting mechanism of shaly cuttings,the computational model of critical starting drilling fluid flow velocity for preventing adhesion was established.The user-defined function(UDF)for judging if cuttings will adhere to bit body when they collided to bit body was compiled.Particle adherence ratio(Ra)was defined as the ratio of the adhesion particles number and total particles number.And it was used as the evaluation parameter of bit balling trend.The effects of cuttings size,nozzle size,nozzle count,nozzle arrangement,blade height,flow rate,and flow guide on bit balling trend were studied.Part of numerical simulation results,including the effect of nozzle count,blade height and flow guide on Ra,were validated using experimental results.According to the results of numerical simulation,the optimum design of anti-balling PDC bit structure was carried out.After optimizing,the Ra was greatly reduced,which means that the trend of bit balling was decreased significantly.