Simulation And Optimization Research Of Hole Bottom Flow Field Of Coring Bits

Coring in the complex broken and formation and stability of the hole wall,such as weak loose formation(referred to soft and broken formation),are always a technical problem in geological drilling engineering.According to so many problems in complex formation of earthquake fault zone,such that coring in this formation is so difficulty that core recovery rate is low.Roundtrip meterage is short and blocked core is serious.The accidents are frequent because of the instability of the hole wall.For these problems,we have some numerical simulation for drill hole bottom flow field using ANSYS CFX software.Taking Wenchuan Earthquake Fault Scientific Drilling project(WFSD)as an example,this paper optimized the hydraulic structure of core bit and drilling parameters,using ANSYS CFX simulation software to simulate hole bottom flow field.Optimizing the hydraulic structure of bits,as electroplating(or hot-press)no-impermeable bits,semi-impermeable bits and whole-impermeable ladder PDC bits in real time and adjusting the amount of drilling parameters of the pumpcapacity at the same time,according to the distribution and size of the velocity field and the pressure field on the coring bit and core recovery rate.If the pumpcapacity is too large,will enhance the hole bottom flow field,reduce the core recovery rate and scour the hole wall easily.If the pump capacity is too small,it will weaken the hole bottom flow field,reduce the up-hole velocity,reduce the capacity of carrying rock,and cause sticking and burning drilling accidents happen.By optimizing the hydraulic structure of the core bit and drilling parameters,coring effect is improved obviously and achieved the desired effect.This paper is divided into three parts.The first part is fluid dynamics theory of the coring drill bit,providing a theoretical basis for numerical simulation of the flow field drill hole bottom;The second part describes the establishment of coring drill fluid equations and mathematical models,numerical simulation of flow field and the drill hole bottom flow field of preliminary application of different hydraulic structures.And the third part describes application of the coring bits in the Wenchuan Earthquake Fault Scientific Drilling project case.This paper optimizes the step height of whole-Impermeable ladder PDC drill bits,for the analysis factors of hole bottom of the flow field.The hydraulics study of coring drill has entered a new era of numerical simulation,which is convenient to optimize the design of coring drill.But it is still need to improve some situation currently.First of all,flow field analysis is like to use Newtonian fluid(such water)as a medium flow field analysis,without regard to viscous shear of the mud.Second,most papers assumed that the drill bit is in a quiescent state,ignoring the effect of rotary drill to flow field.Third,considering the hole bottom is smooth.And the research ofholebottom flow field mostly in qualitative stage and so on.For these problems,using of ANSYS Workbench platform,using CFX technology,this paper analyses the hole bottom flow field of coring drill,non-Newtonian fluids(as mud)for the media and rheological model?=??~nfor the model.The geometric model is divided into two departments of rotating and non-rotating flow fields,considering the influence onhole bottomflow field.The interface of static and dynamic is in the position of 70mm from the drilling plane,as close to the geometric surface existing rotation effect as possible.And set that the bottom of the hole is not smooth and the bit drills into the rock a certain depth.Considering the flow field flow has certain swirl effect,the calculation uses the improved RNG k-?turbulence model.Thisnumerical simulation of coring drill hole bottom flow field makes a very intuitive and quantitative analysis of velocity and pressure distribution,orientation and size in the hole bottom,basically reflected hole the real situation of the underflow field and successfully overcome the short comings of the current flow field simulation.The paper simulates three kinds of three-dimensional mathematical bit models of the not-impermeable core bits,semi-impermeable core bitsand stepped whole impermeable core bits in the drill hole bottom.And numerical simulation and comparative analysis the hole bottom flow field of these three drill bits.According to the results of the simulation,we manufactured a variety of hydraulic coring bit with different structures,used in the WFSD project extensively.We can draw a conclusion:The nonwatertight coring bits not suitable for low core rate;The semi-impermeable coring bits could meet the core rate requirements,when drilling;soft broken formation and the heterogeneous foundation.The stepped whole-impermeable PDC drill bits had good coring,especially for soft and broken formations such as carbonaceous slate,fault gouge and fault breccia.When the waterways size of the stepped core bit is fixed,step heights of nozzles and hydraulic parameters are the main factors affecting the flow field drill hole bottom,which play an important role in core recovery rate of soft broken formation.To the stepped whole impermeable core bits,we simulate hole bottom flow field with the different step height of the waterways,and then a combination of numerical simulation for the same step height and different pump volume are analyzed.The simulation data is analyzed through origin regression analysis,curve inductive analysis and Matlab mapping method.We summarize regression equationand graphical images of the hole bottom flow field,and find the best drill core drilling hydraulic structures-step height and drilling parameters-pump capacity,suitable for soft broken formations.WFSD has proved that the optimum waterways step height of the PDC stepped-whole impermeable drilling bit is 10-20mm.It is very convenient and intuitive for on-site engineering and technical staffto choose the waterways step height and pump volume,according to the regression equation and the graphic imagethe flow field on the holebottom.When the step height is constant,technical staff can adjust the pump capacity to adapt to changes of formation.In summary,the quantitative flow field analysis and comparison of the actual conditions of coring bit in bottom hole are discussed in this paper,where the research is depended on the simulation and optimization of coring bits in bottom hole flow field,which in the simulation three-dimensional mathematical model is established and the parameters of hydraulic and boundary conditions are set.Ether the fixed calculation or the modifying the model in the research is easy to learn and handle,which provide a simple and efficient way to process the calculation of multi-work condition,and provide theoretical basis of hydraulic structure optimization with bits for researchers.In terms of field workers,according to the regression equations,graphs and stratum situation of coring bits in bottom hole flow field,they could efficiently make the best selection of coring bit and reduce the cost.Therefore,design optimization and choice of the core bits for soft broken complex formation provide some reference value,with very important social and economic value.This is the meaning of this paper.