Experimental Study On Hole Cleaning And Cleaning Tool Optimal Design In Highly-deviated Hole

It has been shown by field practice and laboratory experiments that during the drillingoperation of highly-deviated interval of horizontal wells, extended reach wells, multilateralwells, cuttings are likely to accumulate and form cuttings bed, which leads to the phenomenonof high drag, high torque and drag slide of the drill bit, even downhole safety accidents, e.g.,sticking and broken off of the drilling pipe. So we can reach a conclusion that it is necessary toresearch on methods and mechanism of enhancing the hole cleaning efficiency and inhibitingthe formation of cuttings bed. This study is focused on terms of cutting settling velocity indrilling fluids in vertical well interval, cuttings incipient velocity and cuttings suspensionvelocity in highly-deviated interval, etc.In this study, how lithology, bit types and the burial depth of the rock influence the rangeof particle size have been summarized based on previous research achievements. The range ofparticle size, in which the cutting is likely to lead to the formation of cuttings bed in the annulus,have been defined. Based on laboratory experiments, a formula of drag coefficient of thecuttings in different particle size in power-low fluid has been introduced, and a model appliedfor cuttings settling velocity in power-low fluid has been established.In the study, the concept of cuttings bed repose angle has been proposed.The values ofcuttings bed repose angle of different sort of cuttings have been measured during the laboratoryexperiment of cuttings bed repose angle. The friction coefficient of cuttings during the processof slip transport could be obtained by concept of cuttings bed repose angle mechanical balancerelation. Also, initial deviation angle in highly-deviated interval can be obtained by the valueof cuttings bed repose angle. Based on cuttings mechanical balance relationship, three forms ofcuttings incipient velocity model, e.t., cuttings slip transport?rolling transport?lift transporton the surface of cuttings bed, have been built. Numerical analysis of the model showed thatthe critical slip transport and rolling transport incipient velocities exceed0.6m/s generally forcuttings whose diameter range from1mm to3mm.Based on the model of cuttings settling velocity in vertical well section and publishedexperimental data, cuttings suspension velocity models in low, medium and highly deviatedinterval have been built considering impact factors, e.g., rotational speed of drill pipe, cuttingsdiameter, etc.The result of numerical simulation and analysis proves that drilling operation ofhorizontal interval is greatly impacted by deviated angle, rotational speed and effectiveviscosity of drilling fluid. Cuttings diameter and rotational speed of drill pipe impact mainly onhighly-deviated well intervals drilling operation, and flow index and cuttings density influenceon drilling operation in low-deviated well intervals.CFD numerical simulation has been applied to quantitatively evaluate the inhibition effectof drill pipe rotation on cuttings bed. Also, according to numerical simulation regression, the specific area of cuttings bed and a model for pressure loss calculation have been obtained. Thesimulation shows cuttings distribute along the circumferential direction asymmetrically in theannulus owing to the rotation of drill pipe, and thus the tangential velocity of drilling fluidincreases significantly. Then the formation of cuttings bed is inhibited to some extent by thetangent drag force caused by the rotation of drill pipe. As a consequence, increasing rotationalspeed could remarkably enhance the cuttings transport efficiency and hole cleaning efficiencyat medium or low pump rate. However, when drill pipe exceed160RPM at high pump rate, theinhabitation of rotation velocity on cuttings transport efficiency is no longer significant.In this study, basic blade types of mechanism hole cleaning tools are proposed.Rotationhole cleaning assisted mechanisms of different type of blades are obtained according tonumerical simulation and numerical analysis. Based on CFD numerical simulation, it is provedthat spiral-V shaped hybrid hole cleaning tools has a higher efficiency in hole cleaning in theannulus According to mechanical balance of cuttings on the tool face, the motion law of cuttingswhich pass the spiral tool can be evaluated quantitatively, and tangent velocity and axialvelocity can be calculated separately in particular, providing a reference for the design of thissort of tools.