Research Of Cutting Structure And Selection Of PDC Bit For Coiled Tubing Drilling

This paper selected topic from the national science and technology major projects named "Coiled tubing equipment and application technology" sub topic which researched on coiled tubing drilling bit type selection.Coiled tubing drilling is used more and more widely in drilling operation, and it is economical and feasible to use coiled tubing to drill straight or horizontal wells and new wells in "shallow oil layer". The development of coiled tubing drilling operation is still in the primary stage, from the point of view of domestic oil fields, because only the introduction of equipment, no corresponding downhole tools and technology as the support, causes low coiled tube using rate and high construction cost, the advantage doesn’t get sufficient performance. In this paper, the selection of drilling bit in the process of coiled tubing drilling is preliminarily explored, and the efficiency of drilling operation is improved by choosing appropriate PDC bit. As the PDC bit have advantages of no moving parts, adapting to high speed low WOB characteristics of drilling condition and long service life, it is especially suitable for the rock which has high hardness, high grinding that other bits are difficult to effectively break, applied more widely. PDC bits break rocks through the shear force, easy to cause the tube column to produce big anti torque effect, cause the working face of the drill bit bigger fluctuation, directional drilling deviation, pipe string fatigue damage and so on. For coiled tubing in the process of drilling the characteristics of "low WOB and high torque", used for coiled tubing drilling PDC bit in general should have the following conditions:(1) under low WOB and high speed should have enough ability to break rock; (2) in the process of drilling, the bit of low torque requirements, with WOB fluctuation change smaller. In order to meet the above requirements, the PDC bit drilling pressure and torque should be controlled well so that it can minimize the effect of unti-torque of the bit. WOB and torque of the relationship between "aggressive" can be used to represent, that is μ= 36T/(WD), T-drilling torque, W-drilling WOB, D-the bit diameter. But general of PDC bit "aggression" is large, which is 4 to 18 times of roller bit. For rock bit,μ= 0.1-0.2; for the PDC drill bit,μ= 0.6-1.2. "Aggressive" strong PDC bit, easy to cause a lot of drilling string in the torque, especially detrimental for coiled tubing directional drilling. Therefore reducing the PDC bit "aggressive" is an important task.This paper firstly analyzes the underground coiled tube stress, calculate limit of coiled tubing drilling WOB and torque. For vertical wells, horizontal wells and large displacement wells with various borehole trajectory, is divided into three of situations to establish the mechanics models, respectively are the coiled tube mechanical model in vertical well section, the horizontal well section or inclined well section. The results show that the buckling deformation is prone to occur in the vertical well section, then in the horizontal well and the last in the bending well section.In accordance with the need of coiled tubing unaggressive drilling bit, the crown shape is designed to reduce the bit aggressive. According to the researchers on the study of the theory of the crown shape design, it comes out with a new crown shape design principles-equal aggressive principle. In order to ensure that PDC bit cutting teeth aggression is roughly same which has weak aggression, through theoretical analysis, get the "line-arc-straight" type crown contour shape, the aggression of outside cone nodes keep the same level. In order to verify the aggression of the bit with the "line-arc-straight" type crown shape equal, a three-dimensional model of the bit, the finite element analysis, to determine the outer cone angle of the drill bit and aggressive. Within a certain range, with the increase of outside cone angle, bit increases aggression, approximate linear relationship. When the outside cone angle is close to 30°, aggression is minimal. Therefore, designing reasonable outside cone angle can reduce the aggression of PDC bit.Study the influence of the drill cutting structure on the aggression, the main choice of bit single teeth cutting angle (caster angle, side angle), the size of the cutter for single cutter aggressive influence. Using ANSYS/LS-DYNA display dynamics analysis software, simulates PDC bit single cutter cutting process of red sandstone. This paper is aimed at the structure of the single teeth breaking rock, using the ratio of tangential force and axial force to reflect the value of the aggression. The larger the value is, the stronger aggression of the PDC bit has; the smaller the value is, the weaker aggression of the PDC bit has. Through simulation study, changing the caster angle and side angle, the diameter of cutting teeth three parameters, it’s concluded the rule about the three parameters impact on the aggression of single teeth of PDC bit. The caster angle has most profound effect on the aggression, the side angle take second place, the size of the cutting teeth has no obvious influence on aggression. Within a certain range, the aggression of bit cutting teeth decreases with the increase of the caster angle and side angle. Use ANSYS/LS-DYNA finite element analysis software and post-processing LS-PREPOST software to realize the dynamic cutting simulation analysis and the function of cutting simulation animation.Using fuzzy comprehensive evaluation method to select PDC bit for coiled tubing drilling, can consider the use effect of bit and rock mechanics properties and the performance of the bit itself, establish fuzzy comprehensive evaluation rules. The quantitative analysis of the drilling bit selection is realized on the basis of qualitative analysis. First to confirm the model evaluation objects and evaluation factors, single factor evaluation vector and evaluation matrix, and then build the weights for evaluation set, operations by fuzzy algorithm, finally using the maximum membership degree principle to select the optimal bit. Selecting bit aggression as evaluation factors considers the features of coiled tubing drilling which is better targeted.Although this paper has done a certain amount of researches on bit type selection, it is still in the primary exploring stage, and need deeper research. Such as:(1) the tangential force and axial force of the multiple cutting tooth on the bit are superimposed and multiply by the corresponding overlap cutting coefficient caused by cutter density, get the whole PDC bit WOB and torque, then get features of PDC bit cutting teeth for the "aggressive" influence law. (2) the fuzzy comprehensive evaluation method need combine with the actual coiled tubing drilling situation, confirm the selection of evaluation criteria, establish evaluation model and aggregative indicator.