Research On Mechanical Behavior Of Coiled Tubing Operations In Horizontal Well For Shale Gas

“Coiled Tubing”(CT),as a well service tool,has become a key component for many well intervention operations due to its economic advantage and time savings compared with traditional tubular operational technology.CT is typically a kind of thin elastic pipe product manufactured in a continuous length,with its diameters of 25.4mm to 168.275 mm and wall thickness of 2.032 mm to 7.62 mm,while it is extended for thousands of meters underground.However due to the slenderness of CT and graduely increased axial force,the CT would ultimately buckle into a helical configuration resulting in a rapidly increasing friction between the CT and wellbore.The frictional drag will ultimately overcome the axial compressive force resulting in the CT lock-up,so this problem has limited the application of CT technology.In this paper,we first studied the factors affecting coiled tubing operation in a horizontal well,including initial curvature,buckled tubing and the unbuckling behavior,to describe the local mechanical behaviors of coiled tubing.Based on the decoupling solutions of frictional buckling equation,we established coiled tubing integral mechanical model to analyze extended reach of CT in a horizontal well,and provided a method to evaluate fluid hammer effects.We have built a set of prediction models to evaluate extended reach of coiled tubing operation,which provides a theoretical basis for practical application.We selected the coiled tubing section compressed in a horizontal well as the research object to study the effects of initial curvature on CT post buckling,the passibility of CT in buckled tubing,and the comprehensive description of unbuckling state of CT in plug milling operation.Three methods including beam-colum equation,energy method and differential equation were combined to solve these problems.For the neglect of coupling effect between friction and buckling,we solved the frictional buckling equation with perturbation solution to obtain the new axial force distribution.The new expression was compared with traditional results to reveal the effect of coupling effect.Then we substituted the decoupling results into down-hole CT integral model to analyze axial force distribution and extended reach of CT in two-dimensional and three-dimensional wellbores.For the technologies to increase CT extended reach,such as fluid hammer tool,we analyzed the effect of bump pressure and fluid rate on CT axial force and apparent coefficient,and substituted the results into CT integral mechanical model to reveal the effect of bump pressure and fluid rate on injector weight and axial force distribution.The theoretical model proposed in this paper was applied to Jiaoye 2-2HF to analyze the extended reach and parameter rules of CT in plug milling operation.The results are available for practical application.