Mechanics Analysis And Application Of The Snubbing String

Snubbing service is an operation which running or pulling the drillstring without blowout in the condition of the wellbore is under presssure and there is oil,gas or water in the wellbore This method could be widely used in underbalance drilling, sidetracking, slimhole drilling, well completion, testing, fracturing, workover and so on. Domestic and overseas scholars have done a great deal of research on drilling string and production string,but do little research on the behaviour of string in snubbing service.Snubbing service is an operation under pressure,doing mechanical analysis and strength assessment of snubbing string could provide a theoretical foundation for safety operation.Snubbing string does axial movement in wellbore,it is subject to dead weight,inner and outer fluid medium pressure,piston force in the bottom of string.The contaction between snubbing string and hole wall cause contact force and friction. Embrace force and friction exerted on the string by the rubber core of preventer in pulling unit.An downward pressure also exerted on the string by pulling unit.High downward pressure and high piston force may cause destabilizing buckle.This paper based on the actual well path,considered such factors, for the purpose of determining admissible downward pressure .The whole string which from well head to downhole is selected to reasearch,the string is discreted to a certain number beam element. Gap element is used to simulating the contact conditions between string and hole wall.This text analysed the combined deformation which caused by bending and tension compression and torsion by finite element method,an three-dimensional finite element force analysis mode of snubbing string.The model considered the influence of downhole tools and the influence of upward behaviour and downward behaviour,it could reflect the law of bearing forces and deformations.The criterion of determining downward pressure was established based on above mentioned model and theoretical means.The software of analyzing snubbing strings was developed.This paper did numberical simulation some snubbing strings using the software, the result is identical with practice and accord with the mechanism and deformation. The analysis of theory is verified by the simulation of computer.