Mechanical Characterization Of Coiled Tubing In Highly-displaced Horizontal Wells Under Additional Traction Force

Compared with traditional oil wells,large displacement wells with greater horizontal displacement have a larger oil drainage area,enabling them to contact more oil reservoirs in a single drilling operation to achieve higher recovery rates and production efficiency.Currently,in the study of the mechanical properties of Coiled Tubing in horizontal wells both domestically and internationally,most of the research has not considered the influence of frictional forces between Coiled Tubing and the inner wall of the wellbore to simplify calculations.To make the analysis of the downhole behavior of Coiled Tubing more applicable to actual situations and to improve the accuracy of predicting the maximum depth of Coiled Tubing penetration,it is necessary to study the mechanical properties of Coiled Tubing in wells with horizontal displacement while taking into account the effects of friction.This study investigates the critical buckling load of the continuous oil pipe that considers the influence of frictional force using the energy method.The mathematics model of the Coiled Tubing position in the wellbore is described,and the total energy equation and buckling differential equation with the energy of friction in horizontal wells is constructed.Finally,by using perturbation method to asymptotically solve the nonlinear dimensionless total energy extremum and dimensionless axial force,the analytical solution of critical buckling load for the Coiled Tubing with friction coefficient as a variable is derived.By studying the influence of different variable coefficients on critical load values and performing calculations,it is found that the calculated load curve of the Coiled Tubing of different diameters based on the analytical solution of the critical buckling load considering the effect of friction more closely approximates the actual value,which also verifies the significance of considering the effect of friction.The coupling effect of friction coefficient and well inclination angle on the critical buckling load value of the Coiled Tubing shows a three-dimensional convex trend,and the growth rate of critical load decreases by about 79% in the process of friction coefficient increasing from 0.1 to 0.35,indicating that the influence of friction force on the critical buckling load of Coiled Tubing decreases as the friction coefficient increases.By combining the formula of the critical buckling load of the Coiled Tubing considering the effect of friction,the mechanical behavior of the Coiled Tubing and the maximum insertion depth of the Coiled Tubing in the large-displacement horizontal well in different well sections are analyzed.Take the QT800 Coiled Tubing with an outer diameter of 44.5mm as an example,the positions of spiral buckling of the oil pipe in the wellbore are predicted based on different well depths and different axial loads of the released suspension.The concept of "tensilecompressive load dividing point" of the Coiled Tubing in the wellbore is proposed and defined for the first time,and its location is calculated.It is concluded that the Coiled Tubing of this pipe type will not undergo spiral buckling in the vertical section of the well when a 10 k N suspension weight is released at the wellhead.The maximum insertion depth of the Coiled Tubing in the horizontal well is analyzed for four types of pipe diameters of 38.1mm,44.5mm,50.8mm,and 60.3mm at different well depths.As the well depth gradually increases,the length of the vertical section of the wellbore increases,which leads to an increase in the horizontal force of the oil pipe in contact with the inclined section of the wellbore due to the increase in the self-weight of the vertical pipe.The increased compressive load will push the Coiled Tubing into the horizontal section of the wellbore.When the vertical depth of the four types of pipes reaches about 1300 m,1400m,1700 m,and 1900 m,respectively,even if the vertical depth continues to increase significantly,the horizontal insertion depth will only have minimal changes or even stop.When analyzing the maximum insertion depth of the oil pipe in the horizontal well under additional traction force,the maximum insertion depth of four pipe diameters increases by 60%,35%,33% and 28%,respectively.That is,the smaller the pipe diameter,the greater the promoting effect of the traction force on the insertion depth.Kinematic simulation analysis was carried out for the QT800 continuous Coiled Tubing with an outer diameter of 44.5 mm.After releasing 10 k N hanging weight at the top of the wellbore,when the Coiled Tubing model reached the limit of the lower depth,the stress value of the maximum force point was 35.75 MPa,and the location of the spiral buckling occurred near the intersection of the wellbore build section and the horizontal section,which also verified the calculation and prediction of the location of the Coiled Tubing’s spiral buckling.