Study On The Key Issues Of Calculating The Drag And Torque

With the development of drilling technology, the well’s depth and horizontal and apacecurvature become more and more large. So it often happens that the drag and torque overtakesthe bearing capacity maximum of the installation and drilling string and we need to improvethe prediction’s accuracy. Considering the problems in the existing models of calculating thedrag and torque, this paper is to study the three key issues for improving the predictionaccuracy of drag and torque.The planned course is too smooth, the value of calculating drag and torque is smaller. Sothe paper is to simulate the actual well track by adding the track tortuosity ratio to the plannedcourse. In the paper we use the rate of change of inclination and azimuth to describe thetortuosity ratio of the well track and we calculate the distribution law of the tortuosity ratio,then we use the method of the inverse function to generate random number which has someregularity. We can obtain the new value of inclination and azimuth by adding the randomnumber to the planned course; The formation has influences on the drag coefficient, in thepaper we assume that the different formation has the different drag coefficient, according tothe torque at the different bit depths is equal to the drag coefficient multiply the torquecardinal number in every formation we can obtain a series of equations. In the equations thewe can obtain the value of torque from the well logging and calculate the torque cardinalnumber of formation including casing in the models of soft string, so we can calculate thedrag coefficient of the formation or casing by using the method of the least square; For thepipe string of stiffness, we need consider the influence of borehole diameter’s change. Tosolve this problem, we study the contact stress between the drilling string and borehole wall.We use quadratic spline curve to reconstruct the section of borehole diameter vs. depth anduse the gradient to describe the slope gradient of borehole diameter.And to solve the positionof contact point and the force of contact point, we assume a spring to replace the contactpoints. And then we analyse the contact point force to calculate the the drag and axial force’s increment.Based on the above theory, we write the calculate software. In the software, we count thedistribution’s law of well path and handle the planned course. According to the torquecollected, we calculate the casing’s drag coefficient and formation’s drag coefficient andverify the value of drag factor is correct by other well’s torque in the same block, that showthe new method can improve the prediction accuracy. For the new model of drag, we analysethe influence of borehole diameter’s change and evaluate model’s exactness by collecting thehook load and drag of running casing, the results show that the borehole diameter’s changehas the influences on the drag and the predictive value is more close to the measured value inthe new model.