Drillstring Buckling and Drilling Fluid Density Effect on Torque and Drag in Iraqi Oil Wells

Abstract

Drillstring buckling phenomena in oil and gas wells is considered as a critical problem in drilling engineering while it negatively affects the drilling operations. While drilling, drillstring buckling may cause inefficient load transfer to the bit, excessive torque magnitudes, decreases the life of the drillstring significantly and even drillstring failures due to fatigue. In this study, Halliburton's Landmark software (WellPlan) was utilized and two wells; Well-BU (horizontal well) in Buzurgan oilfield, Missan oilfields and Well-Ga (deviated well) in Garraf oilfield, Thi-Qar oilfields were selected. Three main processes are used in torque and drag modeling application. First, utilizing offset wells data, the second process is correcting hookload weight and the third process comprises friction coefficient calibration with actual field data. Calculation of minimum weight on bit (WOB) to initiate buckling and effect of drilling fluid density during different drilling operations in 5 5/8 and 17 1/2 inch open hole sections  in the two wells were studied.The most important conclusion is minimum WOB to initiate buckling is reduced at different drilling operations (sliding and rotating) while drilling operation progresses and both the diameter of the wellbore and drillstring decreases (depth of the well increases). While rotating operation, minimum WOB that caused sinusoidal buckling in 17 ½ and 5 5/8 inch hole diameter were 17.88 and 10.4 tonne respectively. In contrast, during sliding operation, the minimum WOB is less than rotating operation. Significant values of hookload were exhibited in BU-Well and Ga-Well whenever density of drilling fluid was varied, in contrast less effect on torque values were observed. The variations in drilling fluids were 1.21 g/cc to 1.23 g/cc in Well-BU and 1.114 g/cc to 1.138 g/cc in Well-Ga. Not every reduction or rising in torque and hookload means there is a problem.