Three dimensional wellbore feature of air drilling hole and its influence on bit side force

Abstract

In existing drill string mechanics analysis, smooth and regular wellbore is assumed and makes the calculated results sometimes differ greatly from the actual situation. In this study, based on measured data, the well characteristics of air drilling are shown from static and dynamic aspects, which proves the irrationality of this assumption. A six-arm calliper logging tool was applied to measure the wellbore data, and a cubic spline function was firstly adopted to reconstruct the wellbore. Then a 3D dynamic visualization description of the wellbore was realized based on the visualization toolkit. The results indicate that the 3D dynamic wellbore view can intuitively describe the characteristics of a high ovality and notable axial irregularity of the cross section of the wellbore drilled with BHA with an air hammer (AH-BHA), while the wellbore drilled with a pre-bent pendulum bottom hole assembly(PBP-BHA) exhibits low ovality and a smooth and regular shape along the axial direction. This can explain why it is difficult to run casing in a hole section drilled with AH-BHA. The FEM calculation example shows that the regularity of wellbore has a great influence on the bit side force. The BHA, which generates a bit side force that would reduce wellbore deviation in regular wellbore, may even produce a bit side force that increases wellbore inclination in severely irregular wellbore, and can be used to reveal the deviation mechanism in the air/gas drilling process. Importantly, this result indicates that the assumption of irregular wellbore should sometimes be taken seriously in drill string mechanical analysis.