Bit Balling: Causes, Effects and Mitigation Using Bunmi-01 Well in Niger Delta as a Case Study

Abstract

Drilling operations is typically characterized by various operational challenges ranging from surface to subsurface issues. The most prevalent subsurface drilling problems include pipe sticking, lost circulation, borehole instability, bit balling, hole cleaning etc. This paper aims to analyse the causes and effects of bit balling and the mitigation strategies that can be employed in the well planning and drilling phase using Bunmi-01 well in Niger Delta as a case study. Analysis of drilling data gathered from Bunmi-01 well shows that the bit balling is more prominent at depths within the range of 3,000ft – 6,800ft, mostly in the top-hole section (17½") of the well. In the well of interest, the bit balling phenomenon was characterized by a significant drop in average ROP from 78.8ft/hr to 1.5ft/hr (with instantaneous ROP as low as 1ft/hr) and increase in standpipe pressure from 1,200psi to 1,350 psi. Mud logging data also indicated a transition in lithology from predominantly sand (Sand: 70%, Clay: 30%) to predominantly clay (Sand: 20%, Clay: 80%). A total rig time of about 16.5 hrs (NPT) was lost in tripping out of hole, breaking up and making up a new BHA and tripping in hole back to the bottom of the well. This corresponds to about $56,000 in costs incurred. As highlighted above, bit balling poses a threat to achieving desired cost savings through efficient drilling operations, hence it is necessary to put in place effective mitigation strategies both in the well planning and drilling phase to tackle its undesirable effects. This includes optimal bit selection and hydraulics, effective mud conditioning (addition of clay inhibition material – KCl polymer, glycol), proper hole cleaning practices, appropriate monitoring, and control of drilling parameters – ROP, flow rate and weight on bit (WOB) etc.