In a technically challenging & high cost environment such as HTHP wells, careful selection and adoption of advanced technologies for the successful completion of the well is paramount. As per the original well design of the subject well, it was planned to drill with a bit & under reamer which was actuated by a ball drop mechanism. This led to the restriction of placing the under reamer close to the bit as MWD/LWD had limited pass through & hence the under reamer had to be run above the tools leaving approximately 25-30 m of rat hole un-opened along with inability to retrieve radioactive source in an event the BHA was lost in hole. This required two separate BHA runs to open the rat hole in order to place the casing shoe as deep as possible. The additional hole opening trip would have resulted in extra time and hence additional cost, moreover it was prudent in HTHP well to place the casing shoe exactly at right depth to achieve well objectives. Hence, use of Bi-center bit to simultaneously drill & under ream without any pass through restriction was evaluated.
Several critical issues, related to application and BHA design, were faced by the Well Engineering team. Reaming while Drilling (RWD) with bi-center bits has always been challenging in terms of vibration-related inefficiencies. Due to the asymmetrical geometry of the bit, imbalances in force and mass have been responsible for the vibration related problems. Secondly, due to the minimum pass through requirements, these assemblies are virtually always run with under-gauged stabilizers as it allows a certain degree of lateral movement within the constraints of the enlarged hole.
All of these issues could have caused the pilot-bit drilling an over-gauged hole along its axis which results in the bit drilling off center giving an overall under-gauged hole as the pilot of the bit is not centralizing the reamer. A unique eccentric vibration dampening tool (ideally located in the string) was used to mitigate the drillstring vibration & bit whirl. This paper describes the well specific BHA design & placement of the vibration dampening tool used to improve hole opening performance. A total of 4 bi-center bit runs over three hole sections & different sizes i.e.12.¼″× 14.¾″, 10.3/8″ × 12.¼″ & 5.7/8″ × 6.¾″ are also discussed in the paper.
Unusual problems were encountered while drilling shoe track & landing collar for liners in 10.3/8″ × 12.¼″ and 5.7/8″ × 6.¾″ hole section. Lessons learnt, recommendations & contingencies to be planned for successful application of bi-center bit are discussed.