Well M is a vertical shale gas exploration well in Indonesia. The presence of overpressured shale formation dictated the requirement to have a 16-in. intermediate casing string between the 20-in. and 13.375-in. casing to cover an approximately 350- to 1000-m measured depth (MD) interval. The mandatory logging operation in all hole sections required a 12.25-in. pilot hole to be predrilled, to enable the logging tool to get a good measurement quality, before it was enlarged to 17.5 in. for setting the 13.375-in. casing. Several hole opening challenges needed to be considered in this operation: 1) The hole opening tool should pass the 16-in. casing inside diameter (ID) restriction (14.936 in.); 2) the tool needed to be applicable for hole opening with an existing pilot hole (bi-center is not a feasible option); 3) the tool should be able to open the shoe track from 12.25 in. to 17.5 in. for 13.375-in. casing run assurance; and 4) the hole opening needs to be as efficient as possible to eliminate multiple runs.

A dual hydraulically expandable underreamer was proposed to answer all those challenges. The configuration consists of two underreamers with different opening capability, which satisfies the high-ratio opening requirement. The lower reamer opened the hole from 12.25 in. to 14.75 in., and the upper reamer further opened the hole from 14.75 in. to 17.5 in. Because running a tandem underreamer is not a common operation, a thorough vibration analysis must be performed to ensure the tool reams in a stable condition, not only to avoid downhole tool failure but also to produce a smooth borehole. Hydraulics optimization is also critical to ensure both underreamer cutter blocks are fully opened during the run.

The tandem underreamer successfully opened the hole from 12.25 in. to 17.5 in. in one run to the section TD from 980 to 1623 m MD (a 643-m interval). At the surface, both underreamer cutter blocks were still in a good condition with 0-0-NO cutter dull and in gauge condition, which showed a good stability. This is also supported by the success of the 13.375-in. casing run to bottom without any issue, indicating a good and smooth borehole. The tandem underreamer method successfully achieved the high-ratio hole opening objective and saved drilling time by eliminating the unnecessary trip for having two separate underreaming runs.

The underreamer tool mechanism and the criteria by which the dual underreamer configuration became the solution to the main challenges in this case study were documented. The study showed that the prejob engineering work (vibration modeling using dynamic finite element analysis and hydraulics modeling) was a critical part in the success of the operation.

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