The nature of pore pressure profile on deepwater Gulf of Mexico wells dictates the need for hole enlargement while drilling (HEWD) on several intervals. HEWD is a well-established practice in deepwater GoM and allows casing programs with smaller ID clearance between consecutive casing strings. Although the reservoir section is typically drilled as a single diameter, HEWD is required for many of the larger upper hole sections. However, in some cases drilling conditions require the operator to deviate the casing program from plan. On a recent deepwater Gulf of Mexico Lower Tertiary well, pore pressure anomalies, high mud weight and associated equivalent circulating density (ECD) required the operator to raise formation integrity test (FIT) to the maximum limit while drilling the 14½" x 16½" section. To solve the problem it required premature setting of a 14" casing string. The subsequent12¼" section was initially planned as a single diameter drilling interval. However, to account for pore pressure variations experienced on the previous section and to avoid narrowing of the producing casing size the plan was modified to incorporate a contingency liner. The 12¼" section would have to be enlarged to 13½" in case a contingency 11-7/8" liner was needed. Adding to operational complexity, the original drilling program included a plan to core the 12¼" section. To stabilize the coring BHA would necessitate a minimum of 250ft of 12¼" rathole. To achieve all 12¼" objectives, it would require an extra unplanned trip. To solve application intricacies, the operator elected to use a new-type ream-on-demand (RoD) system in the 12¼" section and eliminate multiple trips to change out BHAs. RoD technology is equipped with an innovative hydraulic mechanism that can be opened and closed multiple times as required while drilling. After risk assessment and pre-job planning, the 12¼" section was successfully underreamed to 13½" diameter with the RoD tool. The pressure regime was carefully monitored and it was determined the 11-7/8" contingency liner was not required. The reamer was then closed at the top of the coring section and a 12¼" borehole was drilled an additional 250ft offering the required stabilization for the subsequent coring run. Utilization of the RoD technology eliminated a round trip prior to using the coring BHA and significantly increased operational efficiency while enhancing project economics.
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Yi et al. (2014) studied this question.