An operator was drilling complex big-bore gas extended-reach drilling (ERD) wells from an offshore Sakhalin Island platform. Because of the shallow gas anomaly presence beneath the platform, there was a requirement to set an intermediate casing or liner at ~375-m true vertical depth (TVD), which was between the 30-in. driven conductor at 170 to 175-m TVD and the next casing setting depth of 950 to 1065-m TVD. Due to the well complexity and completion requirements, conventional casing design with no underreaming operations was not an option.

Well reach and complexity significantly increased since the project started in 2007, which called for improvements in wellbore geometry. The wellbore geometry underwent few changes, concluding with the latest most favorable required for a 27 to 28-in. directional tophole out of a 30-in. conductor with the maximum bit size pass-through diameter of 25-in., and setting 24-in. liner at ~375-m TVD. Originally, these types of topholes were delivered in two separate trips. On the first trip, the 24-in. borehole was drilled with a mud motor bottomhole assembly (BHA), and on the second trip, underreaming-only operations were performed to obtain final borehole diameter. This operation required additional rig time and caused excessive vibrations during underreaming. A different type of underreamer was implemented successfully to eliminate vibrations, but it did not reduce the number of trips. The ultimate solution was to run the underreamer below a bent mud motor, enabling simultaneous drilling and underreaming of the directional top hole while steering the trajectory in a crowded subsurface environment. The presence of deviated conductors with 6 to 8° inclination at the shoe in all outer slots played a substantial role in overall success of the operation. It is very unlikely that the same results could be achieved if the outer slot conductors were straight.

Installing the underreamer below the mud motor worked successfully in five recent wells, saving a trip in each well. The tophole trajectory was effectively steered away from the offset wells, creating a no-collision-risk situation. A 24-in. liner was run to the planned depth and cemented. This technique was accepted by the operator for the major offshore project and used as the way forward for the remaining five outer slots.

The successful implementation of the simultaneous drilling and underreaming technique demonstrated the benefits and qualitative acceptance of using an underreamer below the mud motor for the directional tophole in this major Sakhalin offshore project. The knowledge and lessons learned from the project can be applied to other worldwide projects with identical or similar casing design requirements.

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