Enlarge hole in challenging 12 1/4-in × 13 1/2-in section

Hole enlargement while drilling typically results in a long rathole; in some operations, the reamer is positioned above a long, complex LWD string so that the enlarged borehole will not degrade the accuracy of formation evaluation measurements. To open the long rathole to the larger borehole size, the usual practice is to trip the drilling BHA back to surface and perform a dedicated cleanout run, which adds a day or more to the well construction timeline.

During operations in the Statfjord field in the North Sea, Statoil needed to sidetrack, drill, and underream the 601-m interval and 49-m rathole of a 12 1/4-in × 13 1/2-in section of a well. The formation was prone to creeping into open wellbore, so minimizing openhole exposure was critical to minimizing the risk of wellbore collapse. The well section was also known to have blind spots, which created the risk of collision with nearby casing.

Statoil needed a drilling system that would deliver a smooth dogleg and inclination less than 45° in the harsh environment; eliminate the risk of hole collapse; and minimize rathole and time exposure to run and cement an 11 3/4-in liner to the required depth.

Improve drilling efficiency with application-specific BHA

Schlumberger designed an application-specific BHA for this operation that included the Rhino RHE system, PowerDrive Xceed RSS, a C-Link system, TeleScope high-speed telemetry-while-drilling service, and arcVISION array resistivity compensated service. The Rhino RHE system was selected for this operation because it contains a hydraulically actuated reamer positioned above the MLWD tools as well as a near-bit, on-demand reamer positioned below the tools.

Optimal reamer placement and surface operating parameters were determined by modeling the drilling system with i-DRILL integrated dynamic system analysis service. The directional response of the RSS was also modeled to ensure that placement of the near-bit reamer did not interfere with the directional capabilities of the RSS. The Rhino RHE system’s near-bit reamer activates at TD, avoiding the extra rathole cleanout run. Upon reaching TD, the BHA is tripped back to the depth where the hydraulically actuated reamer would be above the pilot hole that was enlarged by the hydraulically expandable reamer. The pumps are cycled to activate the passive reamer blocks, and then the rathole enlargement-while-drilling process begins.

Saved 1.5 days and USD 425,000 using integrated BHA

This operation marked the first time that the C-Link system was used with the Rhino RHE system. The communication system ran for more than 80 hours with no communication or battery issues.

Using the custom BHA, Statoil was able to sidetrack, drill, and underream from 1,136-m MD to 1,737-m MD and the rathole from 1,735 m to 1,784 m in one run. The operator then successfully ran liner and cement to the required depth. Eliminating a dedicated cleanout run saved Statoil 1.5 drilling days and approximately USD 425,000.

The PowerDrive Xceed RSS successfully steered through the loose sand, and MLWD data helped prevent magnetic interference from nearby casing, mitigating collision risks. Real-time communication was achieved throughout the run using the flexible C-Link system.