Closed loop underreaming improves deepwater drilling

Drilling a gauge hole is a basic requirement of wellbore construction, but it is not easy to deliver. To ensure a gauge hole and improve well construction, a novel wellbore diameter and underreaming control system has been developed. The closed loop underreaming system's comparative measurements provide new insight into downhole conditions by measuring wellbore diameter and tracking the downhole status of the underreamer (reamer).

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Wajid Rasheed
Jaime Bernardini
John Thorogood
Smart Wellbore Systems

Drilling a gauge hole is a basic requirement of wellbore construction, but it is not easy to deliver. To ensure a gauge hole and improve well construction, a novel wellbore diameter and underreaming control system has been developed.

The patented closed loop underreaming system's comparative measurements provide new insight into downhole conditions by measuring wellbore diameter and tracking the downhole status of the underreamer (reamer). This gives the operator a clear understanding of hole gauge and issues impacting gauge. The system also allows underreamer or reamer performance to be monitored and remedied in situations where the cutters may be extended but the calliper shows undergauge hole, indicating a radially shrinking formation with time or cutter wear. Ultimately, this data can be used to improve casing running and keep cementing tolerances within specification.

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Deepwater well profile with close tolerance sections and accompanying underreamer sizes. The diameter of the drilled wellbore is increased by around 20% to run casing and cement. This entails construction of the well in pre-determined stages, with each section dependent on successful underreaming of the previous section until the well is fully isolated from the formation from surface to the reservoir.

The closed loop underreaming system monitors the extension and retraction positions of the cutter block, and a telemetry system communicates caliper data and control signals between downhole tools and a surface interface. Dimension data prompt tests and checks on effective deployment of the cutters across a section or a trouble zone subject to radial shrinkage or hard stringers. This can trigger a repeated cycle of expansion, prompting the operator to intervene and activate or deactivate cutters in response to the caliper. The system can control cutter block extension in response to caliper measurements, which can be applied to activate a near bit-reamer after satisfactorily underreaming the section. This ensures the rat-hole is enlarged only when the section is at the pre-determined hole size or any remedial work has been completed. The user can program the system to meet well-specific scenarios such as deactivating a first reamer and then activating the second, which can be either near-bit or string. In addition, a secondary redundant reamer can be activated based on wellbore measurements to save a trip to replace a worn underreamer.

The complex, dynamic interplay between the formation, bit and drillstring during standarddrilling operations is heightened when drilling in deepwater basins exceeding 6,000 ft (1,830 m) water depth, or where the reservoirs underlie massive salt sections.

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Running casing is a decision that should be taken in conjunction with wellbore diameter measurements. Although casing can be run to bottom with a casing shoe or a reamer shoe, this does not ensure gauge hole. There is still uncertainty as to whether the "cased" section has the required cementing tolerance. If the actual hole diameter does not match the planned diameter, casing and cementing tolerances will not be achieved. In this event, the entire cycle of underreaming may need to be repeated or an out-of-specification cement job may result.

In general, underreamer activation depends on flow or pressure to extend cutter blocks. Activation occurs only after the underreamer passes through the casing and the casing shoe is drilled out. Avoiding activation inside casing is understandable, but this precludes surface testing to determine whether the underreamer is functional. Further variations in downhole conditions that influence underreamer functionality are hydraulics, bending moments, formation hardness, directional control, and synchronization with the bit.

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