An oil and gas operator in the Gulf of Mexico planned to drill a deepwater well section in one run by concurrently drilling and enlarging a 12 1/4- to 14 1/2-in. hole while deviating from 60 to 30° inclination and crossing expected depleted formations. At section total depth (TD), the rathole below the under-reamer needed to be eliminated to help ensure successful cementing of the liner. A bottomhole assembly (BHA) was designed to allow achieving these objectives in one run, and the field results obtained with the system are described.

The first step in determining the best BHA design was to compile drilling experiences through the target formations and perform a comprehensive review of all pertinent offset well information. Weaker zones had been encountered in the 12 1/4- × 14 1/2-in. section, and an at-bit reamer (ABR) had to be included in the BHA to allow the liner to be set on bottom of the section, rather than leaving an 85- to 135-ft rathole. Because the ABR placement in the BHA is between the bit and the rotary steerable system (RSS) tool, it was important to ensure directional control could be maintained in the section and make certain no interference existed between the ABR and the wellbore that could compromise control. Stabilization and placement of the under-reamer were crucial also to ensure the necessary directional performance was obtained without overstressing the BHA components, and extensive modeling was performed to optimize the design. Thorough hydraulics and torque and drag modeling ensured the BHA design could drill the depleted zone without premature activation of either reamer.

The detailed modeling and analysis of offset performance resulted in successfully drilling the section and opening the rathole in one run. The BHA was steered to the final desired angle and reached the section TD without incident and at the desired rate of penetration (ROP). Once the section TD was reached, the ABR successfully opened the 12 1/4-in. rathole to the desired 14 in., allowing the liner to be set 3 ft from bottom. This type of operation would normally require a separate dedicated hole-opening run. Using the new design eliminated the additional trip and the time necessary to open the hole, which was estimated at 56 hours.

A BHA solution was developed through detailed modeling that allowed the operator to not only maintain the steerability needed to achieve directional requirements with an ABR between the bit and the RSS while drilling depleted formations but also to concurrently perform under-reaming.

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