Achieve effective hole cleaning at high flow rates

In the Chirag field offshore Azerbaijan, BP's underreaming operation called for a 6 1/2-in rathole to be expanded to 8 in, with a separate cleanout BHA to be used for effective hole cleaning at a high flow rate and with mud conditioning prior to a completion run. Because circulation would continue with a Rhino XS reamer inside the casing, a solution was needed to keep the reamer arms closed. However, the 6375 series Rhino XS reamer does not have a deactivation option; once the arms are activated, they will open each time the pumps are on, which would damage the casing ID while circulating and hole cleaning with the reamer inside the casing.

Perform underreaming and cleanout in one trip

To overcome the reaming and cleanout challenges, Schlumberger recommended drilling and underreaming the 6 1/2-in × 8-in hole section with directional tools below a 6375 series Rhino XS reamer, then underreaming a 6 1/2-in rathole to 8 in with a separate cleanout BHA.

Because the 6375 series Rhino XS reamer cannot be deactivated, Schlumberger suggested using a 5-in WELL COMMANDER valve from M-I SWACO, a Schlumberger company, directly above the reamer to bypass flow to the annulus above, effectively keeping the cutter arms collapsed. Such a configuration would enable circulating at a much higher flow rate than with the reamer alone. A separate 7-in WELL COMMANDER valve was also placed higher up in the string (1,185 m from the bit) and was intended for circulating during hole cleaning above the 7 5/8-in liner top inside the 9 5/8-in casing. This would enable cleaning of the liner top and conditioning of the mud without tripping the 5-in WELL COMMANDER valve up to perform the same operation and then run back to TD as per the completion program.