As offshore drilling progresses into deeper water and highly fractured and depleted formations, the drilling window between pore and fracture margins continues to become narrower and less predictable. Under such conditions, maintaining a wellbore pressure that is high enough to prevent an influx and maintain wellbore stability while remaining low enough to prevent lost circulation, differential sticking and permit kick tolerance can be a significant challenge.

As a result, conventional methods of controlling wellbore pressure by manipulating the drilling mud rheology can result in a significant amount of nonproductive time and additional planned and unplanned casing strings. The end result is escalating project costs and the increased probability of a well control event, sidetracks or project failure.

MPD systems can be utilized to provide opportunities for dynamic pressure control without requiring a change in drilling mud rheology.

To compensate for these challenges, managed-pressure drilling (MPD) systems can be utilized to provide opportunities for dynamic pressure control without requiring a change in drilling mud rheology. To date, the most common form of MPD used to dynamically adjust wellbore pressure is known as applied surface backpressure-MPD (ASBP-MPD). ASBP-MPD systems have a rotating control device (RCD) that forms a wellbore seal around rotating drilling pipe near the top of the riser to divert drilling returns from the riser to a drilling choke on the rig. The drilling choke permits wellbore pressure to be changed in seconds.

While ASBP-MPD systems have been proven to offer significant benefits, a limitation of this equipment and technique is that the operating range of the drilling choke is limited unless a statically underbalanced mud weight is used. Drilling ASBP-MPD with a mud weight below pore pressure can create the operating window necessary for the drilling choke to add backpressure to yield an optimal wellbore pressure. However, under these conditions, a failure of the RCD wellbore seal can immediately result in an underbalanced wellbore with an influx. As such, ASBP-MPD drilling programs that require a statically underbalanced mud are not permitted in the North Sea, Norwegian Sea and Gulf of Mexico.

To combine the benefits of dynamic wellbore pressure control with the security of drilling with a statically balanced mud weight, AFGlobal has developed the Pumped Riser System (PRS). This drilling system controls wellbore pressure by using a variable-speed subsea pump to lift riser returns around an RCD and inject the discharged flow into the riser column above the pump system. The PRS can be deployed through the rotary table as a joint of riser and uses a positive displacement pump system with a low power requirement. As a result, the PRS does not require any rig or riser modifications, resulting in an installation lead time of roughly one week on any deepwater drilling vessel.

The PRS operating philosophy permits an optimal level of dynamic wellbore pressure control while still reducing operational risk and complexity.

In contrast to ASBP-MPD, which adds pressure to a statically underbalanced well with a drilling choke, the PRS is designed to dynamically reduce pressure from a statically balanced well by providing head with a pump. Since the PRS is designed to remove wellbore pressure, a failure of the PRS results in adding pressure back to the wellbore, which is inherently safer than ASBP-MPD, where a failure can result in an influx. In effect, the PRS operating philosophy permits an optimal level of dynamic wellbore pressure control while still reducing operational risk and complexity, which can facilitate both corporate and regulatory approvals for challenging offshore drilling programs.

Based on AFGlobal’s experiences with developing both ASBP-MPD and mid-riser pump systems, the company believes that the PRS approach to drilling and ease of installation will ultimately enable the greatest growth opportunity for offshore MPD technology adoption in the mid- to long-term future.

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