Closed-loop drilling (CLD) systems are the foundation for the evolution of floating rigs able to effectively address the challenges of ever-deeper drilling horizons. A CLD system enables drilling modifications in three key segments: riser gas handling; reactive early kick detection, which is an adjunct to riser gas handling achieved with the addition of mass-flow measurement; and a proactive suite of managed-pressure drilling (MPD) methodologies that are bringing nontraditional solutions to complex deepwater drilling challenges.

These proactive methods, categorized as pressurized mud-cap drilling, constant bottomhole pressure, dual-gradient drilling, and returns flow control, provide a variety of specialized solutions. As these mature, the MPD methods also are proving to be the basis for new technologies that further enhance deepwater drilling capabilities.

Despite the success of CLD systems and the potential presented, the use aboard semisubmersibles and drillships is limited by a host of cost, personnel, and deployment constraints. Fully realizing CLD benefits in deepwater applications requires a focused industry effort to develop guidelines, procedures, and standards for equipment procurement, rig modification and design, and training.

CLD process

CLD refers to a process that circulates drilling fluid within a contained, pressurizable system vs. conventional drilling circulating systems that are open to the atmosphere. A rotating control device (RCD) closes the loop by containing and redirecting annular flow from the wellbore away from the rig floor to an automated MPD choke manifold.

Traditionally, the RCD is installed atop the BOP. On a deepwater vessel, a more sophisticated RCD is installed below the tension ring as an integrated part of the marine riser system. The first RCD designed with below-tension-ring capabilities was recently developed by Weatherford and is being successfully used in deepwater applications.

Within the closed loop created by the RCD, changes in pressure are easily detected and effected. Pressure and mass-flow measurements provide real-time data that inform manual or automated changes in choke settings. Manipulating the MPD choke manifold varies annular backpressure at the surface, which immediately increases or decreases downhole wellbore pressure. Specialized software monitors, analyzes, and precisely controls the process.

Deepwater CLD

This manner of managing wellbore pressure lends itself to a variety of specialized MPD methodologies that are key enablers in drilling wells that challenge or defeat conventional drilling operations. In the Asia-Pacific region, for example, pressurized mud-cap drilling provides the means to drill carbonate formations where total circulation losses defeat the basis of conventional circulating systems.

For deepwater drilling the MPD solution takes the form of constant bottom-hole pressure methods, which provide the precise wellbore pressure control required to navigate extremely narrow drilling windows between pore pressure and fracture gradient. The ability to dial in and hold a specific downhole pressure without changing mud weight provides a high degree of control and a first response to fighting kick/loss cycles, wellbore instability, stuck pipe, and other pressure-related problems.

MPD operations also introduce a new level of well control ahead of traditional BOP and mud weight procedures. While conventional well control capabilities remain fully functional, MPD enables a response that may preclude that use and provides the data for a more informed well control response.

MPD advantages in deep water have been demonstrated in many challenging or otherwise impossible applications around the world. For example, in a recent deepwater well drilled offshore West Africa, an unstable zone and sharp changes in pore pressure and fracture gradient led to the failure of two conventional drilling attempts. MPD operations successfully mitigated the instability and kicks, and the well was successfully drilled without borehole stability issues, underreaming, or contingency liners.

Riser gas handling

As the industry moves toward transitioning deepwater rigs over to CLD-ready status, the modified rigs will have enhanced riser gas handling capabilities. However, it requires more than just a drillstring isolation tool and flow spool to make a rig CLD-capable. Additional considerations in upgrading existing rig systems may include higher volume mud-gas separators, higher circulating rate capabilities, and riser pressure capabilities. Also, any existing architecture constraints of telescopic joints, ball joints, tension rings, etc., must be addressed.

In newbuilds these considerations will become part of the design parameters and requirements, and thus CLD-capable rigs will be the default. The addition of an early kick detection or MPD manifold to the riser gas handling system provides the rig operator with a proactive system that mitigates the specter of unanticipated gas in the riser.

In drilling the West Africa well, MPD provided an effective means of identifying and handling riser gas. Riser gas occurs when a gas influx entrained in an oil-based mud breaks out of solution as it is circulated to the surface. This typically occurs about 610 m to 914 m (2,000 ft to 3,000 ft) below the drill floor. At this point the gas is above the BOP in the riser and beyond conventional containment.

The gas is conventionally vented by using the rig diverter system, but this practice limits control and increases risk. With an MPD system the expanding gas is quickly detected, surface backpressure is proactively applied, and the gas is circulated out to the mud-gas separator.

CLD rigs

Extending these CLD advantages to a broader scope of deepwater wells can offer safety, operational, and economic rewards. But deployment can be hampered by the ability of semisubmersibles and drillships that were originally built for conventional open-to-the-atmosphere circulating systems to readily accommodate a CLD system.

This lack of readiness manifests itself in a number of ways. It is felt in defining the physical parameters such as riser component configurations and inside diameters (IDs), rotary table IDs, deck requirements, the availability of experienced personnel, and basic cost efficiency. Ultimately, this low degree of readiness creates a bottleneck in the deployment of MPD equipment.

Because MPD is used on a limited number of wells, each MPD application typically begins anew. Variables such as rig configuration and type of MPD application complicate the transition. A lengthy training process for rig and operator personnel on safety and operational procedures is often required. The relatively small number of applications also makes MPD capital equipment expensive and lengthens the procurement times even for ancillary equipment.

Increasing industry access to MPD capabilities ideally would be driven by mandates set by local and international governing bodies during applications for deepwater drilling. In lieu of such commitments, a coordinated effort between the E&P companies, drilling contractors, and service companies to create a CLD infrastructure is required. The most obvious means of achieving this would be for the E&P companies to create the demand by requiring drilling contractors to standardize the integration of a riser degasing system with the ability to incorporate an RCD, thus allowing plug-and-play MPD.

Transferring the capital and thus the ownership of the CLD-enabling equipment (a riser degasing system with an RCD body) to the drilling contractor would require a new set of competencies for the drilling crew but would serve as a means to more efficiently use personnel on the rig. Bridging these competencies would allow the service companies to focus on specific subsurface-related problems as defined by the drilling engineering team and allow the correct CLD methodology to be implemented.