Closing the loop

Managed pressure drilling methodologies evolved over the years to address operational and economic problems inherent to conventional circulating systems that are open to the atmosphere. Most recently, the advanced closed-loop technology has swept into deepwater operations to help reduce risk, improve operational capabilities and efficiency, and address new regulatory objectives. Weatherford’s Jim Crenshaw and Brian Grayson have the details.

Real-time information about downhole flow and pressure enables faster, better well drilling decisions. This symbiotic relationship between wellbore event visibility and control is key to advancing closed-loop drilling systems into deepwater operations.

A managed pressure drilling (MPD) system is a simple arrangement of readily recognized rig components that create a closed-loop circulating system and enable its unique properties.

The loop is closed by a rotating control device (RCD) that is typically mounted atop the BOP. In deepwater applications, new RCD technology is integrated with the riser below the water surface. The Model 7875 Below-the-Tension Ring (BTR) device is the first RCD to integrate with the riser system and the first to receive an API 16 monogram. Current RCD research includes subsea technology that will provide the option of connecting to the BOP on the seafloor.

The function of an RCD is to contain and direct annular mud returns. In this environment, the incompressible drilling fluid becomes a medium through which very small pressure and flow fluctuations travel. They travel equally well in either direction – bottomhole influxes or losses of only a couple of barrels are detected at the surface almost immediately. Conversely, a slight change in surface backpressure (achieved by changing choke settings) is felt simultaneously downhole. This backpressure quickly counteracts bottomhole pressure or flow oscillations to mitigate potential problems long before conventional drilling operations can even detect them.

This exceptionally precise degree of control is achieved using Microflux control technology to automatically detect and identify characteristic pressure and flow signatures in real time. ‘Fingerprinting’ common events – such as nuisance gas, or connection gas breaking out at surface, wellbore breathing and ballooning – serves to quickly differentiate them from a kick or loss and enables an appropriate response (either manual or automatic) that can greatly reduce non-productive time.

Data to inform the automatic pressure control system is acquired through a network of pressure sensors and corriolis flow instrumentation. Backpressure control is achieved with an automated choke manifold that enables fast and very fine changes in annular pressure to affect changes in bottomhole equivalent circulating density (ECD) without the cost, delay and imprecision of adjusting mud weight.

Automatic pressure control SCADA instrumentation and choke control algorithms are central to the precision and speed of the choke response. This precise control of the annular pressure profile and equivalent down hole mud weight takes into account thermal gradients, nuisance gas and other issues that affect bottomhole ECD.

A downhole deployment valve is also a common component in MPD operations. Integral to the casing string, the remotely controlled valve isolates the hole below it to prevent swabbing when tripping the drill pipe. The device speeds trip time and protects the rig crew from potential influxes or kicks when the drill string is being tripped out of the hole.

Advancing to deepwater
MPD’s advance into deepwater applications is occurring for several key reasons. The fundamental challenge of many of these wells is to drill the well to target depth while maintaining the planned hole size. MPD provides a way to walk the line in narrow drilling windows and identify common drilling events early to ensure effective corrective actions, thus providing the drilling efficiency to minimize NPT and achieve planned drilling objectives. In these operations, constant bottomhole pressure MPD methods are providing the control and data visibility required to drill previously undrillable wells.

Beyond constant bottomhole pressure, managing wellbore stability and providing early kick-loss detection provide significant advantages in deepwater applications. MPD helps conserve critical hole size by eliminating the need to case troublesome hole sections. As a result, total depth is reached with the optimal hole size required to complete and produce the well.

For deepwater drilling in particular, MPD technology provides the means to detect and control gas in the riser. Formation gases entrained in the oilbased muds come out of solution as they rise in the annulus. When this happens in the riser above the BOP, the gas is effectively beyond the reach of traditional well control techniques. With Microflux technology, the gas presence is detected, controlled and circulated out of the riser and away from the rig.

Regulatory environment
At each step, from constant bottomhole pressure methods to early kick-loss detection and riser gas control, Microflux automatic pressure control significantly reduces risk and non-productive time.

Risk reduction has become more topical as the regulatory environment in deepwater has become more demanding.

Global concerns over deepwater pressure control challenges are focusing increased attention on MPD pressure detection and control capabilities. This emphasis is personified by regulatory changes in the Gulf of Mexico in response to the Macondo accident.

One of the primary drivers of this Gulf of Mexico regulatory change is the US Department of the Interior’s Buffalo Report. Numerous recommendations were presented by the May 2010 report, and in the ensuing months the API and IADC held numerous planning session to communicate how these recommendations affect future Gulf drilling operations.

A key aspect of the report is the adoption of IADC-based safety case requirements for floating drilling operations. These requirements establish risk assessment and mitigation processes related to drilling contractor controls for managing the health, safety and environmental aspects of their operations.

A second document, the well construction interface document, is primarily concerned with operating procedures, the safety management system and the well program. The purpose of the two documents is to ensure one standard for well operations that is specific to the rig and adheres to all regulations and standards.

Closed-loop drilling bridges the gap between the operator/contractor safety case and the well construction interface document with additional barriers and recovery measures over conventional drilling techniques. This bridge is implemented in three stages: RCD and early kick-loss detection; manual pressure control with a manual drilling choke system; and proactive well response with an automated choke.

Through these stages, the technology identifies kicks and losses, diverts annular flow away from the rig, and enhances conventional well control with higher fidelity information. Hazard mitigation benefits from automated pressure control and the ability to maintain a constant equivalent mud weight. A downhole deployment valve eliminates swab and surge forces while tripping and isolates influxes while drilling.

MPD evolution
To reach this point in deepwater capabilities, the preceding technologies and elements of MPD have traveled a decades-long evolutionary path. RCD technology was first introduced in 1968 as a way to divert air-drilling returns. Its adoption by drillers using fluid circulating systems established the first closed-loop environment and set the stage for MPD.

The first offshore use of MPD was in 2004 in the Gulf of Mexico. The application was performed from a jackup rig and used a semi-automatic choke system. 2004 also marked the first MPD application from a floating structure. Aimed at drilling Malaysian carbonate formations where high fluid loss was a problem, the pressurized mud cap variation of MPD was used to successfully drill a three-well program and advanced the adaption of the technology to a floating rig platform. The application used a high-pressure Model 7100 RCD designed for surface applications on land and fixed rigs offshore.

In 2008, MPD success was achieved in drilling HPHT wells in the Mediterranean Sea, where kick-loss scenarios resulted in major NPT incidents with conventional drilling techniques. This led to the first use of the Microflux system from a floating structure in 2008.

The first RCDs purpose-built for harsh offshore environments are Weatherford’s SeaShield RCD series, which were introduced in 2008. Innovations include a remotely operated latching system to enable maintenance without the safety risk of personnel working below the rig floor.

In 2010, the SeaShield series Model 7875 BTR became the industry’s first RCD to be installed aboard a drillship as an integral part of the riser. The Indonesian application installed the RCD about 140ft below the sea level surface.

The integration of the RCD with the riser is a significant advance in RCD technology. Because it is made up below the tension ring, no modifications are required to the riser’s telescoping slip joint or the rig’s mud returns system. With the system in place, drilling operations can easily shift between either conventional or MPD drilling methods. OE

By: Jim Crenshaw and Brian Grayson
Issue: June 2011

About the Authors

Jim Crenshaw serves as North America product line manager, managed pressure drilling at Weatherford International. With over 30 years of petroleum industry experience, he has worked for operators, engineering consultancies and major oilfield services companies in both operations and senior management capacities.

Brian Grayson is global product service line manager for Weatherford’s Secure Drilling Services. He began his career with Weatherford as a R&D engineer and has held multiple positions within the company – from DST equipment design engineer to group product line engineering manager for pressure control systems.

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