Managing The Threat Of Riser Gas

Gas in the riser presents a significant challenge to conventional deepwater drilling operations. Once above the subsea BOP, a gas influx is an immediate and potentially uncontrollable threat to personnel, the environment, the rig, and even the well.

For this reason, the first indication of a gas influx on a deepwater well usually initiates an expensive, time-consuming mitigation process that typically involves weighting-up the mud system and implementing BOP procedures.

This best-practice well-control response commonly is undertaken with imprecise information about the nature of the kick because it is driven by an urgent need to stop the influx and prevent the gas from rising in the well bore. And the step is not without its own risk. In a narrow drilling window, heavier fluid can reverse the balance from mitigating the kick to initiating a loss. Cycles of kicks and losses are a common yet costly problem. If these measures are applied in the reservoir section, heavier mud also can increase skin damage and impair potential production.

To break this reactive well-control dilemma and significantly reduce the risk posed by riser gas, managed pressure drilling (MPD) methods are being used to precisely measure and manage small downhole pressure fluctuations before they become a well-control event. Central to this capability is the growing sophistication of rotating control device (RCD) technology designed specifically for the unique challenges of riser applications.

Riser Integration

The latest advance in MPD riser technology is the industry’s first RCD to be integrated with the riser below the water line. It also is the first RCD to conform to API 16RCD drill-through specifications.

If a gas influx ascends above the BOP and enters the riser, the SeaShield marine series, including the Model 7875 below-tension-ring RCD, enables the operator to contain and bleed off the gas, minimizing potential risks.

Installed for the first time on a drillship in Indonesia, the MPD system successfully provided early kick detection in difficult carbonate formations where kick-loss problems are common. Gas influxes were detected at volumes of only a couple of barrels, and well pressure was managed while drilling, making connections, and tripping. In addition, the MPD system acquired formation pressures in real time and allowed logs to be run in a safe, controlled manner.

Drilling methods were transitioned easily to apply the most appropriate approach to specific wellbore conditions. The upper part of the carbonate section was drilled using constant bottomhole pressure (BHP) MPD methods to manage pressures within a narrow drilling window. When the window closed and losses to natural fractures became total, a shift was made to pressurized mud cap MPD.

To create an MPD system aboard the drillship, a Model 7875 below-tension-ring SeaShield RCD was installed in the MPD riser joint on the top of the upper annular preventer approximately 140 ft (43 m) below the sea level surface.

It is not the first time MPD operations have been implemented aboard a floating structure, but all previous RCD installations have been above the water line and tension ring. The first MPD application from a floater was performed by Weatherford International Ltd. in 2004 using a Model 7100 RCD designed for surface applications.

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 shift easily between either conventional or MPD drilling methods.

The Model 7875 below-tension-ring RCD was installed on a deepwater well offshore Indonesia so the operator could fully enclose the well bore and all drilling fluids in a closed loop and employ advanced drilling techniques without compromising the rig’s heave compensation. (Images courtesy of Weatherford International Ltd.)

Many design changes were made to address riser applications. Location is key to these changes. While surface RCD designs sit atop the BOP and thus require only a bottom flange to bolt them to the stack, integration with the riser requires the RCD to be connected at the bottom and the top. Location also creates challenges for installing, maintaining, and operating the RCD far below the rig floor where conditions make it dangerous and difficult to deploy personnel.

The riser RCD addresses this issue with a number of innovations. Among them, the system has a hydraulic latching system for changing bearing and sealing elements without the need for personnel in the moonpool area. A bearing assembly running tool and ancillary equipment facilitate rig floor positioning and removal. A subsea-rated hydraulic stab plate is used to make hydraulic and electrical connections below the water line, and the multiport connections speed the deployment and makeup of hydraulic and electrical lines and eliminate multiple control cables.

Managing Micro-Fluxes

Measuring and managing downhole pressure fluctuations are core capabilities of MPD methodologies. In a closed-loop MPD circulating system, micro-fluxes in BHP are identified almost immediately in very small increments. Conversely, a little annular pressure applied at the surface is conveyed rapidly to the bottom of the hole, changing equivalent circulating density without altering the mud system.

This takes place with the rig’s BOP and mud management systems in place and ready to take over should a well-control event occur. MPD operations are not a replacement for well-control methods. Rather, they provide an early warning and management capability that stands in front of traditional well-control methods to reduce their use and better inform their application.

In doing so, MPD reduces the threat posed by riser gas in several ways. It provides a highly sensitive instrument for identifying an influx and the means to manage it. This greatly reduces the risk of an influx escalating into a well-control event. If necessary, the gas influx can be managed, circulated out of the hole through the riser and flow spool, and diverted away from the rig floor.

Even when drilling conventionally, the riser-gas handling capabilities of the system remain functional. Should gas enter the riser, circulation is stopped, and the surface and subsea annular BOPs are closed to contain the gas in the riser. Either the MPD or rig manifold can be used to circulate the gas out in a controlled manner.

And should an event occur that calls for well control, MPD measurements enable a much more informed and timely decision.

RCD Basics

The defining MPD technology is the RCD. Its ability to contain and direct annular fluids and gas is what creates the closed-loop circulating system that is the basis for MPD methodologies. The device’s development path spans decades and a scope of applications from basic land operations to drillships and marine risers.

The predecessor of Weatherford’s RCD product line that debuted in 1968 was a small, low-pressure rotating device used for diverting annular returns in air drilling. Drillers using fluid circulating systems soon adopted the technology as a safety device to divert fluids and gas away from the rig floor.

Containing the incompressible drilling fluid created a closed-loop system. That unique environment offered many capabilities to innovative drilling engineers. They were seeking solutions to difficult operational and economic challenges that conventional drilling was unable to resolve effectively. Those pioneering efforts ultimately evolved into the mature set of MPD methodologies being used for land and offshore exploration and development drilling.

These systems employ a set of tools that includes RCDs, flow metering technologies, drilling choke manifolds, and downhole isolation valves. The MPD components are integrated by software that monitors, analyzes, and manages wellbore pressure. A key aspect of this MPD advance has been the growing sophistication of the RCD.

In basic terms, an RCD consists of a bowl or body, a latch mechanism, and a bearing assembly with an elastomeric sealing element. This rotating head is made up directly to the top of the BOP (typically through the rotary table). Drillpipe is then run through the device. The bearing assembly allows the pipe to rotate with the elastomeric element seals so returning annular fluids and gas are contained and redirected to an MPD choke.

This basic functionality has been applied to many RCD designs to handle such factors as larger pipe and higher pressures and temperatures, as well as installation and maintenance advances. The challenges of deepwater drilling have resulted in the next major evolution of RCD design and MPD application.

Deepwater Solutions

The first steps in adapting RCD technology to the marine environment involved land designs deployed on fixed platforms and on floating structures. The most recent advance integrates the RCD in the riser system. By eliminating modifications to the rig system or interference with conventional operations, the new below-tension-ring riser RCD addresses many deepwater issues, including the safety, environmental, and operational challenges presented by riser gas.