Avoiding explosive unloading of gas in a deepwater riser

System handles gas flows that slip past BOP before detection

Colin P. Leach
Well Control & Systems Design
Joseph R. Roche
Hydril Company

A riser gas handling tool has been designed to aid in deepwater well control. In deepwater service, the standard subsea blowout preventer (BOP) stack will be used to deal with kicks that are detected on bottom. The gas handling tool (Gas Handler) can be used to deal with any small kicks that get through the BOP before detection.

The tool also serves to provide an effective means to carry out precautionary circulation. The backpressure put on the riser during well control operations using a gas handling tool is low enough such that an existing riser design can be used.

Hence, cost of implementing this method should be limited to that of the tool itself and relatively minor rig modifications. At any time, the tool is being used, the downhole well can be isolated using the conventional subsea BOP.

The use of oil base mud (OBM) or synthetic oil base mud (SOBM) in deepwater wells intensifies concern over the hazards that result from a small gas influx that is undetected upon entry to the wellbore (typically as a result of swabbing).

Once the gas is in the wellbore, it goes into solution in the mud and there will be no observable sign that the gas exists and is being circulated to the surface until it comes out of solution right below the rotary table. If no action is taken, there can be a violent eruption of mud and gas endangering personnel on the rig floor.

For wells drilled on land or from a platform, a jackup, or a shallow water floater, it is relatively straightforward to close in the BOP and route any potential influx through the choke manifold and surface gas separation system. Indeed, it is accepted practice on many wells where SOBM is in use, to circulate bottoms up to just below the BOP and then route circulation through the choke, simply to avoid any danger to personnel.

Seabed dilemma

This is not possible for a deepwater drilling operation where the BOP is located on the seabed, and where it is quite possible that the influx is only detected once it is above the BOP. In addition, any time the flow is routed through the choke and kill lines, there is a great potential for lost circulation because of the imposed additional choke or kill line friction pressure.

Note that the problem is less severe for water base mud (WBM) because gas migration can occur which inevitably leads to stringing out of the influx. The net result is a less-violent event as the gas reaches surface.

A special tool, derived from annular blowout preventer technology, is aimed at controlling the pressure in the riser, while circulating mud to restore equilibrium. Installed just below the telescopic joint, the GH-21-2000 Gas Handler (Hydril) has an annular packing element that affords shutoff and sealing of the riser annulus on pipe or (with no pipe in the riser) open hole closure.

In the case where formation gas passes through the BOP stack and enters the riser before a BOP can be closed, the long ascent to the surface could cause partial evacuation of the riser (and possible collapse) and/or excessive flow through the diverter system.

Stopping the upward flow at the top of the riser and directing it by way of a side outlet to a choke permits control of volumetric expansion. Meanwhile, circulation of appropriately weighted mud down the riser mud boost line and into the riser annulus (at the lower marine riser package) can restore normal pressure in the fluid column.

The gas handling tool is configured such that the bore of the riser is cleanly shut off without space for stagnant well fluids to solidify and possibly jam components or plug the choke line.

A single hydraulic signal closes the annulus and simultaneously opens the side outlet leading to the choke manifold. After the hazard in the riser has been neutralized, further procedures can be implemented to normalize wellbore pressures using the subsea BOP stack.

Tool deployment

There are three options for deploying the gas handling tool on a riser kick to control pressure and flow:

The diverter is activated and the flow is routed overboard
The flow is routed from the closed diverter to a riser gas separator
The diverter is left open and the flow is allowed to move through the shale shaker flowline and/or to the rotary table.

All three options are identical in the respect that no back pressure can be put on the riser and its contents. The impact of this inability to put any back pressure on the riser was examined using Sidekick, Anadrill's well control simulator.

In one modeled configuration, with 16 ppg mud in the well, a four-bbl kick was introduced at the bottom of the 10,000 ft riser and circulated to surface at a pump rate of 700 gpm with no back pressure held on the well.

No detectable pit gain or flow could be seen until about two minutes before gas arrived at the surface. At this time the flow from the well increased rapidly to about 2500 gpm. Gas then hit the surface, with a maximum gas flow rate of about 4 MMcf/d occurring the instant that the gas arrived (further detailed simulation is required to better establish this figure). The liquid flow rate then fell to 570 gpm before recovering to the 700 gpm being pumped in.

System features

The gas handling tool was designed expressly to deal with riser gas. Installed in the riser system below the slip joint packer, it affords the following:

Backpressure can be used to control otherwise violent expansion of gas as a kick is being circulated out of the riser
Hazardous flow is kept away from personnel
Mud is saved for re-use
Precautionary circulating (circulating bottoms up across the choke) can be carried out at high circulation rates and without imposing choke and kill line friction pressures on an exposed formation
The integral side outlet valve simplifies controls, eliminates sequencing, provides a clean shutoff at the bore
Protection of the diverter system and telescopic joint
The annular packing element closes quickly on pipe or open hole
Configured in a riser joint for convenient installation
Combating of riser collapse or burst
Handles gas trapped in BOP stack
Optional valve in side outlet line enables the option of shut-in, and internal pressure test, if desired.

Specifications

Speicifications of the gas handling tool are the following:

Bore: compatible with 21-in. riser main tube
Working pressure: 2,000 psi
Side outlet(s): 3-in. nominal and larger
Rotary: fits through 60-in.

In several configurations, additional mathematical models were run to demonstrate the effects of using a riser Gas Handler system. The same (10,000 ft) riser and 16 ppg mud combination was used as a "Diverter" example described previously.

In addition, a 100-ft, 3-in. line was used to connect the riser gas handling system to the choke manifold. A constant back pressure of 200 psi was applied at the choke. Again, a 4-bbl gas kick was introduced at the bottom of the riser and it was circulated out at 700 gpm.

As the gas reached the surface, the mud rate increased from 700 gpm to 800 gpm (maximum) and then fell back to 610 gpm before recovering to the 700 gpm being pumped. The maximum gas rate that was seen was 1.0 MMcf/d. The 200 psi choke pressure plus the friction pressure in the line "attenuates" the flow system.

Most importantly, the explosive expansion of the gas that was seen for the diverter-type system (no Gas Handler) was eliminated. For this particular set-up and for the influx size modeled, it is possible to continue circulation at the 700 gpm rate, even as the gas is passing through the choke.

Conclusions

The riser Gas Handling system enables safe handling of reasonable volumes of gas that have by chance entered the riser. The key to this system is the ability to put 300-400 psi total back pressure on to the top of the riser.

This small back pressure "attenuates" the expansion of gas as it reaches the surface. This required backpressure is low enough that existing riser designs are perfectly adequate.

Typical existing diverter systems are not suitable for handling moderate gas "bubbles" in the riser, because no back pressure can be put on the riser and the gas can "explosively unload" when it reaches the top of the riser.

Authors

Colin Leach is the President of Well Control & Systems Design, a company specializing in well-specific well control problem prevention using state-of-the-art simulation. He holds a master's degree in Chemical Engineering from Cambridge University, England and spent about 20 years working for Chevron, Getty, and Conoco prior to his current position.

Joe Roche does product and market development work for Hydril Co. in Houston, and has been with the company for 19 years. He has a BSME from the University of Illinois at Urbana/Champaign. He has done engineering related to deepwater drilling since 1972, holding 21 patents for such equipment. He is chairman of the API task group on controls for well control equipment.