In deepwater dual gradient wells, if gas comes into the riser, a rapid unloading event may occur if removal of the gas is not carried out properly. Oil based muds present an even greater challenge when compared to water based muds to avoid gas in riser events, because the gas dissolves in the mud and may not be detected until it breaks out inside riser near the surface.

A study using a dynamic multiphase flow software simulated a rapid unloading event and determined the gas fraction in the riser annulus and the effect on riser fluid levels. New procedures, such as shut-in the riser or proactively applying surface pressure were simulated to evaluate effectiveness of riser-top equipment to mitigate risks of rapid riser unloading. The effects of gas volume, and mud type (water vs. oil based mud), on water hammer and riser top pressure during the shut-in were analyzed using a simulated case with a riser in 8,000 ft of sea water.

The analysis shows there is no significant pressure increase at the moment of rapid unloading in a shut-in well. The simulation results reliability is verified using a field test conducted in 1,200 ft sea water. The simulation results show it is possible to safely manage gas-in-riser events for large gas fractions using available managed pressure drilling and riser gas-handling equipment.

It should be possible to safely and efficiently manage events involving even relatively large gas influxes in risers by use of new proposed procedures, which mitigate the rapid unloading and can help industry develop new guidelines for deepwater drilling operations.