In deepwater 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 (OBMs) present an even greater challenge when compared with water-based muds (WBMs) to avoid gas-in-riser events, because the gas dissolves in the mud and may not be detected until it breaks out inside the riser near the surface.

A study with 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 shutting in the riser or proactively applying surface pressure were simulated to evaluate the effectiveness of riser-top equipment to mitigate risks of rapid riser unloading. The effects of gas volume and mud type (WBM vs. OBM) on water hammer and riser top pressure during the shut-in were analyzed with a simulated case with a riser in 8,000 ft of seawater.

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