In deepwater drilling where productive horizons are relatively shallow and operators are using horizontal and extended-reach well. profiles, there is a greater chance that a gas influx will be above the subsea blowout preventer (BOP) stack before it is detected. Use of advanced well-control simulators has been beneficial in expanding understanding of the complexity of gas migration. More than 300 simulations covering a broad range of operating depths and conditions were conducted while preparing the full-length paper. Advanced hardware is available to handle gas at the surface by allowing backpressure to be held on the riser. The work reported in the full-length paper applies to waterbased mud (WBM).