A recent Colin Leach comment (below) merits a separate post. Keep in mind that formation gas is highly soluble in oil-based fluids. Under bottomhole temperature/pressure conditions, the gas may be completely dissolved in the drilling fluid, complicating kick detection.
Problem: A small amount of gas is in the riser above the BOP. This gas has been recognized, the BOP has been shut, and the well below the BOP has been killed and circulated.
What to do next? There may be 2000 bbl of mud in the riser. Overboard? In any case if this is done, it may get quite exciting when the (small) gas pocket reaches surface. Do we always send this mud overboard? Whenever there is a doubt?
The alternative of circulating through a “riser gas handler” (if the rig is equipped with one) is quite safe as long as some back-pressure (perhaps 200 psi as a minimum) is held on a choke in order to prevent what was termed “explosive unloading” from happening. Without this back-pressure, the events that follow are disastrous.
Follow-up: It is time to take a step back and re-look at this whole situation. To do so does require the realistic modeling software….OLGA, Drillbench etc.. Simple gas calculations (P1V1=P2V2…..or even allowing for Z) don’t include the dynamic effects that you will see and simply must not be used. The gas rates that come from these simple calculations are misleadingly low.
Certainly this is an area where the industry needs some focused effort.
Bud's Offshore Energy (BOE) 
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