After the pumps are brought up to desired kill rate and drill pipe pressure stabilize, personnel must observe and record the drillpipe pressure. For the first circulation, you must hold drill pipe pressure constant in order to maintain constant bottom hole pressure. To achieve maintain drill pipe pressure constant, personnel must manipulate the choke while circulating.
When gas in drilling fluid is circulated to the surface, the gas will naturally begin to expand because of decrease in pressure. The expansion of gas in mud will result in increasing both pit volume and casing pressure. This time is the most critical period of the well control operation. If personnel panic about it, the operation could be turned out from a good operation to a disaster.
Sometimes, you may have difficulty to bleed the gas off fast enough to keep the drill pipe pressure within required limits. As you know, excessive drill pipe pressure can result in formation breakdown. If you cannot release gas fast enough from the annulus to prevent an increase in drill pipe pressure, you must show down pump rate to compensate the pressure.