An influx of gas into the wellbore can have a significant effect on the annulus pressure.
Since there is such a large difference in density between the gas and the mud a gas bubble entering the well will be subjected to a large buoyancy effect. The gas bubble will therefore rise up the annulus. As the gas rises it will expand and, if the well is open, displace mud from the annulus. If, however, the well is shut in mud cannot be displaced and so the gas cannot expand. The gas influx will rise, due to buoyancy, but will maintain its high pressure since it cannot expand. As a result of this Pann will increase and higher pressures will be exerted all down the wellbore (note the increase in bottom hole pressure). The situation is as shown in Figure 14. This increase in annulus, and therefore bottom hole, pressure will be reflected in the drillpipe pressure (Pph = bhp – ρmd). This situation can, therefore, be identified by a simultaneous rise in drillpipe and annulus pressure.
It is evident that this situation cannot be allowed to develop as it may lead to the problems mentioned earlier (casing bursting or underground blow-out). From the point at which the well is shut in the drillpipe and annulus pressures should be continuously monitored. If Pann and Pdp continue to rise simultaneously it must be assumed that a high pressure gas bubble is rising in the annulus. In this case, the pressure must be bled off from the annulus by opening the choke. Only small volumes (1/4 – 1/2 bbl) should be bled off at a time. By opening and closing the choke the gas is allowed to expand, and the pressure should gradually fall. The process should be continued until Pdp returns to its original shut in value (again Pdp is being used as a bottom hole pressure gauge). This procedure can be carried out until preparations to kill the well are complete. During this procedure no further influx of fluids will occur, provided Pdp remains above its original value.
Institute of Petroleum Engineering, Heriot-Watt University