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How to control gas migration, After closing in the well and lining the annulus up to the choke manifold, the pressure should be monitored on both the annulus and DP gauges until it stabilizes (first build-up). The time required for the pressures to stabilize will depend on the permeability of the producing formation. The stabilized pressures are required for subsequent calculations prior to killing the well. If the influx is gas or contains gas, this gas will migrate up the well. With the well closed in the gas cannot expand and will retain approximately its original pressure.
The column of fluid above the gas will become smaller as the gas rises and
therefore the hydrostatic pressure due to this fluid column will reduce. This will result in an increase in surface pressure on the annulus.
Since the migrating gas bubble retains its original pressure, the bottom hole
pressure will rise. This will also cause the drill pipe pressure (PDP) to rise by roughly the same amount. A crew member must continue to observe the gauges and record the values of Pann, and PDP, against time to check for such pressure increases. These increases as a result of the influx migrating up the annulus are called secondary build ap. Secondary build-up can often be distinguished from the stabilized shut-in pressures on the graph by plotting back along with the buildup.
The aim throughout secondary well control operations is to keep the bottom hole pressure equal to PO. Thus, if the pressure rise due to a secondary build-up becomes significant, the fluid must be bled off from the annulus to allow the gas to expand as it migrates and thus keeps the bottom hole pressure constant. Assuming that the drill pipe contains the original drilling fluid, fluid can be bled off from the annulus until the drill pipe pressure returns to its original value. Accurate measurement of the volume gained in the tank(s) must also be made at this point as this gives the volume of the influx in the well. This value is also necessary for subsequent calculations.
RigWorld Well-control Training Manual