When an influx has occurred and has subsequently been shut-in, the pressures on the
drillpipe and the annulus at surface can be used to determine:
•    The formation pore pressure
•     The mudweight required to kill the well
•     The type of influx

In order to determine the formation pressure, the kill mudweight and the type of
influx the distribution of pressures in the system must be clearly understood. When
the well is shut-in the pressure at the top of the drillstring (the drillpipe pressure)
and in the annulus (the annulus pressure) will rise until:

(i)     The drillpipe pressure plus the hydrostatic pressure due to the fluids in the drillpipe is equal to the pressure in the formation and,
(ii)     The annulus pressure plus the hydrostatic pressure due to the fluids in the annulus is equal to the pressure in the formation.

It should be clearly understood however that the drillpipe and annulus pressure
will be different since, when the influx occurs and the well is shut-in, the drillpipe
will contain drilling fluid but the annulus will now contain both drilling fluid and
the fluid (oil, gas or water) which has flowed into the well. Hence the hydrostatic
pressure of the fluids in the drillstring and the annulus will be different. A critical
assumption that is made in these calculations is that the influx travels up the annulus
between the drillstring and the borehole rather than up the inside of the drillstring.
This is considered to be a reasonable assumption since the influx would be expected
to follow the flow of fluids through the system when they enter the wellbore.

It is convenient to analyse the shut-in pressures by comparing the situation with
that in a U-tube (Figure 9). One arm of the U-tube represents the inner bore of the
drillstring, while the other represents the annulus. A change of pressure in one arm
will affect the pressure in the other arm so as to restore equilibrium.