Page 149 - Hydrocarbon Exploration and Production Second Edition

136 Reservoir Fluids

Pressure
water
table

normal pressures

Depth sealing layer

reservoir

underpressure overpressure
Figure 6.26 Normal and abnormal pressure regimes.

depletion of a sealed or low-permeability reservoir due to production within the
reservoir
depletion due to production in an adjacent field whose pressure drops, with
pressure connection via a common aquifer
phase changes, for example anhydrite into gypsum or alteration of clay mineralogy
overpressures as a result of hydrocarbon columns
inflation of pressure as a result of seal failure, for example a fault between blocks.
This can result in uncontrolled cross-flow between reservoirs.

6.2.8.3. Drilling through abnormal pressures
When drilling through normally pressured formations, the mud weight in the well is
usually controlled to maintain a pressure greater than the formation pressure to
prevent the influx of formation fluid. A typical overbalance would be in the order of
200 psi. A larger overbalance would encourage excessive loss of mud into the
formation, which is both costly, and may damage the reservoir properties. If an
influx of formation fluid into the borehole did occur due to insufficient overbalance,
the lighter formation fluid would reduce the pressure of the mud column, thus
encouraging further influx, and an unstable situation would occur, possibly leading
to a blowout. Hence, it is important to avoid the influx of formation fluid by using
the correct mud weight in the borehole.
When drilling through a shale into an overpressured formation, the mud weight
must be increased to prevent influx. If this increased mud weight would cause large
losses in shallower, normally pressured formations, it is necessary to isolate the
normally pressured formation behind casing before drilling into the overpressured
formation. The prediction of overpressures is therefore important in well design.