Page 68 - Practical Well Planning and Drilling Manual

Section 1 revised 11/00/bc 1/17/01 2:56 PM Page 44

[ ] Well Design
1.4.4

Most instances of overpressures occur in areas of fast deposition of
sediments. Water held in the formation pore spaces does not have time
to move out of the rock matrix as the rock becomes increasingly com-
pressed with growing overburden. This will cause the formation fluids
to bear a larger proportion of the overburden pressure as the grains of
rock are prevented from increasing their contact and taking their share
of the load. As porosity normally decreases with depth, any change in
this trend that slows the rate of porosity decrease with depth is an indi-
cator of possible abnormal pressure. If the formation contains salt
water, then the normally decreasing trend of resistivity with depth will
also slow down or stop. Abnormal pressures may start from the top of
this trend change. Where overpressures are caused by this mechanism
the increase is gradual with depth; ROP trends such as D exponent can
be used to identify this type of overpressure as drilling continues.
Gas generated under an impermeable boundary by decaying organ-
ic matter (biogenic gas) will cause an increase in pore pressure.
Salt domes distort and compress the formations around them and
high abnormal pore pressures can result.
Bad cement jobs on offset wells or faults below a sealing formation
can allow gas migration into higher zones, charging those zones to
abnormal pressures. A similar mechanism is where a long gas column
is normally pressured at the bottom by an aquifer. Due to the low den-
sity of gas, as you move up the gas column the difference between the
gas pressure and the normal pressure will increase and be highest at the
top of the gas column. Since the transition is very sudden if the cap rock
is not leaky, these types of abnormal pressure would not be detected by
D exponent or ROP trends while drilling (see Fig. 1-6).
Some rock transformations can cause significant increases in rock
volume. Montmorillonite changes to illite under pressure, releasing
.
water. Gypsum (CaSO 2H O) also releases water as it changes to anhy-
4
2
drite (CaSO ). If this liberated water is unable to move, pore pressures
4
could increase significantly.
Normal pressure trapped within a boundary may be moved up by tec-
tonic activity. If the pressure cannot reduce within the boundary then it
will become abnormally pressured at shallower depths. Severe kicks can
be taken by drilling into a raft of fractured dolomite within a massive salt
sequence. This is a good example of trapped pressure, originally normal,
which migrates up inside a pressure containing system. Examples are
seen in the Zechstein sequence of the North Sea; saltwater kicks may be
taken at much greater pressure gradients than would otherwise be

44