Page 164 - Well Logging and Formation Evaluation

154 Well Logging and Formation Evaluation

higher than if only water had been present. This is discussed in more
detail in Chapter 2.
• Depletion. Production from a reservoir, where there is insufficient
water drive or injection to replace the drained volumes, will result in
a loss of reservoir pressure. To some extent, this drop in pressure will
be offset by the effects of gas coming out of solution or compaction.
However, these processes will always lag behind production. In some
reservoirs, loss of pressure due to depletion may be very significant
(up to 5000psi) and result in severe problems during drilling of subse-
quent development wells.
• Compaction disequilibrium. During burial under equilibrium condi-
tions, the water in the porespace is free to leave, thus ensuring that as
the overburden increases, it is mainly the rock matrix that takes the
weight of the overburden. If the water is not, or only partially, free to
leave, some of the weight of the overburden is taken up by the
porefluid, resulting in a much higher porefluid pressure than would
otherwise be the case. Although shales are usually impermeable over
production life cycles, the long time periods associated with deposition
and burial usually cause them to expel water quickly enough for equi-
librium to be maintained. Conditions in which such overpressures are
likely to occur are when (a) the permeability of the shale decreases with
compaction, (b) the thickness of the shale is very great, (c) the shale is
structurally weak, or (d) the rate of burial is very fast.
• Aquathermal pressures. Where part of a system becomes isolated so
that it retains a constant volume under burial, a change in temperature
may result in a rapid increase of pressure.
• Phase changes. The volume of water in a system may increase, thereby
resulting in an increase of pore pressure under conditions of (a) dehy-
dration and dewatering of clays, in particular where montmorillonite is
transformed into illite, and (b) conversion of gypsum to anhydrite.
• Osmosis. Where two reservoirs have different salinity and are separated
by a semipermeable membrane (e.g., a clay), water will flow from the
less saline to the more saline, resulting in an increase in pressure. In
theory, such pressure differentials could reach 3000psi, although this
has not been observed in practice.