Page 119 - Hydrocarbon Exploration and Production Second Edition

106 Reservoir Geology

acidity/alkalinity (pH) of the pore water which circulates through the formation.
Consequently, the migration of hydrocarbons and the displacement of water out of
the pore system may end or at least retard diagenetic processes.
Diagenesis will either increase or decrease porosity and permeability and cause a
marked change in reservoir behaviour compared to an unaltered sequence.
The diagenetic processes relevant to field development are compaction,
cementation, dissolution and replacement.
Compaction occurs when continuous sedimentation results in an increase of
overburden which expels pore water from a sediment package. Pore space will be
reduced and the grains will become packed more tightly together. Compaction is
particularly severe in clays which have an extremely high porosity of some 80% when
freshly deposited.
In rare cases, compaction may be artificially initiated by the withdrawal of oil, gas
or water from the reservoir. The pressure exerted by the overburden may actually
help production by ‘squeezing out’ the hydrocarbons. This process is known as
‘compaction drive’, and some shallow accumulations in Venezuela are produced in
this manner in combination with EOR schemes like steam injection.
If compaction occurs as a result of production, careful monitoring is required.
The Ekofisk Field in the Norwegian North Sea made headlines when, as a result of
hydrocarbon production, the pores of the fine-grained carbonate reservoir
‘collapsed’ and the platforms on the seabed started to sink. The situation was later
remedied by inserting steel sections into the platform legs. Compaction effects are
also an issue in the Groningen gas field in Holland where subsidence in the order of
1 m is expected at the surface.
Compaction reduces porosity and permeability. As mentioned earlier during the
introduction of growth faults, if the expulsion of pore water is prevented, over-
pressures may develop.
Cementation describes the ‘glueing’ together of components. The ‘glue’ often
consists of materials like quartz or various carbonate minerals. They may be
introduced to the system by either percolating pore water and/or by precipitation of
minerals as a result of changes in pressure and temperature. Compaction may, for
instance, lead to quartz dissolution at the contact point of individual grains where
pressure is highest. In areas of slightly lower pressure, for example space between the
pores, precipitation of quartz may result (Figure 6.10).
This kind of pressure solution/precipitation is active over prolonged periods of
time and may almost totally destroy the original porosity. Precipitation of material
may also occur in a similar way on the surface of fault planes, thus creating an
effective seal via a process introduced earlier as diagenetic healing.
Dissolution and replacement. Some minerals, in particular carbonates, are not
chemically stable over a range of pressures, temperatures and pH. Therefore, there will
be a tendency over geologic time to change to a more stable variety as shown in
Figure 6.11.
Rainwater, for instance, will pick up atmospheric CO and react with calcium
2
carbonate (limestone) to form a soluble substance, calcium bicarbonate. This reaction
gives water its natural ‘hardness’.
CaCO 3 þ H 2 O þ CO 2 ! CaðHCO 3 Þ
2