POROSITY LOGGING
The porosity of a zone can be estimated either from a single “porosity log”
(sonic, density, neutron, or magnetic resonance log) or a combination of porosity
logs, in order to correct for variable lithology effects in complex reservoirs. In the
carbonates, mineral mixtures are primarily drawn from calcite, dolomite, and
quartz (either as sand grains or as
chert); anhydrite and gypsum may also occur.
When using a single porosity log, the true porosity is calculated from
interpolation between the values for the matrix mineral and the pore fluid
(usually equated with mud filtrate, because of the shallow investigation of the
porosity tools).
Density log:
Porosity is calculated from the mass-balance relationship:
where pb is the bulk density,
0 is the porosity,
pma is the matrix density, and pf
is the pore fluid density. If a sandstone, then the matrix density is 2.65
gm/cc
(quartz), if a limestone, the matrix density is 2.71
gm/cc (calcite); if a dolomite,
then the matrix density is about 2.87
gm/cc. The density log is scaled as bulk
density in grams per cubic centimeter. If a “density porosity log” is displayed,
then it will be an apparent porosity keyed to a specific mineral, usually calcite, in
which case the curve will be indexed as “limestone equivalent porosity”. This
porosity will be in error in all lithologies whose matrix density differs from that
of calcite.
Neutron log.
Older neutron logs were scaled in counts, but modern neutron logs are recorded
in apparent porosity units with respect to a given mineralogy. Calcite is
commonly chosen as a default mineral, in which case the porosity values will be
true porosities in limestone zones. Where zones are not limestone, the
limestone-
equivalent neutron log should be
resealed to the zone matrix mineral or
combined with a density limestone-equivalent porosity in an estimate of the true
porosity.
Neutron-density log combination:
The combination of density and neutron logs is now used commonly as a
means to determine porosity that is largely free of lithology effects. Each
individual log records an apparent porosity that is only true when the zone
lithology matches that used by the logging engineer to scale the log. A
limestone-equivalent porosity is a good choice for both neutron and density logs,
because calcite has properties that are intermediate between dolomite and
quartz. By averaging the apparent neutron and density porosities of a zone,
effects of dolomite and quartz tend to cancel out. The true porosity may be
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