doveton2 - POROSITY LOGGING The porosity of a zone can be...

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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 8
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_ estimated either by taking an average of the two log readings or by applying the equation: where @t and $d are neutron and density porosities. It has been suggested that the square-root equation is preferable as a means of suppressing the effects of any residual gas in the flushed zone.
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doveton2 - POROSITY LOGGING The porosity of a zone can be...

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