Correcting Log Values in Hydrocarbons
The values of density, neutron, and sonic logs recorded in hydrocarbon bearing zones will usually be different than when the zone is filled with water. Invasion of drilling fluid into the reservoir displaces some of the hydrocarbons, so the log readings represent neither the water filled nor the in-situ hydrocarbon cases.

Editing to compensate for hydrocarbon effects will depend on what the log data is to be used for. Reservoir properties calculations, such as porosity and water saturation, no editing is done, but the appropriate petrophysical analysis model is used. In calculation of elastic properties, the analyst might want either water filled or un-invaded values. Seismic modeling requires values for the un-invaded reservoir. The raw log data are inappropriate for many typical analytical problems.

Log modeling, often called fluid replacement modeling, is used to develop edited versions of the logs suitable for the specific situation. Sometimes porosity or lithology are also modeled, so fluid replacement is not the only change we might make. To generate a reasonable synthetic seismograph, the density and sonic data must be reconstructed by calculating the log response for an un-invaded zone. This is done by reversing the usual sonic and density (shale corrected) equations with the correct fluid terms to obtain the interpreted log readings.

Numerous modeling approaches are described in other Sections of this Chapter.
 

Example showing gas effect on density and sonic porosity (reading too high) and on neutron
and resistivity (reading too low).