Re: getting porosity (%) from sonic, density and neutron
My personal preference is to use the desnity log and neutron porosity log in combination to get an apparent porosity corrected for the presence of gas, if any. I would not use the sonic porosity, especially in clastics (shaly sand reservoirs) as the formation may not be well compacted and the presence of shales will also affect the sonic porosity.
Here is the way to get an apparent porosity using the density and neutron porosity log:
Porosity from density log DPHISS = (Rhog - Rhob)/(Rhog - Rhof)
Where, Rhog = 2.65 gm/cc for quartz
Rhob = density log data in gm/cc
Rhof = 1.0 gm/cc assuming the invaded zone is fully flushed with filtrate from water base mud
This porosity from density log will be in decimal fraction. Therefore, you need to multiply it by 100 to get the density porosity in %.
Neutron porosity in sandstone units NPHISS = Neutrom porosity from log + 4%
This is an approximate conversion of the neutron porosity log (which is usually recorded in limestone units). If you want an exact conversion to sandstone units, you can use an appropriate log interpretation chartbook from the respective well logging contractor (i.e. Schlumberger, Baker Hughes or Halliburton).
APHI (apparent porosity) = (2 x DPHISS + NPHISS)/3
This approximate porosity works very well even in gas bearing reservoirs, since it is a weighted average of density porosity and neutron porosity, which are affected by gas in opposite ways. In a gas bearing interval, the density porosity will be too high and the neutron porosity will be too low. Therefore, by taking a weighted average, the gas effect is minimized.
There are also other methods of taking weighted average of the density and neutron porosities.
APHI = SQROOT((DPHISS^2 + NPHISS^2)/2)
This is the Root Mean Square value of the two porosity logs