Re: Porosity and Sw equations in IP
Hi Roambold,
I will try to answer your questions.
1)The reference for calculating Hydrogen Index of Hydrocarbon (used in neutron porosity model of IP software) can be found in the following book:
Chapter 5 Porosity Logs, Page 5-21, Equation 15-5, Log Interpretation Principles/Applications published by Schlumberger, Seventh Edition 1998. Copyright Schlumberger 1991.
There are a whole bunch of references at the end of Chapter 5, but I could not find out from which reference this equation was taken.
2)IP is using the apparent hydrocarbon density, instead of the actual hydrocarbon density, to calculate porosity from the density log, because the actual hydrocarbon density at downhole reservoir conditions is not known and has to be estimated using the density log reading in the invaded zone. Hydrocarbon density at downhole conditions will probably be different from the actual hydrocarbon density measured at surface. The density log is responding to the fluids inside the invaded zone, under downhole reservoir conditions.
3)In WBM environment, the hydrocarbon in the invaded zone is flushed by the mud filtrate (essentially water). Therefore, the Sxo will be higher than the Sw (for a hydrocarbon bearing interval). However, there will still be some residual hydrocarbon left in the invaded zone, meaning Sxo cannot be 100%. This needs to impose a limit on the Sxo computed by the software. Most log analysis softwares have ways to limit the Sxo to a certain value. Some allow the analyst to choose his own Sxo limit. I could not find any reference on why IP software uses Sxo < Sw^Sxo limit.