Dear Milky Way,
As far as I can see from the log parameters table, the HC Holesize Correction has already been applied using the
CALI (caliper from the density log).
Since you have already ticked the "Input log corrected for hole size", you don't need to do the Hole Size correction again. Otherwise, you will be doing it again. Therefore. clear the tick box for Hole Size correction.
There are so many corrections for
NPHI that I normally do not even do any more corrections on the NPHI. As you will have seen from the example in the Schlumberger log interpretation chart book, the various corrections for NPHI tend to go in opposite directions and cancel one another. In the end, the final correction for NPHI is relatively small.
Usually, there is no standoff for the CNT tool, as the detectors and the neutron source (i.e. the whole tool) are pushed against the borehole wall with a bow spring. There is only a small wear ring on the CNT tool to prevent it from being worn down by rubbing against abrasive formation during logging.
I personally do not use the neutron log for computing formation porosity. I use the density log (corrected for gas and light hydrocarbon effects) as my main porosity log. I use the neutron log for fluid identification, lithology component determination and clay volume calculation. In our company, we use the neutron-density crossplot extensively to compute the lithological components, namely sand, silt (silt size sand) and clay. From the dry rock components, we compute a continuous grain density log, which is then used with the corrected density log to compute formation porosity. We have done extensive calibration of our model with core analysis and it is robust.
The IP software does have a sand-silt-clay model, but it uses a different algorithm to compute the silt volume.
Regards,
Ko Ko Kyi