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Dear All,

In Interactive Petrophysics Software, there is a box filled by default the value of GR Tool diameter (3.625 In.). In case that we do not know the diameter of GR Tool, is the corrected values of GR (GrC) acceptable? Or how can we get the information on tool diameter?

Regarding Corrections of Neutron Log labeled NPHI, is it important to convert NPHI to TNPH? What is the input Neutron curve to be selected in CNL Tab? What about the check box (input neutron hole size corrected), does it need to be ticked or left blank? And how can we know the type of the tool?

I am a student having no background neither in Geology and nor in Petrophysics.

Your supports are highly appreciated,

MilkyWay

Dear Milky Way,

The Outside Diameter of the Gamma Ray tool is required to make hole size and mud weight corrections on the GR log. In addition to the OD of the GR tool, you will also need to have a caliper log to make proper hole size correction for the GR log. The OD of 3.625 inches is for the older generation Schlumberger GR logging tool. They also have a GR tool with OD of 3.375 inches. If you don't know the exact OD of the GR tool for your GR log, you can still use the default OD of 3.625 in the Interactive Petrophysics software. Since the GR log is usually used for shale volume calculation, it should be alright, as long as you put in the correct bit size, mud weight and a caliper log.

The CNL (Compensated Neutron Log) is the classical neutron porosity log from the CNT (Compensated Neutron Tool). It has been computed from the ratio of instantaneous (off depth) count rates from the near and far neutron detectors. Usually the hole size correction has already been applied by the logging engineer, either using a constant hole size (i.e. bit size) or a caliper log (from the density tool which is usually run together with the neutron tool). Therefore, keep the "input neutron hole size corrected" ticked.

The TNPH (Thermal Neutron Porosity log) is a later generation neutron porosity log, which has been computed from the ratio of the neutron count rates that are dead time corrected, depth matched and resolution matched. The TNPH is the preferred neutron porosity log. However, we don't have it all the time. Whether the log is CNL or TNPH should be shown on the log header/insert. You can also find out from the log data file (either LAS or LIS/DLIS file). If you don't have the original log print or the data file, it will be difficult to find out what sort of neutron porosity log it is. If it is an old log, it is probably a CNL log, but we cannot say for sure.

Regards,

Ko Ko Kyi

Dear Milky Way,

Usually, for vertical wells the logging tools are run using standard standoffs and eccentralizers etc. The Induction Resistivity Tool is usually run with a 1.5" rubber standard off. The neutron tool is run eccentered with a bow spring, since it needs to be pushed against the borehole wall. The density tool has a caliper arm which pushes the density detector pad against the borehole wall. In the standard tool string the Gamma Ray is eccentered because of the above tools which are run in combination. The GR is seldom run centralized for the standard supercombo or triple combo logs.

If your logs were recorded during the early 1980's, you may not have a tool string sketch on the log. Therefore, you will not know if the GR is run centralized or eccentered. I would use the option of "eccentered" for the GR log. However, in the modern day logs which are recorded using computers, the sketch of the tool string will be shown on the log insert.

As I mentioned earlier, the GrC log will be used for shale volume computation in IP. Therefore, only the relative values (like GR Max and GR Min) are required to make the computation. In other words, all the GR log values will be relative. Therefore, the final shale volume computation should be alright.

Regards,

Ko Ko Kyi

Dear Ko Ko Kyi,

Thank you so much. I have learned a lot from you.

Regarding directional well, I have tried to find the standoff important for environmental corrections of Resistivity and Neutron, but there is no indications. Based on your experienced, is there any standoff in directional wells (the well was logged in 1983)? How can I know the Induction Tool Type for Induction Corrections, and the Tool Combinations for DIL Invasion? (From Log Header: ISF-BHC-MSFL-GR)

Thank you so much in advance.

Best regards,

Milky Way

Dear Milky Way,

The Induction tool will have 1.5" as a standard standoff.
The type of Induction tool can be identified as follows:

ISF - Induction Spherically Focused log uses the old generation 6FF40 Induction Tool. 6FF40 is the code for number of induction coils (6) and spacing (40 inches).
DIT - Dual Induction tool will provide two induction resistivity cures, namely ILD (Deep Induction ) and ILM (Medium Induction).

Therefore for the ISF-BHC-MSFL-GR combination logs, use the 6FF40 Induction tool type.

Regards,

Ko Ko Kyi

Dear Ko Ko Kyi,

It is clear now. Thank you so much.

Best regards,

Milky Way

Dear Ko Ko Kyi,

I'm still having problems with Neutron Corrections; it's confusing me since it is mentioned in the Log Interpretation Chart that the way the "automatic" borehole correction is "backed out", depending on whether the NPHI or TNPH and NPOR curves are used. From the log heading, the tool used is CNL from the combination run LDL-CNL-NGS, and and the curve is labeled NPHI.

If I'm not mistaken Hole Size Correction (HC) was already applied, based on "Parameters". Can you please tell me the term HC (Are mudcake and standoff corrections automatically included)? Would you mind clarifying the process in CNL Tap of IP? Do we need to clear any ticks (please kindly refer to the attached pictures)? What about the standoff (No information from log heading)? How can we assure that the result is acceptable?

Kindly regards,

Milky Way

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

Dear Ko Ko Kyi,

Thank you so much for your clarification.

The result shows relatively high compared to the original log, and even greater than the uncorrected Neutron porosity (using Neutron Conversion to get the uncorrected one) although the same input and output matrix (Limestone to Limestone) are used. Is that logic?

In fact, the zone of interest is dominated by sandstones; that is why I would like to do the environmental corrections. However, the porosity in this study will be computed from the combination of Neutron and Density logs. In practical and based on your experiences, can you tell me which one is more accurate form the three porosity logs (RHOB, NPHI, and DT) and the Neutron-Density combination (No core data are available for porosity and permeability measurements and for core-log calibrations)?

I also have doubt about Default Parameters (Rm, Rmf, Rmc, and their measured temperatures) in IP; when I select the log run to used default parameters, the measured temperatures correspond to each mud resistivity appear the bottom hole temperature, except Rm. If we have the measured values from different sources (Flowline and Press), values from which source should be used as defaults? And the temperatures must be matched with each resistivity value, right?

Thank you so much in advance.

Kindly regards,

Milky Way