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SLB Environmental Corrections in Interactive Petrophysics 
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Joined: Tue Apr 05, 2011 8:38 am
Posts: 80
Post Re: SLB Environmental Corrections in Interactive Petrophysic
Dear Milky Way,

If you use the neutron porosity conversion chart, with input limestone porosity units (as recorded) and output sandstone porosity units, you will get a higher porosity by about 4 to 5 p.u. However, if you input limestone porosity units and obtain output in limestone porosity units, they should be the same, as the limestone porosity is a linear line with one to one correlation.

In my experience, RHOB (bulk density log) is the best log to use for deriving formation porosity. The RHOB log is less affected by clays (clay bound water) than the neutron log. Even if you use a fixed grain density of 2.65 gm/cc for both sand and shale intervals, the computed porosity is not too far off from the actual porosity. We have done extensive calibration with core data and found that the total porosity PHIT derived from the RHOB log matches the core porosity (oven dried porosity) very well. Shell, for example, is a firm believer in RHOB derived porosity. If you use the combination of neutron and density (either apparent porosity or crossplot porosity) you may overestimate the computed porosity. This is due to the fact that the neutron porosity log is very much affected by clays (clay bound water). Of course, the RHOB log has to be corrected for gas or light hydrocarbon effects, by using an appropriate method, before computing porosity.

However, if you wish to use the neutron porosity in combination with the RHOB porosity, both must be in the same lithological units. Therefore, you need to convert the neutron porosity log (which is recorded in limestone units as a standard) into sandstone porosity units. Then only you can use a suitable equation like "Gaymard's equation" or "2/3 + 1/3" averaging equation to compute the gas-corrected porosity.

As for the Rmf, Rm and Rmc values, you should use the ones which come from "Press". This just means that the mud filtrate and mud cake have been obtained from a filter press, to filter out the water from the solids. These values should be converted to downhole temperature as continuous curves, so that they can be extrapolated to the corresponding temperature of the reservoir where you are making your interpretation. If you input the Rmf, Rm and Rmc at the measured temperature (at surface) IP should extrapolate them to downhole temperature. You should also compute a continuous temperature curve using the surface temperature and bottom hole temperature at their corresponding depths. This temperature curve will then be used by IP to extrapolate the mud properties to downhole temperature. The Rmf, Rm and Rmc values need not be at the same temperature as long you correctly input the corresponding temperatures at which they are measured.

Regards,

Ko Ko Kyi


Wed Jul 18, 2012 3:08 pm

Joined: Mon Jul 09, 2012 3:31 pm
Posts: 18
Post Re: SLB Environmental Corrections in Interactive Petrophysic
Dear Ko Ko Kyi,

Thank you so much again for prompt response.

Kindly regards,

Milky Way


Thu Jul 19, 2012 8:44 am

Joined: Mon Jul 09, 2012 3:31 pm
Posts: 18
Post Re: SLB Environmental Corrections in Interactive Petrophysic
Dear Ko Ko Kyi,

If I'm not mistaken, the temperatures corresponding to Rm, Rmf, Rmc are the measured temperatures, right? However IP did not show that although at the beginning the loading PARAMETERS from LAS was selected. Regarding each source as you mentioned earlier, the one from PRESS (their mud resistivities @ measured temperatures, BHT) is more reliable, right? I am sorry about that it seems simple things, but I just want to make sure.

The depth is in feet and the temperature is in degree C; wan we use the temperature gradient in degree C per 100 ft in IP? Or it has to be absolutely in DEG F/100ft or DEG C/100m?

One more thing is the annual meas surface temperature (Tms) for determining geothermal gradient. I do not know exactly the Tms; should we assume or simply take the surface temperature to do the calculation?

Kindly regards,

Milky Way


Fri Jul 20, 2012 12:54 am

Joined: Tue Apr 05, 2011 8:38 am
Posts: 80
Post Re: SLB Environmental Corrections in Interactive Petrophysic
Dear Milky Way,

Yes, sometimes the log interpretation software does not pick up the log header data properly. In that case you will have to key them in yourself.

The reason "PRESS" is taken as the source is not because it is more accurate, but the mud filtrate and mud cake samples are taken from the filter press, hence the source "PRESS".
Of course the mud sample itself is taken from either the mud pit or the flowline, which is more representative of the mud from the well. Choosing either "PRESS" or "flowline" does not affect the accuracy of the measured Rmf, Rm and Rmc values, if they have been done correctly by the logging engineer.

Yes, you can use either degC/100ft or degC/100m, as you wish. The IP software will automatically change the temperature values according to the units of depth measurement. However, to make it consistent, use the same units of measurement for depth and the temperature gradient, so as not to confuse the software.

Regards,

Ko Ko Kyi


Fri Jul 20, 2012 1:39 am

Joined: Mon Jul 09, 2012 3:31 pm
Posts: 18
Post Re: SLB Environmental Corrections in Interactive Petrophysic
Dear Ko Ko Kyi,

Thank you so much indeed for your supports.

Kindly regards,

Milky Way


Fri Jul 20, 2012 7:37 am

Joined: Thu Apr 26, 2012 10:13 am
Posts: 16
Post Re: SLB Environmental Corrections in Interactive Petrophysic
Dear friends,

I have a resistivity measurement in borehole with a HRLT logging tool, with 5 resistivity curves: RLA1, RLA2, ... RLA5. How can I determine true resistivity RT with IP software, it "seems" (for a beginner) that there are no corrections available with IP3.4 and 3.5 ...
Is it possible to make some excel calculation to determine RT?
Any suggestions are welcomed.


Last edited by listmember on Thu Sep 27, 2012 5:36 pm, edited 1 time in total.



Thu Sep 27, 2012 9:04 am

Joined: Thu Apr 26, 2012 10:13 am
Posts: 16
Post Re: SLB Environmental Corrections in Interactive Petrophysic
Dear friends,

I have a resistivity measurement in borehole with a HRLT logging tool, with 5 resistivity curves: RLA1, RLA2, ... RLA5. How can I determine true resistivity RT with IP software, it "seems" (for a beginner) that there are no corrections available with IP3.4 and 3.5 ...
Is it possible to make some excel calculation to determine RT?
Any suggestions are welcomed.


Thu Sep 27, 2012 9:07 am

Joined: Tue Apr 05, 2011 8:38 am
Posts: 80
Post Re: SLB Environmental Corrections in Interactive Petrophysic
You cannot make environmental corrections on HRLA resistivity logs using IP software to obtain Rt values.
The HRLA resistivity curves need to be inverted using Schlumberger proprietary software to obtain Rt.
Usually the Schlumberger logging engineer will provide two additional resistivity curves, namely Rt and Rxo when an HRLT tool is run.
You can request Schlumberger to do the inversion and give you an Rt log.
If that is not possible, just use the RLA5 curve as your Rt. It is the deepest reading resistivity log.


Thu Sep 27, 2012 4:25 pm

Joined: Thu Apr 26, 2012 10:13 am
Posts: 16
Post Re: SLB Environmental Corrections in Interactive Petrophysic
So without an SLB software (Elan or ..) it is impossible to determine RT...
Even from SLB Log interpretation Charts seems not possible to determine Rt, because there in no chart related to ratios of RLA1..5 and Rxo?
Am i right?


Fri Sep 28, 2012 9:24 am

Joined: Tue Apr 05, 2011 8:38 am
Posts: 80
Post Re: SLB Environmental Corrections in Interactive Petrophysic
No, you cannot derive Rt from HRLA resistivity logs without doing inversion using all the HRLA curves. Even a software like ELAN cannot do it.
What ELAN does is to solve for different volumes of minerals and fluid types in a formation using the input log data. It does not do inversion to derive Rt.
The Rt from an HRLT type of resistivity logging tool is derived from the the different HRLA resistivity curves using an invasion model.
Therefore, only a service company like Schlumberger can do the inversion.
This is the same for other service companies which provide multiple array resistivity logs.
I believe you will not be too far off if you use the deepest HRLA (i.e.HRLA5) as your Rt to compute water saturation.
There are uncertainties on many other input parameters for the water saturation equation (depending on the type of model you use) that the end results will not be too far off.
Furthermore, you are going to use the same HRLA log to derive Rw (formation water resistivity). Therefore, the effect of error in the Rt will be somewhat minimized.


Sat Sep 29, 2012 1:54 am
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