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Following are answers to some of the most common client questions we have received over the years.

How do you normalize total gamma ray logs to make sure volume shale output is consistent, well to well?

Typically I use the original gamma ray logs, along with shale and clean endpoints defined for each zone, within each well. I then calculate volume shale using the appropriate equation (linear or non-linear). This approach creates a volume shale result which can be compared well to well, because individual clean and shale lines have been selected for each zone, within each well, eliminating the differences between the gamma ray logs.

The other way that this can be done is to first normalize the GR logs, based on a high reading zone and a low reading zone, and then use the normalized GR logs, along with constant shale and clean endpoint values to calculate volume shale. I have used both approaches over the years and prefer the former to the latter to create a consistent volume shale output.

Why is the shale and organic matter corrected complex lithology density neutron crossplot model your default choice for calculating porosity?

This porosity model usually works well because results are relatively independent of mineralogy and are compensated for gas effects. Porosity calculated this way includes connected porosity as well as isolated micro porosity (if any), but excludes clay bound water and apparent porosity associated with organic matter.

Why does NMR porosity seem to be too low with some fine grained reservoirs?

NMR provides two useful porosity values, generally considered to be total porosity (including clay bound water) and effective porosity (excluding clay bound water). We have found that sometimes the effective porosity from NMR also excludes porosity in very fine grained silt. Since there is often not sufficient clay to account for this anomalously low porosity, the only reasonable conclusion is that the fluids in the tiny pores are not “moveable” in the NMR magnetic field. We have found instead that the NMR total porosity is close to the effective porosity from conventional methods in these reservoir types. This also seems to be tool specific.

Do you distinguish fracture porosity from matrix porosity?

No, we do not try to distinguish fracture porosity from matrix porosity. Our calculated porosity will include a small contribution from fractures, but the contribution from fractures is extremely low. The log response you see over fractured intervals is markedly exaggerated due to bad log data (sonic spikes, etc.). To remedy, we run multiple porosity models to ensure our outputs are reasonable over intervals where the log data is questionable.

Is it OK to use the Modified Simandoux equation for intervals which do not contain shale?

Yes, because this model reverts to the Archie equation in the clean zones (when volume shale goes to zero).

How do you use core measured water saturations in your work for an oil reservoir?

In our petrophysical analysis procedure, we utilize the core water saturation as a guide to the irreducible water saturation in a reservoir above the transition zone. In a core, the difference between residual oil and water saturation is usually the moveable oil fraction of the reservoir fluids, when the reservoir is at initial conditions. In older reservoirs, no longer at initial conditions, there may be some moveable water as well as the moveable oil. This can often be seen on the log analysis results depth plots where log analysis saturation is higher than core SW. The excess water saturation is a measure of potential water production.

Why didn’t you use a Pickett plot to define water saturation parameters?

In order to employ the Pickett plot, a clean (no clay) water zone must be present. No such zone exists in the subject well.

How do you determine irreducible water saturation?

Buckle’s number, along with porosity can be used to determine a theoretical irreducible water saturation (SWBKL). The SWBKL curve is then presented in the water saturation track of the answer depth plot and can be used qualitatively to highlight intervals which may produce water.  Moreover, when SW is greater than SWBKL, there is a chance that water will be produced, with or without hydrocarbon.

Buckle’s number equals the product of porosity and water saturation (BUCKL=PHIE*SW). The relationship is based on the observation that the product of porosity and water saturation remains relatively constant for a specific zone, provided rock texture remains unchanged. Once Buckle’s number is known, the equation can be rearranged to solve for water saturation, independent of resistivity.

Another way to identify moveable fluid is with NMR data, but these data are often not available.

What is your default permeability model for carbonate reservoirs?

We like Lucia’s work based on rock fabric number to calculate matrix permeability in carbonate reservoirs.

How do you calculate fracture permeability?

We don’t calculate fracture permeability, we only calculate matrix permeabiliy. To quantify fracture permeability, a pressure transient test is required.

What is the error associated with core measured water saturations?

Generally, core measured water saturation values contain plus or minus five saturation units in error. However, these values are tied to what we would call good quality data. If your core data set is lower quality, the error may be significantly higher.

What is the difference between x-ray diffraction (XRD) and energy dispersive spectroscopy (EDS)?

XRD and EDS are both quantitative methods of determining rock composition. The main difference between the two methods is that EDS determines the elemental composition near the surface of the sample, while the XRD method quantifies the specific compounds present throughout the sample.

Is core measured porosity affected by the presence of organics?

The solvent used to remove hydrocarbon for Dean-Stark porosity measurement will not dissolve organics; This means Dean-Stark porosity measurement will not be affected by the presence of organics.

What is the error associated with core measured porosity?

Generally, core measured porosity values contain plus or minus one porosity unit in error at higher porosities and plus or minus half a porosity unit at lower porosities. However, these values are tied to what we would call good quality data. If your core data set is lower quality, the error may be significantly higher.

Do you require a well with a shear slowness log to calculate mechanical rock properties?

We can provide mechanical rock properties for wells absent a shear slowness log. Both compressional and shear slowness logs, along with the bulk density log are reconstructed using results from the petrophysical analysis, using the response equation. The model is calibrated against a type well with both sonics available. The type well does not need to be the subject well.

Why do you reconstruct sonic and bulk density logs before calculating mechanical properties?

Reconstructed sonic and density logs are generated to remove bad hole, light hydrocarbon, and organic matter effects from the logs so that accurate water-filled rock mechanical properties can be calculated.

We had Schulmberger run a Platform Express-AIT recently and they did not have a machine to physically measure Rm, Rmf and Rmc. They relied on tool calculated values. Is this acceptable these days? 

This is the standard approach with the AIT tool. Rm is not constant within the borehole and the tool is able to measure Rm during the logging process.

How do petrophysicists define volume shale?

Shale volume is defined as clay bound water plus dry clay matrix.

How did you use the gamma ray logs to calculate volume shale?

Typically I use the original gamma ray logs, along with shale and clean endpoints defined for each zone, within each well. I then calculate volume shale using the appropriate equation (linear or non-linear). This approach creates a volume shale result which can be compared well to well, because individual clean and shale lines have been selected for each zone, within each well, eliminating the differences between the gamma ray logs.

The other way that this can be done is to first normalize the GR logs, based on a high reading zone and a low reading zone, and then use the normalized GR logs, along with constant shale and clean endpoint values to calculate volume shale. I have used both approaches over the years and prefer the former to the latter.

What is your recommended gamma ray log for calculating volume shale?

When available, the uranium corrected gamma ray log should be used for shale volume calculations.

Aptian Technical

 

Client Comments

“We are going to run with your model Dorian. I think your average perms over the intervals are in line with publications for the area. Great work on this set!”

B.L., Principal Geologist

“It’s nice to see your work around the office – always a solid place to start from.”

G.G., President & Principal Geoscientist

“Great work on the last round of data. I threw a few new technical tasks your way and you did a great job integrating them.”

B.L., Principal Geologist

“The merged compressional sonic log that you created now ties the seismic from surface to TD. Great job.”

N. K., Senior Geophysicist

“Thanks Dorian, this is why we keep coming back to you. Hassle free, no BS service, very refreshing.”

Z.J., Senior Geologist

“Many thanks Dorian, excellent work.”

J.C., Lead Geoscientist

“Thanks again Dorian.  And BTW, your analysis helped us enormously with our reserves evaluations. Your company does hold authority.”

P.T., Vice President Exploration

“Thanks Dorian, I knew you would come through. This is one of the reasons we keep coming back to you…you always deliver.”

Z.J., Vice President Exploration

“Hi Dorian, I was just going through the mapping and it looks great so far! Very happy with what I’m seeing.”

J.G., Senior Geologist

“Thanks Dorian for the quick turnaround. Your proposal for petrophysical services looks very good (impressive) and I will be approving.”

A.B., Vice President Geosciences

“Your petrophysical model matches core very closely.”

B.E., Senior Engineer

“Hi Dorian, Thanks for solving those problems. You have a black belt in petrophysics.”

S.R., Senior Geophysicist

“Excellent work! Thank you for the speed!!!”

R. R., Senior Engineer

“Thank you for your petrophysical evaluation. We appreciate that it is ahead of schedule.”

J.M., Vice President Exploration & Production

“Your work is consistently good and your results are very credible.”

R.C., Senior Engineer

“You come highly recommended. We would like you to complete a project for us.”

T.G., Geologist

“We took your advice and had some cuttings analyzed in a few wells. The results support your interpretation. Thanks for pointing us in the right direction.”

D.M., Senior Geophysicist

“We have found your petrophysical evaluations extremely useful and have had good success integrating the work into Petrel.”

A.B., Vice President Geosciences
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