Vclay from Neutron

Summary

The neutron method scales the measured Neutron porosity between a clean-rock value and a clay value to give Clay volume. Clay reads a high neutron porosity because of bound water, so a neutron log in an otherwise tight or clean rock is a sensitive clay indicator. Use it where the neutron log is good and there is no gas.

Inputs and outputs

	Item	Units
Input	Neutron porosity	v/v
Input	Clean neutron porosity	v/v
Input	Clay neutron porosity	v/v
Output	Clay volume	v/v

Equations

The clay volume is the linear position of the neutron porosity between the clean and clay values, clamped to the interval 0 to 1:

\[ \Vcl = \min\!\left(1,\ \max\!\left(0,\ \frac{\phiN - \phiNClean}{\phiNClay - \phiNClean}\right)\right) \]

Symbol	Variable	Units	Typical range
\(\phi_N\)	Neutron porosity	v/v	-0.02 to 0.60
\(\phi_{N,clean}\)	Clean neutron porosity	v/v	-0.05 to 0.15
\(\phi_{N,clay}\)	Clay neutron porosity	v/v	0.25 to 0.50
\(V_{cl}\)	Clay volume	v/v	0 to 1

Single-value calculator

Behavior

Clay volume rises in a straight line from 0 at the clean value to 1 at the clay value. The steepness depends on the clay pick: a lower clay neutron porosity makes the same measured value read as more clay. The plot shows three clay picks.

Parameter guidance

Clean neutron porosity is the neutron porosity in clean rock on the lithology scale of the log. It is close to zero in a tight clean interval, but in a porous reservoir it includes the real porosity, so the method only isolates clay where the clean porosity is known. Clay neutron porosity is read in a thick, representative shale and is usually between 0.25 and 0.50. These picks are shared with the neutron-density method. The general principles are in the Clay Volume page.

Worked example

A neutron porosity of 0.18, with a clean value of 0.0 and a clay value of 0.38:

nphi, nphi_clean, nphi_clay = 0.18, 0.0, 0.38
vcl = min(1.0, max(0.0, (nphi - nphi_clean) / (nphi_clay - nphi_clean)))
print(f"Vcl = ({nphi:g} - {nphi_clean:g}) / ({nphi_clay:g} - {nphi_clean:g}) = {vcl:.3f}")

Output

Vcl = (0.18 - 0) / (0.38 - 0) = 0.474

Assumptions and limitations

All of the neutron porosity above the clean value is due to clay. Real porosity also raises the neutron, so in porous rock the method over-predicts clay unless the clean value includes the porosity.
There is no gas. Gas lowers the neutron response and makes the rock read cleaner than it is.
The neutron log is environmentally corrected, is on a known lithology scale, and is not affected by washouts.
The clay pick is representative of the clay in the reservoir, not only of the shale in the adjacent beds.

QC checks

Clean, tight intervals read near zero and thick shales read close to 1.
Clay volume does not follow the caliper. If it does, the neutron is affected by the borehole.
Gas zones read cleaner than the gamma ray suggests. This is expected, and is a reason not to use this method alone.
Compare with the neutron-density clay volume, which separates clay from porosity.

Going Deeper

A neutron log responds to hydrogen index, which includes the hydrogen in clay-bound water and in hydroxyl groups as well as the hydrogen in pore fluid. That is the physical basis of using it as a clay indicator, and also the reason it cannot separate clay from porosity on its own. The neutron-density method addresses this by using the density log to constrain the porosity, so the neutron-only form is mostly useful as a quick indicator or where the clean porosity is low.

References

References will be added once verified.

Python reference implementation

Python reference implementation

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