CamPetro

Vclay from Neutron-Sonic

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Summary

The neutron-sonic method estimates Clay volume from the position of the measured Neutron porosity and Compressional slowness relative to a clean-rock line and a clay point on the crossplot. It uses the same dual-clay form as the other crossplot methods. Use it when no density log is available and the neutron and sonic logs are good.

Inputs and outputs

Item Units
Input Neutron porosity v/v
Input Compressional slowness µs/ft
Input Neutron porosity at first clean point v/v
Input Slowness at first clean point µs/ft
Input Neutron porosity at second clean point v/v
Input Slowness at second clean point µs/ft
Input Clay neutron porosity v/v
Input Clay slowness µs/ft
Output Clay volume v/v

Equations

The two clean points \(A\) and \(B\) define the clean-rock line on the neutron porosity-compressional slowness crossplot, and the clay point is the 100% clay end. The clay volume is the offset of the measured point from the clean line, as a fraction of the offset of the clay point:

\[ \Vcl = \frac{(\phiNCleanB - \phiNCleanA)(\dtc - \dtCleanA) - (\phiN - \phiNCleanA)(\dtCleanB - \dtCleanA)}{(\phiNCleanB - \phiNCleanA)(\dtClay - \dtCleanA) - (\phiNClay - \phiNCleanA)(\dtCleanB - \dtCleanA)} \]

and the result is clamped to the interval 0 to 1. The numerator and denominator are both proportional to a perpendicular distance from the clean line, so their ratio does not depend on the units or the scaling of either axis.

Symbol Variable Units Typical range
\(\phi_N\) Neutron porosity v/v -0.02 to 0.60
\(\Delta t\) Compressional slowness µs/ft 40 to 140
\(\phi_{N,A}\) Neutron porosity at first clean point v/v -0.05 to 0.05
\(\Delta t_A\) Slowness at first clean point µs/ft 45 to 60
\(\phi_{N,B}\) Neutron porosity at second clean point v/v 0.15 to 0.40
\(\Delta t_B\) Slowness at second clean point µs/ft 70 to 110
\(\phi_{N,clay}\) Clay neutron porosity v/v 0.25 to 0.50
\(\Delta t_{clay}\) Clay slowness µs/ft 80 to 140
\(V_{cl}\) Clay volume v/v 0 to 1

Single-value calculator

Behavior

Clay volume is a straight-line function of each measurement, so the plot lines are straight and are clipped to 0 and 1. In this crossplot the clean line rises steeply, so clay volume is very sensitive to the clean points: a small change in a pick moves the result a lot.

Parameter guidance

Six picks define the method: two points on the clean-rock line and one clay point. The clean line starts near the matrix point (about 0 v/v and 55 µs/ft for sandstone) and a second point is read from the cleanest water-bearing sand in the zone. The clay point is read from a thick shale and must lie clearly off the clean line. The neutron clay value is shared with the other neutron methods. The general principles are in the Clay Volume page.

Worked example

A measured point of 0.20 v/v and 85 µs/ft. The clean line runs through (0.0, 55.5) and (0.27, 92.0), and the clay point is (0.40, 115.0):

A, B, clay = (0.0, 55.5), (0.27, 92.0), (0.40, 115.0)   # (neutron, slowness)
nphi, dt = 0.20, 85.0
num = (B[0] - A[0]) * (dt - A[1]) - (nphi - A[0]) * (B[1] - A[1])
den = (B[0] - A[0]) * (clay[1] - A[1]) - (clay[0] - A[0]) * (B[1] - A[1])
vcl = min(1.0, max(0.0, num / den))
print(f"numerator = {num:.4f}, denominator = {den:.4f}")
print(f"Vcl = {vcl:.3f}")

Output

numerator = 0.6650, denominator = 1.4650
Vcl = 0.454

Assumptions and limitations

  • The clean points lie on a single straight line. Lithology changes move the clean line.
  • The neutron and sonic respond to porosity in different ways, so the clean line is only straight over the range between the two points.
  • There is no gas. Gas lowers the neutron and raises the slowness, and they can offset each other in an unpredictable way.
  • The logs are good, with no cycle skipping and no washouts.
  • The clay point lies off the clean line. If it lies on the clean line the denominator is zero and the method is undefined.

QC checks

  • The result is between 0 and 1 without relying on the clamp.
  • Clean, water-bearing sands read near zero.
  • Check how much the result changes if either clean point is moved by a small amount. A large change means the result is not robust.
  • Compare with the gamma-ray clay volume.

Going Deeper

Of the three crossplot methods this is the least used, because the neutron and sonic both respond to porosity in a similar way and so the clean line and the clay direction are less well separated than on the neutron-density or sonic-density crossplots. It is kept as a fallback for wells with no density log, and its result should be treated as lower confidence than the other two.

References

References will be added once verified.

Python reference implementation

Python reference implementation

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