Total vs Effective Porosity

Summary

Total porosity counts all pore space, including the water bound to clay. Effective porosity counts only the pore space that can hold producible fluid and takes the Clay-bound water volume out. The conversion needs the clay volume from Clay Volume and a clay porosity. Use effective porosity for storage and net pay, and total porosity where the saturation model needs the bound water explicitly.

Inputs and outputs

Equations

Total porosity is the sum of effective porosity and the water bound to clay. The bound water is the clay volume times the porosity of the wet clay:

and, in the other direction, \(\phit = \phie + \Vcl\,\phiCl\). Effective porosity is limited to the interval 0 to 1. The share of the total porosity that is clay-bound water is \(S_{wb} = \Vcbw / \phit\), which dual-water saturation models need.

Single-value calculator

Behavior

Effective porosity falls in a straight line as clay volume increases, with a slope of \(-\phi_{cl}\). The three curves show a total porosity of 0.25 and clay porosities of 0.1, 0.2 and 0.3. At a clay volume of 0.5 the effective porosity is 0.20, 0.15 and 0.10. The curve for 0.3 reaches zero at a clay volume of 0.83 and is held there, because the bound water then equals the whole total porosity. The one parameter that is not measured (the clay porosity) is therefore worth a factor of two in the result at moderate clay volumes.

Parameter guidance

Clay volume. Use the clay volume from Clay Volume, the same value the saturation model uses. The definitions matter: if the quantity passed is shale volume (Shale volume), which includes silt and bound water, then the clay porosity must be the porosity of that shale, not of pure clay. Check which one the clay volume step returns.

Clay porosity. The bound-water porosity of clay depends on clay type and compaction, typically 0.1 to 0.4. It is best taken from the shale point on the porosity crossplots (the total porosity read in a thick shale) or from core, and it must be consistent with the clay endmembers used in a mineral inversion. A single value such as 0.2 is common as a default but is a placeholder.

Total porosity. The total porosity from the density, neutron-density, sonic or inversion pages is the input. Check that kerogen has been removed first (Gas and Organic-Matter Corrections): kerogen is not clay-bound water and is not subtracted here.

Worked example

A total porosity of 0.25 for different clay volumes and clay porosities:

print(f"{'Vcl':>5s} {'phi_cl':>7s} {'bound water':>12s} {'phi_e':>7s} {'phi_e/phi_t':>12s}")
phit = 0.25
for vcl, phi_cl in ((0.0, 0.20), (0.2, 0.20), (0.4, 0.20), (0.4, 0.10), (0.4, 0.30), (0.8, 0.30)):
    bw = vcl * phi_cl
    phie = max(0.0, phit - bw)
    print(f"{vcl:5.2f} {phi_cl:7.2f} {bw:12.3f} {phie:7.3f} {phie / phit:12.2f}")
print()
print("Effective porosity at phi_t = 0.25 and Vcl = 0.5, for different clay porosities:")
for phi_cl in (0.10, 0.20, 0.30, 0.40):
    print(f"  phi_cl = {phi_cl:.2f}: phi_e = {phit - 0.5 * phi_cl:.3f}")

Output

  Vcl  phi_cl  bound water   phi_e  phi_e/phi_t
 0.00    0.20        0.000   0.250         1.00
 0.20    0.20        0.040   0.210         0.84
 0.40    0.20        0.080   0.170         0.68
 0.40    0.10        0.040   0.210         0.84
 0.40    0.30        0.120   0.130         0.52
 0.80    0.30        0.240   0.010         0.04

Effective porosity at phi_t = 0.25 and Vcl = 0.5, for different clay porosities:
  phi_cl = 0.10: phi_e = 0.200
  phi_cl = 0.20: phi_e = 0.150
  phi_cl = 0.30: phi_e = 0.100
  phi_cl = 0.40: phi_e = 0.050

Assumptions and limitations

• All the bound water is in the clay, and the amount is proportional to the clay volume.
• The clay porosity is constant over the interval. In reality it changes with clay type and compaction.
• Effective porosity here means total porosity minus clay-bound water. Other definitions also remove capillary-bound water or isolated pores, and give a smaller number.
• The total porosity includes the bound water. This is true of density and neutron-density porosity in shaly rock, if the matrix density includes the clay solids. A density matrix chosen for clean rock gives a different quantity.
• The clay volume is correct. An error in it is passed to effective porosity multiplied by the clay porosity.

QC checks

• Effective porosity is never above total porosity and is zero or small in thick shale.
• In clean intervals total and effective porosity agree.
• Effective porosity compares with core helium porosity, and total porosity with the core measurement that includes bound water (crushed-rock or high-temperature drying).
• Plotted against clay volume, effective porosity decreases smoothly. A cliff where the clip at zero applies means the clay porosity is too high or the clay volume too large.
• Both porosities are passed to the water saturation step with the matching saturation model: total with a total-porosity model, effective with an effective-porosity model.

Going Deeper

The distinction goes back to the early shaly-sand models, which treated clay as a source of extra conductivity and needed to know how much of the pore space it occupied. Effective porosity is the quantity a producing well can drain, and total porosity is the quantity a volumetric or dual-water model needs. Neither is a measurement: both depend on the clay volume and on the assumed bound-water volume. For laminated sand and shale sequences the linear relation above does not hold, and methods that separate the sand porosity from the shale fraction (Thomas-Stieber type) are used. Some workflows work in total porosity throughout and convert to effective only for reporting.

References

1. Asquith, G. and Krygowski, D., 2004. Basic Well Log Analysis, 2nd edition. AAPG Methods in Exploration Series 16, American Association of Petroleum Geologists, Tulsa, OK.

Python reference implementation

Related methods

• Density Porosity
• Neutron-Density Porosity
• Gas and Organic-Matter Corrections
• Mineral Inversion