Publication History: This article is based on Chapter 7 of "The Log Analysis Handbook" by E. R. Crain, P.Eng., published by Pennwell Books 1986  Updated 2004, 2020. This webpage version is the copyrighted intellectual property of the author.

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Selection of Porosity MethodS
The answers for all the porosity solutions will vary, and in some cases be unreasonable or impossible to calculate due to lack of data. In order of preference, we would choose:

BEST MODEL -- Use whenever possible
  1. Shale Corrected Density Neutron Complex Lithology Crossplot Model
aka "Complex Lith".
Preferred because equations are simple, requires no assumptions, is unaffected by mineral mixtures, gas correction is easy.
The Meta/Kwik spreadsheet for this model is available at Downloads and Spreadsheets.

OTHWER CROSSPLOT MODELS --- Use only within  limitations
  2. Density Neutron Shaly Sand Crossplot Models
Limitations: only for shaly quartz/feldspar sands, matrix offset model needed if other minerals present, complex lith model is easier.

  3. Sonic Density Crossplot
Limitations: gas correction difficult, not as accurate as complex lith.
  4. Sonic Neutron Crossplot
Limitations: OK in carbonates or in bad hole where density is not acceptable, must be shale corrected in shaly sands.

SINGLE LOG MODELS -- Use only if no crossplot method is possible
  5. Density Log Corrected for Shale

Limitations: works in good borehole only, VERY sensitive to mineral and gas parameter assumptions.

  6. Sonic Log Corrected for Shale
Limitations: VERY sensitive to mineral, gas, compaction, and shale parameter assumptions.

  7. Neutron Log Corrected for Shale
Limitations: VERY sensitive to gas and shale parameter assumptions.

  8. Microresistivity, shallow or deep resistivity as a last resort.

Apply maximum porosity and material balance constraints to selected method.

Apply non-porous lithology triggers as needed.
 


Computed Results for Mixed Lithology Example from six different porosity models, compared
to core analysis porosity.

All porosity calculations need to be calibrated to core data at some point, usually at an early stage in an analysis project. Log analysis in isolation from other data is pointless and dangerous.


Maximum Porosity
If no porosity indicating log is available, the maximum porosity model may be useful in shaly sands. It does not work in carbonates.

Discard unreasonable answers, and/or revise shale or matrix assumptions and re-compute if difference between methods is too large.


Material Balance FOR POROSITY
Material balance for porosity is needed to prevent too high a porosity in very shaly sections or in bad hole conditions. Negative values must also be trimmed off to prevent errors in further steps in the calculation sequence..
      1: IF PHIe < 0
      2: THEN PHIe = 0
      3: IF PHIe > PHIMAX * (1.00 - Vsh)
      4: THEN PHIe = PHIMAX * (1.00 - Vsh)

Where:
  PHIe = porosity from any method (fractional)
  Vsh = shale content from any method (fractional)
  PHIMAX = maximum expected porosity in clean rock (fractional)
 
Don't fail to use this routine. It makes computer output look pretty nice even with poor logs.

RECOMMENDED PARAMETERS:
Normal values for PHIMAX:
Very high porosity sandstone (and tar sands) PHIMAX = 0.36 to 0.42
High to medium porosity PHIMAX = 0.25 to 0.36
Low or shaly medium porosity PHIMAX = 0.18 to 0.25
Very low porosity, PHIMAX = 0.10 to 0.18



Normally you will settle upon a method that suits you and the zone under consideration. You will not have time to compute results from all methods. Use the above list as a guide to reduce your effort and to gain a better chance for success on the first pass. Log analysis is seldom satisfactory on the first pass in new areas, so do not be bashful about trying several methods. Then keep a record of which methods worked best in which areas.

Porosity is usually reported to the nearest 1/10 of one percent (or 0.001 fractional) but this can be reduced to the nearest percent (or 0.01 fractional) for hand calculations or for porosity greater than 0.25.


 

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