ABSTRACT

The 2016 AGM of the Aberdeen Formation Evaluation Society was followed by a lively ‘Porosity Debate’, which started by recognizing that there are many definitions of effective porosity in the industry, and specifically that there are six in Wikipedia. An appreciation of these different definitions is fundamental to petrophysics because volumetric results might be generated using one definition while the end-user of those results, maybe a Reservoir Engineer, might assume it was something else. Obviously, this can lead to considerable confusion and significant uncertainty in the STOIIP.

The ‘Porosity Debate’ quickly became a discussion about methodology and ‘should you compute total porosity first and then effective porosity’, or vice-versa? It should not be a case of which to solve first, but rather ensuring ‘consistency’. A ‘consistent’ methodology is one that is based on a valid rock model that sums to 1, yields both total and effective porosity with the same results whichever is solved first, and of course, matches core data.

This paper reviews several deterministic porosity methodologies common in the industry and demonstrates the inconsistencies inherent in many of them.

It would be reasonable to expect consistent definitions of total and effective porosity across all petrophysical workflows, for example, in both deterministic and non-deterministic workflows. However, this is often not the case, for several reasons:

  1. deterministic methods often use Vshale while non-deterministic methods normally use a mineral model and so use Vclay

  2. the user may not be clear which definition were used in their interpretations

  3. some software products use different definitions between methodologies.

One methodology is detailed that is volumetrically consistent regarding total and effective porosity, and that can be implemented in terms of Vclay or Vshale. This methodology is not new or novel, it has been in use within the industry in one shape or another for several decades, but it is often misunderstood and the significance of its consistent approach under-appreciated.

Porosity and water saturation should be solved iteratively. Gus Archie originally proposed that water saturation depends on porosity, which also means the calculation of porosity depends on the water saturation. Without iteration you must assume the water saturation to calculate porosity, and if that assumption is wrong then the porosity, and hence the final water saturation, are wrong.

With a ‘consistent’ methodology porosity and water saturation can be provided in both total and effective systems.

This paper seeks to lift the fog of confusion.

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