The Petrophysicist’s Old Chestnut
PETROPHYSICS Pty Ltd report appendix extract. “The detail of this porosity evaluation and the equality of numerous core to log comparisons suggest a high degree of certainty in the evaluated total porosity (por). This porosity is calibrated to core oven dried helium porosities and is therefore “total porosity”. Core companies often use of the term “effective porosity” to mean these same oven dried helium interconnected pores creating a situation where the same measurement has two different names. However ‘interconnected’ is not the mainstream petrophysical definition. The confusion regarding total vs. effective porosity is due to this longstanding inconsistency between core companies use of the term “effective porosity” to mean interconnected pores penetrated by He atoms in the laboratory vs. mainstream petrophysicists (Shell, Archie, W&S, NMR literature, this report) to mean (total porosity – clay bound water) following Hill, Shirley and Klein’s historic link between lab measured Qv and CBW volume. Hence the same measurement value is called “total” by mainstream petrophysicists and “effective” by many core companies. Herein lies the cause of this dysfunction.
How can a crowd of such logical, scientifically minded folk get so confused? The term effective porosity is also used loosely by others in an intuitive common sense and usually qualitative way, as shown below. Note that non-NMR conventional logs lack the basic data from which to determine a storage capacity “effective” porosity from rocks containing non-clay micro-pores such as carbonates or quartz silts. Non-NMR log based determinations of “effective” porosity in clay and thus CBW free micro-porous rocks are largely meaningless – what tool is providing the information for storage capacity? When you read a clean carbonates petrophysical report and the term effective porosity is used ask yourself how that number was measured. This is the primary reason why carbonates (reservoirs where compaction and clays do not control storage capacity and permeability) are seen as difficult to evaluate with conventional logs, because no log data is available to measure storage capacity – the advantage of NMR in carbonates.
This author conforms with mainstream petrophysics and uses “total porosity” to equate to the basic volumetric reference of conventional, hot oven dried core to constant weight (c.100degC) porosities, as with these plugs dried to 115degC. Reviewers should search the core report for the key words “oven dried to constant weight” to learn if the core reference measurement is total porosity – about 80% of core analysis globally is. In cases of humidity drying (usually 60C 40RH) the evaluation should still be calibrated to that core porosity but all SCAL must have this same measurement as its base or the evaluation becomes “disintegrated petrophysics”. Conventional core, special core and logs must be equal. Logs simply carry the core measurements along the well track, like cargo trains.
Under extremely rare circumstances (pers. comm Dave Bowen) conventional core oven helium dried porosities fail to report all the pore space, where pores are actually closed off as in intra crystalline pores. The essential point for the petrophysicist here is that correct use of the core reported grain density in the fixed point fluid density procedure above will still allow logs to duplicate the core measured porosities – we need a directly measured reference. It does not matter what that porosity is called – the important issue is whether logs are calibrated to a parameter which is directly measured and accurate. This subject has been discussed with rare common sense by Istvan Juhasz: “Porosity systems and petrophysical models used in formation evaluation” SPWLA London Chapter Porosity Seminar, 26th April 1988, which should be read by all aspiring petrophysicists to clarify this vexed issue”
Note: This is a total porosity evaluation in which shale porosity is evaluated as non zero and has an Ro-porosity (m) relationship. One of the many reasons for adopting total porosity is that the relationship between shale volume and Ro is revealed to the log analyst. In effective porosity evaluations the analyst is denied this information because the porosity parameter disappears as shales are approached, denying the petrophysicist powerful, cheap information for shaly sand evaluations. If the petrophysicist wants to understand tool responses – which respond to the whole formation not bits of it – during the petrophysical investigations shale porosity should not be hidden and Sw should not be truncated at 1.00. If your users prefer the cosmetics of effective porosity display it after you have finished the evaluation.
Provide End-Users with Effective Porosity – the Practical Result
“ ’Total’ porosity is useful for the calculation of porosity and water saturation. Core porosity measurements usually give total porosity. E&P managers need to know something different. They need estimates of potential reservoir thickness, ’effective’ porosity, permeability and the volumes of producible hydrocarbon and water.
‘Total’ and ‘effective’ porosity are equal in non-shaly reservoirs, and may be nearly equal in shaly sandstones containing clays (other than smectites) with little clay-bound water in the clay structure. In shaly sandstones containing hydrated clays (smectites), ‘effective’ porosity may be much less than ‘total’. Immature smectite shales with ‘core’ and ‘total’ porosities of 23% are impermeable non-reservoir rock.
To obtain practical results, petrophysicists and log analysts certainly need to understand and evaluate ‘total’ porosity and the volumes of water bound in clays. However, for the end users of the petrophysical results it is more informative to be provided with ‘effective’ porosity. It reduces confusion and give a more practical evaluation of the reservoir. In depth plots of the results, ‘total’ porosity should be de-emphasised and ‘effective’ porosity emphasized.”
Dick Woodhouse, Consultant.