Petrophysics Training & Courses - Effective Porosity

PETROPHYSICS Pty LTD report, appendix excerpt. "The particulars of this porosity analysis and the similarity of a number of log-to-core comparisons indicate that there is a high level of confidence in the assessed total porosity (por). The measurement is calibrated using the core oven dried helium porosities and, therefore, is "total porosity". Core companies typically use"effective porosity" or "effective porosity" to refer to these exact interconnected pores of helium that have been dried in the oven creating a situation where the same measurement can have two distinct names. Interconnected is not the most common definition for porosity that is effective. The confusion over total and. effective porosity stems from this long-standing inconsistency among core companies' usage of "effective porosity" to refer to interconnected pores that are penetrated by He atoms in the laboratory. the mainstream physical physicists (Shell, Archie, W&S, NMR literature, this report) to refer to (total porosity, the clay-bound water) in line with Hill Shirley and Klein's historical link between laboratory measured Qv and the volume of CBW. This is why the same measurement is termed "total" in the mainstream of petrophysicists as well as "effective" by a number of core firms. This is the root of this mishap.

What is the reason that this group of rational scientists and logical people become confused? Effective porosity is also used by other people in a common sense and typically qualitatively, as illustrated below. Also, note that conventional non-NMR logs are not equipped with the necessary information needed to calculate the capacitance to store "effective" porosity in rocks with non-clay micropores like carbonates and quartz silts. Log-based non-NMR determinations regarding "effective" porosity of clay and therefore CBW-free micro-porous carbonates, are not very useful - what instrument is providing the data to determine storage capacity? If you read a clean carbonates Petrophysical report and you see the word effective porosity is used , ask yourself how the number was calculated. This is the principal reason that carbonates (reservoirs in which clays and compaction don't regulate storage capacity or permeability) are considered difficult to measure using standard logs since there is no log data available to assess storage capacity. This is the main benefit of NMR in carbonates.

The author's work is in line with mainstream Petrophysics and utilizes "total porosity" to refer to the fundamental volumetric reference of traditional hot oven dried cores that are the same weight (c.100degC) porosities, which is the case the plugs are that were dried to 115degC. Reviewers should look through in the report's core for the crucial terms "oven dried and weighed at continuous weight" to find out what the reference measurement for the core is total porosity. About 80percent of the analysis of the core is. In the event of drying at high humidity (usually 60C-40RH) the evaluation must be calibrated according to the porosity at the core. However, all SCAL should have the same measurement as their base, or else the evaluation will be "disintegrated petroleum physics". The conventional core, the logs, and special core must be equivalent. Logs carry only all the measurements of the central core along the track of the well as cargo trains.

In extremely rare instances (pers. Comm. Dave Bowen) conventional core oven porosities that have been dried in helium do not accurately reflect the entire pore space where pores are shut off, as they are in intracrystalline pores. The most important thing to remember for the petrophysicist in this case is that the correct application of the core's reported grain density within the fix point liquid density method that is described herein allows logs to reproduce the measured porosity of the core because of the core's smaller grain density and the need for a direct measured reference. The most important thing to consider is how logs are calibrated according to a parameter that is precisely measured and precise. This topic has been debated with uncommon common sense by Istvan Juhasz "Porosity system and models of petrophysical that are used in the evaluation of formation" SPWLA London Chapter Porosity Seminar 26th April 1988. This is a must read for any aspiring petrophysicists in order to better understand this controversial issue.

Note this is a complete porosity analysis where shale porosity is assessed as not zero, and it has an R porosity (m) relationship. One of the reasons for implementing total porosity is the relation between shale volume and Ro is apparent. When conducting effective porosity assessments, the analyst is not able to access this information since the parameter of porosity disappears in shales, giving the petrophysicist a powerful, instant information to evaluate shaly sand. If the researcher seeks to understand the responses of tools that respond to the entire formation and not only fragments of it during the petrophysical investigation, shale porosity should not be concealed from view. Likewise, Sw should not be cut off at 1.00. If your customers prefer the appearance of porosity, show it after you've completed the assessment and have a clear understanding of what's taking place.

Give End-Users Efficacious Porosity. The Practical Results

" Porosity that is 'Total' can be used to calculate the amount of porosity and saturation. The core porosity measurement usually gives the total porosity. E&P managers must know something else. They must have estimates of reservoir thickness, effective porosity, permeability, and the quantities of hydrocarbon that can be produced and water.

Total and effective porosity are the same in non-shaly reservoirs. They could be almost identical in shaly sandstones that contain clays (other than smectites) with a small amount of clay-bound water present in clay structures. For shaly sandstones with clays that have been hydrated (smectites) effective porosity might be lower than total. Shales that are immature smectite with 'core and 'total' porosities that exceed 23% can be considered impermeable and non-reservoir rock.

For real-world results, petrophysicists as well as log analysts must comprehend and evaluate total porosity and the amount of clays' water-bound. For the end people who will benefit from the results of petrophysics it is much more useful to provide 'effective porosity. It helps reduce confusion and provide an easier assessment of the reservoir. In the plots that show the in-depth results, the total porosity should be emphasized and 'effective' porosity highlighted."

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