Review of Existing Shaly-Sand Models and Introduction of a New Method Based on Dry-Clay Parameters
- Max Peeters (Petroskills) | Antony Holmes (Digital Formation)
- Document ID
- Society of Petrophysicists and Well-Log Analysts
- Publication Date
- December 2014
- Document Type
- Journal Paper
- 543 - 553
- 2014. Society of Petrophysicists & Well Log Analysts
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- 245 since 2007
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There are more than a dozen shaly-sand evaluation methods used on a regular basis. The two main groups are based on shale conductivity and cation exchange capacity, respectively. This study shows that they can be linked via the bound-water saturation, which is related both to the shale content and cation exchange capacity.
In this paper, the Archie, modified Simandoux, dualwater, Waxman-Smits, Indonesian, and Juhasz methods are compared, and a novel “difference” method is introduced. The latter, in contrast with all existing “logs only” methods, does not require log readings in adjacent shale beds, provided the clay type can be determined from cuttings, sidewall samples, or cores. For mixed clays X-ray diffraction (XRD) analyses are required to determine the clay types and dry-clay parameters, which in turn are used to calculate the cation exchange capacity.
A field study demonstrated that a judicious selection of input parameters produces water-saturation profiles that almost overlay for most methods. Hence, in this case the choice of shale and clay parameters was more important than the choice of the evaluation method. The Indonesian method could not be reconciled with other methods, because it is, in contrast with all other methods, not based on a parallel resistor network.
A sensitivity study of input parameters highlighted that for the dual-water method, using logs only, small changes in shale density led to discrepancies of more than 10% in water saturation. It is recommended to use the bound-water conductivity relation given in Clavier’s original dual-water paper. The Waxman-Smits, Juhasz, and the novel difference methods were all, as expected, most sensitive to the cation exchange capacity, and the Juhasz method to the neutron porosity of an adjacent shale bed as well.
If for old wells, sidewall samples, cores or even cuttings are still available to determine the clay type, the new difference method promises to give more accurate hydrocarbon saturations.
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