PDVSA INTEVEP undertook a study in a carbonate area located in a gas and condensate reservoir offshore Venezuela, where the main challenge was related to the high heterogeneity of the rock fabric. The geoscientists needed to understand how the pore system pattern reflects good porosities throughout the limestone while having significant permeability variations. The data was collected from wells with cores, as variations in the petrophysical properties are much harder to predict in wells without core data. The main objective was to apply a clustering approach in order to build electrofacies models and integrate them with sedimentological interpretations by extrapolating those models to wells without core data.
The Multi-Resolution Graph-Based Clusters (MRGC) technology clusters dot patterns due to changes in log behavioral responses using geostatistics and neural networks. These dot clusters, also known as electrofacies, are associated with changes in rock quality and are calibrated with core sedimentary facies. The MRGC technology in the Paradigm® Geolog® solution was used to create electrofacies models, textural electrofacies models, and permeability prediction.
Using Paradigm Geolog, an accurate electrofacies reproduction of the sedimentary facies of the core was achieved. A variety of well data, such as conventional logs, special logs, core data and sedimentology information, were integrated. The Geolog modules used included:
- Facimage to build the electrofacies models
- Geolog Correlator to create stratigraphic sections
- Determin to obtain a petrophysical model
- Geolog Image Log Processing & Interpretation to process the resistive image log
Because of the heterogeneity of the carbonate section, two electrofacies models were built: One for the upper unit and one for the lower. In the lower unit, a final model of 5 electrofacies was obtained, with an approximate 60% match with the sedimentary facies. In the upper unit, electrofacies with a similar trend were clustered into a definitive 6-electrofacies model, with an average 50% match with the facies described in the cores.
In general, the electrofacies model with Nuclear Magnetic Resonance data showed good correspondence with the porosity estimated in the petrophysical model, the core porosity and permeability, the sedimentary facies, and the petrofacies.
Texture electrofacies obtained with a resistive image log through a histogram analysis and auto-covariance matrix, followed by MRGC analysis, showed a good correspondence with the porosity estimated from the petrophysical model, the core porosity and the sedimentary facies.
This robust and integrated solution enabled prediction about permeability and facies changes related to rock quality in wells without core data. The changes in rock heterogeneity were also observed in the Nuclear Magnetic Resonance and resistive image logs. Together, the electrofacies models allowed PDVSA INTEVEP to propose a review of the lithostratigraphic tops. Project lead time was reduced through use of the integrated Geolog formation evaluation package.
“Geolog is a versatile tool that enables us to load and manage the latest generation of well logs in a single interface. With Geolog, we can work with more information than with any other platform. We believe Geolog offers the best solution for petrophysical interpretation in a user-friendly software application.”
Zureisy Marin, Petrophysicist, PDVSA INTEVEP
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