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Fractal Dimension and Borehole Images, a New Approach to Enhance the Characterization of Naturally Fractured Reservoirs and its Application in the Catatumbo Basin-Colombia


The Catatumbo Basin is located in the northeast of Colombia and geologically, represents the southwest extension of Maracaibo Basin in Venezuela. Major reservoirs in the Colombian side are the cretaceous calcareous rocks of Aguardiente, Capacho and La Luna Formations, all of them with high fracture density, where borehole image logs have proved to be a powerful tool to characterize naturally fractured hydrocarbon reservoirs. On the other hand, fractal geometry analysis has been applied, in the last two decades, to describe natural systems like fractured and faulted systems where many researchers have pointed out that the fractal dimension has a close relationship with fracture density and each tectonic event has a unique signature which can be recognized using fractal dimension analysis. After image log processing and prior its interpretation, Box Counting method based algorithms were applied to generate a continuous curve representing the fractal dimension variations into the resistive images; the arithmetic mean of the fractal dimension curve showed values ranging from 1.70 to 1.72 at intervals with mineralized natural fractures and between 1.72 and 1.76 for intervals with open natural fractures. By using core white light photographs was possible to perform the morphological classifications of open and mineralized natural fractures. Fractal dimension data was subsequently integrated with gamma ray and resistivity logs to finally implement support vector machines to generate a model to identify the presence of open natural fractures in each well automatically. The predictive model result shows accuracy between 55.4 % and 83.8% in 8 evaluated wells of different fields of the studied area. It means that this fractal dimension can be used as a fracture indicator in this side of the basin. This methodology has shown two main applications. First, in reservoir modeling, the fractal curve can be used as a fracture index and can help in the identification of open fracture in each perforated wells logged with borehole images. The Second application is on field operations, where the fractal curves can be used as indicator of the packer setting for formation test, for improving the setting of the casing depth and to detect zones of perforation with fluid loss. This methodology is highly dependent on the quality images log; where any artifact or issue affecting it, has to be removed prior to fractal dimension curve generation. This proposed approach provides an alternative to evaluate natural fractures system from resistivity images and may have applications not only in the oil industry, but also in other studies related to naturally fractured rock systems.