Abstract
The purpose of this research is to apply a new approach to identify natural fractures in wells in a hydrocarbon reservoir using resistive image logs, fractal dimension and support vector machines (SVMs). The stratigraphic sequence investigated by each well is composed of Cretaceous calcareous rocks from the Catatumbo Basin, Colombia. The box counting method was applied to image logs in order to generate a curve representing variations of fractal dimension in these images throughout each well. The arithmetic mean of fractal dimension showed values ranging from 1,70 to 1,72 at the mineralized fracture intervals, and from 1,72 to 1,76 at the open fracture intervals. Morphological classification between open and mineralized natural fractures is performed using corelogs integration in a pilot well. Fractal dimension of images along with gamma rays and resistivity logs were employed as the input dataset of a SVM model identifying intervals with natural open fractures automatically, shortly after logs acquisition and previous to its interpretation by specialists. Although final results were affected by borehole conditions and logs quality, the SVM model showedaccuracy between 72,3% and 82,2% in 5 wells evaluated in the studied field.
Highlights
The wells investigated during this study are located in an oil field in the municipality of Tibú, Northeast Region of Norte de Santander Department, Republic of Colombia; the oil field examined during this study, which cannot be named due to confidentiality reasons, is approximately 2888 acres in size
True positive “TP” and true negative “TN” terms are correct classifications; “TP” is when the model shows open fractures and they really exist in the image logs; “TN” occurs when the model does not show open fractures and these do not exist in the image logs
The fractal dimension ranges obtained in this investigation are not absolute and will be changing depending on the kind of tool used for image acquisition and parameters applied to transform the image into binary
Summary
The wells investigated during this study are located in an oil field in the municipality of Tibú, Northeast Region of Norte de Santander Department, Republic of Colombia; the oil field examined during this study, which cannot be named due to confidentiality reasons, is approximately 2888 acres in size. In oil reservoirs of the region, identification of natural fractures is important, during drilling and production activities. The knowledge of natural fracture zones prevent drilling fluid losses improve packers setting for pressure tests, and optimize selection of intervals for oil production. This area is within the Catatumbo Basin, where several sections of the Cretaceous formations have shown excellent properties as reservoirs containing natural fractures, wherein resistive image logs have been useful to describe them. Identification of Natural fractures using resistive image logs, fractal dimension and support vector machines.
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