Using Electrical Borehole Images for Reservoir Characterization in Mixed Carbonate/Evaporite Successions
The Electrical Borehole Imaging Tool has proven to be very effective in describing reservoir quality in mixed carbonate/evaporite reservoirs. In wells drilled with water-base mud, the Electrical Borehole Imaging Tool provides a continuous, high-resolution image of the borehole wall by measuring subtle changes in micro-resistivity only a few inches into the formation. Because of its high resolution (0.2 in) the tool is able to identify small-scale features, which would otherwise be overlooked with conventional logging tools. Analyzing the borehole images from Upper Jurassic reservoirs, offshore Abu Dhabi, for mineralogy and rock texture suggests that the Electrical Borehole Imaging Tool should be used for high-resolution reservoir characterization.
Non-reservoir or low quality intervals of the Upper Jurassic are characterized by high background resistivity (light colored areas) on the electrical borehole images, as they indicate anhydrite and/or tight (cemented) carbonates. Anhydrite layers show a distinctive “speckled image appearance” on the dynamically normalized image. Anhydrite cemented carbonates show a similar texture but with less speckles. Dolomite (good reservoir rock), on the other hand, can be separated from the similar looking anhydrite cemented intervals by integrating the electrical borehole images with conventional log readings, such as the density and photo-electric effect. Non-reservoir limestones are characterized by a light colored, smooth image appearance that completely lacks the speckles. Dark colors on the electrical borehole image indicate low resistivity areas, such as porous limestones (grainstones, packstones, etc.), vuggy intervals and bioturbation.
One of the main benefits of the Electrical Borehole Imaging Tool is that subtle and small-scale reservoir heterogeneities that could be easily overlooked in core description and on the lower-resolution conventional wireline log data can be recognized on the image. Changes in mineralogy (dolomite versus limestone layers), firm and hard grounds, stylolites, and tar-filled layers (visible on the statically normalized image) might act as vertical baffles to fluid flow. In addition, other sedimentary and diagenetic features, such as bioturbation, cross-bedding, intraclasts, nodules and vugs, are easily identified. In summary, the electrical borehole images provide a continuous description of the borehole and, once calibrated to the core, enable the geoscientist to obtain detailed reservoir quality information in future wells without the need for expensive coring programs.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009