Comparison of carbonate reservoir fractures from core and modern electrical borehole images

Several methods exist for collecting data about opening-mode fractures (hereinafter fractures) in subsurface reservoir rocks. The datasets, with the exception of core data, are indirect and subject to limited resolutions. This paper includes new results on the detection of fractures in carbonate reservoirs by modern electrical borehole imagers. Barren fractures and calcite-filled fractures (calcite veins) were described and compared from whole core and borehole images. These datasets were obtained from directional drilling into the Arab petroleum reservoirs in a low-relief dome structure in Eastern Arabia. The comparison of fracture abundance resulted in a significant mismatch, because many reservoir fractures in the core could not be detected from corresponding borehole images. Most calcite veins were undetected because of their small widths, below 2 mm. Exceptions include a few calcite veins of 0.5 mm that were detected because of the strong contrast between resistive veins and conductive host rock beds. The barren fractures that are aligned sub-parallel to the maximum horizontal principal stress (SHmax) were detected more abundantly than other non-aligned barren fractures. The present-day SHmax is roughly trending between 60° and 100° azimuth in Eastern Arabia. The aligned fractures have larger apertures in the borehole images than the non-aligned fractures. The apertures diminish in size as the fractures gradually deviate in orientation from the SHmax. Most of the non-aligned barren fractures were undetected, except the fractures with rough and mismatching walls. This indicates that the walls of the non-aligned fractures are mechanically closed by the normal SHmax, and that the remaining apertures were dependent on the availability of roughness mismatches between the closed fracture walls. Electrical borehole images are biased in measuring fracture abundance, but indispensable to detect and measure the orientations and apertures of fractures, especially the large (i.e., more conductive) fractures in reservoir rocks