Many productive carbonates have dual porosity systems with widely varying proportions of primary and secondary porosity. The secondary porosity may contain vugs, molds and fractures. Most interpretation methods rely on traditional resistivity and porosity logs. These often fail to produce results consistent with production because of the complex nature of the dual-porosity carbonate. Borehole electrical images provide both the small-scale resolution and azimuthal borehole coverage to quantitatively resolve the heterogeneous nature of the porosity components. The primary assumption for this technique is that the resistivity data from the electrical images are measured in the flushed zone of the borehole. Then, the electrical images can be transformed into a porosity map of the borehole. Automated analysis of this porosity map, windowed over short intervals, provides a continuous output of the primary and secondary porosity components. This method has been applied to a variety of carbonate formations where conventional log analysis has usually failed. The predicted results compare accurately with the actual production.
Historically, for many carbonate reservoirs, the correlation between hydrocarbon production and neutron-density logs has been inconsistent. Good production has been obtained from intervals showing low log porosity whereas zones having higher porosity have not produced. Also, total production from carbonate reservoirs in mature fields has often been greater than expected from standard porosity logs. Although carbonate porosity often appears somewhat uniformly distributed (Fig. 1), many carbonate sections observed on high-resolution borehole electrical image's exhibit a texture that is apparently vuggy or dominated by patchy porosity (Fig. 2). The prototype software program PoroSpect*, Porosity Spectrum Analysis, which transforms the conductive regions seen on electrical images into equivalent porosities and provides facilities for examining the distribution of porosity values, was developed as a means of better quantifying this patchy porosity.