Carbonate Petrophysics in Wells Drilled with Oil-Based Mud

Existing carbonate classification schemes are based on quantifying rock texture by grain size or pore-throat size. They were developed from visual inspection of cores and cuttings, thin-section microscopy or mercury porosimetry. Recent advances in NMR log and core analysis, complemented by more quantitative use of borehole image logs, have led to the application of log-based porosity partitioning based on some of these earlier models. The foundation of this approach is the link between NMR T, distributions and pore-size distributions obtained from special core analysis. Several case studies record the success of this approach in carbonate formations drilled with water-based mud, where the NMR response is well characterized and has been validated by core analysis. The recently discovered carbonate reservoirs offshore Brazil are typically drilled with oil-based mud to avoid drilling and completion problems in the 2000 m of salt overlying the reservoir. NMR logs are routinely run in the reservoir section and the interpretation methods developed for carbonates drilled with water-based mud have been adapted for evaluating reservoir quality, when oil-based mud is used. These carbonates tend to be oil-wet to the 28-30 degrees API reservoir oil and to the oil-based mud filtrate; this ensures that surface relaxivity is the dominant relaxation mechanism in the NMR response, which enables the correlation of T-2 distributions with the variety of pore sizes in the reservoir zones. Interpreting NMR logs in such conditions requires detailed knowledge of the oil-based mud filtrate and reservoir oil properties, and of the wettability of the formation at downhole conditions. One of the key requirements for correctly interpreting the NMR log data are to perform the lab NMR measurements of fluids and core samples at the same pressure and temperature conditions as the lab PVT measurements. When compared with downhole NMR log data, lab measurements performed on native-state, restored-state, brine-saturated and partially-saturated cores at PVT conditions show that many of the existing carbonate classification schemes can be applied to the presalt carbonates. The common features of these schemes will be examined, and the relevance of these various models to formation evaluation in the presalt carbonates will be reviewed. Options for analyzing borehole image logs in carbonates drilled with oil-based mud will also be presented as an aid to porosity typing with NMR. The key considerations for core analysis and for acquiring, processing and analyzing NMR and logs in carbonates drilled with oil-based mud are summarized.