Analyzing the petrophysics of carbonates drilled with oil-based mud

Several classification schemes have been developed to aid formation evaluation in carbonates, based on quantifying carbonate rock texture by either by grain size,1–2 pore-size3–5 or pore-throat size.6–7 These schemes were developed from visual inspection of cores and cuttings, thin-section microscopy or mercury porosimetry. Recent advances in nuclear magnetic resonance (NMR) log and core analysis, complemented by more quantitative use of borehole image logs, has led to the application of log-based porosity partitioning based on some of these models. The foundation of this approach is the link between NMR transverse relaxation time (T2) distributions and pore-size distributions obtained from special core analysis. Several case studies have now been presented where this approach was successfully used in carbonate formations drilled with water-based mud (WBM), where the NMR response is well characterized and has been validated by core analysis.

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