Improved Digital Rocks-Based Model for NMR Permeability Estimation in Vuggy Deepwater Carbonates

We introduce a new digital rocks-based method for interpreting NMR T2 distributions in well log data acquired in vuggy deep-water carbonate reservoirs. Our method accounts for adverse borehole conditions such as mud invasion and large washouts in vuggy zones, usually neglected in conventional interpretation procedures of NMR logs. The new approach is based on describing the measured distribution of transverse relaxation times as the superposition of a finite set of log-normal components. Each component accounts for specific relaxation rates for drilling mud and original formation fluids. We carefully design our NMR interpretation model after processing whole core X-ray computed tomography (CT) images acquired in whole core samples. Estimation of density and atomic number from dual-energy CT data enabled to directly probe fluid content in the vuggy space, while image segmentation targeting the vuggy space allowed to estimate vuggy porosity and flow properties inside the vug network. Our model was able to explain correlated anomalies shown by caliper, photoelectric, and NMR T2 logarithmic mean logs for the vuggy regions in the dataset studied. The decomposition of inverted NMR T2 distributions in a set of basis functions naturally handles the uncertainty related to inversion parameters, making the task of calculating fluid concentrations and permeability indices more robust with respect to small variations in cutoff values. Permeabilities in vuggy zones estimated from NMR logs using this new method are more accurate than those rendered by conventional techniques based on T2 cutoffs or logarithmic averages, without the need to artificially introduce new fitting parameters. Using this approach, we can also explicitly quantify vuggy porosity, which is in good agreement with values obtained from segmented whole core tomographic images for this particular dataset. The combined use of the above interpretation methods confirms the value of digital rock techniques to improve the evaluation of well logs acquired in complex carbonate formations, specifically in the calculation of permeability across vuggy depth segments. Results can be used to improve well log interpretation in wells devoid of core data and/or high-resolution borehole images.