Carbonate rocks have complex porous structure and conventional approach to characterize two phase flow in the laboratory can be inadequate, especially in vuggy structures. Various laboratory studies indicate that centimeter scale heterogeneities have important effects. An important question is the determination of the 3D permeability map later used in the interpretation of two phase flow experiments.
Our approach is based on the combined use of CT scan 3D porosity maps, NMR relaxometry, tracer experiments and numerical simulations to derive a 3D permeability map consistent with the experimental observations. NMR T2 measurements are used to quantify the average porous volume represented by the vugs while CT scan can reproduce the spatial distribution of porosity at the millimeter scale. The permeability map is mainly deduced by fitting the tracer experiment using 3D numerical simulations where we introduce the CT scan porosity map and where the permeability in each grid cell take only two values, one associated with vugs, and one with the matrix.
The study was performed using a volcanic rock (Andesite) with 22% porosity and low permeability (0.4 mD) to mimic a non fractured vuggy carbonate. The variogram analysis in the flow direction does not indicate a correlation of the porosity map. However, the tracer experiment indicates the existence of a preferential path and the measured flux dispersion behaves as in a layered system (early breakthrough, long tail). For a given volume fraction of vugs (congruent with NMR results), the experiments can be reproduced with numerical simulation by adjusting the permeability contrast between the matrix and vug to 360. The use of a streamline simulator was necessary because of the large number of grid cells used and low numerical dispersion. Finally, miscible displacement experiments using different viscosity fluids are interpreted and discussed using three models considering heterogeneity and viscosity.