Simulation of flow characteristics and development of permeability model in fractured-vuggy carbonate reservoir

Abstract

Fractured-vuggy carbonate reservoirs have complex pore structure, significant heterogeneity, and irregular permeability-porosity relation, which make existing evaluation methods unable to accurately calculate the permeability. To investigate the flow characteristics of carbonate rocks with triple-porosity, 3D digital rock models were constructed from X-ray computed tomography (CT) scanning images of carbonate cores collected in the field, and the physical properties of rocks were obtained by combining digital rock analysis (DRA) and nuclear magnetic resonance (NMR) experiment. The effects of fractures and vugs on the permeability of carbonate rocks was investigated using the Stokes-Brinkman method, and then a new fractured-vuggy carbonate permeability model was developed by artificially adding vugs and fractures. The results indicate that the carbonate pores can be divided into matrix pores, vugs, and fractures according to their sizes by combining digital rock analysis and T2 spectrums. The transport resistance is effectively reduced by fractures and vugs, which leads to the change of the flow path. For tight carbonate rocks, the dominant flow path formed through fractures is a crucial factor affecting permeability. On the contrary, unconnected vugs have less effect on permeability. Finally, the new permeability model was applied to the logging interpretation of field data. The new model is in better agreement with the permeability analysis of field cores, which greatly improves the permeability calculation accuracy of the triple-porosity reservoir in the study area.