Résumé
As standard tools in reservoir engineering and more recently in hydrogeology, downhole logging is used to evaluate reservoir hydrodynamical properties from geophysical measurements. For instance, porosity, water saturation and permeability are often deduced from downhole acoustic or electrical data, through empirical relationships of velocity-porosity, resistivity-porosity and porosity-permeability. Although mostly successful in siliciclastic sediments, these relationships often fails in carbonate rocks, because of their much more complex pore system induced by repetitive post-depositional alteration processes. This paper seeks to highlight the influence of the pore space morphology on reservoir properties in reefal carbonates displaying drastic changes of pore structures at cm scale, focusing on the relationships between acoustic velocity, electrical resistivity, permeability and pore structure. The latter is characterized in terms of pore size distribution as well as connectivity and tortuosity of the 3D pore network using X-ray microtomography (XRMT), while effective parameters (Darcy scale properties such as porosity and permeability) are deduced from borehole logging and core sample measurements. Results show large variability in electrical resistivity, acoustic velocity and permeability for zones displaying equal porosity, due to distinctly different pore structures. For instance, the tightest samples in the cemented zones deviate significantly from traditional Archie, Kozeny-Carman and Wyllie's laws. A specific segmentation procedure is used to retrieve both the macroporosity and microporosity from XRMT and extract the connected cluster and its properties (tortuosity, pore size distribution and specific surface). In our case, the most porous, less resistive and less tortuous sample is not the most permeable. The 3D analysis of this sample's pore structure reveals indeed that the macroporosity is connected to the pore network by the microporosity with a low pore diameter. The most permeable sample has a much bigger dominant pore size and smaller surface-to-volume ratio whereas it exhibits higher values of electrical resistivity, acoustic velocity, tortuosity and a much lower porosity.