Pore network evaluation of the hydrophysical properties of carbonate rocks (coquinas)

Abstract

Carbonate rocks often show nontrivial flow behavior because of their multimodal pore structure. The pore size distribution and pore space topology, the latter describing also how pores are interconnected, can provide much information about the hydraulic behavior of these rocks. Pore Network Modeling (PNM) is an effective method for evaluating petrophysical parameters, including especially the permeability. Pore networks can be generated directly from microCT images to obtain such information as the pore-size distribution, the pore body and pore throat radii distributions, and relevant coordination numbers. The purpose of this paper is to combine microCT, nuclear magnetic resonance (NMR) and mercury intrusion (MICP) techniques to obtain information about petrophysical properties that are not easily measured directly. We were especially interested in comparing microCT, NMR and MICP results, assessing the multi-porosity nature of the coquinas, correlating the permeability with porosity, and obtaining capillary pressure–fluid saturation (Pc-S) relationships. Results were to provide important information about the generally highly heterogeneous nature of carbonate rocks, with as ultimate goal to improve petroleum recovery from oil-bearing carbonate reservoirs. For our study we used four coquina samples from the Morro do Chaves Formation, considered a close analogue of Brazilian Pre-Salt facies. The samples were subjected to routine core analysis, NMR, MICP and microCT scans. The samples had very similar porosities (between 10 and 16%), but different absolute permeabilities (between 5 and 245 mD). Results indicated good correlations, especially between the NMR and microCT data. We further used the mercury intrusion results to obtain estimates of the Pc–S curves. As an example, sample 136.85 showed excellent cross-correlation between the NMR and MICP data, and a well-defined curve of the MICP-derived Pc–S functions for air-water. Plots of the mercury intrusion data and the fitted van Genuchten hydraulic functions indicated a double porosity pore structure.