| Popis: |
The use of Lab NMR, MICP, electrical resistivity measurements and conventional core analysis coupled with petrographic investigations, enabled the authors to understand the pore structure of several carbonate samples and establish a link between pore geometries, Archie's cementation exponent, capillary pressure behavior and NMR T2 response. The lab work was performed on 68 carbonate plugs retrieved from 5 vertical wells completed on a Cretaceous carbonate reservoir, and located at different structural positions (from crest, mid-flank and down-flank areas), capturing different hydrocarbon column heights and, therefore, different degrees of diagenesis and porosity degradation. The equipment used was Magritek 2MHz NMR Rock Analyzer for the T2 distributions, Micromeretics for MICP, Pantera for the ambient resistivity measurements and PORG-200TM for the conventional core analysis. The petrography was done using conventional polarized light microscope. From the combined analysis of NMR T2 spectrum and electrical resistivity measurements we concluded that the magnitude of the Archie's cementation exponent "m" is greater when the samples exhibit variability within the largest pore class ranges (meso to macro scale) than on the smaller pore class range (micro to meso scale). The bi-modality of the pore system tends to increase the cementation exponent "m". The results of this study also shed some light on the rate of porosity degradation with depth which was correlated with the amount of stylolitization (pressure solution) downflank. The novelty of this work is the combined analysis of NMR, electrical resistivity and petrography on the assessment of the pore structure of limestone samples. The main finding of this work has direct application to the computation of water saturation models for rocks with dual porosity systems. If the understanding of the pore structure is not taken into consideration during petrophysical analysis, the volumes of hydrocarbons could be under/overestimated with consequences on the reserves assessment. |
