PETROPHYSICAL MODELING AND NMR ANALYSIS FOR RESERVOIR CHARACTERIZATION OF MIXED CARBONATE-CLASTIC FORMATION OF MUMBAI OFFSHORE BASIN, INDIA

摘要

Carbonate reservoirs of Mumbai High are heterogeneous in nature and comprise of carbonates with shaly intercalations in between. Owing to heterogeneity, the realistic reservoir characterization becomes a challenging task. The T2cutoff values in such formation also vary, thus making understanding of the NMR porosity distribution subjective. Sedimentological studies (thin section, XRD, SEM), petrophysical studies (Ø, k, a, m and n) were carried out on 20 samples and NMR core analysis (T2cutoff, NMR Ø) were carried out on 43 samples. The results of sedimentological and petrophysical studies are used for a realistic petrophysical multi-mineral processing model of the formation. The core derived T2 cutoff values are used to reprocess the CMR data. Two T2 cutoff values of 52ms and 25.5ms are standardized for carbonate and clastic parts of the formation respectively and the CMR data is reprocessed with these values. An innovative logic has been developed to combine the results of CMR processing using variable T2cutoff values and taking the lithology as an input. The continuous FFV, BVI and BVM are created for different lithologies. CMR T2 distribution is used to calculate pore sizes using the Eq, r = CT2, where C= ρs*Fs, is the conversion coefficient. The values of surface relaxivity (ρs=0.069) and shape factor (Fs=2.5) are standardized for the formation and pore sizes are calculated. The results are matched with the pore sizes observed from SEM image data. The study helps in characterizing the complex carbonate reservoir of mixed carbonate-clastic formation of Mumbai High. The standardized petrophysical model gives the realistic reservoir parameters and the reprocessed CMR data shows the realistic porosity distribution within the formation. The pore sizes calculated from the T2 distribution give a good picture of the reservoir quality. The model prepared will help in better reservoir characterization of the wells in the area with similar formation characteristics.