NMR T2 Relaxation Times and Pore Facies Relationships in Carbonate Reservoirs

Coralie Genty, Wayne M. Ahr, and Jerry Jensen
Texas A&M University, College Station, TX

The determination of reservoir quality and its spatial distribution is a key objective in reservoir characterization. This is especially challenging for carbonates because, due to the effects of diagenesis, quality rarely follows depositional patterns. This study utilizes measurements of NMR T2 relaxation times, pore geometry, pore origin, and proportions of different pore types to establish relationships between “pore facies” and reservoir quality. This novel approach to the use of NMR measurements integrates data from thin sections and core analyses and applies geological and statistical analysis to identify relationships between pore characteristics and the T2 data. Samples from forty borehole cores from fields in the Middle East, Alabama and Texas were used in the study. Decomposition of the T2 spectra into several distributions resulted in the definition of 11 parameters directly obtained from the T2 modeling and representing the average, variability and percentage of total porosity of the specific pore volumes present in the sample. Each specific pore volume corresponds to one porosity type among the five used in this study: depositional, dissolution-enhanced, intercrystalline, vuggy and cement-reduced. A significance test run on these 11 parameters helped identify the most reliable discriminators of dominant pore types. Of those 11, we identified three key parameters to develop two pore type predictors. A result of 31 successful predictions out of 40 samples was obtained using the best predictor. These promising results indicate that T2 time can be a useful identifier of carbonate pore types. Application of our method should make it possible to predict the spatial distribution of poroperm facies in carbonates by extrapolating the results of NMR and pore geometry data from pore-to-core- to field scale.