Dolomite cores evaluated by NMR

Abstract

Nuclear Magnetic Resonance (NMR) is a very versatile technique that allows the characterization of several petrophysical properties such as porosity, pore size distribution and fluid saturation and, unlike other techniques, it is possible to be applied on field scale through well loggings and in laboratory scale on the analysis of samples. In this paper, NMR measurements were performed in several samples with different saturation conditions in order to obtain as main information that can be inferred from the application of this technique at the laboratory scale. Silurian dolomites were studied since they are common in many carbonate reservoirs. Several petrophysical properties of these rocks were evaluated from NMR T2 relaxation time distributions at different conditions and compared with common techniques used in laboratory. Three fluids pairs were used: air/brine; mineral oil/brine; and crude oil/brine. The use of different fluids was required depending on the property that was being studied. Nitrogen porosity, commonly used in the petrophysical characterization, was compared to a porosity obtained by the NMR, and an optimum correlation was obtained between the results of these two techniques. T2 relaxation time limits were defined in order to separate micro/mesopores and meso/macropores limits. Using these limit ranges, the pore size distribution was evaluated and it was compared to the pore size distribution obtained when the T2 distribution is scaled by the surface relaxivity value. It was observed that about 80% of the porous medium was composed of macropores, but the evaluation of only the T2 relaxation times provided higher fraction of macropores. The T2cutoff values obtained were in accordance to the values referenced in the literature for vuggy carbonates. From the NMR measurements performed before and after waterflooding was possible to determine the residual oil saturations, which values were similar to the values obtained by mass balance. Finally, comparing the T2 distributions before and after the aging process, it was observed that there was an increase of fluid fraction in lower relaxation times after the aging process, indicating that NMR measurements could be used to evaluate the wettability alteration of the surface. The results obtained in the quantification of the wettability from the T2 relaxation times were very different in relation to the wettability quantification provided by Amott-Harvey Wettability Index.