ABSTRACT

This paper presents a new methodology for performing a cutoff analysis that uses a T1/T2 ratio distribution obtained from two-dimensional (2D) nuclear magnetic resonance (NMR) T1 and T2 measurements. The ability to classify pores and their effect on permeability is noticeably improved compared to a T2-based approach, of which T2 cutoff values vary from a few tens of milliseconds to a few seconds. Based on mercury injection capillary pressure results, the T1/T2 ratio-based cutoff is used to differentiate porosity with a pore throat radius larger than 2 µm from smaller pore throats. The T1/T2 cutoff ranges narrowed to within 1.4 to 1.7 for 100% water-saturated carbonate cores. In addition, the empirical models of NMR-based permeability are enhanced by incorporating the porosity, T1/T2 ratio cutoff, and T2 geometric mean. For the studied data set of 49 carbonate rock samples with a permeability range spanning six orders of magnitude, an excellent correlation coefficient of R2 = 0.9 was observed between the NMR predicted permeability and that measured in the laboratory. This improved permeability prediction technique has the potential to be implemented in applications of downhole NMR logging.

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