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.
Determination of Permeability From NMR T1/T2 Ratio in Carbonates
Cheng, Yuesheng, Chen, Songhua, Eid, Mahmoud, Hursan, Gabor, and Shouxiang Ma. "Determination of Permeability From NMR T1/T2 Ratio in Carbonates." Paper presented at the SPWLA 58th Annual Logging Symposium, Oklahoma City, Oklahoma, USA, June 2017.
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