Evaluation of Low-Field NMR T2 Methods for Residual Oil Saturation Estimation in Carbonate Core Plugs

Nuclear magnetic resonance (NMR) relaxometry is widely used to characterize pore systems and estimate fluid saturations in reservoir rocks. This study evaluates the use of low-field NMR transverse relaxation time (T2) as a standalone method for estimating residual oil saturation in Indiana limestone core plugs following brine flooding. Three core plugs were first saturated with dead light crude oil and then subjected to brine core-flooding experiments at different injection rates to establish different post-flood residual oil saturations. Two NMR T2-based approaches were investigated: (i) a volumetric method based on a calibrated T2 cutoff and (ii) a mathematical method based on the logarithmic mean T2 response. Independent saturation estimates were obtained using X-ray computed tomography (XCT). Overall, the volumetric approach showed closer agreement with XCT than the logarithmic mean approach. However, appropriate cutoff calibration and sensitivity analysis are essential because rock wettability, desaturation conditions, fluid properties, and pore-size distribution can influence cutoff selection and the resulting saturation estimates. At a T2 cutoff of 90 ms, the cutoff-based oil saturation estimates were 42.0%, 38.8%, and 40.1% for Ind03, Ind08, and Ind09, respectively. Increasing the cutoff from 70 to 100 ms increased the estimated saturations by 4.4–5.0 percentage points across the three plugs, highlighting the sensitivity of quantitative saturation estimates to cutoff selection.