A new method to determine surface relaxivity of tight sandstone cores based on LF-NMR and high-speed centrifugation measurements

Low-field nuclear magnetic resonance (LF-NMR) T2 spectrum commonly is converted into pore size distribution (PSD) with accurate calculation of surface relaxivity. Currently, the surface relaxivity for unconventional cores is determined indirectly using average pore radius (ARS) method or surface-to-volume ratio (SVR) method. The ARS method is time consuming, and inevitably it causes permanent damage to core samples because of the injection of mercury. In this study, we proposed a new method, the pseudo T2 cut-off (PTC) method, instead of the ARS method to calculate the surface relaxivity of tight sandstone core samples by correlating a PTC value with the corresponding pore radius. First, we performed LF-NMR and high-speed centrifugation (HSC) measurements on four tight sandstone core samples to determine surface relaxivity using the PTC method. Then, we compared the calculated surface relaxivity using the new method with that determined using the ARS and SVR methods. Then, we converted the T2 spectrum into PSD using appropriate surface relaxivity. Finally, we obtained residual oil distribution. The results showed that we had established an exponential correlation between the PTC value and surface relaxivity. Ultimate surface relaxivity values for tight sandstone core samples were 5.85, 2.98, 4.66, and 2.17 μm/s. The values of ρARS and ρPTC were close but generally larger than that of ρSVR. We still observed a linear correlation between illite content and different surface relaxivity. Moreover, we obtained PSD in mesopores and macropores by jointly using ρPTC (or ρARS) and ρSVR. After centrifugation, the residual oil (62.94 wt%) was mainly distributed in the micropores (~20 wt%) and mesopores (~35 wt%). This study provided a nondestructive method to determine the surface relaxivity of tight sandstone cores and provided information for residual oil distribution. •Proposing pseudo T2 cut-off (PTC) method to calculate surface relaxivity of tight sandstone cores.•Different surface relaxivity were compared using three methods.•T2 spectrum was converted into PSD using appropriate surface relaxivity.