Nuclear magnetic resonance (NMR) relaxometry typically involves the analysis of a relaxation time distribution. The surface relaxivity (⁠ρ⁠) is the key parameter that relates the relaxation time to the pore radius. Only a good estimate of the surface relaxivity enables a reliable determination of the pore radius distribution in a rock or sediment sample. A wide variety of approaches for the estimation of ρ has been proposed; however, the accuracy of ρ determination approaches has rarely been checked. We have compared different approaches of ρ determination for a set of Eocene sandstone samples. Most approaches based on a weighted logarithmic mean of relaxation times or the peak relaxation time result in significant underestimation of ρ⁠. However, the correct weighting of the set of relaxation times has proven to be a crucial approach in ρ determination. The consequent application of geometric rules suggests the application of the weighted harmonic mean (⁠Thm⁠). The specific surface area per unit pore volume (⁠Spor⁠), which results from the gas adsorption method, is another crucial parameter in most approaches for ρ estimation. The quantities Thm and Spor depend on the resolution of the used method. Applying the fractal theory, we adopt an approach that performs an upscaling of Spor to the resolution of the NMR relaxometry. Using equal resolution for Thm and Spor⁠, we obtain more reliable ρ estimates. The resulting ρ values are comparable with the ones determined by using the median relaxation time from NMR and the median pore-throat radius from the mercury injection capillary pressure method.

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