Kerogen is an organic component of unconventional rocks. It is believed that the majority of the pore volume of tight rocks originates from the organic pore systems. Understanding the pore size distribution of these organic pores is instrumental to petrophysicists and geoscientists, as it provides valuable information on the reservoir properties. Nuclear magnetic resonance (NMR) has played a critical role in characterizing the pore systems both in core plugs and in reservoir formations. Although a considerable amount of research has been published focusing on NMR studies of sandstone and carbonate rocks, few studies have been reported for organic pore systems in shale. Of particular interest is the surface relaxivity, which is an essential parameter for estimating pore size distribution via NMR. To date, surface relaxivity of kerogen is only available via estimations based on SEM images of a small area or adsorption/imbibition experiments performed on whole shale core plugs. In this paper, we report a direct measurement of surface relaxivity on isolated kerogen powders.
Kerogen powders were extracted from shale plugs using non-oxidizing reagents. We used the ‘Pulsed Field Gradient’ (PFG) stimulated echo method to measure the apparent diffusion coefficients of the decane molecules, and then calculated the average surface to volume ratio by plotting the diffusion coefficients against the diffusion times. We then obtained the surface relaxivity through the correlation between transverse relaxation time and the calculated surface to volume ratio. The surface relaxivity of Barnett shale kerogen was determined to be 18.9 ± 6.54 μm/s. Based on this result, we further concluded that spherical pores with a diameter of 6 nm or smaller would not be detectable by a lab NMR system with a TE of 0.1 ms. The method was also applied to obtain kerogen surface relaxivity values from other shale formations. The direct measurement of kerogen surface relaxivity is of great importance in the determination of organic pore size distribution in tight rocks, which will in turn facilitate the calculation of capillary pressure, relative permeability and other important petrophysical properties.