Evaluating the surface relaxivity and movable fluid of low-permeability sandstones based on low-field nuclear magnetic resonance

Comprehensive characterization of pore structure and fluid distribution is beneficial for efficiently exploring and developing low-permeability sandstone reservoirs. As a conversion parameter, the surface relaxivity is significant for characterizing the pore structure of porous media and evaluating fluid mobility. The surface relaxivity indicates the strength of the interaction between the fluid and the solid during the relaxation process. This paper conducts mercury intrusion porosimetry, low-temperature nitrogen adsorption, and nuclear magnetic resonance-centrifugation experiments on low-permeability sandstones, providing insight into the evolution of pore size and water content distribution. Combining mercury intrusion porosimetry with nuclear magnetic resonance, the surface relaxivity of samples is measured to be 9.57-23.79 mu m/s. The surface relaxivity ranges from 0.70 to 3.72 mu m/s utilizing low-temperature nitrogen adsorption and nuclear magnetic resonance. Based on the movable water saturation through the critical radius, the calculated surface relaxivities using two methods are compared. The result indicates that surface relaxivity determined by low-temperature nitrogen adsorption is smaller than that obtained through mercury intrusion porosimetry. This is attributed to overestimating the ratio of pore surface and pore volume in the low-temperature nitrogen adsorption, which is difficult to capture information about macropores. Conversely, the similar principle between mercury intrusion porosimetry and centrifugation leads to consistent movable water saturation, minimizing discrepancies in evaluating surface relaxivity. Therefore, the surface relaxivity determined by mercury intrusion porosimetry-nuclear magnetic resonance is more suitable for characterizing the pore structure and fluid mobility of low-permeability sandstones. In addition, the ink-bottle effect retains water in the macropore during centrifugation experiments.