Oil migration in nanometer to micrometer sized pores of tight oil sandstone during dynamic surfactant imbibition with online NMR

Dynamic imbibition is an imbibition phenomenon considering fluid flow around the surface of porous media and plays an important role in the development of tight oil reservoirs. In this paper, the behavior of oil migration in nanometer to micrometer sized pores during dynamic imbibition was studied using nuclear magnetic resonance (NMR). Firstly, the basic properties of surfactant and core samples were characterized. The boundary of cores were semi-sealed to simulate practical conditions. The dynamic imbibition experiments were carried out in the core flooding system using low-field NMR. The T-2 spectrum and magnetic resonance imaging (MRI) were scanned, and the T-2 relaxation time was converted to pore sizes using the mercury injection curve. The oil recovery factor and capillary number in nanopores and macropores were calculated. Four characteristic points of T-2 spectrum were selected, and the recovery of these characteristic pores (0.3, 0.6, 18.3, and 116.7 mu m) were found to be 20.0%, 19.8%, 11.9%, 30.1%, respectively. Furthermore, the effects of injection rate, interfacial tension (IFT), soaking time, and permeability on oil recovery from different typical pore sizes were investigated in detail. The recovery factor ratio (r), which was defined as the ratio of oil recovery in nanometer pores to that of macrometer pores, increased with the increase in soaking time, IFT, and permeability, while it decreased with the increase in flow rate.