Characterizations of phase behavior and miscibility of CO2-hydrocarbon mixtures in bulk and porous media using low-field NMR technique

Low-filed Nuclear Magnetic Resonance (LF-NMR) technique has been widely used to characterize rock properties and to investigate performance of CO 2 EOR . However, rare studies have been found to characterize the complex phase behavior variations in CO 2 -oil systems using the LF-NMR technique, i.e. interfacial tension reduction, viscosity reduction, oil swelling, hydrocarbon extraction and miscibility between gas-oil phases when pressure exceeds the minimum miscible pressure (MMP). This study explores the phase behavior and miscibility of CO 2 -hydrocarbon mixtures using the low-field nuclear magnetic resonance (NMR) techniques. First, the transverse relaxation time (T 2 ) of n-dodecane (C12) and n-hexadecane (C16) at different pressure and temperature conditions were measured as a baseline. Results show that the logarithm mean of T 2 (T 2lm ) has a good linear relationship with viscosity. Thereafter, T 2 distributions of CO 2 -hydrocarbon mixtures were continuously collected to investigate the evolutions of phase behavior induced by CO 2 -hydrocarbon interactions. Three stages were categorized from the T 2lm of CO 2 -hydrocarbon mixtures and pressure. As pressure increases for all cases, there is an observable rise in T 2lm values due to the viscosity reduction after CO 2 dissolution. At the subsequent higher pressures, as more CO 2 dissolves into the oil phase, the molecular mobility of oil components decreases due to increased intermolecular interactions and reduced free mean path . This effect causes a decline in T 2lm values, as the motion of oil molecules becomes more restricted. Finally, T 2lm stabilizes when miscibility is achieved, as the CO 2 -oil system transitions into a single-phase state Similar findings were observed for CO 2 -C12/C16 mixtures in glass-bead packs. In addition, interactions between CO 2 and light oil mixtures were investigated to validate the new technique. Results show that the MMP of the mixture is 8.5 MPa. As for CO 2 -oil in core plug , T 2lm increases first and stabilizes below and above the MMP, while higher pressure accelerates the stabilization process. This might be because oil close to the gaseous CO 2 becomes miscible, while for oil in the smaller pore spaces , a fully CO 2 -saturated phase exits as multi-contact miscible cannot be achieved simply through molecular diffusion . The findings in this study reveal the dynamic interactions between CO 2 and oil and provide an effective method for estimating the MMP, illustrating a significant potential application for commercial-scale CO 2 EOR and storage projects. • Low-field NMR T 2 relaxations of CO 2 -oil in bulk and porous media were measured for tracking phase changes at different injection pressure. • Relationship between T 2 relaxation and viscosity of CO 2 -oil mixtures were evaluated. • The effects of temperature and oil component on T 2 relaxation of CO 2 -oil mixtures were investigated. • Variations of logarithm mean of T 2 were used to estimate the minimum miscibility pressure.