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

Abstract

Low-filed Nuclear Magnetic Resonance (LF-NMR) technique has been widely used to characterize rock properties and to investigate performance of CO₂ EOR. However, rare studies have been found to characterize the complex phase behavior variations in CO₂-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 CO₂-hydrocarbon mixtures using the low-field nuclear magnetic resonance (NMR) techniques. First, the transverse relaxation time (T₂) 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 T₂ (T_2lm) has a good linear relationship with viscosity. Thereafter, T₂ distributions of CO₂-hydrocarbon mixtures were continuously collected to investigate the evolutions of phase behavior induced by CO₂-hydrocarbon interactions. Three stages were categorized from the T_2lm of CO₂-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 CO₂ dissolution. At the subsequent higher pressures, as more CO₂ 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 CO₂-oil system transitions into a single-phase state Similar findings were observed for CO₂-C12/C16 mixtures in glass-bead packs. In addition, interactions between CO₂ 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 CO₂-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 CO₂ becomes miscible, while for oil in the smaller pore spaces, a fully CO₂-saturated phase exits as multi-contact miscible cannot be achieved simply through molecular diffusion. The findings in this study reveal the dynamic interactions between CO₂ and oil and provide an effective method for estimating the MMP, illustrating a significant potential application for commercial-scale CO₂ EOR and storage projects.