Characterization of CO2 Transport in Low-Permeability Reservoirs using Low-Field Nuclear-Magnetic-Resonance and Mass-Transfer Theory: Experimental Applications on Evaluation of CO2 Sequestration

Summary Carbon dioxide (CO2) flooding provides a promising approach to enhance oil recovery and achieve carbon sequestration in low-permeability reservoirs. Nevertheless, the storage capacity and longevity of CO2 flooding are significantly influenced by the states of CO2 within the porous media. We conducted a series of experiments to characterize the mixing of the CO2-oil system and elucidate the underlying mechanisms influenced by injection pressure. n-Alkanes were positioned on the T1-T2 maps through T1/T2 values calculated using the Stokes-Einstein equation and the Bloembergen-Purcell-Pound (BPP) model. CO2 flooding experiments were conducted at varying backpressure values, and the T1-T2 maps of n-alkanes were obtained. By applying two-phase mass-transfer theory, “pseudo T1-T2 maps” of CO2 were derived. The maps of dissolved and independent CO2 were acquired according to the solubility of CO2 in n-alkanes. The correlation of oil recovery and the ratio of trapped CO2 in different states were finally obtained, and the trapping mechanisms were revealed. The result of n-alkane zoning indicates that pressure variation will cause component shifting in the T1-T2 maps. The comparison with oil composition tested by gas chromatograph (GC) proves high reliability and accuracy of low-field nuclear magnetic resonance (LF-NMR) to analyze the fluid components variation in the CO2 flooding. A detailed analysis on T1-T2 maps of hydrocarbon in CO2 flooding shows stronger extraction effect of CO2 on oil components in the miscible flooding. Moreover, the “pseudo T1-T2 maps” of CO2 provide valuable insights into the mass-transfer process between the displacing and displaced phases. By dividing the 2D map into distinct regions, CO2 was effectively differentiated into independent and dissolved states based on its interaction with hydrocarbon components. Subsequently, key metrics such as oil recovery factor, storage mass, storage efficiency, and solubility trapping ratio were computed, revealing that the oil recovery and solubility ratio show consistency in the immiscible flooding and display contradictoriness in the miscible flooding. Notably, this study reveals complex correlations among storage capacity, stability, and adjustable injection pressure. The increase in injection pressure has a positive effect on CO2 storage capacity and stability in the immiscible flooding, and this effect is negative for CO2 storage stability in the miscible flooding. These findings offer valuable insights for optimizing large-volume and long-term CO2 sequestration strategies.