Quantitative analysis on distribution of microcosmic residual oil in reservoirs by frozen phase and nuclear magnetic resonance (NMR) technology

Understanding and characterizing the distribution and occurrence state of microcosmic residual oil is helpful to optimize judicious strategy for enhancing oil recovery (EOR). In this paper, frozen phase technology was developed to realize accurately characterize the oil and water saturation and their occurrence state. First, the core was placed in liquid nitrogen to frozen the oil and water inside it, and the solidified oil and water guarantees no destroying of their original distribution of oil and water, and 0.05 mm slice could be obtained. Second, taking advantage of different freezing point of oil and water, the low-field NMR is employed to achieve high-resolution single oil signal under freezing point of water and both oil and water signal at room temperature, thus, the oil and water saturation could be accessed, and their distribution in different-sized pore could be achieved based on conversion calculation of transverse relaxation time of NMR. Third, the displacing performance of water flooding and CO2 flooding was investigated. The results characterized by laser confocal indicates that the water flooding has strong scouring capability for free-state oil and CO2 flooding has strong stripping capability for bound-state oil, suggesting CO2 water-alternating-gas injection has unique advantage for EOR. Our work develops a frozen phase technique to realize accurate characterization the residual oil and water saturation and give their original distribution, which could provide basic foundation for EOR design.