Exploration of the Migration, Miscibility, and Storage Characteristics of Impure CO2 in Offshore Low-Permeability Oilfields Based on Online LF-NMR

The co-reinjection of oilfield-produced gas and CO₂ will enhance the economic efficiency of greenhouse gas (CO₂, CH₄) storage, which is conducive to the green transformation of the upstream petroleum industry. However, the actual migration and storage characteristics of impure CO₂ have not yet been clarified. This paper conducts a comprehensive study on the migration, miscibility, and storage characteristics of impure CO₂ in offshore low-permeability oilfields based on online LF-NMR. The results show that for impure CO₂ flooding, the miscible state affects the recovery by more than 14.8% under the same injection parameters, which is the fundamental cause affecting migration and recovery. Impurity gases affect the displacement process through a dual mechanism of reducing the degree of miscibility and enhancing the pressure gradient, which accelerates gas breakthrough and channeling. The impact of low CH₄ concentration on recovery is less than 1.9% under miscible conditions, showing the potential for co-displacement and storage with CO₂. The migration of oil and gas in porous media shows staged characteristics. In the initial stage, the large and medium pores are preferentially utilized through a physical driving force, and impurity gas can enhance this process. In the middle stage, the gas breaks through the lower limit of small-pore utilization through the diffusion-extraction-miscibility effect. After breakthrough, it mainly maintains the continuous utilization capacity of the medium and small pores through miscibility, extraction, and the carrying of oil in the pores around the channeling channel, while impurity gas weakens this process. The sweep of the injected gas in the porous medium showed spatial heterogeneity. The gas can be more sweep-balanced in the front section due to the resistance effect, with a sweep efficiency exceeding 85%, contributing to the main recovery and storage space. Due to the influence of fingering and gravity, the mesopores and macropores in the middle and rear regions have difficulty providing excessive storage and recovery space. This article provides valuable insights for understanding the migration and storage characteristics in the impure CO₂ flooding process.