Performance and applicable limits of multi-stage gas channeling control system for CO2 flooding in ultra-low permeability reservoirs

Abstract CO2 flooding is an effective enhancing oil recovery (EOR) method that is suitable for ultra-low permeability reservoirs. Generally, high heterogeneity and natural fracture development are the major characteristic of ultra-low permeability reservoirs, which can induce serious gas channeling issues. In this study, a systematic and comprehensive gas channeling control system has been formed to enhance the mobility control and sweep efficiency of CO2 flooding in ultra-low permeability reservoirs with different degrees of heterogeneity and fracture scales. Physical models are designed and fabricated to perform the laboratory experiments for investigating the performance and applicable limits of multi-stage gas channeling control system. Experimental results indicate that continuous CO2 flooding is more suitable for applying in ultra-low permeability reservoirs compared with water flooding. CO2 flooding is a preferred EOR method applied in the homogeneous model with the permeability from 10 × 10−3 μm2 to 0.1 × 10−3 μm2 or heterogeneous model with the permeability ratio (the ratio between the permeability values of each half layer) of less than or equal to 5, whose elevated oil recovery is all more than 20%. As the permeability ratio is from 5 to 30, the primary-stage gas channeling control system is a much suitable method which depends on CO2 mobility control to improve sweep efficiency and oil production, and the oil recovery can be improved from 16.88% to 22.46%. Besides, CO2-thickener flooding can mitigate the gas channeling to some extent by controlling CO2 mobility in weakly heterogeneous models. To the permeability ratio of more than 30 or even to small-scale fracture, secondary-stage gas channeling control system can effectively reduce the flowing capacity of CO2 in the microcosmic or macroscopic small-scale gas channeling path and play a role in profile control, which improves oil recovery for more than 12%. To high permeability path and intermediate-scale or large-scale fractures, tertiary-stage gas channeling control system is selected as a blocking agent which can availably block off both macroscopic and microcosmic gas channeling path, and its elevated oil recovery is all more than 34%.