Performance and gas breakthrough during CO2 immiscible flooding in ultra-low permeability reservoirs

The performance data, water/gas injection profile monitoring, tracer monitoring, and composition of crude oil and formation water are analyzed in the CO2 immiscible flood pilot area of Yaoyingtai Oilfield, Jilin Province, to examine the well production characteristics and gas breakthrough law of CO2 immiscible flooding. In the pilot, the characteristics of CO2 extraction, viscosity reducing, oil volume distension and effective formation opening increment are combined, reflecting obvious effects of recovery enhancement. For the ultra-low permeability and strong heterogeneity reservoir, monolayer breakthrough and plane fingering may take place in either water flooding or gas drive. The alterative water/gas injection can increase the sweep volume effectively and help to mitigate gas breakthrough. The increment of oil production is less when the injection shifts from water to gas than when the injection shifts from gas to water. So, the adjustment of water injection half-cycle plays a dominant role in improving gas drive effect. In the Yaoyingtai Oilfield, gas show is mainly observed along fractures, along the high permeability bands, and in low-permeability zones. According to the established standard of gas breakthrough in oil well, the key to improving the immiscible flooding effect is to keep the oil well to see gas but avoid gas channeling and also to slow down the gas/oil ratio ascending. Once gas breakthrough occurs in a well, it is necessary and effective to shut down the well, adjust the bottom hole flowing pressure in production well and implement the alternative water/gas injection.