The Injection Pattern of Immiscible Nitrogen Displacement After Water Flooding in an Ultra-low Permeability Reservoir

With the nature ultra-low permeable cores from Changqing Oilfield, the simulated experiments of nitrogen drive after water flooding have been accomplished for the ultra–low-permeability reservoir. The injection pattern of nitrogen includes direct gas injection, water/gas alternating injection and pulse gas injection. The results show that direct injection could not develop the ability that nitrogen enters into the local tight area to improve the microsweep efficiency, because the good mobility generates gas channeling in some local high-permeable zone. Subsequent direct nitrogen injection after water drive only enhances the oil recovery ratio by 3.65%. The dispersion degree of oil, water, and gas is much higher for water/gas alternating injection after water injection, and the capillary force is the main resistance to suppress the rapid gas flow validly in local high-permeable zone. Nitrogen is enforced to drive the residual oil in local tight area or micro pore system. Water/gas alternating injection after water flooding enhances oil recovery ratio by 16.37%. Pulsing injection can take advantage of the elastic energy of the high-pressure nitrogen and pressure disturbance effect, which makes pressure redistribution and fluid deallocation between local high-permeability zone and local compact area. With pulse injection, oil in local tight region also can be driven by nitrogen, and the recovery ratio is enhanced by 15.94%.