Oil Recovery Enhancement by Composite Flooding Technology for Gasi N₁–N₂¹ Ultra-High-Salinity Reservoir in Qinghai Oilfield

The salinity and the content of calcium and magnesium ions are ultra-high in the formation water of Gasi N₁–N₂¹ reservoir in Qinghai Oilfield. While using gel and surfactant composite flooding, conventional gels are prone to dehydrate and break, showing poor long-term stability. Meanwhile, conventional surfactants are easy to react with the calcium and magnesium ions in formation water to cause precipitation. In view of this, a high-salinity-resistant organogel suitable for Gasi N₁–N₂¹ reservoir was developed, which consisted of polymer (0.3%–0.4%) + crosslinking agent (0.2%–0.3%) + stabilizer (0.1%–0.2%). The initial setting time of the system was longer than 70 h at 68 ℃, and the viscosity after gelling was greater than 1.0×10⁴ mPa·s. What's more, a high-salinity-resistant surfactant QH-1 was optimized, and the interfacial tension and oil displacement effect were evaluated, witha finding that the QH-1 solution with a mass fraction of 0.4% could enhance the oil recovery by 18.72%. The laboratory test results indicated that alternate injection of the high-salinity-resistant organogel and QH-1 could effectively curb the ineffective water circulation and improve the oil displacement efficiency in the low and medium permeability areas. Notably, the optimized “gel + QH-1” composite flooding was capable of enhancing oil recovery by more than 27.6%. The composite flooding was applied to 9 water-injection wells in Gasi N₁–N₂¹ reservoir. As a result, the average water cut of these oil wells decreased from 80% to 70%, and the oil production increased by 2.41 × 10⁴ t. The research results show that the oil recovery enhancement by “gel + QH-1” composite flooding is effective in enhancing oil production and decreasing water cut in Gasi N₁–N₂¹ ultra-high-salinity reservoir, so it is worthy of promotion and application.