Objective To address the issues of high water cut, high recovery factor, and the difficulty in effectively using of residual oil in remote wellbore zones caused by long-term waterflooding in Bohai Oilfield, an ultra-long delayed crosslinking system was proposed. This system delays the crosslinking reaction time and enhances the deep fluid diversion, thereby expanding the swept volume of profile control and flooding, mobilizing residual oil in remote wellbore zones, and significantly improving waterflooding development efficiency.

Method Scanning electron microscopy, a six-speed rotational viscometer, and physical simulation experiments were used to analyze the gel-forming properties, temperature and salt resistance, injectivity and plugging properties of the ultra-long delayed crosslinking system, as well as its effect of enhanced oil recovery.

Result The ultra-long delayed crosslinking system began gelling after 15 days at 55 and 65 ℃, exhibiting a long gelling time that effectively plugged the high permeability channel and used the residual oil in the remote wellbore zone. The gelling time accelerated sharply at 75 ℃, and the viscosity increased with increasing temperature and salinity. The injection and plugging experiments showed that as the concentrations of polymer and crosslinker increased, the injection pressure gradually increased, reaching a maximum of only 0.09 MPa, indicating good injectivity. The plugging rate remained above 84%, demonstrating good plugging performance. Microscopic oil displacement experiments showed that after injecting the ultra-long delayed crosslinking system, the diversion rate of the high permeability layer decreased significantly, while that of the low permeability layer increased markedly, leading to a substantial improvement in oil recovery.

Conclusion The ultra-long delayed crosslinking system can effectively improve waterflooding performance and increase the oil utilization degree of residual oil in remote wellbore zones. The system showed good gel-forming performance and plugging effectiveness in high-temperature and high-salinity environments, making it suitable for long-term plugging requirements under complex reservoir conditions. By optimizing injection parameters, the system can significantly improve waterflooding efficiency.