Urea Delays High-Temperature Crosslinking of Polyacrylamide for in Situ Preparation of an Organic/Inorganic Composite Gel

To address the rapid crosslinking reaction and short stability duration of polyacrylamide gel under high salinity and temperature conditions, this paper proposes the use of urea to delay the nucleophilic substitution crosslinking reaction among polyacrylamide, hydroquinone, and formaldehyde. At the same time, urea also regulates the precipitation of calcium and magnesium ions, enabling the in situ preparation of an organic/inorganic composite gel of crosslinked polyacrylamide and carbonate particles. With calcium and magnesium ion concentrations at 6817 mg/L and total salinity at 15×104 mg/L, the gelation time can be controlled to range from 6.6 to 14.1 days at 95 °C and from 2.9 to 6.5 days at 120 °C. The corresponding composite gel can remain stable for up to 155 days and 135 days, respectively. The delayed gelation facilitates longer-distance diffusion of the gelling agent into the formation, and the enhancements in gel strength and stability provide a solid foundation for improving the effectiveness of profile control and water shut-off in oilfields. This innovative approach promotes the comprehensive utilization of mineral resources within the formation.