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

## TL;DR

Urea is used to delay the crosslinking of polyacrylamide gels under high temperature and salinity, enabling the formation of stable organic/inorganic composite gels for oilfield applications.

## Contribution

A novel urea-based method is introduced to control high-temperature crosslinking and in situ formation of composite gels.

## Key findings

- Gelation time can be controlled from 6.6 to 14.1 days at 95 °C and 2.9 to 6.5 days at 120 °C.
- Composite gels remain stable for up to 155 days at 95 °C and 135 days at 120 °C.
- Urea converts calcium and magnesium ions into inorganic particles, enhancing gel strength and stability.

## Abstract

To address the rapid crosslinking reaction and short stability duration of polyacrylamide gel under high salinity and temperature conditions, this paper proposes utilizing urea to delay the nucleophilic substitution crosslinking reaction among polyacrylamide, hydroquinone, and formaldehyde. Additionally, urea regulates the precipitation of calcium and magnesium ions, enabling the in situ preparation of an organic/inorganic composite gel consisting 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 resulting composite gel can remain stable for up to 155 days at 95 °C and 135 days at 120 °C. The delayed gelation facilitates longer-distance diffusion of the gelling agent into the formation, while the enhancements in gel strength and stability provide a solid foundation for improving the effectiveness of profile control and water shut-off in oilfields. The urea-controlling method is novel and effective in extending the high-temperature cross-linking reaction time of polyacrylamide. By converting calcium and magnesium ions into inorganic particles, it enables the in situ preparation of organic/inorganic composite gels, enhancing their strength and stability.