In situ crosslinked weak gels with ultralong and tunable gelation times for improving oil recovery

Polymer-based in situ cross-linked gels have been widely used in improving the oil recovery process; however, the insufficient and uncontrollable gelation time still deteriorates the successful in-depth water shutoff and conformance control for oil reservoirs due to the short and uncertain moving distance underground. Herein, we report weak gels formed by in situ crosslinking of partially hydrolyzed polyacrylamide with hexamethylenetetramine and resorcinol as binary crosslinker pairs. These gels exhibit ultralong and adjustable gelation times between 3 and 47 days at 45 ℃. The chemical principles of controllable gelation time and microstructure evolution during gel formation are well elaborated using nuclear magnetic resonance spectroscopy and scanning electron microscopy. The crosslinker pairs and temperature greatly affect gelation kinetics, and polymer molecular weight and concentration as well as salinity also affect the gelation process. Fine-tuning the hexamethylenetetramine and resorcinol concentrations affords a series of gels with gelation times in a large range. The activation energy for the system with the longest gelation time was calculated to be 95.33 kJ/mol, much higher than similar gel systems reported previously, accounting for the ultralong gelation time. Core plugging and flooding tests showed that this gel system can be easily injected into porous media and form in situ gels that selectively block highly permeable layers and force subsequent injected water to pass through low permeability zones, improving sweep efficiency and oil recovery. These in situ cross-linked gels could find wide applications for water shutoff and conformance control in heterogeneous oil reservoirs, and the knowledge obtained provides new insight into gelling process control and would help in developing novel systems with desirable gelation time under various conditions.