Evidence of the Gelation Acceleration Mechanism of HPAM Gel With Ammonium Salt at Ultralow Temperature by SEM Study

Abstract

Summary Previously reported phenol-formaldehyde-based gel formulas with ammonium salt can provide a gelation time between 2 hours and 2 days at 25 °C, which has great potential to serve as water-shutoff or conformance-control agents in ultralow- to low-temperature reservoirs. The gelation performance can be adjusted by changing the concentration of hydrolyzed polyacrylamide (HPAM), crosslinker, and ammonium salt. Over the last decades, the working behavior of a gel system in a reservoir has been evaluated simply through the gelation-performance bottle test and the effectiveness of gel treatment at the macro level. However, the nature of a gel microstructure greatly dominates its working performance in subterranean reservoir conditions. The objective of this work is to investigate the relationship between gelation performance and microstructures of target gel by scanning-electron-microscope (SEM) analysis at different test conditions, including the effects of components concentration, aqueous-phase salinity, and porous-medium shearing, and to reveal evidence of the gelation-acceleration mechanism. Results show that ammonium salts act as bridging branch chains between polymer and crosslinker to accelerate the gelation progress. We reveal the gelation-process indicator through the discovery of several “coverings” that appeared among the polymer branch chains. The gelation-acceleration mechanism should be the embedding and growth of crystalline ammonium salts in the polymer skeleton to promote the extension of these coverings. The polymer skeleton covered by crystalline ammonium salts can increase the gel strength and salinity tolerance as a result of the unique structure of “crab's big claws.” The gelation performance is not seriously affected by the shearing effect in porous media, while the final gel strength still can reach Code I and exhibit a favorable long-term stability. Some application recommendations of the target gel are also discussed in this paper.