Acrylamide copolymer hydrogels used in profile modification applications in an oilfield share a common problem, termed syneresis, which decreases the efficiency of profile modification. In this paper, sodium tripolyphosphate (STPP) was found to be an effective syneresis inhibitor for the hydrogel formulated with acrylamide copolymer of acryloyloxyethyl trimethylammonium chloride (AM/DAC), and an ultrastable hydrogel for enhanced oil recovery in high-temperature and high-salinity petroleum reservoirs was obtained on the basis of double-groups cross-linking. Experimental investigations, including DLS, FTIR, NMR, SEM, and core flood test, have been conducted to elucidate the mechanism of STPP inhibiting the hydrogel syneresis in the aspect of the reaction between STPP and AM/DAC. The result showed that the AM/DAC cross-links with STPP on the basis of the hydrolysis reaction of the ester group and STPP, whereby the new bond of C–O–P is formed. For this reason, the viscosity and hydrophilicity of AM/DAC were significantly increased by STPP, and the generated double-groups cross-linking (AM/DAC cross-linked with phenol-formaldehyde, AM/DAC cross-linked with STPP) made more AM/DAC molecule chains cross-linked together; therefore, the stronger grid structure was formed. Therefore, the increase of the hydrophilicity and the improved stability of grid structure enhanced the water-holding capacity of the hydrogel, leading to the decrease of the hydrogel syneresis and the increase of the water-shutoff efficiency.

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Polymer gels have been widely used in high water cut oilfields for profile control and water plugging. It is urgent to develop a gel suitable for the Tahe Oilfield (Temperature: 130 °C, salinity: 2.2 × 105 mg/L) in China. A stable gel was prepared by using an acrylamide (AM)/2-acrylamide-2-methyl propanesulfonic acid (AMPS) copolymer crosslinked with urotropin (HMTA), hydroquinone (HQ), thiourea and Nano-SiO2. This paper covers a step-by-step process for designing gels based on experience with preparing gels. A wide range of combinations between polymers and crosslinking agents with and without stabilizers were investigated, and the results indicated that there is an optimal value of AMPS content of AM/AMPS copolymers in the preparation of gels. Increasing the mass fraction of copolymer and using stabilizer enhanced the performance of gel, but an excessive amount of crosslinking agent was not conducive to the stability of gel. The work optimized the formula of plugging agent suitable for the high temperature and high salt (HTHS) condition in the Tahe Oilfield. The gelling solution had a long gelation time of 20 h. The gel had high strength (Sydansk’s gel-strength code of “G”) with storage modulus of 12.9 Pa and could be stable for half a year at 130 °C and 2.2 × 105 mg/L of salinity. The plate model that could be heated and pressurized was used to simulate the oil flooding and profile modification under the condition of the Tahe Oilfield for the first time. The experiment results showed that the oil recovery could be increased by 13.22% by subsequent water flooding under heterogeneous formation condition. Therefore, it was fully confirmed that the plugging performance of AM/AMPS phenolic gel prepared in the work was excellent. The information provided in the study could be used as a reference for the design and evaluation of polymer gels in other HTHS reservoirs.

As a temperature-resistant and salt-resistant polymer, acrylamide and 2-acrylamide-2-methylpropane sulfonic acid (abbreviated as AM-AMPS) copolymer is currently widely used in drilling, water control and oil production stabilization, enhanced oil recovery and other fields, but its stability under high temperature has been less studied. The degradation process of the AM-AMPS copolymer solution was studied by measuring viscosity, the degree of hydrolysis, and weight-average molecular weight at different temperatures and aging time. During the high-temperature aging process, the viscosity of the AM-AMPS copolymer saline solution first increases and then decreases. The combined action of the hydrolysis reaction and the oxidative thermal degradation leads to the change of the viscosity of the AM-AMPS copolymer saline solution. The hydrolysis reaction of the AM-AMPS copolymer mainly affects the structural viscosity of its saline solution through intramolecular and intermolecular electrostatic interactions, while the oxidative thermal degradation mainly reduces its molecular weight by breaking the main chain of the copolymer molecules, reducing the viscosity of the AM-AMPS copolymer saline solution. The content of AM and AMPS groups in the AM-AMPS copolymer solution at various temperatures and aging time was analyzed using liquid nuclear magnetic resonance carbon spectroscopy, demonstrating that the hydrolysis reaction rate constant of AM groups was significantly higher than that of AMPS groups. The contribution values of hydrolysis reaction and oxidative thermal degradation of the AM-AMPS copolymer at different aging time to viscosity were quantitatively calculated at temperatures ranging from 104.5 °C to 140 °C. It was determined that the higher the heat treatment temperature, the smaller the contribution of hydrolysis reaction to viscosity, while the bigger the contribution of oxidative thermal degradation to the viscosity of the AM-AMPS copolymer solution.

Experimental investigations have been conducted to elucidate the mechanism of sodium tripolyphosphate (STPP) inhibiting hydrogel syneresis with respect to the reaction between STPP and partly hydrolyzed polyacrylamide (HPAM).