Phenolic resin gel suitable for medium-temperature and high-salinity reservoirs

• Two phenolic resin gels suitable for medium-temperature and high-salinity reservoirs were prepared. • The properties of the two phenolic resin gels were investigated systematically. • Part gel can be stable for more than 5 months under harsh conditions. • A piece of advice should bear in mind: keep the cross-linker dosage appropriate to avoid overloading it. • The syneresis mechanism of phenolic resin gel was proposed. Gel has been widely applied as a plugging agent for enhanced oil recovery. In this study, partially hydrolyzed polyacrylamide (HPAM) or co-polymers of acrylamide and 2-acrylamido-2-methylpropanesulfonic acid (AM-AMPS) cross-linked with water-soluble phenolic resin (WSPR) forming two phenolic resin gels with mineralization of 41529 mg/L at 90℃. The properties of the two gels were systematically evaluated in this work. Under the same dosage of polymer and cross-linker conditions, HPAM gel exhibits a faster crosslinking rate and higher strength than AM-AMPS gel, but is less thermal stable than AM-AMPS gel. Part of the two phenolic resin gels was prone to premature syneresis at higher crosslinker dosages, which has not been reported in published literature . Severe syneresis was observed in the HPAM gel with the cross-linker concentration ranges from 0.8∼1.0 wt % when heated up to 60 days. Fortunately, stable gels were developed when the loading of HPAM and cross-linker in the gelant ranges from 0.8∼1.0 wt % and 0.4 ∼ 0.6 wt % respectively. Besides, the dehydration ratio of some stable HPAM gels was still less than 8% when these gels aged for 150 days. For AM-AMPS gel, syneresis was observed in the gels with 0.8∼1.0 wt % cross-linker and 0.4 wt % polymer when heated up to 120 days. Luckily, when the polymer dosage is over 0.4 wt %, the crosslinker loading had little effect on thermal stability, so some stable AM-AMPS gels can be acquired. Premature syneresis of higher cross-linker loading gels, which is counter to our goal of increasing crosslinker dosage to improve the crosslinking density and further enhance thermal stability. Based on XPS, FT-IR, and Cryo-SEM testing results, two syneresis mechanisms of the higher crosslinker gels were proposed, which ascribes to the decrease in hydrophilicity and excessive crosslinking of gel systems. A piece of advice should bear in mind when design a phenolic resin gel: keep the cross-linker dosage appropriate to avoid overloading it. As a result, our findings may guide the development of stable phenolic gels under similar conditions, and part of the two gels could provide a solution for plugging in medium-temperature and high-salinity reservoirs.