Nanoparticles have recently been widely used in polymer gel systems to enhance gel strength. However, it is yet unclear how various types of nanoparticles affect the gelation characteristics of polymer gels. In addition, nanomaterials are rarely studied for enhancing the temperature resistance of polyethyleneimine (PEI) cross-linked polyacrylamide gels (i.e., HPAM/PEI gels). Under the urgent requirements of global carbon neutrality, this paper intends to use carbon nanofibers as gel performanceenhancing materials. Furthermore, the mechanism of their influence on the gelation properties and temperature resistance of HPAM/PEI gels was investigated. We evaluated the gelation time of the carbon nanofiber-enhanced HPAM/PEI gels at both 80 and 150 °C by the Sydansk method. The gelation properties and microscopic network structure of the gels were evaluated by rheometer, differential calorimetry scanning/thermogravimetry, and scanning electron microscopy. Results show that adding carbon nanofibers could delay the gelation time at 80 °C, and the gelation time of the polymer gel system reaching F was more than 6 days. On the other hand, the addition of carbon nanofibers could slightly shorten the gelation time of the polymer gel at an ultrahigh temperature of 150 °C. Moreover, the gel strength reached the highest when the addition amount of carbon nanofibers was 0.1%. The gel strength (G′) was as high as 12 Pa, and its stability was as long as 30 days, which was more temperature-resistant than that without adding nanofibers. The microstructure of carbon nanofiber-enhanced HPAM/PEI gels showed many tiny pore-like structures, which might be more conducive to heat dissipation and prevent dehydration. As a result, the carbon nanofiber-enhanced HPAM/PEI gel system had outstanding thermal stability and could be applied in ultrahigh-temperature reservoir conditions while maintaining high plugging strength. This study can present new ideas for improving the nanoparticle-enhanced HPAM/PEI gels in high-temperature petroleum reservoirs and further improving oil recovery, which has essential reference significance.

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Lithologic reservoirs, such as conglomerate reservoirs, exhibit stronger heterogeneity compared with conventional sandstone reservoirs. Their early stage development usually requires hydraulic fracturing to increase the stimulated reservoir volume (SRV). As a result, high-conductivity channels are generated during waterflooding, which further limit the sweep volume and oil recovery. Traditional particle gels and in situ cross-linked polymer gels exhibit limited field application due to their low plugging strength. In this paper, a high-strength preformed particle–gel (HSPPG) with sticking-together capability was studied through evaluating different factors such as particle size, swelling ratio, injection volume, and their effects on the swelling performance, gel strength, and plugging performance in fractures. The results show that the swollen HSPPGs had strong water swelling and sticking-together ability, exhibited good viscoelasticity, and formed high-strength adhesive bonds on rock surfaces. In addition, as the particle size decreased, the gel’s maximum swelling ratio increased. However, as the swelling ratio of HSPPG increased, the storage modulus G′ and the loss modulus G″ both decreased, but they still displayed excellent elasticity and viscosity. The greater the filling volume of HSPPG in fractured conglomerate rock cores, the stronger the overall plugging efficiency. Oil displacement experiments showed that the oil recovery of fractured conglomerate rock cores treated with HSPPG increased by 22.12 percentage points on the basis of primary waterflooding. Therefore, HSPPG has a promising application prospect in the conformance improvement of strong heterogeneity conglomerate reservoirs.

Temporary plugging agents (TPAs) have been widely used in unconventional petroleum reservoirs to increase the stimulated reservoir volume and generate an easier flow of oil and gas into the production well. The properties of particle-gel-based TPAs include a straightforward injection procedure, excellent deformation, strong plugging strength, and total self-degradation. This study prepared degradable preformed particle gel (DPPG) samples polymerized by two types of crosslinkers (i.e., alcohol ester and alkenoic acid ester) for low-temperature (less than 40 °C) reservoirs. The effects of different crosslinkers in the DPPG compositions on the temporary plugging performance were investigated. Infrared spectrum analysis, differential calorimetry scanning evaluation, static particle gel swelling and degradation performance assessment experiments, and dynamic temporary plugging performance tests were all employed. The results showed that the swelling ratio of DPPGs prepared by the crosslinker containing alcohol ester was larger than that of alkenoate acid ester, but the degradation rate was slower. Among them, when the amount of crosslinker was 0.01 g, the swelling ratio of DPPG prepared using the alcohol ester crosslinker was as high as 40 times, and the complete degradation time could be regulated between 80 and 360 h. When the type of crosslinker was the same, the influence of its molecular weight also significantly affected the performance of the particle gel. The plugging strength of DPPG prepared by different crosslinkers in cores was as high as 20 MPa. The DPPG prepared from the alcohol ester crosslinker caused less damage to the core after being self-degraded and was used as an ideal crosslinker. Infrared spectroscopy and differential scanning calorimetry analysis at the molecular level verified that different types of crosslinkers had apparent effects on the chemical structure stability of the DPPG. In this paper, the influence of the type of crosslinker on the performance of the DPPG was studied, which provides a new idea and theoretical basis for developing a particle gel-based TPA system with programmed self-degradation time at low temperatures.