HPAM/PEI gel is a promising material for conformance control in hydrocarbon reservoirs. However, its use in low-permeability reservoirs is limited by the high polymer concentrations present. In this study, the gelation performance of an HPAM/PEI system with HPAM < 2.0 wt.% was systematically investigated. The gelation time for HPAM concentrations ranging from 0.4 to 2.0 wt.% varied from less than 1 h to 23 days, with the highest gel strength identified as grade H. The hydrodynamic radius manifested the primary effect of HPAM on the gelation performance. Branched PEI provided superior gelation performance over linear PEI, and the gelation performance was only affected when the molecular weight of the PEI varied significantly. The optimal number ratio of the PEI-provided imine groups and the HPAM-provided carboxylic acid functional groups was approximately 1.6:1~5:1. Regarding the reservoir conditions, the temperature had a crucial effect on the hydrodynamic radius of HPAM. Salts delayed the gelation process, and the order of ionic influence was Ca²⁺ > Na⁺ > K⁺. The pH controlled the crosslinking reaction, primarily due to the protonation degree of PEI and the hydrolysis degree of HPAM, and the most suitable pH was approximately 10.5. Plugging experiments based on a through-type fracture showed that multi-slug plugging could significantly improve the plugging performance of the system, being favorable for its application in fractured low-permeability reservoirs.

The potential of magnetic nanofluids as fracturing fluids in tight oil reservoir development is investigated. Hydrophobic magnetic nanoparticles (MNPs) are synthesized using a chemical co-precipitation method and characterized by Fourier transform infrared spectroscopy, dynamic light scattering, and scanning electron microscopy. A magnetic nanofluid is prepared as a pad fracturing fluid in accordance with the aforementioned methodology. Significantly, the effects of homogeneous and heterogeneous magnetic fields were evaluated on oil recovery and formation damage through core flooding experiments. The synthesized MNPs are hydrophobic, 20 nm in diameter, and stabilized by one layer of surfactant molecules in an aqueous solution. The nanofluid exhibits a relatively higher oil recovery than the surfactant solution because of its superior function of wettability alteration. The chaining behavior of MNPs under homogeneous magnetic fields leads to severe particle retention in the pore throats, which results in a significant decrease in core permeability. In contrast, the magnetic force gradient is unique to heterogeneous magnetic fields, resulting in a 40% increase in oil recovery at a magnetic intensity of 900 G near the core, while the permeability reduction is negligible. In conclusion, the retention of nanoparticles in pore throats represents a significant obstacle to the application of nanotechnology in tight reservoirs. The utilization of hydrophobic MNPs in conjunction with heterogeneous magnetic fields presents a potential solution to this conundrum.

Summary: In an ideal hydraulic fracturing project, the viscosity of hydraulic fracturing fluid (HFF) should drop to facilitate an efficient and quick fracture cleanup once proppants have been placed. Ammonium persulfate (APS) is widely used as an oxidizer breaker to degrade the insoluble residue of the HFF. However, a complex network of artificial fractures with sizes ranging from millimeters to micrometers restricts the transportation application and reaction time of standard breakers and current millimeter-sized capsule breakers. In this study, we introduce an APS nanocapsule (APS-NC) breaker, fabricated via a simple miniemulsion technique, that is capable of addressing the degradation of insoluble residue of the HFF in deep fractures. The nanocapsules are produced through precise control of the nanoprecipitation of poly(methyl methacrylate) (PMMA) onto aqueous APS nanodroplets. Stable aqueous nanodroplets are generated by means of an inverse miniemulsion using Pluronic P-123 as a specific surfactant to form a large and stable water-oil internal surface referring to the long chain and amphipathic property of Pluronic P-123 molecule. Pluronic P-123 is included in the process not only to stabilize the miniemulsion and increase the precipitation efficiency of PMMA but also to function as a thermo-responsive switch for the delayed burst release of APS. The size of the resulting nanocapsules can be controlled within the 207 and 821 nm range, yielding APS up to 92%. The release rate of APS can be controlled by the initial amount of PMMA (150–300 mg), environmental temperature (40–80°C), and environmental pH (3.4–7). When the temperature was 80℃, the initial amount of Pluronic P-123 dominated the release dynamic of APS. Compared with the APS breaker in a dynamic degradation experiment, the APS-NC had a more controllable degradation profile that could cause hydrolyzed polyacrylamide (HPAM) to burst degrade after 6 hours of heating at 80°C.

Summary: The objective of this paper is to investigate the effect of the addition of zirconium crosslinker on retarding the degradation of the partially hydrolyzed polyacrylamide/polyethyleneimine (HPAM/PEI) gel system. Dehydration and degradation under salinity conditions pose serious challenges to the effectiveness of the HPAM/PEI system used for water control treatment in heterogeneous reservoirs. In this study, rheological tests, Fourier transform infrared (FTIR) spectroscopy, and 13C nuclear magnetic resonance (NMR) spectroscopy are used to elucidate the characteristics and mechanisms of dehydration and degradation of HPAM/PEI gel in medium-salinity conditions. The effects of three types of zirconium crosslinkers on the gelation performance of the HPAM/PEI system are evaluated. Furthermore, the mechanism by which an organic zirconium complex (ZrOr complex) with triethanolamine (TEA) and lactic acid (LA) as ligands improves the stability of the HPAM/PEI system under medium-salinity conditions is revealed. The results indicate that ZrOr complex notably retards the degradation of the HPAM/PEI system under medium-salinity conditions (50 g/L) for more than 60 days while improving its gel strength. TEA and LA as ligands have a positive effect on crosslinking of HPAM with the organic Zr4+ complex by modulating the availability of Zr4+ and the homogeneity of the crosslinking reaction. Specifically, our data suggest that TEA stabilized Zr4+ and retarded its release through its chelating effect, while LA improved the efficiency and effectiveness of the crosslinking reaction by increasing the solubility of Zr4+ and providing additional carboxyl groups. The synergistic effect of the two ligands significantly improved the properties of the final crosslinked product. Evidence from FTIR, 13C NMR, and microstructural tests supports the conclusion that the retardation of degradation and enhancement of gelation performance in the HPAM/PEI system by ZrOr complex are associated with its crosslinking with carboxyl groups produced by HPAM hydrolysis, leading to the formation of a more uniform and compact network structure. A field trial conducted in the Beiyao Block demonstrated the potential of the ZrOr complex to extend the treatment lifetime of the HPAM/PEI gel system in medium-salinity reservoirs.