Development of a High-Temperature-Resistant Encapsulated Gel Breaker for Polymer Gels and Evaluation of Its Performance

To address the poor temperature resistance of conventional gel breakers, the uncontrollable gel-breaking time, and the risk of secondary reservoir damage during temporary plugging of fractured formations with polymer gels, a high-temperature-resistant double-shell encapsulated gel breaker, UF-EC/SA, was prepared using oil-phase phase separation combined with in situ polymerization. In this material, urea-formaldehyde resin (UF) served as the outer shell, ethyl cellulose (EC) as the inner shell, and sulfamic acid (SA) as the core. Unlike conventional single-shell persulfate or directly added acid breakers, this double shell design integrates a thermally resistant UF barrier, a diffusion-controlling EC layer, and an acid core to delay premature gel degradation while enabling subsequent cleanup. The physical structure and sustained-release behavior of the capsules were characterized by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), powder X-ray diffraction (XRD), and conductivity measurements. The compatibility between the encapsulated breaker and the polymer gel, as well as the effects of salinity and breaker dosage on the rheological properties of the gel, were investigated. The regulatory effects of temperature and capsule dosage on gel-breaking performance were studied in detail. In addition, high-temperature/high-pressure displacement experiments were conducted to evaluate the temporary plugging performance of the gel containing the encapsulated breaker in fractured cores and packed-sand tubes. The results showed that the prepared capsules had good sphericity and a dense shell structure, with an encapsulation efficiency of 76.7%. The capsules exhibited temperature resistance up to 150 °C and favorable sustained-release characteristics. The UF-EC/SA breaker showed good compatibility with the polymer gel and did not inhibit gelation within the temperature range of 80–150 °C or at dosages of 0–16 wt.%. The gel maintained good mechanical strength even in highly mineralized brines. At 150 °C and a capsule dosage of 16 wt.%, the gel was completely broken within 2.5 d; the residue concentration was only 351 mg/L, and the residue size was mainly distributed within 100–500 um. The high-temperature/high-pressure displacement tests demonstrated that the gel containing 16 wt.% capsules achieved a maximum breakthrough pressure of 5.16 MPa in a 3 mm wedge-shaped fracture core, and the pressure remained stable for 5 d. After gel breaking, the residue could be readily flowed back, indicating excellent synergy between temporary plugging and subsequent gel breaking. Therefore, the UF-EC/SA encapsulated breaker provides a new technical option for efficient gel breaking in high-temperature fractured formations.