A high-stability polymer gel system for selective water shutoff: degradation inhibition and long-term stability in high-temperature, high-salinity bottom-water gas reservoirs

High-temperature and high-salinity (HTHS) bottom-water gas reservoirs are prone to rapid gas–water contact rise and severe water coning, resulting in early water breakthrough and low gas recovery. However, the long-term stability and selective plugging performance of conventional polymer gel systems are severely compromised under such harsh conditions. In this study, a high-stability polymer gel system was formulated for selective water shutoff in HTHS bottom-water gas reservoirs. The system consists of PM134 (PM-5) polymer crosslinked with polyethyleneimine (CA-6), with thiourea (AD-1) and nano-SiO₂ (AD-5) incorporated as stabilizing and reinforcing additives. Under simulated reservoir conditions (110°C and salinity of 16 × 10⁴ mg/L), the gel exhibited a controllable gelation time of 2.5–3.5 days and achieved a high gel strength (Grade H). Long-term aging tests demonstrated excellent stability, with the gel maintaining at least Grade G strength and a syneresis ratio below 10% after 60 days. Rheological measurements indicated predominantly elastic behavior (G′ > G″) throughout the aging period, confirming the integrity of the gel network under HTHS conditions. The incorporation of thiourea (AD-1) effectively inhibited thermal–oxidative degradation, while nano-SiO₂ (AD-5) reinforced the three-dimensional gel structure. Core flooding experiments revealed strong selective plugging performance, with water-phase plugging efficiency exceeding 95% and gas-phase plugging efficiency remaining below 35%. Gel treatment significantly delayed water breakthrough, increased the breakthrough pressure from 6 MPa to 13 MPa, and enhanced ultimate gas recovery by 12.84%. These results demonstrate that the proposed gel system provides a reliable and effective solution for selective water shutoff in HTHS bottom-water gas reservoirs, offering strong potential for field application under extreme reservoir conditions.