Abstract: 【Objective】Water-sealed underground oil and gas storage caverns are the core infrastructure for national strategic energy reserves, with the most stringent industry control requirements for cavern water inflow. Under deep burial conditions, the surrounding rock of the caverns is mostly in a high in-situ stress state, and the fractured rock mass generally presents the technical challenge of "water seepage occurs while grout can hardly be injected". Existing conventional grouting materials and processes are difficult to adapt to the seepage control requirements of high-stress compacted fractures. The industry has insufficient understanding of the anti-seepage grouting mechanism of this type of rock mass, and lacks targeted technical approaches and engineering implementation strategies, which has become a core bottleneck restricting the safe and efficient construction of water-sealed storage caverns.【Methods】Therefore, to address the above engineering challenges, this paper is based on a deep-buried water-sealed underground oil storage cavern project. This paper first systematically analyzed the seepage characteristics and anti-seepage grouting mechanism of high-stress fractured rock mass, and clarified the core causes of its poor groutability. On this basis, three typical high-stress water-seepage tunnel sections were selected, and field comparative pre-grouting tests were carried out with ordinary Portland cement and new ultra-fine cement material respectively. Taking unit grout consumption, water inflow of inspection holes, and post-excavation seepage characteristics as the core evaluation indicators, the anti-seepage effect and engineering applicability of different grouting schemes were quantitatively compared.【Results】The test results showed that ordinary Portland cement had extremely poor groutability in high-stress fractured rock mass, which was characterized by low unit grout consumption, an unobvious decreasing law of grout consumption along with the grouting sequence, and a limited grout diffusion range. After multiple sequences of grouting, the maximum water inflow of the inspection holes was still far beyond the qualified standard specified in the design. The application of the new ultra-fine cement material significantly improved the groutability of the rock mass: the unit grout consumption was greatly increased compared with that of ordinary Portland cement, the decreasing law of grout consumption along with the grouting sequence was distinct, and the number of water seepage positions and total water inflow of the grouted tunnel section after excavation were significantly reduced compared with those of the section grouted with ordinary Portland cement. However, optimizing the grouting material alone could not achieve the ideal anti-seepage effect. The insufficient grouting pressure, which failed to overcome the high in-situ stress and open the compacted fractures, was the core bottleneck restricting the grouting effect from meeting the design standard.【Conclusion】This paper clarifies the core technical approach that compaction-expansion grouting should be adopted for high-stress fractured rock mass, and proposes the engineering implementation strategy of "solving the water control problem in one go through pre-grouting". The research results can provide key technical support for the anti-seepage grouting design and construction of similar high-stress underground engineering projects.