Abstract: To address the challenge of rapid bottom water breakthrough through fractures during water injection in fracture-cavity reservoirs, which results in shielded “attic oil” being difficult to exploit, this study proposes a novel strategy termed “synergistic gel-inorganic particle damming for water control and oil enhancement.” The method leverages the synergistic effect of gel blocking high-permeability channels and inorganic particles vertically stacking to construct a stable dam with specific height and slope within near-wellbore cavities. This elevates the overflow point, diverts bottom water during secondary flooding, expands the sweep volume, and mobilizes remaining oil at the reservoir top. Based on field well cases and geological models, a large-scale visual physical model of a fracture-cavity reservoir was developed. Utilizing similarity principles, equivalent plugging agents and injection parameters were designed to simulate the in-reservoir damming process. The study investigates the migration and distribution patterns of plugging agents under different injection modes and evaluates the impact of agent combinations, slug numbers, total agent volume, agent ratio, injection rate, and cavity filling degree on dam morphology and water-oil control efficacy. A predictive model based on the Back Propagation (BP) neural network was established to forecast dam height and enhanced oil recovery (EOR). Experimental results demonstrate: ①Synergistic damming using gel and inorganic particles effectively mobilizes attic oil, increasing recovery by 14.4% with significant water control and oil enhancement; ②Plugging agent combinations directly determine dam morphology and height, while injection parameters critically influence agent migration patterns and overall efficacy; ③The BP neural network model, after rigorous training, accurately predicts dam height and EOR under various injection modes, achieving mean squared errors (MSE) of 22.24 and 2.92, respectively. This research elucidates the mechanism of synergistic damming, identifies optimal operational parameters, and provides a novel and practical approach for improving recovery in mature waterflooded fracture-cavity reservoirs.

Key words: fracture-cavity reservoir, attic oil, damming for water control and oil increment, gel, inorganic particles, EOR