This study employed high-resolution CT scanning and Avizo software algorithms to analyze the microscopic pore structure of the clastic rock reservoir in the Yingcheng Formation, Block D, southeastern Songliao Basin, China, and its impact on fluid flow, constructing a high-precision three-dimensional digital rock model. The optimal representative elementary volume (REV) was identified as 400 × 400 × 400 voxels. Using this as a basis, the ″maximum ball″ algorithm extracted the pore network model, allowing for quantitative characterization of its topological features and PNM-based single-phase flow simulations. Furthermore, two-phase flow simulations of CO₂ displacing CH₄ were performed via the AVIZO-FLUENT interactive coupling technology. Results show that the reservoir exhibits significant heterogeneity, with pore radii mainly between 2 and 7 μm and throat radii from 0.5 to 3 μm, reflecting a typical ″large pore, small throat″ characteristic. Throat lengths are primarily between 10 and 25 μm, while the coordination number is mainly within 1–4. Permeability correlates positively with pore-throat radius and throat length but negatively with tortuosity and fractal dimension. Single-phase flow simulations indicate that pore structure complexity significantly affects seepage behavior. The numerically simulated permeability deviates from experimental measurements by no more than 12.5%, confirming the reliability of the simulation approach. Two-phase flow simulations show that the CO₂ displacement is significantly affected by pore structure complexity and heterogeneity, progressing through three stages: uniform front advancement, fingering development, and residual gas retention. Residual CH₄ mainly accumulates at pore edges and corners, with final CO₂ and residual CH₄ saturations stabilizing at 94.74 and 5.26%, respectively. Integrating digital rock technology with multiscale flow simulations, this study refines the characterization of complex reservoir pore structures and elucidates their control over fluid migration. These findings provide a scientific basis for optimizing CO₂ geological storage and enhancing gas reservoir recovery while also supporting development strategies for low-permeability reservoirs.

Polymers derived from renewable polysaccharides offer promising avenues for the development of high-temperature, environmentally friendly drilling fluids. However, their industrial application remains limited by inadequate thermal stability and poor colloidal compatibility in complex mud systems. In this study, we report the rational design and synthesis of epichlorohydrin-crosslinked carboxymethyl xylan (ECX), developed through a synergistic strategy combining covalent crosslinking with hydrophilic functionalization. When incorporated into water-based drilling fluid base slurries, ECX facilitates the formation of a robust gel suspension. Comprehensive structural analyses (FT-IR, XRD, TGA/DSC) reveal that dual carboxymethylation and ether crosslinking impart a 10 °C increase in glass transition temperature and a 15% boost in crystallinity, forming a rigid–flexible three-dimensional network. ECX-modified drilling fluids demonstrate excellent colloidal stability, as evidenced by an enhancement in zeta potential from −25 mV to −52 mV, which significantly improves dispersion and interparticle electrostatic repulsion. In practical formulation (1.0 wt%), ECX achieves a 620% rise in yield point and a 71.6% reduction in fluid loss at room temperature, maintaining 70% of rheological performance and 57.5% of filtration control following dynamic aging at 150 °C. Tribological tests show friction reduction up to 68.2%, efficiently retained after thermal treatment. SEM analysis further confirms the formation of dense and uniform polymer–clay composite filter cakes, elucidating the mechanism behind its high-temperature resilience and effective sealing performance. Furthermore, ECX demonstrates high biodegradability (BOD₅/COD = 21.3%) and low aquatic toxicity (EC₅₀ = 14 mg/L), aligning with sustainable development goals. This work elucidates the correlation between molecular engineering, gel microstructure, and macroscopic function, underscoring the great potential of eco-friendly polysaccharide-based crosslinked polymers for industrial gel-based fluid design in harsh environments.