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<p>The impact of long-term low-salinity waterflooding on reservoirs of differing permeability was investigated through reservoir-condition corefloods and multi-scale characterization. Representative cores of low, medium, and high initial permeability from the Chang 4 + 5 reservoir in the Nanyang Oilfield, Ordos Basin were subjected to prolonged injection under simulated temperature and stress, and the evolution of permeability and pore structure was quantified. Scanning electron microscopy (SEM; imaging of mineral textures and pore surfaces), cast thin-section petrography (impregnated thin sections for mineralogical and pore-type observation), high-pressure mercury intrusion (HPMI; porosimetry to derive pore–throat size distributions), nuclear magnetic resonance (NMR; T2-relaxation-based estimation of pore-size classes), and confocal microscopy (optical 3D imaging of pore networks) were integrated to resolve mechanisms. A significant reduction in permeability was observed for all core types, with decreases of 26–54% as measured herein. The temporal behavior was characterized by an initial rapid decline followed by stabilization. No simple monotonic dependence on initial permeability was detected, indicating that impairment is co-controlled by petrophysical properties, mineralogical composition, and pore-network architecture; systems dominated by fine pore throats were particularly susceptible. Microstructural evidence showed decreased pore–throat radii, a shift of pore-size distributions toward smaller sizes, blockage of preferential flow channels, and diminished connectivity. The principal mechanisms comprised: (i) physical plugging, including hydration-induced swelling of clay minerals, generation, transport, and retention of fines (fines are mobilized small detrital or clay particles), which was most pronounced in low-permeability rocks; and (ii) chemical reactions, including ion exchange that promotes localized mineral dissolution and re-precipitation and the formation of carbonate scale. The water-chemistry contrast was substantial in this study, with injected water total dissolved solids (TDS) of approximately 1,558&#xa0;mg/L versus formation water TDS of approximately 128,288&#xa0;mg/L, i.e., an ~ 82-fold difference, which is conducive to clay swelling, fines dispersion, and scaling risks. These findings refine the understanding of reservoir impairment under low-salinity waterflooding and provide an experimental basis and theoretical guidance for optimization of injection water quality, operating parameters, and plugging-mitigation strategies.</p>

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Evolution of pore structure and permeability damage mechanisms in reservoirs with different permeabilities under long-term low-salinity water flooding

  • 期刊论文

  • 发表日期：

    2025年10月25日

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  • Jin Pang,
  • Haotian Chen,
  • Yang Yang,
  • Jiaao Gao,
  • Chunxi Zhou,
  • Shihao Chen,
  • Guixiao Li,
  • Tongtong Wu

摘要

The impact of long-term low-salinity waterflooding on reservoirs of differing permeability was investigated through reservoir-condition corefloods and multi-scale characterization. Representative cores of low, medium, and high initial permeability from the Chang 4 + 5 reservoir in the Nanyang Oilfield, Ordos Basin were subjected to prolonged injection under simulated temperature and stress, and the evolution of permeability and pore structure was quantified. Scanning electron microscopy (SEM; imaging of mineral textures and pore surfaces), cast thin-section petrography (impregnated thin sections for mineralogical and pore-type observation), high-pressure mercury intrusion (HPMI; porosimetry to derive pore–throat size distributions), nuclear magnetic resonance (NMR; T2-relaxation-based estimation of pore-size classes), and confocal microscopy (optical 3D imaging of pore networks) were integrated to resolve mechanisms. A significant reduction in permeability was observed for all core types, with decreases of 26–54% as measured herein. The temporal behavior was characterized by an initial rapid decline followed by stabilization. No simple monotonic dependence on initial permeability was detected, indicating that impairment is co-controlled by petrophysical properties, mineralogical composition, and pore-network architecture; systems dominated by fine pore throats were particularly susceptible. Microstructural evidence showed decreased pore–throat radii, a shift of pore-size distributions toward smaller sizes, blockage of preferential flow channels, and diminished connectivity. The principal mechanisms comprised: (i) physical plugging, including hydration-induced swelling of clay minerals, generation, transport, and retention of fines (fines are mobilized small detrital or clay particles), which was most pronounced in low-permeability rocks; and (ii) chemical reactions, including ion exchange that promotes localized mineral dissolution and re-precipitation and the formation of carbonate scale. The water-chemistry contrast was substantial in this study, with injected water total dissolved solids (TDS) of approximately 1,558 mg/L versus formation water TDS of approximately 128,288 mg/L, i.e., an ~ 82-fold difference, which is conducive to clay swelling, fines dispersion, and scaling risks. These findings refine the understanding of reservoir impairment under low-salinity waterflooding and provide an experimental basis and theoretical guidance for optimization of injection water quality, operating parameters, and plugging-mitigation strategies.