基于ScCO₂-水耦合作用的页岩微观力学特性分析——以四川盆地龙马溪组露头页岩为例

Analysis of micromechanical properties of shale under ScCO₂-water coupling: a case study of outcrop shale from Longmaxi Formation in Sichuan Basin

摘要

超临界二氧化碳(ScCO₂)技术具有高效压裂增产与有效保护储层等特点,成为页岩气绿色高效开发的优势方案。为研究ScCO₂-水耦合作用对页岩微观力学性质的影响,对四川盆地龙马溪组露头页岩进行干燥、水湿及ScCO₂-水耦合处理,应用纳米压痕连续刚度实验测试弹性模量与硬度;采用能量分析法与扫描电镜观测实验确定页岩断裂韧性,分析微观力学参数间的关系;应用X线衍射与扫描电镜实验,揭示矿物组分对力学特性的影响规律。结果表明:受水湿与ScCO₂-水耦合作用影响,页岩发生明显的弹性损伤,弹性模量、硬度与断裂韧性下降显著,降幅分别为14.01%、20.99%和16.30%;弹性模量、硬度与断裂韧性的理论计算结果与实验结果拟合呈一致的线性正相关关系。龙马溪组页岩具有高脆性矿物含量比,由ScCO₂-水耦合作用引起的钙质与硅质矿物溶解沉淀,导致页岩基质损伤加剧、弹性指数下降、脆性指数增大。该结果为页岩微观力学性质表征及CO₂提高采收率措施设计提供参考。

Abstract

Supercritical carbon dioxide (ScCO₂) technology has emerged as an advantageous solution for the green and efficient development of shale gas due to its significant advantages, such as effective fracturing for production enhancement and effective protection of reservoirs. To investigate the influence of ScCO₂-water coupling on the micromechanical properties of shale, outcrop samples from the Longmaxi Formation in the Sichuan Basin were subjected to drying, water-wet, and ScCO₂-water coupling conditions. The elastic modulus and hardness were obtained through nanoindentation continuous stiffness measurement tests, and the fracture toughness was determined based on energy analysis and scanning electron microscopy observations. The correlations among various micromechanical parameters were analyzed. Combining X-ray diffraction and scanning electron microscopy, the influence patterns of mineral composition on mechanical properties were revealed. The results indicate that both water-wet and ScCO₂-water coupling cause significant elastic deterioration to shale. The reductions in elastic modulus, hardness, and fracture toughness are more pronounced under ScCO₂-water coupling, with decreases of 14.01%, 20.99%, and 16.30%, respectively. Both theoretical derivation and experimental fitting demonstrate a consistent linear positive correlation among elastic modulus, hardness, and fracture toughness. The Longmaxi Formation shale exhibits a high proportion of brittle minerals. ScCO₂-water coupling induces dissolution and precipitation of calcareous and siliceous minerals, leading to intensified matrix damage, decreased elasticity index, and increased brittleness index. These findings provide a reference for characterizing the micromechanical properties of shale and designing CO₂ enhanced recovery measures.

关键词

页岩 / 纳米压痕 / 超临界二氧化碳 / ScCO₂-水耦合作用 / 微观力学参数 / 断裂韧性 / 龙马溪组 / 四川盆地

Key words

shale / nanoindentation / supercritical carbon dioxide / ScCO₂-water coupling effect / micromechanical parameters / fracture toughness / Longmaxi Formation / Sichuan Basin

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基金资助

国家自然科学基金联合基金项目(U19B6003-03-04)

国家自然科学基金面上项目(51774298)