CO ₂-水-岩作用下冀南深部咸水层砂岩组构演化与微观力学劣化机制

Fabric evolution and microscale mechanical degradation mechanisms of sandstones in deep saline aquifers of Southern Hebei under CO ₂-water-rock interactions

摘要

CO₂地质封存过程中,CO₂-水-岩作用显著影响砂岩储层的矿物组成及力学性质,进而影响储层的可注性和安全性。为探讨封存过程中CO₂-水-岩作用对储层稳定性的影响,以冀南地区晚古生代-中生代砂岩为研究对象,开展CO₂-水-岩反应模拟试验,结合pH测试、X射线衍射(XRD)、扫描电子显微镜(SEM)及纳米压痕技术,系统剖析反应前后不同类型砂岩的矿物组成、微观结构及微观力学性质的演化规律与内在机制。结果表明:CO₂-水-岩反应后,砂岩中的方解石、菱铁矿等易溶矿物完全或部分溶解,长石(尤其是斜长石)蚀变增强并生成黏土矿物、铁白云石和针铁矿等次生矿物,而石英质量分数相对下降但总体保持稳定;反应液的pH值呈现“先升高后回落”的动态变化规律,反映矿物溶解释放碱性离子与次生矿物沉淀消耗碱性离子的平衡过程,但不同样品的峰值出现时间和幅度存在差异;CO₂-水-岩作用后,各类砂岩的孔隙结构均呈现孔喉扩大、连通性增强的趋势,其中黏土矿物表面出现蜂窝状和条带状溶蚀孔,且次生矿物多以松散片状、颗粒状或絮状形式充填孔隙;所有砂岩的微观弹性模量和硬度均显著降低,降幅分别在20.5%~41.4%和10.2%~41.2%,其中下三叠统细砂岩因方解石胶结物的大量溶解,粒间支撑度降低,微观力学性质劣化最为严重并形成局部弱化区域;易溶矿物特别是胶结物(如方解石)溶解对岩石骨架的破坏,低强度次生矿物(如黏土)重填孔隙但支撑度不足,以及溶蚀作用引发的孔隙结构重塑与应力集中加剧等因素的耦合作用,是导致CO₂地质封存中砂岩微观力学性质劣化和非均质性增强的主要原因,同时也构成封存过程中储层潜在的力学稳定性风险。

Abstract

During geological CO ₂ sequestration, CO ₂-water-rock interactions can significantly alter the mineral composition and mechanical properties of sandstone reservoirs, thereby affecting injectivity and long-term stability. To investigate these effects, Late Paleozoic-Mesozoic sandstones from Southern Hebei were subjected to CO ₂-water-rock reaction experiments. The experiments were complemented by pH monitoring, X-ray diffraction (XRD), scanning electron microscopy (SEM), and nanoindentation analyses. The results show that after the reactions, soluble minerals, including calcite and siderite, were completely or partially dissolved. Feldspars, particularly plagioclase, experienced enhanced alteration accompanied by the formation of secondary minerals such as clay minerals, ankerite, and goethite. In contrast, quartz content decreased only slightly and remained relatively stable. The pH of the reaction solution exhibited a dynamic rise-fall trend, reflecting the balance between the release of alkaline ions during mineral dissolution and their consumption during secondary mineral precipitation, with variations in timing and magnitude of the pH among lithologies. Microstructural observations reveal enlarged pore throats and improved pore connectivity in all sandstone types. Honeycomb-like and elongated dissolution pores developed on clay mineral surfaces, while secondary minerals precipitated as loosely distributed flakes, granules, or flocculent fillings. Microscale elastic modulus and hardness decreased by 20.5%-41.4% and 10.2%-41.2%, respectively. The greatest mechanical deterioration occurred in Lower Triassic fine-grained sandstones, where extensive dissolution of calcite cement weakened intergranular support and generated localized mechanically weak zones. These findings suggest that the degradation of microscale mechanical properties during CO ₂ geological sequestration is primarily controlled by the coupled effects of dissolution of soluble minerals, particularly carbonated cements, resulting in framework weakening; pore refilling by mechanically weak secondary minerals, such as clays, which provide limited structural support; and pore structure remodeling and stress concentration induced by dissolution. Together, these processes enhance the mechanical heterogeneity of sandstone reservoirs and may compromise their long-term mechanical stability during CO ₂ storage.

关键词

CO ₂地质封存 / 冀南地区 / 水-岩相互作用 / 矿物组成 / 孔隙性 / 微观力学性质

Key words

CO ₂ geological sequestration / Southern Hebei region / water-rock interaction / mineral composition / porosity / microscopic mechanical properties

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基金资助

国家自然科学基金项目(42172198)

国家自然科学基金项目(42202210)