Research on Influencing Factors and an Evaluation Method for the Breakthrough Pressure of CO ₂ Geological Storage Caprock: A Case Study of the Chang 4 + 5 Member in the Ansai Area

Breakthrough pressure (BP) is the core index used to evaluate caprock sealing, which plays a key role in the long-term safety of CO₂ geological storage. This study focuses on the caprock of the Yanchang Formation in the Ansai area of the Ordos Basin. By integrating X-ray diffraction, casting thin sections, scanning electron microscopy, high-pressure mercury injection (HPMI), nuclear magnetic resonance (NMR), CO₂-brine-rock interaction, and BP experiments, in combination with fractal theory, the pore-throat structure characteristics of caprock, the influencing factors of BP, and its evaluation methods are systematically analyzed. The results reveal that the caprock in the study area is predominantly argillaceous siltstone, with micronano pores developed and high clay mineral content, and a dense network formed by nonuniform distribution of micropores effectively inhibits upward CO₂ migration, leading to elevated BP. The pore-throat fractal characteristics are quantitatively characterized by HPMI and NMR, and it is found that the heterogeneity is the key factor affecting BP. BP increases with a higher proportion of micropores and decreases with a larger proportion of macropores. R35 more effectively characterizes the control of the pore-throat structure over sealing capacity than R50. Quartz and feldspar indirectly influence BP by affecting porosity and permeability; illite bridges the pore-throat, segmenting macropores into nanopores, which reduces porosity and permeability and increases BP; chlorite promotes macropore development, forming CO₂ preferential pathways, thereby reducing BP; an illite/smectite mixed layer shows no significant correlation with BP. The CO₂-brine-rock interaction triggers the transformation of caprock minerals; however, the absolute content of minerals changes slightly. The mineral filling pore effect generated by precipitation effectively compensates for the localized structural weakening caused by dissolution, exerting a positive influence on the BP of the caprock. As the porosity and permeability increase, BP decreases. There are linear correlations between BP and the reciprocal of porosity (<i>A</i>) and the -0.155 power of permeability (<i>B</i>). Based on <i>A</i> and <i>B</i>, the BP calculation formula is fitted using the ridge regression approach of MATLAB software, and calculated values are in good agreement with measured values. This study provides a research idea and a BP mathematical calculation model for evaluating and predicting caprock sealing, offering certain reference significance for CO₂ geological storage site selection in the Ordos Basin.