Progressive Alterations of Shale Properties under Staged Long-Term CO2 Exposure: Insights from a Multicycle Experimental Study

The long-term integrity of shale formations upon exposure to CO2 is paramount for carbon geologic sequestration and enhanced hydrocarbon recovery. However, predicting the property alterations of shale remains challenging due to the complex and inconsistent responses reported under varying experimental conditions. This study investigated the progressive alterations in mineral composition, pore structure, and fracture network of three lacustrine shale samples subjected to a multicycle (3×45 days) CO2 exposure experiment at 80°C and 30 MPa. Results revealed that the evolution of shale properties was nonlinear and time-dependent, governed by the competitive and coupled effects of mineral dissolution and secondary precipitation. Concretely, Sample 1 (carbonate rich) exhibited cyclical fluctuations in carbonate and clay content, yet its multiscale pore structure remained surprisingly stable. Sample 2 (carbonate poor, quartz and clay rich) showed dissolution of feldspar and clays, leading to a reduction in micropores and mesopores but an enlargement of macropores, primarily in the first cycle. Sample 3 (carbonate poor, clay rich) demonstrated continuous clay dissolution throughout the cycles. Micro–computed tomography (CT) scanning analysis further revealed that mineral precipitation could preferentially fill smaller microfractures, altering their volume distribution. The findings highlight lithology-dependent responses to long-term CO2 exposure: carbonate-rich shales maintain structural stability, while carbonate-poor shales undergo significant pore structure alteration. The study provides critical insights into the long-term evolution of shale properties, which are essential for assessing the risks and viability of CO2 storage projects in shale formations.