Comparative Analysis of CO₂ Sequestration Potential in Shale Reservoirs: Insights from the Longmaxi and Niutitang Formations

Abstract

The rapid escalation of global warming, driven by anthropogenic carbon dioxide (CO₂) emissions, underscores the necessity of carbon capture and storage (CCS) technologies as a critical strategy for mitigating atmospheric CO₂ levels. Shale reservoirs, characterized by their extensive nanopore networks and heterogeneous pore structures, hold significant promise for CO₂ sequestration. This study investigates the storage and sequestration potential of shales from two distinct formations: the Lower Silurian Longmaxi Formation (TY1 group) and the Lower Cambrian Niutitang Formation (N206 group). A comprehensive suite of experiments, including XRD analysis, mercury intrusion porosimetry (MIP), low-pressure gas adsorption (N₂ and CO₂), field-emission scanning electron microscopy (FE-SEM), and mineralogical analysis, was employed to characterize pore structure, adsorption behaviour, and mineralogical controls on CO₂ storage. Moreover, a novel fractal parameter, succolarity along with conventional mass and surface fractal dimensions were used to depict the pore systems of the two groups.

Results reveal that the TY1 samples exhibit higher total organic carbon (TOC; up to 7.58%), greater microporosity, and stronger CO₂ adsorption energies (up to 34 kJ/mol) compared to the N206 samples, which display a more mesopore-dominated system and lower adsorption energies (28–30 kJ/mol). The Longmaxi Formation demonstrates superior pore connectivity and pore size distribution (PSD) homogeneity, enhancing both CO₂ retention and transport. Its higher carbonate content also suggests potential for mineral trapping through carbonation reactions. In contrast, the Niutitang Formation is characterized by higher total porosity (up to 2.4%) and mesoporous contributions, favouring rapid injection but limiting long-term retention. Meanwhile, the FE-SEM observations revealed that many authogenic minerals such as quartz, pyrite and rutile occupy the pore space in organic matters. It is much more prevalent in the N206 samples, which may be responsible for its lower microporosity.

Key findings include a strong correlation between TOC and micropore volume, as well as between clay minerals and mesopore-macropore attributes. These correlations highlight the dual role of organic matter and mineral content in determining gas adsorption capacity and flow dynamics. The TY1 group’s balanced micropore and mesopore contributions make it ideal for long-term CO₂ sequestration, while the N206 group’s larger pore sizes enhance its suitability for rapid injection and enhanced gas recovery (EGR) applications.

This study provides critical insights into the interplay of organic matter, mineral composition, and pore structure in controlling CO₂ storage potential in shale reservoirs. The findings emphasize the Longmaxi Formation's superior suitability for CO₂ storage and EGR, with implications for optimizing CCS strategies in similar shale systems globally.