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Description
Understanding the mineralogical and structural responses of reservoir rocks to acidic fluids is essential for predicting the long-term stability of geological CO$_2$ storage sites. In this study, the dissolution mechanisms within Rio Bonito Formation sandstones were systematically investigated under acidic conditions using a multi-technique, time-resolved synchrotron approach. X-ray microtomography (4D $\mu$-CT), time-resolved X-ray diffraction (TR-XRD), and time-resolved X-ray fluorescence (TR-XRF) were employed to characterize porosity evolution and mineral reactivity across a range of pH conditions relevant to CO$_2$ sequestration scenarios. Experiments were conducted at the Brazilian Synchrotron Light Laboratory (LNLS) utilizing custom-designed sample environments to enable real-time fluid injection during imaging and spectroscopy. Acid solutions of varying pH were injected through the samples while continuously acquiring datasets. 4D $\mu$-CT revealed a front-like dissolution pattern, primarily affecting cement-rich regions. These regions dissolved preferentially before the acid infiltrated the intrinsic pore structure, leading to early-stage heterogeneity in porosity evolution. Under higher pH conditions, designed to simulate CO$_2$-rich brines at reservoir conditions, complete dissolution of cement phases was observed, destabilizing the rock matrix. This behavior is attributed to the acid volume exceeding the buffering capacity of the cement minerals, preventing early saturation and promoting continued dissolution. TR-XRD and TR-XRF analyses confirmed the progressive dissolution of key mineral phases such as calcite and microcline, with concurrent release of Ca$^{2+}$, Al$^{3+}$, and K$^+$ ions. The quartz framework remained largely inert, maintaining the mechanical stability of the porous matrix as reactive phases dissolved. The dissolution rate demonstrated an approximately exponential decrease with increasing pH, consistent with theoretical predictions and previous flow-through experiments in carbonate-bearing rocks. The findings reinforce that mineral reactivity is strongly governed by pH, spatial distribution of reactive phases, and fluid accessibility. Comparative analysis with prior studies supports that such exponential behavior is expected during acid-rock interactions in real-world scenarios. While direct HCl injection used here differs from the gradual acidification expected in CO$_2$-brine systems, it effectively simulates a wide range of pH conditions, providing critical insights into reactive transport phenomena. Overall, this work highlights the effectiveness of time-resolved synchrotron techniques in capturing the dynamic processes of mineral dissolution and offers a framework for future studies under reservoir-relevant pressure and temperature conditions. The results contribute to a better understanding of CO₂ mineralization pathways and underscore the importance of mineralogical buffering in the mechanical and chemical stability of geological storage sites.
| Country | Brazil |
|---|---|
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