Impact of hydrogen sulfide on CO₂ mineralization and carbon storage efficiency

Although CO₂ mineralization in the presence of H₂S has been observed in pilot projects, the mechanisms controlling chelating agent degradation and cation availability under sour conditions remain poorly understood. This study presents a novel integration of lab-scale optimization, speciation chemistry, and DFT calculations to uncover key operational thresholds and molecular interactions crucial for secure mineralization in sour environments. It investigates subsurface field-relevant conditions, including temperatures between 50 and 80 °C, pressures exceeding 1500 psi, and H₂S concentrations of up to 200 ppm, to simulate realistic scenarios for CO₂ mineralization in sour reservoirs. The results showed that, in a sour environment, mineralization still takes place (ensuring storage security), although EDTA's structural integrity is compromised through interactions with H₂S speciation products. This occurs particularly at higher pH where S²⁻ dominates. DFT calculations reveal that these interactions result in molecular cleavage, diminishing the chelating agent's ability to stabilize and deliver Ca²⁺ for carbonate formation. However, pre-chelation of EDTA with Ca²⁺ before exposure to H₂S significantly mitigates degradation, preserving function and enabling mineralization. These insights support field strategies involving injecting pre-chelated solutions and maintaining brine pH below 8.0 to ensure effective and sustained carbonate precipitation in sour reservoirs. Additionally, the findings provide new insights by (1) identifying the minimum cation and chelating agent concentrations necessary for mineralization under sour conditions, (2) elucidating the pH- and speciation-dependent degradation pathways of EDTA using DFT simulations, and (3) proposing a field-applicable strategy to preserve chelating agent integrity. These mechanistic and operational insights offer practical guidance for CO₂ storage in H₂S-rich environments.