Role of Bicarbonate Speciation and Brine Chemistry in Chalk Formations for CO ₂ Storage Efficiency

Chalk compaction during seawater injection has been attributed to calcite dissolution and magnesite precipitation, yet the geochemical impact of CO2 for carbon storage remains uncertain. In this study, a simplified field-scale geochemical model was developed using Valhall reservoir data to simulate 20 years of CO2 injection and 980 years of postinjection evolution. Compaction mechanisms and CO2 solubility behavior were validated, including a stepwise qualitative solubility assessment under high pressure and temperature conditions. Correlation mapping using the statistical analysis confirmed a strong negative correlation (r = −0.92) between two parameters of calcite dissolution and magnesite precipitation under carbonated seawater conditions, supporting a coupled weakening mechanism. Contrary to common assumptions, higher salinity and temperature were found to increase system acidity and promote greater calcite dissolution, driven by magnesite supersaturation and faster precipitation kinetics. Aqueous HCO3– complexation contributed up to 40% of total alkalinity, highlighting its role in buffering and CO2 speciation. Alkalinity was shown to reflect both buffering capacity and evolving carbon species, with complex HCO3– increasing at elevated temperatures. Among the tested brines, 3× diluted seawater offered the best performance at 130 °C, balancing weakening with effective mineral and ionic trapping. Lower-temperature North Sea reservoirs are expected to exhibit reduced magnesite precipitation-induced weakening and enhanced solubility trapping. These findings underscore the role of bicarbonate speciation and buffering in CO2 storage design in chalk formations, while highlighting the need to better distinguish between precipitation- and dissolution-driven weakening processes.