Optimising Long-Term CO2 Storage Security in Sandstone Reservoirs: Petrophysical and Geochemical Dynamics for Clean Energy Transitions

Abstract Geological carbon storage in deep saline sandstone aquifers is central to net-zero targets, yet the long-term geochemical stability of stored CO2 in far-field reservoir zones—where the plume interacts with brine rather than with dry supercritical CO2—remains a critical uncertainty for storage security. This study investigates how the quantity of CO2 available to acidify the pore fluid, a variable operators influence through injection design, governs the petrophysical and geochemical evolution of sandstone under simulated far-field conditions (60 °C, 35,000 mg/L NaCl brine, static no-flow). Cores sectioned from a single parent block were aged for 1, 15 and 30 days in cells charged at CO2:brine volumetric ratios of 30:100 and 60:100 (the 30% and 60% conditions), with effective porosity tracked gravimetrically and surface mineralogy characterised by scanning electron microscopy-energy-dispersive X-ray spectroscopy. The response bifurcates with the CO2 inventory charged. In the 60% condition, an initial decline (15.90% to 11.02% within 24 hours), attributed to fines mobilisation and handling artefacts, was followed by recovery to 12.00% through sustained ferroan-carbonate dissolution—a tentative “self-stimulation” evidenced by a monotonic rise in surface Fe rather than by porosity alone, since the brine-only control recovered comparably without CO2. In the 30% condition, a transient gain to 14.46% at day 15 reversed to 12.35% by day 30—the only reversal in the dataset—accompanied by microcrystalline pore-throat precipitates whose elemental signature is consistent with secondary carbonate: a “self-sealing” response that could reduce injectivity. Equilibrium PHREEQC calculations reproduce all three stages and confirm that partial depletion of the CO2 inventory drives a calcite-undersaturated brine into supersaturation. Critically, the dissolution that benefits reservoir porosity could compromise seal integrity if extended to caprock interfaces. Deriving from short-duration, unreplicated, sub-critical-pressure experiments without permeability measurement, aqueous geochemistry or mineralogy analysis by X-ray diffraction/Raman spectroscopy, these findings are mechanistic hypotheses requiring validation.