Stress Corrosion Cracking in Carbonates Initiated by Carbonic Acid, A Reactive Molecular Dynamic Simulation

ABSTRACT: As the greenhouse effect gradually intensifies, addressing CO2 as the primary greenhouse gas has become a pressing concern. Currently, the predominant carbon sequestration technique involves storing CO2 in reservoirs or saline aquifers. However, CO2 readily dissolves in water, forming carbonic acid, which can compromise geological formations and lead to stress corrosion cracking. Stress corrosion cracking (SCC), a chemical activation mechanism, involves fluid adsorption on crack surfaces and fluid-assisted crack propagation. While there is extensive research on the stress corrosion cracking of CO2 in quartz crystals, investigations on stress corrosion cracking in gas-water-carbonate rock systems remain limited. Moreover, the carbonic acid formed when CO2 dissolves in water can engage in complex chemical reactions with carbonate rocks, exacerbating the corrosion and cracking of carbonate rocks. This study focuses on carbonate reservoirs, representing them using calcite crystals. Molecular dynamics simulation is employed to investigate stress corrosion cracking in pre-cracked calcite crystals under fluid interaction. By comparing the cracking behavior of calcite crystals in different environments, the intrinsic mechanisms of crack propagation are analyzed. This study indicates that carbonate fluids have a strong impact on the subcritical expansion of cracks. During the stress corrosion cracking process, Ca-O bonds are the first to break. H atoms from the external environment readily form H-O bonds and C-H bonds on the surface of calcite, with H-O bonds binding to Ca atoms, accelerating the fracture of Ca-O bonds. In a carbonate environment, H atoms infiltrate the interior of calcite more rapidly, binding with C and O atoms, creating crystal vacancies, and intensifying the extent of stress corrosion cracking. This study aims to explore the damage to carbonate rock reservoirs during the CO2 storage process and analyze the gas-water-rock interactions in enhanced oil recovery with CO2 injection or during CO2 fracturing processes. These findings contribute to describing the details of rock fracturing and provide insights for the development of effective oil and gas extraction and carbon sequestration technologies.