Study On Fracture Failure Risk In The Process Of CO₂ Geological Storage

In the process of carbon dioxide storage,cap rock and cement sheath integrity is the key to ensure its safe storage for a long time.However,during a long storage time cap rock and cement sheath have a potential failure risk.Although cap rock is fine and close,there are some weak links,such as natural micro cracks or small pore,when carbon dioxide is injected into the cap rock,increasing the pore pressure,crack extension may occur,form a long cracks,especially the long vertical cracks,provide a smooth channel for the carbon dioxide emitted to the ground.Micro crack inside cement sheath or at the interface is one of the main escaping ways,cement sheath was in carbon dioxide environment for a long time,it would cause seriously corrosion degradation of the cement,which would have a certain influence on permeability or transport capacity of these micro cracks.Therefore,before carbon dioxide geological storage,it is necessary to analyse the potential risks in the process of carbon dioxide storage.This paper makes a thorough analysis about the main escaping ways and the buried failure mechanism of carbon dioxide,vertical cracks in the cap rock and fracture at cement sheath interface are two of the most dangerous escape channeles.To assess cap rock sealing ability,established cracks formation and propagation forecast model of cap rock,obtained cracks formation and propagation probability and the average crack radius of cap rock,and studied the single fracture extension and the formation of the joint network.Established oil well cement corrosion change model,the model divided reaction zone into three different layers,structure composition changed after reaction of the three different layers,and young's elasticity modulus declined in varying degrees.By cement water flooding experiment and numerical simulation method,using solid state nuclear magnetic resonance(NMR)spectroscopy,scanning electron microscope(SEM),X-ray powder diffraction(XRD)and nano indentation method to determine,when the interface between cement-cap layer existes micro cracks,coupling the chemical corrosion effect and surrounding formation pressure force,whether cement reaction zone permeability and the change of mechanical properties enhanced the escape ability of carbon dioxide or not.Study results of this article provide theoretical guidance for carbon dioxide geological storage technology,powerfully promote the application of carbon dioxide geological storage technology in our country,realize the primary goal of carbon dioxide emissions reduction.