Study On Mechanism Of Integrity Change And Risk Assessment Of Leakage Of Well-bore During CO₂ Storage

Geological storage of CO₂ is a kind of technology that can effectively mitigate the greenhouse effect and the integrity of the storage system is the key issue to ensure a successful CCS.The integrity of the storage system can be divided into well-bore integrity and cap-rock integrity,and it is considered that well-bore integrity serves as the most dangerous part to CO₂ leakage.Therefore,in this paper,the factors affecting well integrity are analyzed such as alternating working stress,cyclic temperature loading and chemical corrosion by theoretical analysis and the CO₂ corrosion experiment of cement sample.It is concluded that CO₂corrosion mainly leads to the dissolution of alkaline minerals of cement sheath such as Ca?OH?2,C-S-H,AFt and CaCO₃ mineral precipitation.In addition that,when the corrosion temperature increased from 60°C to 90°C,the CO₂ partial pressure increased from 0.5MPa to 3MPa and the corrosion time increased from 30 d to 120 d,the permeability of cement sheath increasing rate,strength loss rate and corrosion rate all increases significantly.This experimental phenomenon indicates,with the increase of CO₂ corrosion temperature,carbon dioxide partial pressure and corrosion time,the corrosion rate of the cement sheath increases.Then,an experiment is designed to study the well integrity changes when the caprock is salt rock.It is found that when rock salt is used as a cap rock,the capillary permeability of the cement sample decreases from about 10^-15and 10^-13m²to 10^-18and 10^-22m²from the initial permeability baseline after etching.The CO₂ leakage paths is blocked resulting in self-healing of the storage wellbore,thereby reducing the risk of CO2 leakage.In addition,this paper also discusses the principle of THMC coupling numerical simulation and conducts coupling numerical simulation study of CO₂ sequestration.By numerical simulation study,the variation law of well-bore cement ring integrity is predicted.This comparative study has also effectively compensated the shortcomings of laboratory's small time scale.At the end,this paper systematically evaluates the leakage risk of CO₂ storage system by introducing the method of fuzzy comprehensive evaluation and analytic hierarchy,thus a CO₂ storage leak risk evaluation system is formed by theoretical analysis,laboratory experiments,and numerical simulation combined with system leakage risk assessment.