Study On Corrosion Law And Control Mechanism Of Oil Well Cement Paste By CO₂

As one of the most effective ways to reduce greenhouse gas emissions,geological storage of carbon dioxide is an important technology for China to achieve the emission reduction target of the Paris Agreement signed with the international in 2015.The wellbore integrity in the process of long-term storage determines whether the storage can be implemented smoothly.In view of the phenomenon that the cement ring of oil well is corroded and the local physical and chemical properties of cement ring are destroyed during carbon dioxide storage,the corrosion mechanism and corrosion process control mechanism of oil well cement stone corroded by carbon dioxide are studied in this paper.Through the formula of cement slurry used in cementing,the hydration reaction of cement paste during underground consolidation is analyzed,and the different composition distribution ratio of cementing cement stone is obtained.According to the different reaction processes between cement paste and saturated carbon dioxide formation water,the carbonation process,neutralization process and leaching decalcification process in the whole corrosion process,as well as the effects of each reaction on cement stone were analyzed.Based on the comprehensive analysis of each corrosion process,the causes of four different regions formed after cement paste corrosion are revealed accurately,and the reaction mechanism of carbon dioxide corrosion of oil well cement stone is obtained.In order to verify the corrosion process analyzed in the mechanism study,an indoor experiment simulating the underground sealing of carbon dioxide by cement ring was designed,and the cement corrosion depth varying with time under different temperature and pressure was obtained.The cement samples were analyzed by means of X-ray diffraction and scanning electron microscope,and the phase composition and microstructure changes in different areas of cement paste with time were obtained.By comparing the amount of corrosion of cement paste when the reaction is sufficient with the corroded area obtained in the experiment,the amount of corroded material in the experiment is much less than the amount of cement matrix that should be consumed in the full reaction,and it is confirmed that the main control mechanism of corrosion is mass transfer.Through the corrosion depth at different time obtained from the experiment,the mass transfer control mechanism in the process of carbon dioxide corrosion cementing paste was analyzed.Firstly,the whole mass transfer process is summarized as percolation process and diffusion process,and the Knudsen diffusion of cement stone in micro channel is studied.It is determined that the critical pore diameter of Knudsen diffusion and Fick diffusion of ions in oil well cement stone is 3.55.10^-9 m under experimental conditions.Based on the experimental results of corrosion depth of cement paste at different times,the change of corrosion depth per unit time is calculated,and the relationship between this change and diffusion velocity parameters is established,thus the diffusion contribution in the advancing process of corrosion leading edge is obtained.by comparing it with the seepage contribution obtained from the corrosion depth under different pressure,it is obtained that the critical time when the diffusion velocity is equal to the percolation velocity is 117 days.As a result,the main controlling factors of the corrosion process at different times are judged,the hydrogen ion concentration field in the corrosion area formed by the combination of mass transfer is analyzed,and the boundary conditions of several corrosion areas are determined combined with the kinetic reaction.according to the hydrogen ion concentration field,the thickness of each corrosion area is obtained,and the corrosion law under several action mechanisms of carbon dioxide on oil well cement stone is clarified.