Corrosion Behavior Of P110 Steel/high-salt Cementing Cement System In CO₂ Formation Water

In the oil and gas industry,high salt cementing cement is generally used to solve the cementing problem in the salt-gypsum layer.Compared with other oil well cements,high-salt cements contain more erosive anions such as Cl-,and casing steels are easier to be corroded during use.There are some reports on the corrosion of oil well cement and casing,but little research has been done on high-salt cement and casing as a whole.In this paper,the steel is coated with high-salt cementing cement to prepare the cement/casing system and corrosion research is carried out under the simulated actual service condition of downhole.In the present work,P110 steel and saturated/semi-saturated brine cement slurry are selected to prepare the cement/casing system..With the help of electrochemical workstation,SEM,XRD,XPS,hydraulic servo control universal testing machine and other equipments,the corrosion mechanism of two brine cement/casing systems under different pressures and different corrosion time immersed in simulated formation water with saturated CO2 were carried out.The following main conclusions were obtained:(1)The cement/casing interface was a weak zone.The hydration has the same effect on the two systems,while the high pressure has different effects,in the hydration period;The interface between oil well cement stone and casing was a weak zone,where the hydration process of this region was slower than the cement matrix.The prolongation of hydration time and high pressure both were favorable to the stability of the interface.High pressure could slow down the passivation speed of the casing steel in the saturated brine cement/casing system,and the passivation film in the semi-saturated brine cement/casing system is more stable.(2)In the immersion corrosion test,the system was affected by both hydration and corrosion.High pressure promotes hydration and corrosion;The performance changes of the two brine cement/casing systems in simulated formation solution containing CO2 were affected by both cement hydration and corrosion of corrosive media such as CO2 and Cl-,which make the performance of cement stone and the stability of cement/casing interface strengthened first and then weakened.High pressure could promote the hydration of cement,but at the same time,high pressure could improve the penetration corrosion rate of CO2 in cement stone and the speed of Cl-gathering on the surface of casing steel.(3)The corrosion degradation trend of the cement matrix and the interface transition zone is consistent,and the cement/casing interface will be unstable before the cement stone is completely corroded.The corrosive medium are preferentially act at the local pores,and then gradually spread to the entire interface to cause the interface performance to decrease.The penetration of CO2 is faster than the reaction step with cement,so it will accumulate in the interface transition zone,which leads to more serious corrosion on the interface transition area.As a result,the cement/casing interface will be unstable before the cement stone is completely corroded.(4)When the corrosion time of 10 days,the casing steel is passivation state,and then the blunt and corrosion occurs.But the corrosion causes of the casing steel in the two systems are inconsistent;With the corrosion time of 10 days,it can be seen from the AC impedance spectrum that the casing steel is passivated state,and then the transmission resistance of the double electric layer on the surface of the casing steel gradually decreases with the increase of corrosion time.The difference is that Cl-is the main factor in the saturated brine cement/casing system,and the main factor the semi-saturated brine cement/casing system is CO2.(5)During the immersion corrosion process,the casing steel is corroded by CO2 and Cl-,the cement stone is mainly affected by carbonization corrosion of CO2.