Study On Corrosion Rule Of Oil Well Cement Slurry In H₂S And CO₂ Environment

The problem that oil well cement was corroded by H₂S and CO₂ has a long history and has been paid more and more attention to.In high acid oil and well field,cement sheath is corroded day after day,which results in poor cementation and poor strength even complete loss,causing interlayer crossflow and wellbore collapse.Crack of cement sheath provide flow channels for acidic medium,which make acidic medium corroding casing further.The loss of casing strength will seriously affect the production of oil and gas wells,even lead to the abandonment of oil and gas wells,resulting in serious economic losses.This paper surveys the status and existing study of H₂S and CO₂ corrosion of cement,it is found that most of studies about affacting the corrosion of cement stone in acidic medium,focuse on the corrosion behavior,few studies relate to key internal factors that influenced cement corrosion.One corrosion environment was limited in most of studies,insteading of existence of more corrosion environment including gas and liquid in actual underground.New preservatives is lacked in the design of corrosion resistant cement slurry system.In this paper,the key internal factor affecting cement corrosion is discussed,it is definited that compactness of cement is the key factor to influence the corrosion of cement in acid medium.Based on the comparison about the change of cement properties in different corrosion time,the basis has been proposed for the change of different cement properties.The experiment shows that the cement which have smaller permeability,higher density,bigger specific surface area,smaller average pore have better corrosion resistance.With corrosion time prolonging,the permeability is very important to the corrosion resistance of the cement in the later stage of corrosion,which is argued further through the cement slurry system with different formulations from three large companies.Two kinds of corrosion environment which may encountered in the actual underground has been simulated,the corrosion behavior of cement with different permeability was studied in the water wet gas environment and saturated acid gas liquid environment,the corrosion development law of cement in different corrosion environment was definited,The changes of microstructure,permeability,average pore size,phase and compressive strength after cement corrosion were compared.Experiments show that three kind of permeability cement in the water wet gas environment have shown a greater degree of corrosion,the permeability increased faster,larger average pore,strength loss faster,corrosion depth increases faster.Basing on the study about corrosion behavior of comprehensive experiment,the corrosion mechanism of H₂S and CO₂ was discussed,and several corrosion resistant methods were put forward.A kind of corrosion resistant cement slurry system was prepared by self developed preservative FH.The main conclusions are as follows:?1?the compactness is the key factor affecting the corrosion of cement stone.The initial permeability and initial density are the most important factors which affect the compactness.?2?under the condition of the same corrosion temperature,pressure and time,the cement is more susceptible to be corroded by H₂S and CO₂ in the wet gas environment than the liquid envoriment saturated acid gas.?3?FH can effectively improve the microstructure of cement and improve the corrosion resistance of cement.