Study On The Corrosion Mechanism Of CO₂ Driven Surface Gathering Pipelines In Shaanxi North Oilfield

With the widespread use of CO₂oil drive technology,a large amount of CO₂has been introduced into the recovery fluid,leading to serious corrosion of oilfield gathering pipelines and posing a significant safety hazard to oilfield safety production.Based on the nature of the gathering pipeline and the composition of the transported material in Shaanxi North oilfield,this paper investigates the influence of pipeline surface properties on the corrosion behaviour of CO₂in gathering pipelines in terms of surface roughness,wettability and surface scaling.Corrosion specimens with different surface properties were firstly prepared,and then the corrosion rate of carbon steel in a simulated extraction fluid with 70%water content was measured by the loss-in-weight method,and the corrosion morphology and composition of corrosion products were observed and measured by scanning electron microscopy（SEM）,energy spectrum analysis（EDS）and OLYM PUSDSX-500 automatic 3D imaging microscope.The main findings are as follows:（1）In the simulated recovery fluid at 65℃,Pco₂=1.5MPa and water content of 70%,the average corrosion rate of carbon steel decreases and then increases with increasing surface roughness,indicating primarily uniform corrosion.The corrosion products are mainly Fe CO₃,Fe C₃and a small amount of Ca CO₃.The surface roughness primarily impacts the corrosion rate and maximum corrosion depth of carbon steel by affecting the adhesion of crude oil to the specimen’s surface and the formation of a corrosion product film on the substrate surface.（2）In the simulated recovery fluid at 65℃,Pco₂=1.5MPa and water content of 70%,the average corrosion rate of carbon steel decreases rapidly to 0.205mm/a with the increase of emulsifier OP-10 content,and then does not change much,mainly showing uniform corrosion.The corrosion products were mainly Fe CO₃and a small amount of Ca CO₃.The addition of the emulsifier improved the wettability of the oil with respect to the corrosion specimen,increased the hydrophobicity of the corrosion specimen surface,strengthened the interaction between the corrosion specimen and the crude oil,and played a role in corrosion inhibition.（3）The average corrosion rate of carbon steel increased with the increase of calcium carbonate scale on the surface at 65℃,Pco₂=1.5MPa and 70%water content in the simulated recovery fluid,with the average corrosion rate increasing from 0.803mm/a to 1.08mm/a.The corrosion type was uniform corrosion.The main composition of the corrosion product film is Fe CO₃.As the calcium carbonate scale layer is hydrophilic,it is not easy for the crude oil to adhere to the surface of the specimen to form a protective layer.At the same time,the water containing CO₂has an erosive effect on the calcium carbonate scale layer,which makes the calcium carbonate scale layer also has no protective effect on the metal substrate.（4）Orthogonal experiments have shown that the order of influence of surface properties on carbon steel at 65℃,Pco₂=1.5MPa,and with a water content of 70%in the simulated recovery fluid on corrosion rate is:emulsifier additive>surface scaling amount>surface roughness.The order of influence on maximum corrosion depth is:emulsifier additive>surface roughness>surface scaling amount.The effect of the amount of emulsifier is significant,surface wettability is the main factor affecting the CO₂corrosion rate and the maximum corrosion depth of the collector pipeline.