Research On Corrosion Mechanism And Prevention Of Re-injection Water In Xinmin Oilfield

The oilfield produced water extracted in the process of crude oil treatment has the characteristics of high oil content,high salinity,large number of bacteria and so on.Oilfield produced water is treated and then injected back into the formation to maintain formation pressure and increase crude oil recovery.Oilfield produced water reinjection process often causes pipeline corrosion.Under normal circumstances,dissolved oxygen,corrosive carbon dioxide,hydrogen sulfide,bacteria,dissolved salt,pH,temperature,etc.are the main factors leading to the corrosion of oilfield reinjection system.For this reason,the oilfield generally adopts the method of adding chemical agent for the anti-corrosion of the reinjection water pipeline.The high degree of salinity in the produced water of Xinmin Oilfield has caused serious corrosion and perforation leakage to the reinjection water pipeline system,resulting in environmental pollution.Frequent replacement of the reinjection water pipeline will cause serious economic losses.Therefore,research on the corrosion control of reinjection water pipeline in Xinmin Oilfield is of great significance to the environmental protection,economic benefits and sustainable development of oil and gas field development.In order to further solve the corrosion problem of oil field water,the quality of produced water from Xinmin Oilfield was analyzed.The content of anions,dissolved gas and sulfide in the produced water of the oilfield was measured by titration,the content of cations was measured by atomic absorption spectrometer,and the content of bacteria was measured by extinction dilution method.The results show that the average content of Ca²⁺and Mg²⁺in the produced water of Xinmin Oilfield is generally low,there is a certain content of HCO₃^-ions,and the content of Cl^-ions is high,and the content of dissolved oxygen is higher than the specified index.Since the pH value of the water samples at each node of the Xinmin reinjection system is alkaline,and the dissolved oxygen corrosion usually occurs in the alkaline or neutral environment of the water quality,high concentrations of Cl^-ions can destroy the protective layer of the metal oxide film,forming Pitting.Therefore,it is preliminarily judged that Cl^-ions and dissolved oxygen corrosion are the main controlling factors that cause the corrosion rate to exceed the standard.The corrosion rate of each reinjection water point in Xinmin Oilfield was measured through laboratory experiments,and the corrosion of the oilfield reinjection water system was systematically studied by changing the concentration of Cl^-ions,dissolved oxygen,the pH value of the solution,the experimental temperature and the flow rate in the reinjection water.Due to the strong adsorption of chloride ions and the relatively small radius,they can destroy the passivation film of the metal and corrode the metal,while the reaction of dissolved oxygen and iron ions in the solution to form iron oxides will corrode the metal.Combining the morphology of the corroded coupons,the XRD and SEM analysis of the corrosion mechanism,the results show that the local pitting of the coupons after corrosion is obvious.There are a lot of iron oxides in the corrosion products of the coupons,and the overall spatial structure is loose and porous.The corrosion characteristics of Cl^-ions and dissolved oxygen prove that they are the main factors that cause corrosion of the reinjection water pipeline,and further explore the corrosion mechanism of oilfield reinjection water.Through corrosion inhibition experiments,the five kinds of oxygen scavengers,Na₂SO₃,hydrazine hydrate,dimethyl ketoxime,diethylhydroxylamine and gallic acid,were optimized.Different concentrations of deoxidizer(50mg/L?100mg/L?150mg/L?200mg/L)were added to the water sample.The experimental results show that when the concentration of Na₂SO₃ reaches 100mg/L,the average corrosion rate of the water sample after removing dissolved oxygen is reduced by nearly 50%,and the effect on the corrosion rate of coupons is more obvious.Imidazolines,Mannich bases,quaternary ammonium salts and Schiff bases were selected as corrosion inhibitors,and the dosage of corrosion inhibitors was 100mg/L.The experimental results show that the imidazoline corrosion inhibitor has a better corrosion inhibition effect on the reinjection water of Xinmin Oilfield,with a corrosion inhibition rate of 57.4%.In order to further reduce the corrosion rate of the reinjection water in Xinmin Oilfield,experiments were carried out using a synergistic corrosion inhibition method of oxygen scavenger and corrosion inhibitor to study the relationship between the multi-element composite corrosion inhibitor solution with different concentration ratios and the corrosion rates of 0.5:1?1:1?1:1.5?1:2?1.5:1?2:1.The results show that the inhibition rate with the cooperation of deoxidizer and inhibitor is more effective than that with monomer dosage.When the optimum concentration ratio range is 1:1 and the dosage is 100mg/L,the corrosion inhibition effect is the best,the corrosion inhibition rate is as high as 81.4%,the corrosion rate is less than the corrosion standard 0.076 mm/a,and the overall anti-corrosion control effect of the Xinmin Oilfield reinjection system is better.