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Completion Fluids Corrosion Properties in Petroleum Production System
Corrosion is defined as natural and continuous degradation of materials due to chemical or electrochemical and mechanical reactions. Presence of electrolytes, oxygen, impurities of metallic surface etc. can result in corrosion, which still remains as one of the pertinent worldwide challenges in the Petroleum Industry. This page addresses the performance of effective corrosion inhibitors, mainly phosphate and sulfite based inhibitors under increasing temperature and pressure conditions, in an economically feasible manner.
The most common well completion and workover fluids in the Petroleum Industry are brines such as chlorides, bromides and formates. The selection of these fluids is based on their ability to keep the formation pressure in control via their inherent fluid density. These fluids are expected to prevent filtrate invasion by solids that could result in formation damage as well as prevent the casing from collapsing under over pressured conditions.
Factors affecting the performance of completion fluids
The most important parameter while selecting the appropriate completion fluid is density amongst others. When temperature or pressure increases, the increase in concentration of salt beyond a certain value can result in crystallization. These crystals are solids that can eventually result in formation damage.
An ideal completion fluid is characterized by high specific gravity and temperature stability and also a pH level in the range of 9 – 11. pH and salinity levels are two major properties that have an impact on the performance of the completion fluids. However, an increase in pH levels in brines can result in formation damage as well as cause clotting of mud that can eventually result in de-flocculation related problems.  High pH can also cause Stress Corrosion Cracking (SCC) in the pipeline which is a localized form of corrosion that produces cracks in metals as a result of being under continuous tensile stress . Additionally, greater the pH, higher the chances of sweet corrosion, i.e. carbon dioxide (CO2) corrosion. Crude oils are made of hydrocarbons but they also contain non-hydrocarbon elements like sulfur, nitrogen and oxygen.
Effect of non-hydrocarbon elements in completion fluids
The presence of CO2 in crude oils can lead to corrosive reaction which can be explained as a chemical reaction between the CO2 present and the water (present in completion fluids) forming bicarbonates as seen in Equation 1 below:
2CO2 + 2H2O + 2e- = 2HCO-3 + H2 (1)
It is imperative that the acidic nature of bicarbonates be controlled since they can enhance corrosion and also cause pitting issues within pipes and is the dominant cause for pipeline failures, release of flammable media, fires and explosions.  With sweet corrosion also comes sour corrosion which is due to Hydrogen Sulfide (H2S). Reaction of H2S with water results in the formation of weak acid resulting in hydrogen ions as represented by Equation 2 below: 
H2S + ½ O2 = H2O + 2S (2)
Lower the concentration of H2S, greater and severe the corrosion and vice-versa. But if both H2S and CO2 are present, then the material will crack and fail due to higher severity in the damage caused by localized corrosion.  The presence of dissolved oxygen (O2) in the fluid can form ferric hydroxide, i.e. if the pipe used is made of steel. Ferric hydroxide in common language is known as rust and this does not dissolve in water. Equation 3 below describes this chemical reaction:
2Fe++ + ½ O2 + H2O = 2Fe+++ + 2OH- (3)
The relation of corrosion due to O2 is similar to that of H2S; smaller the concentration, greater the corrosion rate. The presence of oxygen is also related to an external factor that causes corrosion, i.e. temperature. In a closed system, the amount of oxygen will be limited as in the case of an open system where infinite oxygen will be present. Under high temperature conditions, oxidation increases resulting in corrosion.
Corrosion inhibitors now come into play and are chemical substances that are added to an environment suspected of potentially corroding substances. A scientific definition in terms of its use in the oil and gas industries as explained by Popoola et al. is a substance that can restrict the rate of anodic or cathodic process by blocking active sites on the surface of the metal. Several corrosion inhibitors have been tried and tested for specific brines and other alternatives like corrosion-resistant alloys (CRA’s) have also been adopted to overcome corrosion. Adding to that, suitable coatings (organic, inorganic or metallic) have been chosen, using biocides, cathodic protection and also attempting to remove the corrosive impurities like H2O, CO2 and O2.
Corrosion inhibitors are more common and because there are more options available in the market, it becomes easier to compare and choose which one would be better and efficient. In gas systems, the most common inhibitor used is “filming amine” that create a protective film on the surface.  The inhibitors injected in pipleines carrying natural gas are nitrogen-organic compounds. It has to be noted that at temperatures above 200 will cause the inhibitors to degrade and breakdown in natural gas systems as they are organic. Predicting the corrosiveness of crude oils is more complicated. They are defined by Total Acid Number (TAN), sulfur, water and salt content and also presence of microorganisms. But their corrosivity are not fixed based on the presence of above substances rather they act differently based on the stages of crude oil production, extraction, transportation, storage etc. For instance, water in great amount can inhibit corrosion and in some cases make crude corrosive at small amounts. 
Corrosion management comprises of proper planning and execution of operations, leverage of correct selection of materials in the production system like completion fluids and design of materials, corrosion protection and monitoring tools. Enhancing corrosion management can help reduce the harmful effects on the environment as well as grant long production life of the wells.
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