Corrosion Inhibitor Selection for High Temperature Sweet Gas Wells

Corrosion protection of production tubulars in high-pressure, high-temperature (HPHT) sweet gas wells is among the challenges encountered by corrosion engineers. Corrosion control by inhibitors in these wells has always been a challenge for the developers of oilfield chemicals. This is mainly due to the poor stability of the chemicals at elevated temperatures. The inhibitor components tend to degrade at high temperatures, losing the corrosion control function and, in some cases, these degraded products can pose adverse effects by promoting higher corrosion rates. This paper will focus on a laboratory study on various inhibitor chemistries under simulated HPHT sweet gas wells conditions. As part of the investigation, the corrosion behavior of the tubular carbon steel materials along the well depth at various temperatures was studied to measure the inhibitor capacity to protect the entire production tubulars from top to bottom sections. The study confirmed that temperature in a CO₂ environment was the main factor controlling the corrosion process with significantly higher rates at the upper sections of the wells. None of the investigated inhibitor chemistries exhibited adequate corrosion control efficiency.

CO₂ is well recognized to be a major factor accelerating the corrosion process. When CO₂ dissolves in water, it forms carbonic acid which is a weak acid and it dissociates only slightly to form bicarbonate ion followed by further dissociation to form carbonate ions (Figure 1). At pH levels lower than 5.5, the bicarbonate ions (HCO3-) are the main carbonic species in solution. As the pH increases, carbonate ions (CO₃2-) concentration increases. When in contact with a steel surface, the anodic reaction proceeds to produce iron carbonate (FeCO₃) as a corrosion product. The solubility of FeCO₃ in solution is low and decreases with increasing temperature. The stability and hence the protectiveness of the carbonate depends on various factors with temperature, fluid hydrodynamics, pH, and steel microstructure are the main factors. The effect of temperature is very complex and the corrosion rates at different temperatures with CO₂ can vary, as the temperature can change the gas solubility in the solution. In addition, temperature can affect H₂CO₃ ionization and change the solution pH. Lastly, temperature can also affect the formation, coverage, density and degree of protectiveness of the corrosion product film ^[1-4].