Application Of Simulation Techniques For Internal Corrosion Prediction

Characterization of corrosion in the oil and gas industry is becoming of increasing importance for safety reasons as well as for the preservation of production facilities; to prevent down time and damage to the environment. This article presents the methodology used by our company to characterize the corrosion behavior of the whole production facility, taking into consideration the hydrodynamic and thermodynamic conditions of the produced fluids (flow velocities, flow pattern, liquid holdup, pressure, temperature, etc.) as they flow from the reservoir through the surface installations (flowlines, gas/oil gathering and transmission lines, gas processing plants, artificial lift systems, etc.). The methodology uses Petroleum Engineering and Two-Phase modeling techniques to: 1) optimize the entire production system to obtain the most efficient objective flow rate taking into consideration the corrosive/erosive nature of the produced fluid and 2) characterize the corrosive nature of the produced fluid as it flows through the above mentioned installations. The modeling techniques were performed using commercially available simulators and CO₂ corrosion rates were determined using well known published correlations. For H₂S corrosion, NACE MRO175 criteria is applied. The application of this methodology has allowed corrosion control strategies, protection and monitoring criteria, inhibitor optimization and increased the effectiveness of already existing corrosion control systems.

Velocity enhanced corrosion problems in oil and gas production equipment are common. These often occur when produced fluids are accompanied by carbon dioxide (sweet corrosion) and/or hydrogen sulfide (sour corrosion). In recent years the presence of carbon dioxide in the produced fluids is encountered more frequently due to the use of enhanced oil recovery techniques involving CO₂injection into reservoirs, due to the occurrence of sweet/sour gas production from deeper wells, and increased exploitation of heavy crudes with higher H₂S and CO₂ content. The effect of flow velocities on corrosion is more severe and more frequently encountered in multiphase flows and in many cases corrosion rates depend on the flow patterns. These are enhanced in the presence of sand inducing the phenomenon called erosion/corrosion.

Carbon dioxide dissolves in the presence of a water phase forming a weak acid (carbonic acid) which ionizes, thus reducing the pH and corroding carbon steel pipes. As the carbon steel corrodes, it forms a corrosion product scale (ferrous carbonate, FeCO₃) which provides some degree of protection for the steel from further corrosion. The protectiveness of the scale depends on environmental factors and the characteristics of the steel. The environmental factors are those that define the medium in which the steel is corroding such as temperature, CO₂partial pressure, solution chemistry, fluid velocity, single-phase or multi-phase flows, flow pattern, pipe geometry, solution pH, and the ratio CO₂/H2S^1-4 Steel characteristics include chemical composition and heat treatment, which determine the microstructure of the steel^1,5 . As the partial pressure of CO₂ increases, the pH decreases, thus making the environment more corrosive. Temperature directly influences the solubility of the FeCO, scale; as it increases, corrosion rate decreases because of the formation of a more tenacious scale.