1) NACE2005 Paper #05636

Inhibitor for CO2/H2S environments at HT/HP

However, mitigating corrosion in systems that produce high levels of H2S with CO2 is difficult because these systems produce elemental sulfur and

polysulfides, which tend to cause localized rather than general corrosion. The corrosion problem becomes a huge challenge if the system is also at elevated temperatures.

In sour gas reservoirs, elemental sulfur, polysulfides, water, and CO2 exist with hydrogen sulfide. Elemental sulfur thus can be carried out with hydrogen sulfide by dissolving in H2S or by chemically binding to

hydrogen sulfide gas as H2Sx. Elemental sulfur dissolved in sour gas can be released as elemental sulfur by changes in temperature and pressure4. When flowlines are partially filled with formation solids, corrosion by-products and sulfur, the resulting problem is equally serious. Controlling deposition of elemental sulfur is thus as important as mitigating corrosion in flowlines. Elemental sulfur exists as a stable crown below 950C as depicted below. Above 1140C cyclooctasulfur (S8) polymerizes to yield zigzag chains with S-S bonds with a bond length 0.24 nm5.

Elemental sulfur reacts with H2S in sour gas systems and forms polysulfides at high temperatures as shown below. It is believed that the formation of polysulfide has a

greater significance at high levels of hydrogen sulfide in sour gases, which prove to be the dominant mechanism by which elemental sulfur is transported in high sour gas fluids.

Higher temperatures, and pressures, along with the partial pressures from hydrogen sulfide could drive the chemical equilibrium to the right. Once the pressure is released

and fluids are cooled, the equilibrium will shift to the left, releasing elemental sulfur into the flowlines.

Sx + H2S = H2Sx+1

elemental sulfur sour gas liquid polysulfideIt is accordingly important to control the formation and deposition of elemental sulfur in flowlines, especially in view of the need to control corrosion on iron surfaces.

With increasing world energydemands and as a result of total supply getting lower, the petroleum industry has been forced to drill deeper into hostile environments in search of critically needed fuel. This

created the emergence of a number of high-pressure and high-temperature petroleum wells around the globe.

Theseexperiments indicate why some of the commercially available corrosion inhibitors do not perform at high-temperature. They can undergo polymerization or decomposition to

yield insoluble materials, which can clog the flowlines.

2) NACE2005 Paper #05637

Corrosion Monitoring Options

Monitoring of internal corrosion in oilfield environments has been a topic of technical conversation and argument the past half century. During this period the options have included the following methods:

1. Manual Inspection – During periods of planned or unplanned downtime, field personnel manually inspect certain locations in the system. This type of inspection is usually limited to visual (e.g. assessment of corrosion modality – general or localized (pitting) attack) and/or manual measurement of remaining wall thickness.

2. Non-Destructive Testing (NDT) – During downtime or during production periods, inspection can be conducted using non-destructive methods which commonly include: radiography or ultrasonics. These techniques are often the best for assessment of general attack but, with advanced techniques, higher precision can be obtained along with better sensitivity to localized forms of corrosion. Attempts have been made to make these conventionally offline techniques online however costs are usually high and the value questionable in all but the most extreme cases.

3. Corrosion Coupons – Corrosion coupons are normally small unstressed metal specimens that are inserted in the line. They can be removed during periods of downtime or extracted under pressure using specialized equipment. These are similar to manual inspection since they allow visual characterization of corrosion modality. Additionally, the coupons can, like NDT, provide an assessment of rate of attack, but it is usually assessment by mass loss (gravimetric) techniques. Also, of possible interest in coupons analysis is the characterization of scale weight and composition.

4. Electric Resistance (ER) – This method, like coupons, uses insertion devices to expose a probe and sensing element. As the thickness of the probe element corrodes, the resistance in the circuit increases and produces an indication of metal loss. It is best in identification of uniform corrosion and does not provide quantitative indication of localized attack. In recent years, attempts have been made to increase the sensitivity of ER devices to produce quicker response and sensitivity.

5. Electrochemical Methods – Electrochemical methods provide an indication of the corrosion rate through measurement of the current produced as a result of corrosion, for example using linear polarization resistance (LPR) measurements. Until recently, field instruments were rather rudimentary and mainly provided only semiquantitative indications of corrosion and no indication of corrosion modality. However, with the advent of multi-technique devices that integrate LPR, electrochemical noise (EN) and harmonic distortion analysis (HDA) for measurement of Tafel slope, Stern-Geary relationships and instrument corrosion

proportionality constants (B values – See Appendix I), electrochemical monitoring offers the potential for online, real-time measurement of corrosion with increased corrosion rate accuracy and differentiation between general and localized corrosion. The first four measurement methods are basically historical (i.e. after-the-fact) measurements which provide an indication of the cumulative damage that has occurred over time. Advanced electrochemical methods offer the capability to assess corrosion on a real-time basis (measurement cycle of less than 10 minutes) in a manner consistent with modern process control

methodologies (e.g. temperature, pressure, flow). Therefore, it is now possible to readily interface electrochemical corrosion measurements with established facilities control systems and to link operations and specialists to optimize productivity while minimizing costly damage to system assets.