The purpose of this laboratory study was to assess the corrosivity of fluids transported through a huge network of gas transmission lines associated with nine (9) Gas Compression Plants (GCPs) and six (6) Liquid Separation Stations (LSSs). The CO2 content ranges from 0.94 mol% to 3.9 mol%, while the H2S content varies from 0 to 1.9 mol%. The lines range in diameter from 20 to 36 inches. The highest gas flow rate in these lines is 757 MMscfd, and the highest estimated temperature and pressure are 140°F (60°C) and 1050 psig (72 bar), respectively. The study consisted of three parts: 1) uniform corrosion in liquid and gas phases; 2) pitting corrosion in liquid and gas phases; and 3) gas phase corrosion. The absence of dissolved solids and bacteria eliminated the need to study for scaling and microbiologicallyinfluenced-corrosion (MIC). The study was conducted using a rotating cage assembly with low alloy carbon steel test coupons and distilled water with 0, 150, and 1000 mg/L chloride.
The obtained results show that corrosion in the liquid phase is more severe under sweet conditions than under sour conditions. In both cases, however, corrosion can be managed through the use of effective corrosion inhibition program. On the other hand, corrosion in the gas phase is more significant under sour conditions than under sweet environments. Therefore, the use of both continuous and batch corrosion inhibitor treatments is necessary under these conditions. The presence of chlorides showed an impact under sour conditions while under sweet conditions their impact was not as obvious. Pitting attacks were observed under sour test conditions in the liquid phase when left running for a period of 14 days. Pits were observed in the presence and absence of chloride ions. Coupons tested under sweet conditions didn't show any observable pits.
Gas phase corrosion was studied at low velocities to avoid water splashing. The wall shear stresses in these tests were close to 1 Pa. Similar trend was observed as in tests conducted at higher rotation speeds. Under sour conditions, the results indicated that corrosion rates were comparable in both liquid and gas phases while under sweet conditions corrosion rates in the gas phase were negligible. Based on these results, if corrosion inhibition is selected as the corrosion control method, then the applied inhibitors must be effective in protecting the whole internal circumference of the line, i.e. internal surfaces exposed to both liquid and gas phases.
