Gas Field Gathering Pipelines Material Selection And Corrosion Protection

Kera-2 gas field is located at Baicheng county of Xinjiang Tarim Autonomous Region. It has a small-scaled gas-bearing zone and only 10 producing wells. The design productivity of single well is large up to 1100×10⁴m³/d. The producing wells are distributed averagely from east to west along the gas field. The maximum distance of wells from east to west is 12km, and it is 1.5km from south to north. So the single well gas gathering process and centralized processing are adopted. Kera-2 gas field provides eighty percents gas for the West to East project. So it's very important to ensure the long-term safety and reliability of the pipeline networks.From the internal and external corrosion prevention, writer discussed material selection and corrosion prevention of Kera-2 gas gathering and transmitting system. Internal corrosion prevention: There are ninety-eight percents CH₄ in Kera-2 gas source and no H₂S in it, but 0.7 percents CO₂ is in it. At the late stage, produced water in gas field is CaCl₂ type with 100000mg/l of Cl^- content. At the beginning of operate, gas pressure is high and temperature up to 70⁸0℃. which accelerated corrosion rate. The writer has discussed the material selection and corrosion control aimed at Kera-2 gas. At first ,the writer introduced CO₂ corrosion tenet and all the various aspects which influences CO₂ corrosion. Selecting some carbon steel, martensitic steel, austenitic steel and Duplex stainless steel, the writer did the Corrosion resistance experiment with simulated corrosion condition in Kela 2 Gas Gathering System. It can be concluded that 22Cr Duplex stainless steel's spot corrosion resistance is very good, it can be adopted in Kera-2 gas gathering and transmitting system. It is the first time to adopt Duplex stainless steel in gas gathering and transmitting system. Further more, adopting Duplex stainless steel is more economical than carbon steel + inhibitor option.External corrosion prevention: Saliferous clay is broadly distributed more eighty percents in Kera-2 gas field. According to our survey and the introduction from the information concerned, the fusible salt content of this area is 0.35% - 6.08%, it is mainly medium - strong saliferous clay, hydrochloride - chlorite type. The soil resistivity 5.15Ω .m - 15.08Ω.m. In accordance with the regulation of "the code of corrosion control for engineering design of steel pipe and storage tank", if the resistivity of soil is less than 20 Ω .m, it belongs to strong corrosion degree. So the soil of this area belongs strong corrosion to steel structure. At present the trend for the external anti-corrosion coat of pipeline is developed to the direction of plastic anti-corrosion coat. The developed plastic anti-corrosion coating includes the following: PE anti-corrosion coat, PP anti-corrosion coat, fusion bonded epoxy powder coating, PE tapes anti-corrosion coat, and polyurea anti-corrosion coat, etc.. PP is the better anti-corrosion coat under the condition of high temperature. The temperature can reach 100°C if use it for a long time, the improved three layer of PP has the following advantages: strong adhesive power, good performance for low temperature resistance and soil stress resistance, with high hardness and low water-intake rate, small cathodic disbonding radius, good stability, and long servicelife, etc.. it is the developed anti-corrosion coat for buried pipeline used under the condition of high temperature at present abroad. During the transportation and construction it is difficult to protect the corrosion proof coating from damage and wear, the joint acting force of corrosion proof coating and cathodic protection can be the economic and effective way in anti-corrosion. The cathodic protection is also good for reducing stress corrosion cracking. So it is adopted that the joint method of anti-corrosion coating and cathodic protection for this pipeline.