Comparative study on corrosion between drill stem S135 and valve plate Cr13 indiesel tail gas drilling environment

The underbalanced drilling technology on diesel tail gas has the advantages of protecting oil and gas reservoirs and improving the rate of penetration. However, it may result in the severe corrosion of down hole drilling stems and the failure of compressor valve plates on the ground, due to the complicated composition of the diesel tail gas. The simulating test of the corrosion in the diesel tail gas drilling environment shows that under the same conditions the average corrosion rate of Cr13 is far behind the S135. On average, the corrosion rate of Cr13 in dynamic liquid phase greatly exceeds that in dynamic gas phase. The corrosion rates of the two kinds of steel reach the highest when the temperature is 100 ℃, the total pressure is 10MPa, the response time is 120 h, and the fluid velocity is two meters per second (m/s). Particles of corrosion product films on the surface of Cr13 steel are fine and uniform, while those of S135 stell are coarse and uneven. So the corrosion rate of S135 is much higher than that of Cr13. The corrosion products of S135 steel in liquid phase feature in large porosity, poor compactness, and great looseness, so they can efficiently cover the surface of samples and to protect the base. However, the corrosion products of S135 stell in gas phase can uniformly cover the surface of samples, and can effectively obstruct corrosion media. Typical uniform corrosion occurs. Subsequently, the corrosion rate of S135 steel in gas phase is in relatively lower. Diesel tail gas may lead to severe corrosion of drill stems, so the technology is not suitable for water-production gas wells and high pressure gas wells. Valve plates failure can be alleviated by taking rational preventing measures.