Corrosion Behavior of Drilling Pipe Steels for High Sour Gas Field

Abstract

A polymer drilling fluid containing high content of hydrogen sulfide was used as the corrosive medium to investigate the effects of temperature, flow velocity, pH value and partial pressure ratio of CO2/ H2S on the corrosion behavior of high strength drill pipe steel S135 and G105. The morphology and composition of the corrosion products were analyzed as well. It was found that the average corrosion rate of the two types of steel increased with increasing temperature of the corrosive medium, with the corrosion rate to decrease slightly within 60°C-80°C and keep almost unchanged above 120°C. At the same time, the corrosion rate of the drill pipe steels had little to do with the flow rate but increased with decreasing pH value of the corrosive medium. Moreover, the partial pressure ratio of CO2/ H2S had a slight effect on the corrosion behavior of the drill pipe steel. However, the two types of drill pipe steel showed a larger corrosion rate in gas phase than in liquid phase corrosive medium, which was contrary to what were observed in gas and liquid phases corrosion tests of conventional acidic drilling fluids. In addition, it was confirmed by sulfide stress corrosion test that the drill pipe steel of a higher strength had smaller critical stress, and the resistance of the drill pipe to stress attack was ranked as G105(C)＞G105(D)＞S135(B)＞S135(A). It was anticipated that the present research results could be used to guide the selection of materials for drilling pipe steels used in natural gas field of high acidity.