Corrosion analysis and anti-corrosion measures of oil casing of sulfur content gas wells: A case study of Daniudi gas field in the Ordos Basin

This study is designed to determine the H2S origin in the Ordovician Ma5 Member of Daniudi gas field and study the corrosion behavior of N80, P110 and N80S tubing and casings in medium-sulfur content HE16, micro-sulfur HE3 and low-sulfur HE1 gas wells. Therefore, method of weight loss of coupon on site were used to determine the corrosion rate. Similarly, SEM, XRD and EDS analyses were employed to know the corrosion products while establishing the relationship with H2S formation conditions. Results of this study show that, as the production time increases, there was initial increase in the volume of H2S before it finally decreases. Among the different sulfur gas wells, the corrosion rate of the HE16 is the largest, followed by the low-sulfur HE3 and the micro-sulfur HE1 (smallest). Within a gas well there exist a proportional relationship where higher partial pressure of H2S leads to higher corrosion rate. Generally, there is a trend of initial increase followed by decrease in the corrosion temperature of the gas wells in order of decreasing sulfur content (HE16, HE3 and HE1 well). Thus, the average corrosion temperature changing-rate of N80, P110, N80S pipe in the medium sulfur content well area, low sulfur content and micro-sulfur wells are 15.13%, 31.01% and 49.45% respectively. Although, the wells have some coupon corrosion products in common, there is still some basic differences in the composition of the HE1 wells (FeCO3 and CaCO3), HE3 wells (FeCO3, FeS and CaCO3) and HE16 wells (FeCO3, FeS0.9, FeS and CaCO3). These corrosion products are clearly stratified in the HE16 and HE3 wells, while no stratification occur in the HE1 wells. The densely structured FeCO3 is well-distributed within just as the loose and poorly-converted sulfides are distributed on the surface. Both uniform and local corrosion exist simultaneously in the study area. The galvanic corrosion between the product substrate as well as the pitting caused by the inorganic salt result in local corrosion. In the micro-sulfur gas well, only uniform corrosion occur to form FeCO3 as the corrosion product. For future development of the M5 member of Daniudi gas field, N80 tubing and casing is recommended for the micro-sulfur gas well, P110 or N80S material tubing and casing for the low sulfur gas well and N80S oil jacket for the sulfur gas well. It is good to note that special attention should be paid to the influence of temperature changes on the corrosion rate of the tubing and casing in the low-sulfur and medium-sulfur well zones. (C) 2021 The Authors. Published by Elsevier Ltd.