Effect of type of oil or gas condensate on carbonic acid corrosion

Abstract

Corrosion rates to be expected on carbon steel in oil and gas production are often estimated using the De Waard and Milliams equation, which is based on experimental corrosion data obtained in CO2-containing saline water only, in the absence of liquid hydrocarbons (crude or gas condensate). Under certain conditions, liquid hydrocarbons may greatly reduce corrosion. Then the use of the original nomogram is to be regarded as a worst case approximation. The aim of this study is to quantify the extent to which liquid hydrocarbons can decrease corrosion, with specific reference to the hydrocarbon composition. The hydrocarbon can be classified on the basis of its 'kerogen' type, with 'maturity' expressed in terms of density. Both electrochemical impedance spectroscopy (EIS) and coupon mass loss were used to estimate corrosion under static and dynamic conditions. At lower water cuts (up to some 45 wt%) in fully dispersed mixtures with liquid hydrocarbons, corrosion is fully suppressed, regardless of the type of hydrocarbon. At higher water content, however, the reduction in corrosion depends on the type of oil, and on its geological history in terms of time, temperature, and pressure (thermal gradient). Apparently, the 'immature' oils behave as if they contain natural corrosion inhibitors.