Corrosion in Multiphase Slug Flow Loop in Deep-Water Oil and Gas Exploitation

Abstract

The necessity to reduce cost in handling fluids requires a high flow rates with an inherent risk for turbulence influencing the corrosion process of the involved equipment. These conditions occur especially in offshore environments, as oil and gas production, and in chemical processes and energy production industries. There are several ways to discuss corrosion control associated to multiphase-flow- induced corrosion. One possibility is the mechanistic approach and the other makes use of professionals' practical experiences. A pre-requisite for any effective control practices in multiphase flow induced corrosion is a basic understanding of the involved flow mechanisms. This work presents the study of the influence of slug multiphase flow on API 5L X80 carbon steel pipe corrosion in an environment that simulates oil wells. As a simulated environment, an 80 %water-cut mixture, comprised of 20 % light oil (10 cP) or heavy oil (150 cP) plus 15 % of sodium chloride and 80 % of deionized water, was used. The tests were performed at 40 oC under a different CO2 and H2S partial pressure balanced by N2 to reach a total pressure of 2.5 bar. The tests were conducted in a multiphase simulating loop. All test parameters were monitored to guarantee a slug flow pattern. The conceptual design of the loop allowed the simultaneous evaluation of the influence of the horizontal (0°) and inclined (45°) position of the flow to verify the influence of hydrodynamic effects on the weight loss. The conducted laboratory test results allow the establishment of empirical correlations between the corrosion rate and specific exposure conditions. These correlations may be used in corrosion prediction softwares, which can help an engineer to design and to monitor life expectancies of industrial pipes. Studies conducted in autoclaves and in glass cells produce results which, despite here being useful on the understanding of corrosion mechanisms, are not able to produce multiphase-flow regime results like those related to the conditions of pipelines in oil and gas exploitations. A smaller scale corrosion loop provides a suitable environment for better reproducing hydrodynamic effects on the corrosion of pipe walls. In tests performed in the presence of only CO2, concavities on the carbon steel surface were observed. This was attributed to the flow regime, which allowed the formation of a higher disperse-bubble volume and of an emulsion, either by the formation of an oil-in-water emulsion or a water-in-oil emulsion. However, the increase in CO2 partial pressure promoted, in all tested conditions, an increase of the weight loss rate. Thus, the work allowed scoring basic mechanisms of multiphase-flow-induced corrosion which intensifies the damages of carbon steel pipes in environments containing CO2 and H2S, in different concentrations and in controlled conditions of pressure and temperature.