Hydrogeochemical factors influencing external corrosion of petroleum pipelines in Tilenga Area, Western Uganda

Abstract

The integrity of petroleum pipelines is critical for the safe and efficient transportation of hydrocarbons, yet external corrosion remains a significant threat, particularly in buried pipelines. This study investigates the hydrogeochemical factors influencing the external corrosion of petroleum pipelines in the Tilenga area of western Uganda, a region which is planned as a hub for oil and gas production. The research aims to identify key soil and shallow groundwater parameters that contribute to pipeline corrosion and to develop a soil corrosivity index (SCI) to guide mitigation and monitoring strategies by pipeline operators. Field activities involved the excavation of 82 trial pits to a depth of 3.0 meters below ground level (mbgl) and drilling of 17 boreholes to 10.0 mbgl. Apparent soil resistivity was measured using the Wenner four-electrode method at trial pit locations. Hydrogeochemical properties included texture, pH, moisture content, organic matter, chlorides, sulphates, and carbonates were analysed following standard procedures for sampling and laboratory analyses, and quality control. The results revealed that apparent soil resistivity ranged from 3.2 Ωm to 3,465.8 Ωm, with lower resistivity values correlating with higher moisture content and clayey soils, indicating increased corrosivity. The pH of the soil and groundwater varied from 4.9 to 6.9, with acidic conditions (pH <5.5) observed in specific locations, further exacerbating corrosion risks. Chloride and sulphate concentrations were generally below critical thresholds of 500 ppm and 150 ppm, respectively, however, exhibited seasonal variability, with higher concentrations exceeding thresholds (3%) recorded during dry periods. Developing SCI was by integrating seven key factors: pH, soil organic matter, sulphate and carbonate content, moisture content, soil texture, and resistivity. The SCI classified the study area into four corrosivity levels: non-corrosive (0–10), partially corrosive (11–20), corrosive (21–30), and severely corrosive (31–40). Spatial analysis highlighted that approximately 18% of the pipeline routes in Buliisa district, particularly along the NGR02-NGR01-CPF flowlines, are prone to moderate to severe corrosion primarily due to clay-rich soils, high moisture, and low apparent resistivity. The findings underscore the importance of site-specific corrosion risk assessments prior to pipeline installation. Recommendations include the use of protective coatings, cathodic protection systems, and regular monitoring, especially in identified high-risk zones. The SCI model provides a practical tool for pipeline operators to enhance the longevity and safety of petroleum transportation systems in the Tilenga region and similar hydrogeochemical environments globally.