Detailed analysis of gaseous components in soil gases around petroleum wells - An effective tool for evaluation of their integrity

Recently methane has been regarded as one of the most hazardous gases in the Earth's atmosphere. Among important sources of methane emissions are leaking petroleum wells, both production and abandoned. The following paper presents an example of an optimal solution to the planning, execution, and interpretation of the results of surface geochemical surveys around decommissioned or abandoned, as well as production petroleum wells. The research consisted of the analytical results of the molecular composition of 94 soil gas samples collected around 3 wells in one of the oil and gas fields in the Polish Outer Carpathians. Using a patented sampling kit, soil gas was taken at 1.2 m depth at sampling sites located within a radius of 60 m from a particular well. Samples were analyzed chromatographically for their molecular composition. The results revealed the following maximum concentrations of selected gaseous components of soil gas: methane – almost 51 vol%, total alkanes C2–C5 – 0.51 vol%, total alkenes C2–C4 – 158.4 ppm, hydrogen – 0.1 vol%, carbon dioxide – 11.2 vol%, and nitrogen - 90.5 vol%. Detailed statistical analysis of the results demonstrated the presence of anomalous concentrations of methane and/or higher hydrocarbons that originated from leaking wells but also from the complex and complicated microbial processes that may be related to spills of crude oil or hydrocarbon fuels. The methodology of interpretation of the geochemical results applied may be useful in the evaluation of hydrocarbon distribution patterns and may provide an effective tool for the evaluation of the integrity of oil wells. •Soil gas samples in the vicinity of oil wells should be collected using a soil probe.•The composition of soil gases should be tested at least 60 m from the well.•Combined analysis of soil gas composition and hydrocarbon ratios is necessary.•Not every anomalous methane concentration is indicative of a borehole leak.•Hydrocarbon pollution and microbial methane may appear in the vicinity of boreholes.