Analysis of potential acid gas leakage from wellbores in Alberta, Canada

Deep geological disposal of acid gas reduces green house gas emissions and removes potentially toxic hydrogen sulphide from the accessible environment. Leakage of acid gas through abandoned wellbores could compromise greenhouse gas mitigation and could be toxic. In this study, simple analytical equations are developed to predict the worst case, post injection leakage rate from wellbores with intact casings such that the acid gas moves directly from the reservoir to the surface with no dissolution into surrounding formations. The U.S. EPA code, ALOHA and Lawrence Livermore code, SLAB, are used to estimate atmospheric concentrations of hydrogen sulphide for worst case acid gas leakage at two sites west of Edmonton, Alberta; one in the Pembina-Wabamun area and the other in Brazeau-Nisku area. Results indicate detrimental atmospheric concentrations at low wind speeds would be confined to within about 3.5 km downwind for the Nisku site. Lethal concentrations would be confined to within about 10 m downwind for the Wabamun site and about 30 m downwind for the Nisku site.A semi-analytical model is used to predict long-term leakage rates and concentration of dissolved acid gas in formation fluids surrounding a wellbore with leakage through a failed casing at a site near Wabamun Lake. Two sequestration processes: geochemical reaction and convective mixing in the reservoir are shown to be important in decreasing the duration of long-term leakage from thousands of years to hundreds or years. Leakage confined to the annulus of the wellbore could result in complete acid gas dissolution into surrounding formations with no atmospheric leakage. The Joule–Thomson cooling effect is shown to be unimportant for passive leakage. Concentrations of dissolved acid gas above the odour and taste threshold could occur in potable aquifers such as the Paskapoo. Contamination of potable aquifers with heavy metals could not be ruled out.