Potential for environmental impact due to acid gas leakage from wellbores at EOR injection sites near Zama Lake, Alberta

The potential for environmental impact from future leakage of toxic hydrogen sulfide (H 2 S) at an enhanced oil recovery (EOR) and acid gas sequestration site near Zama Lake, Alberta, is examined. Over 800 pinnacle reefs are potentially suitable for EOR by injection of acid gas. Leakage rate as a function of time through a reference wellbore is determined for various scenarios including leakage through the annulus of the wellbore and leakage through the central plug seal of the wellbore with an intact and failed wellbore casing. Potential plumes of H 2 S in the air and in shallow aquifers emanating from a single reference wellbore and from 350 wellbores are modeled. Leakage rates from the 350 wellbores are calculated from randomly sampled wellbore seal failure times, reservoir permeability, and initial amounts of acid gas, and from reference values of other reservoir parameters. Results indicate that for hundreds of years after injection, the entire Zama Lake area of 12 000 km-super-2 could have lethal concentrations of H 2 S over each of the leaking wellbores. The shallow aquifers over the entire Zama area and over 30 kilometres in the direction of aquifer flow could be undesirably tainted with dissolved H 2 S. The entire Zama Lake area and hundreds of kilometres beyond could become uninhabitable for more than 1000 years after injection due to toxic plumes of H 2 S in the air and in shallow aquifers. This analysis has implications for the potential use of acid gas for EOR and for subsurface sequestration in general in areas with large numbers of abandoned wells. © 2012 Society of Chemical Industry and John Wiley & Sons, Ltd