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

Potential for environmental impact due to acid gas leakage from wellbores at EOR injection sites near Zama Lake, Alberta by D.M. LeNeveu, LeNeveu Simulations Inc., Manitoba, Canada in Greenhouse Gases: Science and Technology Volume 2, Issue 2, Article first published online: March 1, 2012, onlinelibrary.wilely
Abstract: The potential for environmental impact from future leakage of toxic hydrogen sulfi de (H2S) 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 H2S 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 km2  could have lethal concentrations of H2S 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 H2S. 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 H2S 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.

The structural shape of the pinnacle reefs at Zama Lake confines the acid gas to relatively large heights of about 120 meters. In the four Alberta acid-gas injection sites and the Weyburn site, the free phase was not structurally confined and was estimated to spread out with the final smaller heights being determined by the capillary pressure with the surrounding brine rather being determined by the structure of the formation. This initial large height of the acid gas at Zama Lake results in a high buoyancy driving force. The higher permeability of the pinnacle reefs results in less resistance to the large buoyancy driving force. These two effects  combine to give larger calculated leakage rates for Zama Lake. …

The results indicate that lethal concentrations from a single leaking wellbore could extend about 75 meters downwind for a calm wind of 2 miles per hour and about 15 meters downwind for a moderate wind speed of 20 miles per hour. For calm winds of 2 miles per hour, acute health effects (H2S above 30 ppb or 0.03 ppm) could be experienced up to 40 km downwind. For the acute effcts to penetrate that far, the atmospheric stability class and low wind would have to persist in one direction for about 12 h. For a moderate wind speed of 20 miles per hour, acute health ef ects could occur up to 4 km downwind and chronic health effects (H2S above 1 ppb) could occur up to 45 km downwind. At 20 miles per hour the plume would extend 45 km downwind in about 1.4 h; however for chronic health effects, the yearly average concentration should be used. ….

Plumes of dissolved H2S in shallow aquifers in the Zama Lake field and in the direction of aquifer flow are illustrated in Fig. 17 at times of 120, 200, 400, and 1000 years after cessation of injection operations. The plume concentration and extent becomes progressively larger as time increases.

At 120 years after injection, the plume has not yet coalesced over the entire Zama field (Fig. 17).  Concentrations are above the taste and odor threshold (0.05 mg/l) over most of the field. By 200 years, the plume above threshold has coalesced over the entire field. At 400 years, the plume above threshold has spread about 10 km in the direction of aquifer flow and the concentrations over the Zama field have increased. After 1000 years, the plume above threshold has expanded to about 30 km downstream, and the concentrations in the interior of the plume are still increasing. 

Overall, the results of this modeling analysis predict unacceptable future consequences from acid gas disposal at Zama Lake. The high leakage rates are primarily a result of high calculated wellbore leakage rates and the assumed distribution of seal failure times. An alternative analysis using more gradual seal failure, failed to eliminate the adverse consequence. [Emphasis added]

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