Preliminary Evaluation of Geochemical and Hydrodynamic Effects of Deep Injection of Residual Water at the AOSTRA Underground Test Facility

This report presents the results of a preliminary evaluation of the geochemical and hydrodynamic effects of deep injection of residual water at the Underground Test Facility (UTF) being expanded by the Alberta Oil Sands Technology and Research Authority (AOSTRA) to a pilot operation. AOSTRA is proposing to inject over a period of two years up to 900,000 m³ of produced, blow down and regeneration water into thin Cretaceous Wabiskaw sands, until a long-term disposal solution is found. Environment Canada and the Alberta Research Council initiated in 1990 a collaborative study on the hydrogeological effects of deep injection of residual water at the UTF site; with data support and cooperation form AOSTRA. As part of this effort, the baseline hydrogeological and geochemical conditions at the site were previously identified and described starting from the regional scale and zooming-in to the total UTF scale. Numerical modeling of the geochemical and hydrodynamic processes relate to the deep injection of residual water were used in the present study to analyze the effects of injection and the suitability for disposal of all the aquifers in the sedimentary succession at the UTF site.

The results of the analysis conform that the Wabiskaw aquifer is the only suitable one capable of accepting the proposed injection volumes. All the Devonian aquifers (Winnipegosis, Slave Point, Calumet and Moberly) have low porosity and permeability. These aquifers also have a relatively high potential for formation plugging by precipitation of silica, dolomite and calcite. Mineral precipitation resulting from geochemical reactions between the injected water and the formation water and rocks will lead to a further decrease in porosity and permeability. Because of the low permeability of rocks in these aquifers, the pressure will increase rapidly, probably reaching the fracturing threshold in several weeks. The regulations for deep disposal of residual water require that the injection pressure be less than 90% of the fracturing pressure. Thus excluding these aquifers as potential host zones for the projected volumes. The only available Cretaceous aquifer, the Wabiskaw, has very high porosity and permeability. The numerical simulations indicate that the pressure induced by injection will diffuse rapidly, with the effect that the pressure buildup will most probably remain at all times below the fracturing threshold. The radius of influence (pressure buildup) is estimated to be approximately 4000 m from the injection well at the end of the disposal operation. Geochemically, the Wabiskaw aquifer has the lowest potential and probable rates of mineral precipitation. It is expected that any mineral precipitation will occur far from the injection well and will not significantly alter the porosity and permeability of the Wabiskaw aquifer.