Analysis of Hydrodynamic, Thermal and Salinity Effects of Deep Injection of Residual Water at the AOSTRA Underground Test Facility

This report presents the results of a computer-based evaluation of hydrodynamic, thermal and salinity effects of deep injection of residual water in the Wabiskaw aquifer at the Underground Test facility (UTF) operated by the Alberta Oil Sands Technology and Research Authority (AOSTRA). The valuation is based on numerical simulations of injecting up to 900,000 m³ of produced, blowdown and regeneration water over a period of two years. The salinity of the injected water is lower than that of formation water, while its temperature is higher. Thus, the main purpose if the evaluation is to assess to what extent the natural pressure, temperature and concentration fields in the Wabiskaw aquifer will be affected and modified by residual-water injection. This report is a result of a collaborative agreement between Environment Canada and the Alberta Research Council to study the hydrogeological effects of deep injection at the UTF site. AOSTRA's cooperation and support with data is gratefully acknowledged.

Mathematical and numerical modeling of coupled processes of fluid flow, heat transfer and mass (solute) transport in porous media was used in the present study to evaluate the hydrodynamic, thermal and salinity effects of deep injection in the Wabiskaw aquifer. In addition, the spatial variability in aquifer properties, particularly porosity, was taken into account. The porosity distribution was obtained from geophysical logs calibrated on core analyses, resulting in a better representation than that obtained previously from core analyses only. The numerical simulations, based on forecasted rates of injection, indicate that the pressure buildup at the injection site will remain below the accepted limit of 90% of the rock-fracturing threshold. After cessation of injection, the pressure buildup will rapidly decay due to the high hydraulic diffusivity of the aquifer. The areal spread of pressure buildup will be asymmetric, reaching between 4 km in the northwest and 6 km in the southeast because of lower aquifer porosity southeast of the injection site. The temperature is not likely to increase significantly because of heat transfer from the injected water to the aquifer water and rock, and because of heat losses by conduction through the overlying and underlying aquitards. The thermal effects of injection are likely to be felt at no more than 400 m from the injection well. After cessation of injection, the temperature will slowly decay because of low thermal diffusivity of aquifer water and rocks. Because the salinity of the injected residual water is actually lower than that of aquifer water, a dilution plume will form as a result of injection. The dilution plume will be slightly asymmetric, at the end of the injection period reaching approximately 500 m northeast from the injection site, but only approximately 300 - 350 m in other directions. This asymmetry is attributed to the effect of the northeastward flow of aquifer water. After cessation of injection, the initial pressure regime in the aquifer will be rapidly re-established, and the dilution plume will be carried by the natural flow of aquifer water with a velocity estimated to be approximately 2.5 m/a. Thus, overall, the numerical simulations show that the injection of the proposed volume of residual water at the UTF site is not likely to have any significant hydrogeological effect in the Wabiskaw aquifer.