Geochemical Effects of Storing CO2 in the Basal Aquifer that Underlies the Prairie Region in Canada

AbstractIn the area underlain by the Basal Aquifer in the Prairie region of Canada, there are 20 large CO2 sources (coal-fired power plants, oil sands and heavy oil production and upgraders, refineries, chemical and petrochemical plants, fertilizer plants and cement plants) that emit more than 1 Mt CO2/year each, for a total of 83 Mt CO2/year,which represents 12% of canada ‘s anual greenhouse gas emissions. If post-combustion capture technologies are used, 75 Gt CO2/year can be captured from these sources, with a composition (% mass) estimated to be 99.95% CO2, 0.02% N2/Ar/O2, and 0.03% H2O. The Basal Aquifer comprises Middle Cambrian sandstones that overlie the crystalline Precambrian basement in the Alberta basin and the Canadian part of the Williston basin. Shales constitute the primary caprock of the Basal Aquifer. Pressures in the aquifer generally follow a gradient of 10.8kPa/m. Temperatures in the aquifer vary between >150°C in the deepest part of the Alberta Basin to less than 10°C in outcrop areas. Water salinity ranges from > 300,000mg/L in central Alberta to < 10,000mg/L in the southwest and in the east. The formation water in the aquifer region suitable for CO2 storage is NaCl dominated. Generally, at in-situ conditions, these waters are saturated with CaSO4 (anhydrite), with some at NaCl (halite) saturation. The mineralogy of the aquifer rocks and caprock were determined using a suite of laboratory analyses and normative calculations on core samples. The dominant mineral in the Basal Aquifer formation in regions suitable for CO2 storage is quartz, which is generally present in the 65% to 95% range, while potassium feldspar is the next most common silicate. Pyrite is present in trace amounts in many of the samples. Calcite and illite, when present, are primarily pore- filling minerals. Calcium sulphate (anhydrite) is present in many of the samples, and, when present, is a pore-filling phase. In regard to the caprock, quartz is still a predominant mineral phase, comprising of at least one third of the rock for all samples. Potassium feldspar, illite and kaolinite are the next most common minerals. The ability of the rocks of the Basal Aquifer to react with CO2 is limited, with potassium feldspar and complex clays providing the bulk the reactive capacity via the formation of kaolinite, or potentially a coupled reaction forming alunite and calcite in anhydrite-bearing zones. The effect of these reactions on CO2 storage capacity, aquifer porosity and permeability is limited, allowing decoupling of flow from geochemical processes. Caprock reactivity towards CO2 is much greater due to its more complex mineralogy, however they will not propagate far into the caprock due to its low permeability, thus allowing decoupling of flow and geochemical processes