REACTIVE TRANSPORT MODELING OF EFFECTS OF CO2 INJECTION WITH VARIABLE IMPURITIES ON DEEP SALINE RESERVOIRS

CO₂ streams from the emitters contain various impurities that upon injection into deep saline reservoirs may cause physical and chemical changes in the formation properties, and thus influence CO₂ injection and geological storage capacity and dynamics. Interactions between variable CO₂ stream compositions and potential host reservoirs were assessed by a reactive transport modeling to support the site selection and planning of storage strategies in Denmark. Simulations were conducted using ECO2N and TOUGHREACT. CO₂ streams with oxidizing (SO₂, NO₂, O₂) and reducing (H₂S, CS, COS) impurities mix were injected into model domains representing water saturated reservoirs with various mineralogy. The models contained kinetically controlled reactions for dissolution and precipitation of minerals and oxidation and reduction of impurities, including interactions between impurities, e.g. SO₂ and NO₂, and impurities and water.

Effects of injection of impurities on the reservoir mineralogy and brine chemistry are more pronounced upon injection of the oxidizing mix. Simulations without rock-water interactions predict that low pH of around 1 will occur in the vicinity of the injection well and SO₄^2- and NO₃^- will accumulate at the boundary between very high gas saturation and mixed saturation. When minerals are present, concentration of NO₂^- increases at the expense of SO₄^2- due to the pH dependence of the reaction:

2 SO₂ (aq) + 2 NO₂- + H₂O -> 2 SO₄^2- + 2 H⁺ + N₂O.

In addition, some of the SO₄^2- precipitates as anhydrite, where calcite dissolution occurs. Negligible amount of O₂ is consumed by oxidation of Fe(II) in the water. Calcite dissolution effectively buffers pH that remains close to the pH of 4.6 generated by CO₂ dissolution. While the abundance of calcite varies in the formation, calculations with just siderite and K-feldspar result in minimum pH values of 3.4. Thus, the acidification from impurities where calcite is present will not pose a critical problem for the durability of infrastructure or induce reactions that could cause deterioration of the formation strength. However, the impact of impurities on the reservoir properties could scale with the amount of impurities injected, or different injection strategies (i.e., several injection/production cycles, alternating injection of oxidizing/reducing impurities).