Impacts of mineralogical compositions on different trapping mechanisms during long-term CO₂ storage in deep saline aquifers

Abstract

Deep saline aquifers in sedimentary basins are considered to have the greatest potential for CO₂ geological storage in order to reduce carbon emissions. CO₂ injected into a saline sandstone aquifer tends to migrate upwards toward the caprock because the density of the supercritical CO₂ phase is lower than that of formation water. The accumulated CO₂ in the upper portions of the reservoir gradually dissolves into brine, lowers pH and changes the aqueous complexation, whereby induces mineral alteration. In turn, the mineralogical composition could impose significant effects on the evolution of solution, further on the mineralized CO₂. The high density of aqueous phase will then move downward due to gravity, give rise to “convective mixing,” which facilitate the transformation of CO₂ from the supercritical phase to the aqueous phase and then to the solid phase. In order to determine the impacts of mineralogical compositions on trapping amounts in different mechanisms for CO₂ geological storage, a 2D radial model was developed. The mineralogical composition for the base case was taken from a deep saline formation of the Ordos Basin, China. Three additional models with varying mineralogical compositions were carried out. Results indicate that the mineralogical composition had very obvious effects on different CO₂ trapping mechanisms. Specific to our cases, the dissolution of chlorite provided Mg²⁺ and Fe²⁺ for the formation of secondary carbonate minerals (ankerite, siderite and magnesite). When chlorite was absent in the saline aquifer, the dominant secondary carbon sequestration mineral was dawsonite, and the amount of CO₂ mineral trapping increased with an increase in the concentration of chlorite. After 3000 years, 69.08, 76.93, 83.52 and 87.24 % of the injected CO₂ can be trapped in the solid (mineral) phase, 16.05, 11.86, 8.82 and 6.99 % in the aqueous phase, and 14.87, 11.21, 7.66 and 5.77 % in the gas phase for Case 1 through 4, respectively.

Appendices

Appendix 1

For kinetically controlled mineral dissolution and precipitation, a general form of rate law [31, 54] is used:

$$r_{m} = \pm k_{m} A_{m} \alpha_{H}^{n} \left| {\left[ {\left( {\frac{{Q_{m} }}{{K_{m} }}} \right)^{\mu } - 1} \right]} \right|^{\nu }$$

(6)

where m is mineral index, r _m is the dissolution/precipitation rate (positive values indicate dissolution, and negative values precipitation), k _m is the rate constant (moles per unit mineral surface area and unit time) which is temperature dependent, A _m is the specific reactive surface area per kg H₂O, $\alpha_{H}^{n}$ is the activity of H⁺ and n is empirical reaction order accounting for catalysis by H⁺ in solution. K _m is the equilibrium constant for the mineral–water reaction written for the destruction of one mole of mineral m, Q _m is the reaction quotient, the parameters μ and ν are two positive numbers normally determined by experiment and are usually taken equal to unity (as in this work). For many minerals, the kinetic rate constant k (T) can be summed from three mechanisms [31, 54]:

$$\begin{aligned} k =& \,k_{25}^{nu} \exp \left[ {\frac{{ - E_{a}^{nu} }}{R}\left( {\frac{1}{T} - \frac{1}{298.15}} \right)} \right] + k_{25}^{H} \exp \left[ {\frac{{ - E_{a}^{H} }}{R}\left( {\frac{1}{T} - \frac{1}{298.15}} \right)} \right]a_{H}^{{n_{H} }} \\ &+\, k_{25}^{OH} \exp \left[ {\frac{{ - E_{a}^{OH} }}{R}\left( {\frac{1}{T} - \frac{1}{298.15}} \right)} \right]a_{OH}^{{n_{OH} }} \hfill \\ \end{aligned}$$

(7)

where E is the activation energy, the subscripts nu, H and OH denote the neutral, acid and base mechanisms, respectively, k ₂₅ is the rate constant at 25 °C, R is the gas constant, T is absolute temperature, α is the activity of the species and n is the reaction order.

Mineral dissolution and precipitation rates are a product of the kinetic rate constant and reactive surface area (Eq. 6). Notice that parameters μ and ν (see Eq. 6) are assumed to be the same for each mechanism. For all minerals, it is assumed that the precipitation rate equals the dissolution rate. The temperature-dependent kinetic rate constants are calculated from Eq. 7. More details of the symbols used in the rate law are given in Xu et al. [67].

Appendix 2

The parameters used for the kinetic rate expression are given in Table 4. Calcite is assumed to react at equilibrium because its reaction rate is typically quite rapid.

Table 4 List of parameters for calculating kinetic rate constants of minerals