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Summary

Modeling of supercritical CO₂ injection into a deep saline carbonate formation (calcite and dolomite with minor anhydrite) was performed using TOUGHREACT (Xu et al. 2006) with Pitzer ion-interaction-model implementation for handling high-salinity problems (Zhang et al. 2006). The formation-brine salinity is approximately 225,000 ppm (NaCl dominant), the temperature is 102°C, and the pressure is 225 bar. The CO₂ is injected through a horizontal well in a 3D model domain at a constant rate for a period of 1 year. The carbonate formation was assumed to have homogeneous porosity and permeability and to be overlain by an impermeable seal. The effect of a high-permeability fault with orientation perpendicular to the horizontal well and bounded by the impermeable overburden was evaluated. The changes in mineralogy and rock property during the injection have been assessed. The simulation results illustrate that (1) the high-permeability fault acts as a CO₂ conduit; (2) a dry-out zone is developed within a few meters from the injection well because of displacement by supercritical CO₂ and evaporation of water into the CO₂ stream; (3) at the front of the dry-out zone, brine is further concentrated because of water evaporation into the supercritical CO₂, the pH is lowered from 5.5 to 3.1, halite (NaCl) and anhydrite (CaSO₄) precipitate, and the brine becomes CaCl₂ dominant; (4) near-wellbore porosity reduces by approximately 5 - 17% (1 - 3 pu) because of halite precipitation in the dry-out zone; (5) HCl gas is generated from the dry-out front; (6) calcite and dolomite dissolve as the CO₂ plume advances during injection; (7) anhydrite, however, slightly dissolves along the CO₂ front but precipitates in the area corresponding to the CO₂ plume, with higher proportions of this mineral precipitated near the wellbore dry-out zone.

These findings are valuable for the assessment of injectivity changes and near-wellbore stability of saline aquifers in carbonate formations during injection of CO₂. The overall mineral trapping in hundreds of years is not the focus of this paper. The method of this study is useful for further evaluation of engineering options to enhance immobile trapping of CO₂ and mitigation measures for potential injectivity impairment.