2.1 Background

The natural subsurface is composed of rocks, soils, as well as other forms of porous materials that contain various types of minerals. Minerals dissolve and precipitate when interacting with water. As minerals dissolve, chemicals in the solid phase transform into ions in water, resulting in a decrease in mineral mass and volume. For example, calcite (CaCO₃) dissolves into carbonate species (H₂CO₃, HCO₃^-, CO₃^2-) and Ca(II)-containing species in water. In contrast, mineral precipitation occurs when aqueous species transform to become solid phases, therefore leading to mass and volume increase in solid phases while decrease in aqueous concentrations.

Mineral dissolution and precipitation are important in both natural and engineered processes. They influence water chemistry, soil formation, contaminant transport and fate, acid stimulation in reservoir engineering, environmental remediation, and global carbon cycling. For example, acid stimulation accelerates mineral dissolution, therefore increasing reservoir porosity and permeability and enhancing oil production. On the other hand, mineral precipitation during hydrocarbon production results in wellbore clogging. The dissolution of carbonate caprocks can potentially result in CO₂ leakage from CO₂ storage reservoirs. Alteration of flow fields induced by mineral dissolution and precipitation in geothermal systems could affect the long-term production of geothermal energy. Over geological time scale, the CO₂ consumption during chemical weathering (through mineral dissolution) helps maintain the clement conditions for life on earth.