Hydrogeochemical modelling of mineral precipitation potentials in carbon capture and storage (CCS)

Reduction of atmospheric greenhouse gas emissions have become a main focus of research and policy debates and are most likely among the primary environmental concerns of the upcoming decades. One of several options is carbon capture and storage (CCS) after electricity production. Storage of carbon dioxide in geological reservoirs is attributed to three different processes, i) filling of pore space within the reservoir by gaseous or supercritical CO2 (pore trapping), ii) dissolution of the CO2 into the formation water (solubility trapping) and iii) precipitation of carbonate as mineral phase (mineral trapping). The potential of the latter is considerably uncertain, but has probably the most long-term potential of carbon sequestration in the subsurface. Underlying concepts of geochemical equilibria computation are discribed for the conditions of pressure and temperature in deep reservoirs up to 300°C and 1000 atmospheres. The geochemical codes Phreeqc and MIN3P have recently been upgraded for that purpose and were applied in the study. Models using field data from the Heletz oil field (Israel) sandstone formation are presented, focusing on the shift of saturation index (SI) of carbonates due to injection of CO2. Alterations of the mineral phase over time become visible and potentials to precipitate were observed for the minerals Ankerite > Dolomite > Siderite ~ Calcite > Magnesite, and for the mineral Dawsonite during early stages when only Na+ is present in high ionic concentrations. Observed variability of water chemistry and the database records provide an amount of uncertainty, which was used as input to delineate the range of the mineralization potential. Simple approaches of principal component analysis leading to sensitivity coefficients are shown.