Quantification of solubility trapping in natural and engineered CO2 reservoirs

Secure retention of CO ₂ in geological reservoirs is essential for effective storage. Solubility trapping, the dissolution of CO ₂ into formation water, is a major sink on geological timescales in natural CO ₂ reservoirs. Observations during CO ₂ injection, combined with models of CO ₂ reservoirs, indicate the immediate onset of solubility trapping. There is uncertainty regarding the evolution of dissolution rates between the observable engineered timescale of years and decades, and the >10 kyr state represented by natural CO ₂ reservoirs. A small number of studies have constrained dissolution rates within natural analogues. The studies show that solubility trapping is the principal storage mechanism after structural trapping, removing 10– 50% of CO ₂ across whole reservoirs. Natural analogues, engineered reservoirs and model studies produce a wide range of estimates on the fraction of CO ₂ dissolved and the dissolution rate. Analogue and engineered reservoirs do not show the high fractions of dissolved CO ₂ seen in several models. Evidence from natural analogues supports a model of most dissolution occurring during emplacement and migration, before the establishment of a stable gas–water contact. A rapid decline in CO ₂ dissolution rate over time suggests that analogue reservoirs are in dissolution equilibrium for most of the CO ₂ residence time.