Carbon dioxide dissolution 1 in structural and stratigraphic traps

The geologic sequestration of carbon dioxide (CO2) in structural and stratigraphic 9 traps is a viable option to reduce anthropogenic emissions. While dissolution of the 10 CO2 stored in these traps reduces the long-term leakage risk, the dissolution process 11 remains poorly understood in systems that reflect the appropriate subsurface geometry. 12 Here, we study dissolution in a porous layer that exhibits a feature relevant for CO2 13 storage in structural and stratigraphic traps: a finite CO2 source along the top boundary 14 that extends only part way into the layer. This feature represents the finite extent of 15 the interface between free-phase CO2 pooled in a trap and the underlying brine. Using 16 theory and simulations, we describe the dissolution mechanisms in this system for a wide 17 range of times and Rayleigh numbers, and classify the behavior into seven regimes. For 18 each regime, we quantify the dissolution flux numerically and model it analytically, with 19 the goal of providing simple expressions to estimate the dissolution rate in real systems. 20 We find that, at late times, the dissolution flux decreases relative to early times as the 21 flow of unsaturated water to the CO2 source becomes constrained by a lateral exchange 22 flow though the reservoir. Application of the models to several representative reservoirs 23 indicates that dissolution is strongly affected by the reservoir properties; however, we 24 find that reservoirs with high permeabilities (k > 1 Darcy) that are tens of meters thick 25 and several kilometers wide could potentially dissolve hundreds of megatons of CO2 in 26 tens of years. 27