Experimental study of gas evolution in heterogeneous shallow subsurface formations during leakage of stored CO2

A concern for geologic carbon sequestration is the potential for stored CO2 to leak upward into valuable shallow aquifers where it can cause potentially detrimental impacts to groundwater resources. Understanding the mechanisms of CO2 migration and predicting its movement in shallow aquifers is a critical part of determining those impacts. During leakage, CO2 dissolved in brines may travel upward, potentially causing the gas to be released from solution (exsolve). Exsolved gas may accumulate at soil layer transitions, or flow into the vadose zone and ultimately the atmosphere. For this study, a series of intermediate-scale laboratory experiments were conducted to observe CO2 gas evolution in heterogeneous porous media. Results indicate that: (1) heterogeneous interfaces as well as flow constrictions through discontinuities in low-permeability layers enhance the evolution of gas phase, provided the water pressure at those layers is less than the pressure at which the flowing water was saturated with CO2, (2) higher contrast between the sands in a 1-D heterogeneous system leads to faster gas evolution, and (3) the effects of water flow rate on the evolution of the gas phase are sensitive to two-dimensional water flow pattern fluctuations.