Enhanced solubility trapping of CO2 in fractured reservoirs

AbstractThe dissolution of supercritical CO2 in formation water is one of the main trapping mechanisms for CO2 storage in saline aquifers. We develop an analytical solution for one-dimensional flow of CO2 in a fracture with dissolution into the surrounding matrix. The solute in the matrix is transported perpendicular to the fracture by molecular diffusion. We show that there is a time-scale when the mass transfer from dissolution and advection in the fractures is comparable - after this, there is significant dissolution of the injected CO2 and the transport through the fracture is slowed down. This time-scale is typically of order 1 day, implying that dissolution is significant for CO2 injection into fractured aquifers. This analysis suggests that dissolution may be a significant long-term trapping mechanism for fractured reservoirs. Although the reservoir volumes into which the CO2 must be injected will be huge, the CO2 will rapidly dissolve and hence is stored safely. We conclude by discussing dissolution as a potential permanent storage mechanism in fractured aquifers and the design of CO2 injection in such systems. We compare analytical and numerical solutions for a single fracture. The numerical solution also accommodates transport of the solute in the fracture plane and confirms that the analytical analysis is a very close approximation of the full three-dimensional problem for simple fracture geometries. The numerical model allows for two-phase flow and accounts for mutual dissolution of CO2 and water. The solution to this problem is mathematically similar to work on single-phase tracer flow and heat propagation in a fractured environment, but is new in its application to CO2 storage in geological formations