The role of natural fractures of finite double-porosity aquifers on diffusive leakage of brine during geological storage of CO2

The diffusive leakage rate (DLR) of brine from a finite fractured aquifer into intact overlying and underlying layers during geosequestration of carbon dioxide is modeled using a two-dimensional radial domain. To handle the interaction between aquifer and caprock or overlying layer (underlying layer), continuities of pressures and fluid fluxes are taken into account at the aquifer-caprock interface. In order to solve this problem, Laplace transform and finite Hankel transform are adopted. The early-time and late-time solutions for DLR of brine from a double-porosity aquifer are also examined using the developed general solution. The solutions are used to analyze DLR from the fractured aquifer into overlying formation and the average pressures in the fractured aquifer and the caprock during injection and post injection periods. Variations of DLR and the average pressures in the fractured aquifer and the caprock with the double-porosity parameters are characterized. It is shown that DLR of brine and the average pressures in the aquifer and the caprock are sensitive to double-porosity parameters during CO2 injection when ratio of the horizontal fracture to the caprock permeabilities (k(fh1D)) is small. However, they remain practically insensitive to double-porosity parameters for a large k(fh1D). The results also confirm that DLR of brine reaches to a fixed pseudo-steady state value at late times during CO2 injection. In addition, the post injection DLR for low to moderate values of k(fh1D) is found to be negligible while for high value of k(fh1D) DLR is more evident. These results find applications in geological storage of CO2 in fractured saline aquifers.