Modeling and analysis: Deterioration of a fractured carbonate caprock exposed to CO ₂-acidified brine flow

A flow-through experiment was performed to investigate evolution of a fractured carbonate caprock during flow of CO ₂-acidified brine. A core was taken from the Amherstburg limestone, a caprock formation overlying the Bois Blanc and Bass Islands formations, which have been used to demonstrate CO ₂storage in the Michigan basin. The inlet brine was representative of deep saline brines saturated with CO ₂, resulting in a starting pH of 4.4. Experimental conditions were 27 °C and 10 MPa. X-ray computed tomography and scanning electron microscopy were used to observe evolution of fracture geometry and to investigate mineralogical changes along the fracture surface. The initial brine flow corresponded to an average fluid velocity of 110 cm hr?1. After one week, substantial mineral dissolution caused the average cross-sectional area of the fracture to increase from 0.09 cm2 to 0.24 cm2. This demonstrates that carbonate caprocks, if fractured, can erode quickly and may jeopardize sealing integrity when hydrodynamic conditions promote flow of CO ₂-acidified brine. However, changes to fracture permeability due to mineral dissolution may be offset by unaltered constrictions along the flow path and by increases in surface roughness. In this experiment, preferential dissolution of calcite over dolomite led to uneven erosion of the fracture surface and an increase in roughness. In areas with clay minerals, calcite dissolution left behind a silicate mineral-rich microporous coating along the fracture wall. Thus, the evolution of fracture permeability will depend in a complex way on the carbonate content, as well as the heterogeneity of the minerals and their spatial patterning.