Effects of CO2-Saturated Brine on the Injectivity and Integrity of Chalk Reservoirs

Underground storage of CO 2 in geological structures is very often accompanied by chemical interactions between the storage rock formation, existing fluids (e.g. brine) and injected CO 2 . Depending on the mineralogy and initial petrophysical properties of the rock formation, such reactions may also alter the petrophysical properties of the rock through dissolution, precipitation, fines migration and compaction mechanisms. In fact, carbonate formations are often highly reactive with carbonated brine and the extent of any reaction often depends on the precise rock composition as well as the accessible surface area with the fluid; a higher surface area will typically increase the reaction rate for heterogeneous systems between solids and liquids. Furthermore, fracturing and weakening of oil-bearing chalk reservoirs are approaches that have been implemented to improve oil recovery from various fields worldwide. In this paper, we present the results of an experimental study on a heterogeneous chalk sample (calcite concentration > 98.9 wt%) which has been cut in half (to form an inlet and outlet sample) subjected to carbonated brine flooding under in situ reservoir conditions. The results show a significant increase in the post-flood permeability of the inlet plug and a slight decrease in the outlet plug. The increase in permeability of the inlet sample is supported by X-ray CT and SEM images which reveal significant mineral dissolution and establishment of preferential flow paths (or wormholes). On the other hand, dissolution is not observed in the outlet sample. This suggests that the fluid has reached equilibrium (i.e. achieved solute saturation) with the rock samples after traversing the first sample (i.e. there is no further mineral dissolution). This is strong evidence for the existence a dissolution front that forms during the core flooding process. With continued flooding of CO 2 -saturated brine, this front eventually traverses the whole sample and the dissolution becomes more substantial along the entire length of the core. As a result of the dissolution process, there is some degree of fines migration (induced by the dissolution in the first samples) into the outlet sample which has negative impacted its permeability. This change in permeability could also be caused by later precipitation of minerals from the brine, but this is likely a minor effect as the pore pressure/temperature conditions (for example, causing a pH change) do not vary significant