Impacts of Limestone Vertical Permeability Heterogeneity on Fluid–Rock Interaction During CCS

Abstract

Within the context of subsurface CO₂ storage, “live brine” (formed by the mixing of CO₂ and brine) in limestone rock environments can cause mineral dissolution resulting in elevated levels of Mg²⁺/Ca²⁺ and carbonate/bicarbonate ions. Furthermore, vertical heterogeneity (where different layers in the formation of differing thicknesses exhibit significant permeability variation) is common within these types of formations and is an important factor that impacts subsequent fluid–rock interactions. To evaluate the impact of vertical permeability variation on the evolution of the rock microstructure experimentally, we conducted a novel core flooding experimental on a composite limestone rock core sample. This composite was constructed from two outcrop Indiana limestone plugs of nearly identical mineralogy with different porosity and different permeabilities which were cut/polished into identically sized lateral halves. Pre- and post-flooding X-ray computed tomoraphy and nuclear magnetic resonance measurements (NMR) were conducted to evaluate pore topology and the changes (location and intensity) induced by CO₂–brine–rock reactions. Analysis of the X-ray CT images of the half plug with a higher permeability obtained after flooding indicates significant calcite dissolution and evolution of preferential flow paths which are evolving into wormholes; these would eventually span the entire length given a sufficiently large live brine injection volume. On the contrary, the second half of the plug with a lower permeability shows relatively little reactivity with any macroscopic changes only occuring near the flooding inlet. Furthermore, NMR results of the higher permeability half core show that the dissolution is dominant in the larger pores representing a 3.5% increase in porosity. In contrast, most of the porosity increase in the lower permeability half plug can be attributed to the smaller pores, with a 1.3% overall increase in porosity measured. In both core halves, the T₂ relaxation distribution indicates the formation of smaller pores suggesting that a secondary mechanical compaction mechanism may also be occurring. Furthermore, portions of the core where dissolution occurs are indicative of preference flow of live brine as the rock surface can interact with unsaturated live brine. We believe this study improves our understanding of the dynamic rock–brine interactions occurring in field-scale CO₂ sequestration projects in heterogeneous limestone reservoirs.