Permeability of wormholes created by CO2-acidized water flow through stressed carbonate rocks

Abstract

Sequestration of CO2 relies on the storage capabilities of the deep geologic setting throughout the lifetime of the storage activity. Preferred storage horizons are largely composed of sandstone, which is considered to be chemically inert to the injected CO2. Carbonate rocks and carbonate zones existing as seams or lenses in sandstone formations are, however, prone to chemical alteration during reactive flows of CO2-acidized water that can be created by the mixing of the injected CO2 with either fresh or saline water present in a storage horizon. Reactive flows can erode the fabric of carbonate rocks leading to the creation of high permeability pathways that are referred to as wormholes. The paper first examines the generation of wormholes in cylindrical samples of calcium carbonate-rich Indiana Limestone that are subjected to geostatic stress states representative of deep sequestration sites. The leakage potential of the wormhole is examined by appeal to computational fluid dynamics simulations of Stokes’ flow in wormhole features and an elementary approach involving Stokes’ flow-based hydraulic diameter concept in cylindrical pathways with deviating segments, representing the passages for flow in a wormhole.