Understanding Stratigraphic Heterogeneity

Abstract

The subsurface geological storage of CO 2is influenced by many variables. Stratigraphic architecture and reservoir heterogeneity primarily affect the migration pathway of CO 2. Therefore, an understanding of these parameters can assist with devising an injection strategy to maximize the efficiency of the geological storage of CO 2. An example is presented from the Kingfish field area in the offshore Gippsland Basin, southeastern Australia.

The potential injection targets are interbedded sandstones of the Paleocene–Eocene upper Latrobe Group, which are sealed regionally by the Lakes Entrance Formation. Sequence stratigraphy suggests that several packages of sandstone reservoirs exist separated by locally effective, but regionally nonextensive, intraformational seals. Seal capacity analyses indicate that the intraformational seals can retain an average CO 2column height of around 500 m (1640 ft). Thus, the interbedded siltstones and shales will behave as flow baffles and barriers that will hinder or slow vertical migration, encouraging the CO 2to migrate laterally, and create localized traps throughout the stratigraphy, which reduces the reliance on the top seal.

Numerical simulations demonstrate how these siltstone and shale baffles reduce the effective vertical permeability, thereby creating a more tortuous pathway for CO 2migration. This increased pathway length enables a greater volume of pore space to be accessed, increasing the potential for residual gas trapping and dissolution of CO 2to occur along the migration pathway, and may provide more time for geochemical reactions to occur. These effects all increase the potential CO 2storage capacity and containment security and should be considered when devising injection scenarios to optimize the CO 2geological storage process.