Effect of CO2 sorption-induced anisotropic swelling on caprock sealing efficiency

Abstract

Caprock sealing integrity is a key issue to CO2 sequestration in a saline aquifer over a long period. Caprock as a sealing layer is defined as water-saturated formation with a sufficient capillary entry pressure to prevent the upward migration of a buoyant fluid. Most caprocks are naturally anisotropic, hence the effect of CO2 sorption-induced anisotropic swelling may heavily impact their sealing efficiency.This paper proposes a numerical model based on a conceptual model for the investigation of the sealing efficiency of anisotropic caprocks, where caprock is a composite body of fracture network and shale matrix. Two-phase flow of brine water and CO2 is observed only in the fracture network but the CO2 in the fractures further diffuses into shale matrix through a much slower diffusion process and makes the shale matrix anisotropically swell or shrink, thus significantly altering the directional porosity and permeability of the fracture network. This numerical model is verified by a storage reservoir and applied to a caprock layer to explore the mechanism for self-enhancement or self-limiting in the CO2–brine mixing zone if anisotropic swelling is considered. These examples demonstrate that this model is able to numerically simulate the CO2 storage relevant geological systems within anisotropic shale. The sorption-induced anisotropic swelling of shale matrix has significant impacts on the caprock sealing efficiency. This work provides an alternative tool to enrich the numerical modeling for the assessment of CO2 caprock sealing efficiency in natural shale caprocks.