Assessing Conditions for CO2 migration through a fractured shale, inspired by Sleipner

Carbon capture and sequestration (CCS) is one of the key geo-energy solutions essential to mitigate the acceleration of climate change. Presence of a secure caprock formation that serves as a seal for injected CO₂ – such as a low permeability shale – is vital for ensuring safe long-term storage of CO₂. The effectiveness of a seal is controlled by its petrophysical properties which can change due to compaction, diagenesis and the in-situ stress field. The geological history and the changing stress field could reduce the caprock’s effectiveness as a flow barrier, by enabling localized migration through the primary seal, leakage along faults and fractures, or diffusive flow through the rock system. However, the relative effect of associated processes on CO₂ migration is difficult to investigate individually due to the complexity of the interactions that may require modelling of coupled processes. Faults and fractures are common in geological formations and may act as conduits for flow. Over the course of injection and storage, reactivation or closure of pre-existing discontinuities or injection-induced microfractures may occur.

In this paper, we assess the conditions for CO₂ migration through a shale layer by investigating end-member scenarios to improve the accuracy of simulations of the complex physical and chemical interactions involved. A reservoir-caprock system model, based on the 2019 Sleipner Benchmark dataset, was utilized with implementation of regionally determined petrophysical parameters to evaluate the effects of different conditions for CO₂ migration. We generated two-phase flow models with inclusion of the effects of capillary pressure and relative permeability appropriate for the Utsira sand and intra-reservoir shales. By using PFLOTRAN, we have simulated a hypothetical fault with high resolution gridding to evaluate the criteria which are likely to control flow through a faulted caprock.

The results demonstrate how a CO₂ plume with realistic buoyancy pressure only migrates along fracture zones of relatively high effective permeability (affected by fracture width), while lower permeability fractures (1 mD and below) act as a capillary barrier to two phase flow. Comparison with the latest insights into the actual migration routes at Sleipner, based on Full-Waveform Imaging, allows us to infer the outer bounds for properties of faults that have acted as CO₂ migration pathways at Sleipner.