Evaluation of roof integrity above a CO2 storage site in a coal mine

Sorptive CO₂ storage on mining wastes succeeded by their injection into gob areas can contribute to world-wide greenhouse gas control. CO₂ migration into mine drifts and further workings can be controlled by the installation of sealing systems, such as road side packs, at the level of the storage sites. Additionally, the rising of hydrostatic pressure resulting from the mine flooding after its abandonment increases the CO₂ storage security by strengthening the sorptive linkage. However, CO₂ migrating into the roof above such storage sites preceding the mine flooding can represent a potential leakage path and therewith a storage risk.
A numerical analyis was performed using the MUFTE_UG software package to quantify the CO₂ migration through the roof above such a storage site. Based on the two-phase three-component model (CH₄/air/water) developed by Dogan (2004) and the two-phase two-component model (CO₂/water) implemented by Bielinski (2006) a new two-phase three-component model involving the components water, CO₂ and air was set up. This adapted model can be applied for the analysis of CO₂ migration through cap rock formations saturated with a mixture of the components air and water, such as the roof above a gob area of a longwall working.
Benchmarks involving CO₂ sorption using the two-phase two-component model (CO₂/water) were carried out to verify the numerical results of the adapted model (CO₂/water). Both models show identical results regarding flux and mass balance within their boundaries.
Furthermore, a sensitivity analysis of the roof integrity problem was conducted taking into account variations of gas and water saturations, absolute and relative permeabilities, porosity and initial pressure conditions of the gaseous phase.