Effective Gas Migrations in Low Permeability Shales: Application to CO2 Storage Sites
Large amounts of carbon dioxide are envisaged to be stored in depleted oil and gas reservoirs, deep saline aquifers or coal seams as a solution for reducing greenhouse gas emissions. Secure containment from natural overlying impermeable cap-rocks is thus a key element.
This paper discusses experiments of gas migration in water saturated compacted powders and rock plugs with specific mineralogies, porosities and permeabilities. Natural shales were selected from clayey cap-rocks and reservoir transition zones from a petroleum field where CO2 has been injected for EOR purposes for many years. The dissolved gases, here carbon dioxide, oxygen and noble gases migrated across mineral assemblages like kaolinites, smectites, carbonates and sandstones. Gas aliquotes were collected regularly for the production assessment. Samples were characterized by X-Ray diffraction, MEB image analysis, low field NMR for pore areas and connectivity and porosity assessment, and X ray scanner for fluid saturation. Permeabilities were measured by conventional and unconventional methods.
Results provide key elements for the understanding of the reactivity of gas molecules through diffusive flows which, no doubts in our experiments, include other processes such as mineral interactions, precipitation or dissolution of carbonates and adsorption onto mineral surfaces. Effective diffusions for each gas species in water at given temperature and pressure conditions were then modelled.
If carbon dioxide is always found to migrate fastest as a product of solubility and diffusion as compared to helium or neon, the latter do not easily retain in the pore network. This is because CO2 is more reactive towards some minerals, carbonates for example, than to others. We observed that in a pure smectite at comparable porosity range or in a natural heterogeneous but dominated clay mineral, the CO2 hardly traverses the rock, while helium expectedly migrates with lower interactions and was in agreement to the tortuosity electrical measurement. On the other hand, results showed that heavier molecular weight noble gases had non monotonous flow patterns and were subjected to adsorptive mechanisms, due probably to their sizes (Krypton and Xenon).
The objective of this research is to develop monitoring tools for detecting leaks out of an injection site by using noble gases and CO2 ratios which would highlight and quantify anomalous content of gases in aquifers or overlying sediments.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009