Rapid gas transport in the near-surface of the earth

Summary

Conventional models of gas transport in the earth centres on diffusion, convection and advection. Anecdotal and experimental evidence suggests that sometimes gas transport can be orders of magnitude faster. A model of microbubble transport with vertical velocities of the order of 1 - 1000 m/day, dependant on microfracture size, provides a mechanism for these observations. Because the force acting on a microbubble is buoyancy, the flow will be vertical with little dispersion. This presentation will review this theory and some of the tests conducted to confirm such rapid vertical velocities. Fast gas transport also has applications. For example, radon (²²²Rn, t_½ = 3.8 days) is often used for locating uranium but can only work with rapid movement of a carrier gas. Radon has also been used for fault location. Measurement of radon gas flux on the surface has also been used to locate in-situ fires in coal seams in the Hunter Valley, fires which are located at 300 – 450m depth. Whether the radon is from the source (burning coal) or picked up by the flowing gas (CO₂), the surface measurements of radon show an anomaly located vertically over the fires and which may then be used to locate such events.

The occurrence of rapid gas movement gives cause for concern over geosequestration of CO₂ as zones of high flux must be mapped and avoided.

Key words::

• microbubble
• gas
• transport
• radon
• geosequestration