Limited studies are available for modeling gas migration in risers. Outdated and small-scale models provide insufficient reliability, and a thorough mechanistic description of the problem is still not available. A significant part of the problem concerns understanding how pressure, temperature, liquid properties, and gas-liquid dynamics effect gas expansion during migration.
This paper provides information on Computational Fluid Dynamics (CFD) simulations performed on gas injections in three static and dynamic vertical fluid columns, with and without back pressure measuring 27-ft. and 330-ft. tall with 6, 12, 19.5 in. diameter. These CFD simulations analyzed the recorded gas expansion, change in pressure and temperature, and the volume fraction of the gas throughout the riser. In addition, these simulations also analyzed the change in flow rate, velocity, and the unloading effect at the inlet and outlet.
The 330-ft. pipe simulation demonstrated explosive unloading behavior with maximum discharge velocity and flow rate of over 2.8-ft./sec. and 6617.5-gpm., while the shorter pipes demonstrated relatively slower overflow. The case with a 330-ft. pipe also recorded a rapid change in temperature close to the top. Back pressure application at the surface minimized the effects of unloading and slowed down expansion.