The Institute of Gas Technology's Sustaining Membershp Program is currently sponsoring research to investigate the use of inert base gas for natural gas storage fields. As part of that program, an apparatus was constructed in which to measure longitudinal dispersion of gases flowing through porous media. Whole core samples from several underground storage reservoirs were confined in a Hassler type coreholder. Two different gases (methane and nitrogen) were successively flowed through the core and the effluent concentration profile was recorded. Superficial pore velocities varied from 0.25 to 3.5 ft/hr [0.02 − 0.3 mm/s], and pore pressures were in the range of 500 − 1000 psi [3450 − 6890 kPa]. A single parameter diffusion-type equation was used to correlate experimental results. This model is used to generate longitudinal dispersion coefficients and "scale of dispersion" (dispersion coefficient divided by velocity).
The rate and degree of mixing of two gases in porous media seems to be predominantly controlled by three factors: the porous media itself, the viscosity ratio of the two gases, and the superficial pore velocity of the gases. Specifically:
Mixing parameters were found to vary widely with different porous media, but do not see to be directly related to permeability, porosity, or tortuosity.
In most cases, the values of dispersion coefficient and mixing length for an unfavorable viscosity ratio were greater than those for the favorable case. The magnitude of the difference between the two cases varies with different porous media.
Dispersion coefficient increases with increasing velocity. This dependence may be linear for favorable viscosity ratios and some porous media. However, it is often not linear and needs to be measured for each porous media and set of gases.