The Institute of Gas Technology's Sustaining Membership 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 seem 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.
During natural gas storage operations, a large fraction (typically, about half) of the gas in a given reservoir is base gas. This base gas remains in storage to insure deliverability at the end of the withdrawal season and is not normally cycled. When a storage field is abandoned, much of the base gas is not recoverable.
Recent increases in gas prices have resulted in base gas costs becoming a major cost item for companies that develop new storage fields. The Institute of Gas Technology's research program will examine the technical feasibility of using less expensive, inert gas as a portion of the base gas required. Replacement of base gas in existing storage fields may also prove feasible. Additionally, inert gases may be used when fields are abandoned to "sweep" more valuable natural gas prior to pressure reductions during abandonment.
In order to minimize risk and maximize potential savings, the degree to which injected inert gas mixes with natural gas in a storage field needs to be understood. Mixing has been found to be controlled by several factors, including: molecular diffusion, pore geometry, turbulence, stagnant fraction of pore space, presence of an immobile fluid, viscous fingering, adsorption/desorption, and gravity segregation. The cumulative effect of all such factors may be referred to as dispersion. Longitudinal dispersion, or dispersion in the direction of flow, is often described by a single-parameter diffusion-type equation or a three-parameter capacitance model.