It has been proposed in the literature that certain economies might result from the use of an inert cushion gas, such as flue gas, during the storage of natural gas in aquifers. Where cushion gas is used, two types of displacement occur:
- miscible displacement at the natural gas-cushion gas boundary and
- immiscible displacement at the cushion gas-water interface.
Mixing and gravitational segregation occur during miscible displacement with the result that not all of the stored natural gas can subsequently be recovered. A calculation procedure, which considers the miscible boundary only, has been developed to calculate the recovery efficiency. This is tantamount to assuming that the aquifer water contacts only cushion gas throughout the operation and has no effect on the dynamics of the mixed zone between the natural gas and cushion gas. The calculation procedure was validated by displacements in a horizontal three-dimension radial consolidated sand model utilizing miscible liquids of differing densities. Excellent agreement was found over two complete injection-production cycles. Thus validated in the laboratory, the mathematical model was applied to a hypothetical storage aquifer to investigate the effect on recovery efficiency of several of the more important variables, including aquifer permeability and thickness and the density difference between the gases. The recovery efficiency during the second cycle was greater than the efficiency obtained during the first storage cycle. The per cent of injected gas recovered during a per cent of injected gas recovered during a particular storage cycle should increase with particular storage cycle should increase with the number of years of operation of the storage project.
The underground storage of natural gas during the summer months is widely used by the natural gas industry to supplement the increased winter demand in urban areas. Furthermore, it enables pipe lines to be operated at nearly a 100 per cent load factor throughout the year. The increasing scarcity of depleted oil and gas fields has directed attention to the use of aquifers for such storage.
When aquifers are used, gas injected from the surface under sufficient pressure will desaturate the pore space and provide storage for the gas. Ultimately, the water saturation in the gas swept region may approach an irreducible minimum value.