This paper focuses on an important problem of gas storage projects; the mixing of the cushion gas and the storage gas. Due to mixing, the reproduced gas may not meet customer specifications or valuable gas becomes unrecoverable. In this work we address the effect of shales on the mixing process.

The shales in the gas storage site belong to the 200 m thick Rotliegend sandstone and occur at the base of aeolian facies sequences or are associated with fluviatile intervals between the aeolian dune facies. The horizontal continuity of the shale is strongly facies dependent.

The objective of this paper is to investigate field scale dispersion during cyclic injection and production. Based on shale size distribution we calculate the transfer of gas through shale breaks. This results in an effective dispersion depending on permeability contrast, injected volume and number of cycles.

We observed that shales smaller than the characteristic displacement length contribute to the effective dispersion, while the larger shales do not affect the mixing.

For narrow shale size distributions, the effective mixing is very sensitive to small changes in injection or production volume.

We conclude that shales play an important role in field scale dispersion, and could account for the high dispersivities measured in field tests.

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