Aquifer gas storage

An aquifer is an underground body of rock, sand, or gravel that holds groundwater. In some areas natural aquifers have been converted to natural gas storage reservoirs. An aquifer is suitable for gas storage if the water-bearing sedimentary rock formation is overlaid with an impermeable cap rock. Aquifers are only appropriate for gas storage use if the formation is not connected to an aquifer used for producing water. 

Although the geology of aquifers is similar to depleted gas reservoirs used for storage, aquifers usually require more cushion gas and allow less flexibility in injecting and withdrawing of gas than depleted reservoirs. Deliverability rates may be enhanced by the presence of an active water drive, which supports the reservoir pressure. Because of the need to fill aquifer storage slowly while pushing the water back, it is most commonly operated with one injection and withdrawal cycle per year.