Reitbrook oil field, located in the North of Germany, has produced oil since 1937 and has an impressive production history of more than 77 years. Reitbrook field life can be divided into three main periods: Oil Production (1937-1963), Gas Blowdown (1963-1973) and Underground Gas Storage (UGS) (1973-2014).

The oil production did not stop during the Gas Storage period. On the contrary, the gas injection and withdrawal periods linked to the UGS activity allowed to enhance the oil production of this mature asset. After a brief presentation of the field, the specific mechanisms enhancing the oil production during the UGS period will be detailed and compared to the traditional gas injection technique.

Reitbrook is a shallow (top reservoir at 640 m/bsl) oil field with an initial reservoir pressure of 81 bars and a specific oil gravity of 22°API. Some uncertainty remains on the STOOIP which is in the 31-41 106 Sm3 range. Before being converted into an UGS in 1973, the field had a cumulative oil production of 2.35 106 Sm3, corresponding to a recovery factor around 6.5%. During the UGS period, circa 0.5 106 Sm3 of additional oil has been produced, increasing the recovery factor from 6.5% to 7.8%. The increase in oil production during the UGS period is linked to various physical mechanisms, both at the pore scale and at the reservoir scale. The conversion of an oil field into an UGS makes a specific EOR process which can be contemplated to extend the life of a mature oil field.

Through Reitbrook case study, the oil production mechanisms linked to the UGS activity will be presented, showing that this combination could be considered as an effective EOR technique. The conversion of a mature oilfield into an UGS while enhancing its oil production is also interesting from an economic perspective since it cumulates the value of the increase in oil recovery and the value of the gas storage activity while sharing the cost of gas compression / treatment and the benefits from the wells already in place. In addition, the implementation of such projects also allows to extend the economic life of installations and to postpone the decommissioning costs.

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