ENHANCED OIL RECOVERY BY CO_2 FLOODING

摘要

CO_2 injection in Hungary has achieved a wide range of field application based on natural CO_2 resources. Carbon dioxide injection has been applied in different types of reservoirs: e.g. sandstone-, karstic-, and metamorphic. The flooding technology depends on oil properties and reservoir parameters and it is based on laboratory measurements, pilot tests and practical results.The technology in the case of sandstone reservoirs: the pressure of the reservoirs exploited by primary and/or secondary methods is increased by CO_2 gas injection which is about 20% above the hydrostatic pressure. During the injection moderate production takes place for the sake of better flooding. Afterwards carbon dioxide and water are injected periodically when the gas-water ratio is 1 :1 under reservoir conditions. The amount of CO_2 is 0.2 Vp (referring to the flooded pore volume), and the total amount together with the gas during the pressure increase period is about 0.5 Vp. At the end of WAG the amount of injected water is 0.5 Vp. It is followed by reservoir depletion. The process is immiscible. The oil is light, it has low viscosity and it is saturated. Flooding pressure is 100-200 bar. The additional recovery factor is 5-9%.The technology in the case of karstic reservoirs: the limestone-dolomite reservoirs were produced with active karstic water drive at the original reservoir pressure. The water cut was 97-98% when CO_2 injection was started. During the establishment of gas cap an oil belt is formed (due to gravity segregation) under the gas-liquid contact, which was displaced downward. A part of this oil can be recovered through the production wells until gas breaks through. During the blowdown of the gas cap and after it the oil moves upwards, and it can be recovered due to the karstic water drive. The oil is heavy, it has moderate viscosity and is completely under-saturated. Flooding pressure is 200 bar. The process is immiscible. The additional recovery factor is 7-13%.The technology is the case of a metamorphite reservoir: the reservoir is a massive type (brecciazed metamorp-hites with lime and limestone). The CO_2 is injected at the top of the structure after natural waterflooding. The quantity of injected carbon dioxide is 0.37 Vp. During the injection, the production goes as normal. When the CO_2 gas breaks through, gas injection is ceased and produced back through the injection wells. The process is miscible. The oil is light, it has low viscosity, and is moderately undersatured. The expected additional recovery factor is about 5%-6%.