Marathon Oil Company

James L Bowzer

Eugene E Wadleigh

Douglas E Kenyon

This invention relates to the recovery of oil from an oil-bearing formation having a natural fracture network with substantial vertical communication and wherein gravity drainage is the primary means of recovery. A downwardly inflating gas-cap is pressured up with a chase gas having a density less than that of CO.sub.2. CO.sub.2 is injected and a CO.sub.2 -rich displacing slug is formed at the gas-liquid hydrocarbon contact. The chase gas is injected to facilitate displacing downwardly the CO.sub.2 -rich displacing slug to recover hydrocarbon from the reservoir. CO.sub.2 is replaced in the displacing slug as the CO.sub.2 is solubilized into the oil, including matrix oil, to facilitate recovery thereof. The oil is recovered through production wells in fluid communication with the reservoir, preferably the inlet to the well is below the water-liquid hydrocarbon contact at such a level to prevent free-gas production. The chase gas has a density less than that of the CO.sub.2 and is comprised mostly of nitrogen; however, it can contain other gases such as methane, ethane, CO.sub.2, and miscellaneous gases. The chase gas is injected at a rate to minimize mixing of the chase gas with the CO.sub.2 and to facilitate gravity segregation of the CO.sub.2 from the chase gas. The CO.sub.2 in the CO.sub.2 -rich displacing slug can be replenished by incorporating CO.sub.2 into the chase gas and permitting the CO.sub.2 to gravity segregate downwardly while the less dense gases move upwardly.

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Houston, TX, United States of America

James L Bowzer