Most field applications and pilot tests of CO2 injection are for miscible drive, and are therefore confined to light oil reservoirs. However, it has been known for decades that as CO2 dissolves in heavy oil, it swells the oil and reduces its viscosity. Core displacement tests in the fifties indicated that substantial additional recovery of heavy oil, and in some cases light oil, was possible with carbonated water. Until recently, unfavorable economics discouraged field application, and yet in 1976 a field project was commenced in southern Arkansas.

Two techniques have been field-tested. These are "huff-and-puff" CO2 injection for stimulation of the producing wells and either alternate or simultaneous injection of CO2 and water.

Both process mechanisms are discussed and are illustrated, where possible, by the results of field tests in the United States. These have been in sandstone reservoirs containing light oils and heavy oils. Several other pilot or field projects are in the planning stage. A planned field demonstration project of both processes, in different test areas of a fractured limestone reservoir (oil 9° - 15°) in Turkey, is summarized.

The possible range of candidate reservoirs is discussed, and general economic comparison with steam injection is given. The obvious candidates are heavy oil reservoirs that are too deep for current steam injection technology and economics.

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