Residual oil

Residual oil is oil found in low concentrations naturally or in exhausted oil fields. Often mixed with water, it cannot be recovered by conventional techniques. However, part of it can be recovered using carbon dioxide-enhanced oil recovery (CO2 EOR) which involves injecting carbon dioxide into the well reducing viscosity and enhancing flow of the oil. The technique is not new but has not been used extensively on residual oil zones, low-grade deposits of petroleum such as the 38 square miles in the Permian Basin of Texas leased by Elliott Roosevelt Jr. The technique is limited by availability of carbon dioxide. It does not result in sequestration of the carbon dioxide but the gas is recycled as much as possible. United States reserves of residual oil are estimated to be 100 billion barrels.