Production of gas from waterflood residual oil: gas saturation build-up and relative permeabilities

When solution gas is released from waterflooded residual oil by depressurization, the gas becomes mobilized after the critical gas saturation has been reached, but the gas saturation continues to increase as more gas is generated. The rate of gas saturation change plus the unrecoverable gas (gas saturation remaining at the end of the depressurization) are important in determining the economic performance of the process. The values depend upon the rate of gas generation and the gas relative permeability. Experiments are reported to measure water and gas relative permeabilities when gas is released from waterflooded residual oil. The values for the gas phase were extremely low, approximately an order of magnitude lower than would be predicted using a Corey-type equation suggesting that gas relative permeabilities measured in normal gas-water displacement experiments are not applicable to the depressurization of waterflooded reservoirs. The low values of gas relative permeabilities can be explained by the fact that the filaments which form the channels for gas flow are not stable, and that the flow is intermittent. Visual observations of the flow behaviour in micromodels has e confirmed this. As a result, the effective total area for flow at any particular gas saturation is substantially reduced, thus reducing the permeability to the gas. New algorithms need to be introduced into the network models for estimating relative permeabilities in these situations before they can be used to represent the cyclical nature of filament rupture and reformation.