Enhanced oil recovery methods are usually used to increase production of depleted fields. High pressure CO2 is pumped into the ground at the periphery of the field and diffuses through the formation to drive residual oil towards the oil wells. Large quantities of CO2 are required in the initial stages of the process. In time the CO2 arrives at the well head and the concentration of CO2 in the gas associated with the well will increase to values as high as 70% to 90%. The objective of the gas separation is to recover the natural gas and also the CO2. The CO2 is recompressed and re-used in the process. The variable composition of CO2 in the gas makes membrane separation more attractive than amine absorption, especially when high CO2 concentrations are encountered. The operation of the system requires optimization of the permeate pressure - high permeate pressure minimizes recompression costs but decreases membrane efficiency