A method of storing CO2 in a porous and permeable hydrocarbon reservoir having at least one injection well and at least one production well penetrating said reservoir, which method comprises the steps of: (a) recovering a produced fluid stream comprising produced hydrocarbons, produced water, and produced CO2 from the production well; (b) passing the produced fluid stream to a production facility where a produced vapour stream comprising carbon dioxide and volatile hydrocarbons is separated from the produced fluid stream; (c) compressing the produced vapour stream to above the cricondenbar for the composition of the produced vapour stream; (d) cooling the compressed stream thereby forming a cooled stream that is in a dense phase state; (e) importing a CO2 stream to an injection facility wherein the imported CO2 is either in a liquid state or a supercritical state; (f) mixing the cooled stream from step (d) with the imported CO2 stream thereby forming a co-injection stream; and (g) injecting the co-injection stream into the hydrocarbon bearing reservoir from said injection well.