Reservoir-tailored CO.sub.2 -aided oil recovery process

A process for formulating and injecting a CO.sub.2 -containing fluid for displacing oil within an oil and brine-containing subterranean reservoir comprising:

• determining the reservoir temperature and properties of the oil and brine contained within the reservoir;
• selecting a surfactant which both consists essentially of at least one polyalkoxyalcoholic surfactant and is capable of inducing the formation of substantially homogeneous dispersions of each of the specified types when said surfactant is mixed with the specified fluids at the reservoir temperature and a pressure of at least about 2000 psi at which CO.sub.2 is substantially liquid at the reservoir temperature;
• said types of dispersions being (1) CO.sub.2 dispersed in a solution of surfactant dissolved in brine having a salinity substantially equivalent to that of the brine in the reservoir, when the volumes of the CO.sub.2 and aqueous liquid are substantially equal and the mixture is substantially free of oil, (2) CO.sub.2 and oil having properties at least substantially equivalent to those of the reservoir oil dispersed in a solution of the same surfactant in brine where the volume of oil is smaller than the total volume of CO.sub.2 and brine, and (3) CO.sub.2 and a solution of the same surfactant in brine dispersed in the same oil where the volume of oil exceeds the total volume of CO.sub.2 and brine;
• and
• injecting into the reservoir, at a pressure greater than about 2000 psi at which the CO.sub.2 is substantially liquid at the reservoir temperature, CO.sub.2, the selected surfactant, and a brine having a salinity at least substantially equivalent to that of the brine in the reservoir, in a sequence and proportion such that before, or soon after, entering the reservoir (a) the CO.sub.2 is initially dispersed in a solution of the surfactant in brine, (b) advancing portions of the initial dispersion are converted to a dispersion of CO.sub.2 and the reservoir oil in the solution of surfactant in brine, and (c) advancing portions of the latter dispersion are converted to a dispersion of CO.sub.2 and a solution of surfactant in brine in the reservoir oil.