Method for improving sweep efficiency in co.sub.2 oil recovery

Abstract

Volumetric sweep efficiency and oil recovery by CO.sub.2 flooding processes may be increased by injecting a surfactant solution into the formation which will form a gel in-situ in the high permeability zones via interaction of the surfactant with formation brine, oil, and CO.sub.2 under formation conditions. Thereafter, improved oil recovery efficiency is realized in displacing oil from the lesser permeability zones. The surfactant solution is selected from phase behavior experiments which show gel precipitation at conditions of temperature, salinity, oil composition, and CO.sub.2 pressure which are expected to exist or may be practically established in the particular oil-containing formation. Preferably, the gel is precipitated at CO.sub.2 pressures above the minimum miscibility pressure for CO.sub.2 miscible flooding, and below the prevailing formation pressure during the CO.sub.2 flooding. The surfactant solution may be injected into the reservoir as an aqueous solution or as a microemulsion solution prepared in surface mixing facilities. The surfactant solution may also be injected prior to injection of CO.sub.2, or may be alternately injected in WAG cycles following CO.sub.2 breakthrough into production wells.