This paper was prepared for the Oilfield Chemistry Symposium of the Society of Petroleum Engineers of AIME, to be held in Denver, Colo., May 24–25, 1973. Permission to copy is restricted to an abstract of not more than 300 words. Illustrations may not be copied. The abstract should contain conspicuous acknowledgment of where and by whom the paper is presented. Publication elsewhere after publication in the JOURNAL paper is presented. Publication elsewhere after publication in the JOURNAL OF PETROLEUM TECHNOLOGY or the SOCIETY OF PETROLEUM ENGINEERS JOURNAL is usually granted upon requested to the Editor of the appropriate journal, provided agreement to give proper credit is made. provided agreement to give proper credit is made. Discussion of this paper is invited. Three copies of any discussion should be sent to the Society of Petroleum Engineers office. Such discussions may be presented at the above meeting and, with the paper, may be considered for publication in one of the two SPE magazines. Abstract Construction of an oleotropic, or oil seeking, post primary fluid is described. Multiple viscosity levels are possible for a polymer solution depending upon the degree of crosslinking of the polymer. Cross-linking is dependent upon the presence of a highly oil-soluble, barely water-soluble "trigger." Extraction of the "trigger" at the oil-water interface effects viscosity reversal. Introduction Two limitations have generally limited the use of fluids designed to effect the post primary recovery of oil. one of these is economic, the other technical. then the product to be recovered can be sold for very low price, the product injected to price, the product injected to replace it must be purchased for markedly less. This constraint has all too frequently been ignored. Techniques for recovering oil, for example, miscible displacement and surfactant recovery methods, have been mainly concerned with reducing oil saturation in the regions of the reservoir traversed by the injected fluid. Most petroleum engineers would agree that at the termination of a water flood, most of the oil remaining in the reservoir is in those portions of the formation not traversed by water. The primary function of a post primary recovery fluid ought to be the complete traverse of the entire oil-bearing region. Reduction of oil saturation within that region must be considered a secondary objective. SYSTEM DESIGN AND CONSTRUCTION The goal of the research effort to be described was the creation of a self-conforming post primary fluid which was to cost not more than 25 cents per barrel. The fluid was planned to per barrel. The fluid was planned to demonstrate a variable tendency to flow in different portions of the formation.