Abstract Multi-lateral horizontal drilling technology has been employed since late 1996 in the mature carbonate waterflood of the Ratherford Unit, Greater Aneth Field, San Juan County, Utah, in preparation for the implementation of a CO2 Water-Alternating-Gas (WAG) tertiary recovery project, planned for initiation in 2000. The program of multi-lateral reentries of 21 existing producers and 24 existing injectors over a two-year period (late-1996 through late-1998) was comprised of 100 laterals targeting the highly-stratified layers of the Upper Desert Creek. Miscible flooding of the low-permeability Pennsylvanian Upper Desert Creek Zone I has been shown by analysis of the adjacent McElmo Creek Unit to be economic only through increased vertical/areal sweep efficiencies and processing rate enhancements obtained by the application of horizontal injector-producer pairs. The results of this study were confirmed by the 1994 - 1995 Ratherford Unit horizontal injection well drilling program, the 1996 Ratherford Unit multi-lateral production well drilling program, and the 1995 horizontal CO2 pilot pattern in the adjacent McElmo Creek Unit. This paper will review the basic geologic and engineering concepts behind the application of horizontal drilling technology within the Ratherford Unit, detail the multi-lateral techniques employed and process improvements made through time, and summarize the actual field results.