The technical performances of horizontal and vertical wells were examined for a tertiary, carbon dioxide miscible flood in a 240-acre (97 ha) area of a west Texas field using a black-oil, pseudo-miscible simulator. Although a 40 acre (16 ha), inverted five-spot pattern was used initially for both vertical and horizontal wells, the spacing was increased to 80 acres (32 ha) for the horizontal wells to maintain the miscibility pressure in the reservoir. Horizontal injection and production wells, 1,320 feet (402 m) in length, completed at the bottom of the formation, recovered 14 to 22 percent more oil than vertical wells. Economic analyses for the horizontal injectors and producers were compared to economic analyses conducted for vertical injectors and producers. Project economics were significantly effected by the capital expense for drilling new wells. The vertical wells provided the better rate of return if no new drilling- or only one new well for every six patterns was required. If horizontal wells could be drilled from existing vertical wells- or from the surface at 1.5 times the cost of vertical wells, the rate of return was comparable- or better than vertical wells requiring one new well for every three patterns. Horizontal wells drilled from the surface at twice the cost of vertical wells provided the lowest rate of return.