A compositional equation-of-state simulator has been used to make comparisons of CO2 process performance with several combinations of horizontal and vertical wells and various alternative reservoir descriptions. This is the first time that the performance of CO2 flooding using horizontal wells has been reported. We have used the reservoir and fluid properties of an actual West Texas carbonate reservoir that is currently being waterflooded. The phase behavior and properties of the CO2 used in these simulations are typical of those of multiple-contact-miscible field conditions of West Texas. A layered-type reservoir description was initially used to investigate the effect of water-alternating-gas (WAG) ratio with various combinations of well types. We then investigated the effect of vertical permeability, length, and position of the horizontal injector using the same reservoir description and a horizontal injector-vertical producer combination. Similar investigations using an unconditioned stochastic permeability field having a Dykstra Parsons coefficient (VDP) of 0.81 were made. Our final three-phase flow simulation was performed using a stochastic permeability field conditioned with core data. Since three nonaqueous phases can occur at low temperatures typical of West Texas floods resulting in up to four-phase flow with mobile water, we investigated the impact of four-phase flow in one of these simulations. This is the first time that three-dimensional, four-phase flow reservoir simulations have been reported with either vertical or horizontal wells. Introduction During the last five years the petroleum industry has experienced a rapid increase in the number of horizontal wells being drilled and completed, primarily because of the following reasons. First, recent advances in drilling technology, which have resulted in substantially reduced drilling and completion costs, have made the drilling of horizontal wells an economically viable alternative. A large U.S. independent has reported an average cost of about $1.25 million per well (average measured depth of 13,400 ft), which is comparable to the average cost of a vertical well. The second important factor that has contributed to the recent upsurge in horizontal well activity is because horizontal wells may be able to deliver two to ten times the performance of conventional wells due to their larger surface area. Therefore, the higher productivity coupled with favorable drilling and completion costs appears to have considerably increased the potential of horizontal wells to produce more oil. Finally, horizontal wells offer solutions to the problem of producing oil or gas in reservoirs where conventional technology either fails or is uneconomic, for example,reservoirs where conventional wells have low productivity,reservoirs with vertical fractures,oil reservoirs where recovery is limited by water or gas coning, andthick continuous heavy oil and bitumen reservoirs where steam-assisted gravity drainage is practical. The current target of horizontal wells is mainly primary oil, although there do exist some applications of horizontal wells in the recovery of heavy oil by steam injection. Because U.S. domestic oil reserves are declining, and the chances of discovering large fields are diminishing, various forms of enhanced oil recovery (EOR) processes, for example, carbon dioxide (CO2) miscible floods. have recently gained considerable attention as a means of increasing the reserves base. Because of the better sweep efficiencies and higher injectivities possible with horizontal wells, all EOR methods should benefit by their use. P. 753^