A horizontal and slanted well model was developed and incorporated into BOAST, a black oil simulator, to predict the potential production rates for such wells. The slanted/horizontal well model can be used to calculate the productivity index, based on the length and location of the wellbore within the block, for each reservoir grid block penetrated by the slanted/horizontal wellbore. The well model can be run under either pressures or rate constraints in which wellbore pressures can be calculated as an option of infinite-conductivity. The model is easy to use and can simulate the performance of multiple horizontal/slanted wells in any geometric combination within reservoirs. The model was checked against the analytical formulas of transient wellbore pressure in an infinite slab reservoir. Production rates from vertical wells, horizontal wells, and slanted wells of variable well lengths in a field of 40-acre well spacing were also compared. A field simulation of a horizontal well and its three offset conventional vertical wells demonstrated the application of the developed well model. 31 refs., 18 figs., 15 tabs.