A horizontal and slanted well model was developed and incorporated into BOAST, a black oil simulator, to predict 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 pressure 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. A close match having an average deviation of only ±4% was found between the analytical formula and the fully penetrating slanted well model after a production period of early radial flow. A good agreement was also obtained between the simulation results of the horizontal well model and published analytical curves. This is the first reported mathematical model that has been validated for slanted well simulation.

Production rates from vertical wells, horizontal wells, and slanted wells of variable well lengths in a field of 40-acre well spacing were compared. During the pressure-depletion phase, about the same improvements in production rates over those of vertical wells were observed for horizontal and slanted wells which extended the same length from the plan view.

Horizontal/slanted wells showed more advantage over vertical wells at lower formation permeability (≤ 1 md) than at higher permeability (≥ 100 md). The effects of eccentricity on horizontal well performance were examined. The effects of vertical permeability on the production rates of horizontal and slanted wells were also studied. As expected, the vertical discontinuity of reservoirs showed insignificant effects on the production from slanted wells.

Field simulations of a horizontal well and of multiple slanted/horizontal wells demonstrated the application of the developed well model.

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