Numerical Modeling of Condensate Buildup Extending to Well Drainage Region Using Effective Upstream Mobility

Development of a computationally efficient numerical scheme to model condensate banking extending to drainage region is presented. A unified near wellbore modelling approach described uses effective upstream mobility to resolve condensate banking; non-Darcy flow; and capillary number dependence of relative permeability. This work will provide an understanding on how to accurately model and improve forecast of gas and/or condensate reservoir. Potential drop in well drainage region across the grid block interfaces can be significant. This is true in particular for gas condensate reservoirs with low to moderate permeability (poor rocks), and in the vicinity of gas-condensate wells with high productivity indices consistent with pressure transient analysis. In gas condensate wells due to fracking, wells can have high productivity index, which results in relatively small drawdown. Due to poor rock connectivity in neighbouring cells, interface drawdown can be significant. Consequently, the use of conventional pseudo-pressure integral (limited to 1/5 of perforated grid block dimension) partially resolves pressure dependence of upstream mobility and/or upstream fluxes. One approach to reduce pressure dependence of upstream mobility would require the use of fine grids, i.e., (local) grid refinement. Nevertheless, grid refinement comes with significant added computational cost, this is impractical for large scale field simulations with thousands of wells. The method presented here involves identifying drainage region around wellbores, whereby to simulate condensate banking (extending to drainage region) effective upstream mobility is computed numerically. Further, in a gas-condensate system, as condensate drops out available pore spaces are reduced, and velocity of free gaseous phase increases, consequently non-Darcy flow effects and velocity dependence of relative permeability become more pronounced. Concept of pseudo-pressure integral evaluation is extended to drainage region, where non-Darcy flow effects and capillary number dependence of relative permeability both naturally fit. Drainage pseudo-pressure replaces the traditional single point upstream mobility with an integrated form that more accurately predicts condensate banking in the high drawdown regimes around the wellbore, and is independent of well and grid geometry. Numerical results are validated against fine grid results, demonstrating accuracy and consistency of the approach. Drainage pseudo-pressure method is tested on coarse meshes and demonstrated to yield numerical results comparable with those obtained on (non-uniform) grids with local refinements, and comes with much less computational expense. The new method improves accuracy of reservoir simulation, and compared to its counterpart local grid refinement method is computationally efficient. For selected test case(s), the new approach is demonstrated to be 4X faster, and this gain in computational performance is directly proportional to number of wells and/or model size.