Well Productivity of Gas Condensate Reservoirs

The objective of this work was to provide more accurate models for predicting well and reservoir performance for gas condensate reservoirs by investigating near-wellbore effects and to explore ways of improving well productivity. Specifically, the effects of non-Darcy flow, changes in relative permeability due to interfacial tension, gravity and flow rate (modeled based on capillary number) and heterogeneity on well productivity were investigated. These effects are shown to be important factors affecting the productivity index (PI) of gas condensate wells. Remediation strategies for improving the PI of a well impaired by condensate dropout using injection of dry gas were also investigated. An analytical method for calculating an effective non-Darcy flow coefficient for a heterogeneous formation is presented. The method presented here can be used to calculate an effective non-Darcy flow coefficient for large gridblocks in reservoir simulators. Based on this method, it is shown that the non-Darcy flow coefficient for a heterogeneous formation is larger than the non-Darcy flow coefficient for an equivalent homogeneous formation. This result was confirmed using numerical simulations with fine grids. Sensitivity studies of non-Darcy flow effects on the productivity of a single gas condensate well indicate that when only non-Darcy effects are considered, the condensate bank near the well can cause an order of magnitude reduction in the PI. Hence, immediate remediation steps might be necessary for formations with a high non-Darcy flow coefficient. The reduction in PI with increasing non-Darcy flow coefficient is non-linear. Careful comparisons of relative permeability data at various capillary numbers with a capillary trapping model show that these data can be matched with a simple two-parameter model. A compositional simulation study of a single well with a capillary number included was done. The relative permeability improvement obtained at high capillary numbers counteracts the PI reduction due to non-Darcy flow effects. This effect reduces the drop in PI due to condensate buildup and makes the drop in PI more gradual. Hence, both effects should be considered while studying the production performance of a gas condensate reservoir. The capillary number effect is often found to completely overshadow the two-phase non-Darcy effects. A case study of a lean-gas well similar to those in the Arun field was considered and a history match of the production data was performed. Simulation results are presented which clearly show that accurate prediction of PI based on laboratory measurements is possible using both non-Darcy and capillary number effects. The effect of heterogeneity was demonstrated using simulations of various stochastically generated permeability fields.