Decline Curve Analysis for Low Permeability Gas Condensate Reservoirs: Effect of Fluid Richness, Inertia and Coupling

Decline curve analysis has been proposed as an efficient approach for estimating reservoir and well parameters especially in unconventional gas reservoirs where the use of conventional pressure-transient analysis is often technically and economically challenging. This study investigates the applicability of the widely accepted Fetkovich type curves to low permeability gas-condensate reservoirs under various operating conditions. A synthetic reservoir model based on one of the Fetkovich et al. (1987) case histories was first constructed and validated. Then the impacts of condensate gas richness, different rock types, relative permeability (kr) including coupling or capillary number (increase in kr as flow velocity increases and/or interfacial tension decreases) and inertia (decrease in kr as flow velocity increases) on estimated parameters were examined. The results show that the quality of match (and parameter estimates) between the generated decline curve and type curves is dependent on the condensate saturation level around the wellbore and the nature of the kr curves. In those cases where non-Darcy effects are strong enough to affect the results, use of Fetkovich type curves in the presence of inertia (coupling) results in pessimistic (optimistic) estimates of permeability, and higher (lower) skin estimates than the non-rate-dependent values input into the simulations.