Decline curve analysis for two-phase flow in tight gas condensate reservoirs

Traditionally, decline curve analysis (DCA) has been used by reservoir engineers to estimate hydrocarbon-in-place and expected ultimate recovery. Most modern DCA type curve techniques, which make it possible to determine reservoir and well productivity parameters such as permeability and skin, were originally developed for the analysis of single-phase flow. The presence of two-phase conditions in a gas condensate reservoir (GCR) operating below dew point pressure introduces severe non-linearities in the diffusivity equation, which make the use of single-phase techniques debatable. Effective linearization requires the use of pseudo-pressure and pseudo-time functions, which account for the multi-phase conditions. In this study, synthetic production data, generated using a fine-grid GCR simulation model, with a permeability of 0.1md, was analysed with the widely-used Blasingame type curves. An “equivalent phase” approach is introduced in the calculation of the pseudo-variables for two-phase conditions. The results demonstrate that when DCA type-curve techniques are used in GCRs where operating conditions result in significant condensate saturations (more than 15%, as observed in this study), the use of two-phase pseudo-pressures and an equivalent-phase-based material balance pseudo-time help to improve reservoir and well productivity parameter estimations.