Gas condensate relative permeability for well calculations : Behaviour of Retrograde Gas-Condendate Mixtures in Porous Media

This paper addresses several issues related to the modeling and experimental design of relative permeabilities used for simulating gas condensate well deliverability. Based on the properties of compositional flow equations, we make use of the fact that relative permeability ratio k rg /k ro is a purely thermodynamic variable, replacing saturation, when flow is steady-state. The key relation defining steady-state flow in gas condensate wells is relative permeability k rg as a function of k rg /k ro . Consequently, determination of saturation and k r as a function of saturation is not important for this specific calculation. Once the k rg = f(k rg /k ro ) relationship is experimentally established and correlated with capillary number (Nc), accurate modeling of condensate blockage is possible. A generalized model is developed for relative permeability as the function of k rg /k ro and capillary number. This model enables us to link the 'immiscible' or 'rock' curves with 'miscible' or 'straight-line' curves by a transition function dependent on the capillary number. This model is also extended to the case of high-rate, inertial gas flow within the steady-state condensate blockage region and locally at the wellbore. We have paid particular attention to the effect of hysteresis on the relation k rg = f(k rg /k ro ), based on our observation that many repeated cycles of partial/complete imbibition and drainage occur in the near-well region during the life of a gas condensate well. Finally, the composite effect of condensate blockage is handled using a 'Muskat' pseudopressure model, where relative permeabilities are corrected for the positive effect of capillary number dependence and the negative effect of inertial high velocity flow. Special steady-state experimental procedures have been developed to measure k rg as a function of k rg /k ro and Nc. Saturations, though they can be measured, are not necessary. An approach for fitting steady-state gas condensate relative permeability data has been developed and used for modeling relative permeability curves.