Modeling Non-Equilibrium Mass Transfer Effects for a Gas Condensate Field

Abstract

To assess the effect of non-equilibrium mass transfer on the productivity ofa single weI1 producing horn a gas condensate fiel~ a model incorporating non-equilibrium mass transfm * was implemented into an equation-of-state @OS) compositional reservoir simulator developed at The University of Texas at Austin. A comlation from the literature was used to account for the effect of variables smh as gas velocity and difision coefficients on the mass -fer coefficient. However, no mass transfer data were available for gas condensates, so a sensitivity study on the mass transfer coefficient was performed. Several simulations have been performed to evaluate the effm of the non-equilibrium mass transfer on the flow behavior in the region near the welIbore, The results tim these runs w= comparet with those obtained under the local equilibrium assumption. Such comparisons reveal that non-equilibrium phase behavior lead to a reduction in the condensate saturation in the region near the wellbore. The mole fractions for light and heavy components in the oil phase are noticeably different. In the high veIocity layers, these ditinces become more significant. In generaI, non-equilibrium eti lead to slower reductions in well productivity due to the fict that condensate dropout was reduced near the wellbore. INTRODUCTION Most reservoir simulation studies to date assume local equilibrium between the fluid phases. For high rate gas wells, the residence time for fluids in the gridblocks near wellbore is Society of Petroleum E~eers for a Gas Condensate Pope and Mukul M. Sharma University of Texas at Austin exmcted to be of the order of seconds. Field This is unlikely to fivide sufficient time for the fluids to reach equilibrium with each other. Some laboratory experiments have supported this hypothesis (Burger et al., 1996). The purpose of this work is to assess the effa of the nonequilibrium mass transfer on the productivity of a single well producing tim a gas condensate field. To do so, we have implemented a model dealing with non-equilibrium mass transfer into the equation-of-state (EOS) compositional reservoir simulator UTCOMP, developed at The University cf Texas at Austin. This model is based upon data and correlation, published by Wilkins et al. (1995). To our knowledge, there are no data on mass transfer coefficients under high temperature and high-pressure gas condensate reservoir conditions. Therefor, we performed a sensitivity study on the mass transfer coefficient to assess the large uncertainty in it. Nghiem et a/. (1997) recently presented a simulation study cf dry gas displacing a light oil with mass transfer limitations and a constant mass Wfer coefflcien~ but no comparisons with data were made. Burger and Mohan~ (1997) studied the effect of diffision on gas displacing oil, but each gridblock was assumed to be at local equilibrium, so this is not nonequilibrium mass transfer in the local sense. Several simulations have been performed to evaluate the effof the non-equilibrium mass transfer on the flow behavior in the region near the wellbore. The dependence cf relative permeability and residual saturations on the capillary number was not used so that we could assess just this one effect, but in future simulations the combined eff~ should be included since separate studies have shown that capillary number eff~ ca be very large under these conditions. The results from these simulations we~ compared with those obtained under the local equilibrium assumption. THE UTCOMP SIMULATOR The UTCOMP simulator is a three-dimensional, EOS compositional reservoir simulator (Chang et al., 1990). The formulation follows the one by Acs et a/. (1985) with some changes. The solution scheme is analogous to IMPEC (implicit pressure and explicit composition). A higher-order finite-difference method with the total variation diminishing