Modeling Non-Equilibrium Dynamics of Condensate Recovery for Mature Gas-Condensate Fields

Abstract

Abstract Theoretical and experimental studies, as well as actual data of gas-condensate fields development, indicate non-equilibrium phase behavior at low reservoir pressures. However, matching of pVT-models and prediction of condensate (group C5+) recovery dynamics are performed on the basis of equilibrium constant volume depletion (CVD) experiments and simulations. This results in unreasonably high expected values of the condensate recovery factor and in selection of unappropriate technological regimes for reservoir development. To simulate properly the actual dynamics of C5+ content in reservoir gas during revaporization (regular evaporation) at low pressures, one should consider non-equilibrium phase behavior of hydrocarbon mixtures. In this study, a numerical algorithm for simulation of the non-equilibrium constant volume depletion (NCVD) process is developed on the basis of the previously proposed model for non-equilibrium phase behavior and the non-equilibrium flash algorithm for calculation of phase fractions and compositions. The NCVD algorithm and two pVT-models of reservoir hydrocarbons are used to simulate the dynamics of condensate recovery at Vuktylskoye field. A special procedure is used to match the pVT-models to lab and field data. During the development of Vuktylskoye field, reservoir pressure has decreased from 36.6 MPa to 2.5 MPa, with the dew-point at 32.3 MPa. Non-equilibrium phase behavior is essential for gas-condensate reservoirs at pressures below the maximum condensation pressure. For the case considered, non-equilibrium effects are expressed in significant deviation of the actual curve of C5+-content in reservoir gas from the results of equilibrium experiments for pressures less than 5 MPa. The NCVD algorithm and the two pVT-models are used to simulate the dynamics of C5+-content in the reservoir gas at low pressures. A satisfactory match to the actual data is achieved for both the models by adjustment of characteristic relaxation time of the gas-condensate reservoir system to the value of about 30 years. This value, considering rescaling, is in good agreement with the experimental calorimetry data for non-equilibrium phase behavior. Equal values of characteristic relaxation time for both the pVT-models certify the possibility of physical simulation of nonequilibrium condensate revaporization by the NCVD algorithm regardless of specifics of a pVT-model previously matched to equilibrium experimental data.