Measurement of Gas Condensate, Near-Critical and Volatile Oil Densities and Viscosities at Reservoir Conditions

Abstract

Abstract Gas condensate liquid dropout can occur in gas reservoirs, especially near the wellbore when the pressure falls below the dew point. In near-critical and highly volatile oils, extreme phase behavior effects can occur, especially near the critical temperature. Liquid dropout phenomena in these fluids affect well productivities. To simulate the liquid dropout phenomenon, as well as the flow of fluids in these reservoirs, viscosity values of reservoir fluids at high pressures and temperatures are required. Viscosities of gas condensate and near-critical fluids at elevated pressures and temperatures are not measured routinely due to inherent problems associated with their capture, measurement difficulties, and considerable time and effort required. Therefore, it is frequently not possible to obtain measured values, and viscosity values must be estimated from correlations. Empirical correlations have been published for determining viscosity as a function of pressure, temperature, gas-liquid ratio, and gas composition. Their accuracy for gas condensates and near-critical fluids has not been evaluated due to lack of data. This paper presents some new and unique viscosity data for gas condensate and near-critical systems at elevated pressures and temperatures. A new, state-of-the-art pressure-volume-temperature, PVT, apparatus was designed and commissioned to measure the densities and viscosities of Saudi Arabian gas condensate, near-critical and highly volatile oils at reservoir conditions. The data include viscosity and density values above and below the saturation pressure. The data were used to evaluate existing correlations that are typically used for predicting viscosity in PVT software. Introduction Viscosity is an important fluid property and is required in reservoir simulation and engineering calculations. In a recent paper1 it was shown that a 1% error in reservoir fluid viscosity resulted in a 1% error in cumulative production. This can be substantial for large reservoirs. In rich and wet gas reservoirs, near-critical and highly volatile oils, large amounts of hydrocarbon liquids can condense in the near wellbore region. This phenomenon is called liquid banking and can result in moderate to severe productivity declines2–3. To simulate the effect of banking in numerical reservoir simulation studies, accurate values of liquid viscosities are required. The viscosity of liquid condensates at any pressure and temperature is difficult to measure experimentally and are generally estimated through correlations. The accuracy of these correlations in predicting the viscosity, especially for gas condensate is uncertain due to lack of measured viscosities at high pressures and high temperatures (HPHT). Inaccurate viscosity values can have a detrimental effect on reservoir simulation results, leading to errors in reserves and cumulative production. The common assumption in reservoir simulations, that the accuracy of fluid properties has an insignificant effect on reservoir performance, is inaccurate. Uncertainties in reservoir fluid properties, particularly viscosity, can lead to large errors in reservoir performance and can influence the economics of reservoir exploitation. At the beginning of this project, a concerted effort was made to investigate the availability of experimental viscosity data for gas condensates, near-critical and highly volatile oils at reservoir conditions. To our knowledge, there are no reported data for gas condensate liquid viscosities, especially below the dew point pressure. It is inherently difficult to measure the viscosity of gas condensate liquids below the dew point pressure because:Liquid dropout is generally very small due to the small size of the commercial PVT cells.Lack of HPHT small-volume cell viscometers.Time and effort required to generate liquid phase viscosities as a function of pressure and temperature. This paper presents a novel method to measure the density and viscosity of gas condensate liquids and near-critical fluids. The main purpose of this study was to utilize a specially modified PVT apparatus to measure the viscosities of typical Saudi Arabian gas condensate and near-critical fluids at elevated pressures and temperatures. These data can be used in reservoir simulation as well as other reservoir engineering calculations. The data are also used to compare and evaluate existing viscosity correlations and suggest improvements.