Synthetic Fluid Selection for High-Pressure Gas Condensate Flow Tests

Abstract High-pressure steady-state flow tests on gas condensates are needed to determine relative permeabilities and their dependence on capillary number. Such experiments require large quantities of reservoir fluids. Other laboratory challenges using reservoir fluids include H2S content and high temperatures. This paper presents a method that has been used successfully to create synthetic hydrocarbon mixtures that closely mimic reservoir fluid PVT, viscosity and IFT behavior at relevant reservoir pressures. The synthetic mixtures typically consist of 3-4 hydrocarbon compounds, and are readily created in the laboratory in large quantities. The selection of a synthetic mixture starts with a known description of reservoir fluid PVT properties, from laboratory measurements and/or EOS modeling. Typical PVT include constant composition and constant volume depletion data, viscosities, and gas-oil interfacial tensions (IFT). The procedure for creating an appropriate synthetic fluid that mimics the reservoir fluid PVT behavior is selection of 3-4 available hydrocarbon compounds, always consisting of methane, at least one light intermediate (C2 to C10), and at least one heavy compound (most often Diphenylmethane DPH-C1). An automated selection process has been developed. We used the SRK EOS with zero BIPs (binary interaction parameters) to describe phase and volumetric behavior of the synthetic fluid system. With a given group of selected compounds, the amount of each compound is determined by regression to minimize the mismatch between synthetic fluid PVT and reservoir fluid PVT. The synthetic mixture component slate that gives the best match is then chosen from the many possible combinations. This method has been used for some twenty reservoir gas condensates during the past 15 years. In this paper we illustrate the method for some twelve "public" reservoir gas condensates ranging from lean- to rich fluids, with some containing significant H2S and CO2 content. The accuracy of synthetic fluid mixtures to mimic actual reservoir gas condensate behavior is surprisingly good. Laboratory applications of the methods presented in this paper have been made without experimental difficulties. In general, the modeled synthetic fluid PVT behavior predicted by an EOS are quite close to the measured laboratory PVT of the synthetic fluids. The paper provides, for the first time, documentation that 3-4 component synthetic mixtures can be used to represent reservoir gas condensate fluids covering a wide range of composition and PVT behavior.