A new technique for simultaneous measurement of PVT and phase equilibria properties of fluids at high temperatures and pressures

Abstract

This paper describes the development and use of a technique to measure simultaneously pressure-volume-temperature (PVT) and phase equilibria compositions of fluid mixtures. This technique enables the analysis of reservoir fluids encountered in depletion recovery and EOR processes at pressures and temperatures commonly found in petroleum reservoirs. These fluids may include hydrocarbons, CO 2 and water. A mercury free Ruska PVT Model 2370 is assembled to a Hewlett Packard 5880 gas chromatograph (GC) and a specially designed sampling device capable of measuring compositions on-line at high pressures and temperatures. The significance of this experimental setup is: (1) a superior on-line sampling technique for compositional analysis, (2) capability of simultaneous measurement of all phase volumes, and (3) a closed sampling loop part of the PVT system. A negligible volume (0.1–0.5 μl) is withdrawn for compositional analysis and sent to the GC through specialized sampling valves. Because the volumes used for analysis represent less than 1 × 10 −5% of the sample volume, the mass withdrawn for analysis is negligible as well and does not affect the overall composition or the equilibrium of the system. This is referred to as a PVT-VLLE (vapor-liquid-liquid-equilibrium) apparatus. Although commissioning and validating experiments are worthy, they are generally not the most exciting papers to read and are probably considered minimal in nature. This paper, however, has some exciting potential. It describes a one-of-a-kind apparatus that could greatly contribute to our knowledge of process and mechanism in miscible- and steamflooding. This setup was validated by obtaining a good agreement (better than 0.1%) between the measured and reported compositions from standard mixtures. Phase compositions and densities of a binary mixture of n- hexane (n-C6) n- pentane (n-C 5) were also measured and compared with the predicted values from the Soave-Redlich-Kwong Equation-of-State (EOS). Deviations of the experimental vapor and liquid compositions from the EOS predictions were less than 1%. Phase equilibria compositions of hydrocarbon mixtures of similar molecular weight and structure are well predicted with an EOS. The deviations on the molar liquid and vapor volumes were less than 20%; however, it is well known that cubic EOS's do not predict volumetric properties as accurately as compositions. The phase compositions and three-phase pressures of a binary system of n-octane/water are measured and compared with data published by Heidman et al. (1985). The measured three-phase pressures and compositions agree well with the published data. This confirms that this experimental procedure can be well suited for multiphase analysis of hydrocarbon/water mixtures. The quantitative determination of water in all the equilibrium phases at high pressures and temperatures requires detailed and novel procedures, but these are not the focus of this paper at this point.