Experimental determination of the dew point pressure for bulk and confined gas mixtures using an isochoric apparatus

Abstract A robust high precision experimental approach to determine the Dew Point Pressure (DPP) of the gas condensates in a nano-porous medium is presented in this study. Gas condensate reservoirs have been the center of attention for numerous numerical and experimental studies for decades. Therefore, accurate measurement of DPP is crucial in developing long-term production plans for these reservoirs. This paper presents for the first time a proof of concept for a procedure to study the effect of the pore size distribution on the degree and direction of the shift in the saturation pressure of hydrocarbon gas mixtures under confinement over a relevant range of pressures and temperatures. Isochoric method, an indirect high-precision way of phase transition point determination, is commonly used in other disciplines where a clear non-visual determination of phase transition of a fixed volume of fluid is needed. This study provides an insight into using this approach for determining DPP of gas mixtures inside and outside of the porous media. A semi-automated apparatus for measuring and monitoring equilibrium conditions along with fluid properties is designed based on the isochoric method. The apparatus provides constant volume, variable pressure (0–104 bar), and variable temperature (290–410 K) experimental conditions. Pressure and temperature measurements provide a way to detect the phase transition point along the constant-mole-constant-volume line based on the change in the slope of this line at the phase transition point. A packed bed of BaTiO3 nanoparticles, providing a homogenous porous medium with pores of 1–70 nm is used as a representative nano-scale porous medium. The synthesized porous medium is very helpful in uncoupling the effect of pore size from the effect of mineralogy on the observed deviations in behavior, providing a volume more than 1000 times larger than the typical nano channels. The result is a set of isochoric lines for bulk and confined sample, plotted on the mixture's corresponding phase envelope to demonstrate the change in the saturation pressure. Phase envelopes (P-T diagrams) of the same mixture using different equations of state are created and the accuracy of each of these equations of state in providing an estimate of the experimentally detected DPP is discussed. Many attempts in explaining the shift in saturation pressures of the reservoir fluid confined in the narrow pores of unconventional reservoirs compared to those of the bulk can be found in the literature. However, there are some contradictions between the predicted behavior using different mathematical approaches. Experimental data could be substantially helpful in both validating the models and improving the understanding of the fluid behavior in these formations. Contrary to what many published models proposed, our results show that confinement effect shifts the DPP towards higher values compared to the bulk for a fixed temperature in the retrograde region. Capillary condensation is identified as the main source of the deviations observed in the behavior of fluids inside the nanopores. We evaluated some published models, including those based on EoS modifications, by comparing those to the experimental results which provides a quantification of their accuracy in estimating saturation pressure values for the confined mixtures.