Use of differential scanning calorimetry to study phase behavior of hydrocarbon mixtures in nano-scale porous media

Abstract

The phase behavior of petroleum fluids is a challenging problem in shale oil and gas production. Due to strong surface-fluid interactions and complex pore geometries in shale nanopores, PVT properties of fluids in shale are altered from those of conventional reservoirs and cannot be described by bulk-phase thermodynamics. To our best knowledge, the experimental data for hydrocarbon phase behavior in shale systems is severely absent. The experimental difficulty lies in introducing the nanoporous environment and finding an effective technique to study the fluid-nanoporous media system.In this work, we investigate the phase change of mixtures of hydrocarbons in nano-scale capillaries using a thermal laboratory technique, differential scanning calorimetry (DSC), which probes the thermal properties by measuring the heat exchange evolution of a sample during a temperature ramp. Porous media controlled-pore glasses (CPGs) are applied to model the nano-porous structure of shale reservoirs. CPGs (pore diameters 4.1 and 37.9nm) infiltrated with hydrocarbons (octane and decane binary mixtures) are subjected to DSC. It is observed that at 37.9nm the bubble point shift is negligible, but at 4.1nm two distinct vaporization incidents occur with deviations as great as ∖xB120K relative to the bulk. The unusual observation of bubble points for mixtures in 4.1nm pores suggests compositional heterogeneity in the nanopore.The bubble point is modeled using Peng-Robinson equation of state (PR EOS) with the capillary pressure considered. The flash calculation is performed based on isofugacity and an interfacial tension model is accommodated. The modeling gives an exaggerated bubble point temperature shift and does not match the experimental results. This suggests the poor accuracy of the PR EOS/capillary pressure model in describing the behavior of nanoconfined fluid mixtures.