As stated by the classical Thomson equation, the pore scale thermodynamics of a solvent is different from bulk conditions, being critically controlled by capillary characteristics. This equation shows that the boiling point temperatures decrease remarkably as the pore size becomes smaller, after a threshold value. This paper experimentally investigates this phenomenon for hydrocarbon solvents and compares the results with the values, obtained from the Thomson equation, to test its applicability in modeling heavy-oil recovery by solvents under nonisothermal conditions. As an initial step, the boiling point temperatures of two single-component solvents (heptane and decane) were measured by saturating Hele-Shaw type cells with variable apertures (ranging from 0.04 mm to 5 mm) and monitoring the boiling process. One experiment was run with a thickness of 12 mm to represent the bulk case. As the aperture (pore size) became smaller, the boiling point temperature decreased. For example, the measured boiling temperatures of heptane and decane were approximately 58 °C and 107 °C for the aperture values less than 0.15 mm, which were considerably lower than the “bulk” values (around 40%). In the next step, the same experiments were repeated using micromodels, representing porous media. Using the Thomson equation, the boiling points of the selected liquids were mathematically computed and compared with the experimental results from Hele-Shaw and micromodel experiments. Finally, modifications to the Thomson equation and alternative formulations were suggested.

Abstract

The thermodynamics of fluids in confined (capillary) media is different from the bulk conditions due to the effects of the surface tension, wettability, and pore radius as described by the classical Kelvin equation. This study provides experimental data showing the deviation of propane vapour pressures in capillary media from the bulk conditions. Comparisons were also made with the vapour pressures calculated by the Peng–Robinson equation-of-state (PR-EOS). While the propane vapour pressures measured using synthetic capillary medium models (Hele–Shaw cells and microfluidic chips) were comparable with those measured at bulk conditions, the measured vapour pressures in the rock samples (sandstone, limestone, tight sandstone, and shale) were 15% (on average) less than those modelled by PR-EOS.

Tight rock reservoirs have gained popularity and become a subject of great interest due to their huge recovery potential. A substantial portion of the potential hydrocarbon could be removed from the reservoir by injecting solvent gases (hydrocarbon or CO2) as an Enhanced Oil Recovery (EOR) application. Achieving a precise modeling of these processes and accurate description of hydrocarbon dynamics requires a clear understanding of phase-change behaviour in confined (capillary) medium. It was previously shown that early vaporization of liquids could occur in channels that were larger than 1000 nanometers. The surface wettability plays a critical role in influencing the vaporization and condensation nature in confined systems. This paper studies the influence of the medium wettability on phase-transition temperatures of liquid hydrocarbons in macro (> 1000 nm) and nano (< 500 nm) channels by using different types of rock samples. The boiling temperature of hydrocarbon solvents was measured in two extreme wetting conditions: (1) strongly water wet, and (2) strongly oil wet. Boiling temperatures of heptane and octane in sandstone, limestone, and tight sandstone were observed to be lower than their bulk boiling points by closely 13%, on average. Altering rock wettability characteristically changes the average hydrocarbon nucleation temperatures being as critical as the pore size. The experimental outcomes also showed that reducing the solvent adsorption on clays in Berea sandstone lowers the nucleation temperature of heptane and octane from their normal phase-change temperatures by 30%.

Phase behavior of fluids at capillary conditions differs from that in bulk media. Therefore, understanding the thermodynamics of solvents in confined media is essential for modeling thermal EOR applications. The Thomson equation states that pore sizes have a control on boiling points of liquids in capillary channels. As pore spaces become smaller, boiling points become lower than normal boiling temperatures of the same liquids. The target of this paper is to inspect this phenomenon by physically measuring the boiling points of several solvents and compare them with the calculated boiling temperatures for different capillary structures. Furthermore, the feasibility and accuracy of the Thomson equation is investigated to check its applicability in heavy-oil recovery modelling. To do so, Hele-Shaw cells with several gap thicknesses (0.04, 0.45, 1.02, and 12 mm) are used to measure the boiling points of heptane, heptane-decane mixture, and naphtha. Experiments are repeated for the same solvents on homogeneous and heterogeneous micromodels to observe the phase behavior in a more realistic porous medium. Finally, the effect of surface wettability on boiling temperatures is examined in Hele-Shaw and micromodel experiments.