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Reports: ND954593-ND9: Confinement Effect on Solubility of Oilfield Fluids in Sub-Micron Pores

Keith E. Gubbins, PhD, North Carolina State University

            The effect of confinement in a narrow pore on the solubility of a dilute solute in a solvent is poorly understood.  A few scattered studies based on experiment, density functional theory and molecular simulation have been reported, but they present a confusing picture with little general understanding of the phenomenon.  In most reported cases confinement seems to lead to an increase in solubility, but in others there is a decrease.

            Under this project we have studied the effect of confinement in a slit-shaped carbon pore on the solubility of a non-polar gas, methane, in a liquid solvent that is modeled on benzene.  This system was chosen because of relevance to oil and gas exploration applications, and also because extensive experimental data is available for methane solubility in the bulk liquid phase. The method used was Gibbs Ensemble Monte Carlo (GEMC) simulation, and involved molecular exchange between three samples representing the bulk liquid, bulk gas and pore phases (Fig. 1).  On reaching equilibrium amongst these three phases the chemical potentials of the methane are equal in each phase, as are those of benzene. 

Figure 1.  Schematic of the GEMC molecular simulation system

The solubility was studied as a function of the pore width, temperature and bulk phase pressure.  The solubility in the pore was observed to be either greater or less than that in the corresponding bulk phase, depending on the pore width.

            The solubility, and also the location of the methane molecules in the pore, were found to be strongly affected by the pore width, H. At certain pore widths the solvent molecules are tightly packed in the pore, leaving little room for solute methane molecules, but as H is increased there are ranges of width where the pore is not wide enough to allow an additional layer of solvent to form, and there is more vacant space to accommodate methane molecules. Typical results are shown in Figs. 2 and 3.
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Figure 2. Results for methane solubility for a pore width of H = 3.0σbenzene at 323 K and 15 bar bulk gas pressure.  Left: density distribution for methane and benzene molecules; the abscissa is the distance from the center of the pore in terms of benzene molecular diameters.  Right: snapshot of the simulation showing orange methane molecules in the pore center; benzene molecules are shown as blue spheres.

Figure 3. Methane solubility as a function of the pore width (expressed in terms of benzene diameters) for the confined mixture (red points) compared with the solubility in the bulk liquid (black points), from GEMC simulations.

            We have studied the effect of temperature on the solubility for the range 270 to 323 K.  When the solubility is small, as is the case here, the influence of temperature on solubility, xCH4, is given by the thermodynamic equation

                                                      

where R is the gas constant and  is the heat evolved on solution of the methane.  We found that the heat of solution was greater for the confined mixture than for the bulk one, due to the strong forces from the pore walls.  Thus the solubility for the confined phase increases more rapidly than for the bulk mixture as temperature is lowered.

            We also studied the effect of the bulk pressure on the solubility, and found that the solubility increased linearly with pressure for the lower pressures, up to about 20 bar, for all pore widths, for both the confined and bulk mixtures.  This suggests that Henry's law (solubility is proportional to the partial pressure of methane in the gas phase) is obeyed for the confined system for this pressure range. The range of pressures over which Henry's Law is obeyed appears to be smaller for the confined mixture than for the bulk one.  This is the subject of our ongoing studies.