Interfaces, driving pressure

The contiguous resei"voirs of gas or water that contribute the drive pressures for an oil well can cause a problem. Gas and, in some cases, water, are more mobile than oil in an oil reservoir. As a result, during production the oil-gas or oil-water surface can move toward the region of reduced pressure around the borehole. If the interface reaches the borehole, the driver fluid will enter the well and be produced along with the oil. Since gas is not as valu-... 

An interface has an effective driving pressure if its displacement decreases the total system s free energy. This effective pressure can derive from any mechanism by which a material stores energy, but for many cases it arises from only two sources the volumetric free-energy differences between the interface s adjacent phases, and mechanical pressure differences due to reduction of the interfacial energy. 

Figure 13.7 Spherical particle pinning an interface between grains 1 and 2. The interface is subjected to a driving pressure that tends to move it in the y direction. From Nes et al. [11].

The driving force for liquid penetration is governed by the pressure gradient, P, across the curved liquid]gas interface. The pressure drop across the interface in a uniform capillary of radius r is given by ... 

Chapter 3, there is often a region immediately preceding the lower closure point, in which increased adsorption is brought about by reversible capillary condensation. The meniscus now tends to be somewhat ill defined owing to its small dimensions (p. 153), but the mechanism can still be thought of in Kelvin terms, where the driving force is the pressure difference across an interface. 

Disjoining Pressure. A static pressure difference can be imposed between the interior and exterior of a soap film by several means including, for example, gravity. In such cases the equiHbrium film thickness depends on the imposed pressure difference as weU as on the effective interface potential. When the film thickness does not minimize lV(f), there arises a disjoining pressure II = —dV/(U which drives the system towards mechanical equiHbrium. 

Expressions of this type can be written for both gas and liquid films in which the absorption coefficients are the gas- and liquid-film coefficients, respectively. The driving force across the gas film is given by the difference between the actual partial pressure of the soluble gas and that at the interface, v/hile the driving force across the liquid film is given by the difference between the concentration of the soluble gas at the interface and that in the main bulk of liquid. 

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