The Surface Chemistry Of Drilling Fluids

We have shown how surface forces so largely control the behavior of clay suspensions. But surface forces also affect many other aspects of drilling fluid technology, such as the formation of emulsions and foams; bit balling by-plastic clays; and formation damage by drilling mud filtrates. In this chapter, therefore, we will discuss the basics of surface chemistry.

Surface Tension

The interface between a liquid and a gas behaves as if it were a stretched elastic membrane. The contractile force of this imaginary membrane is known as surfac e tension. Surface tension also occurs at the boundary between a solid and a gas, between a solid and a liquid, and between two immiscible liquids. In the last instance, the tension is referred to as interfaciai tension.

To illustrate the contractile nature of surface tension, consider two glass plates separated by a thin film of water (see Figure 7 1). Force must be applied to pull the two plates apart because of the contractile meniscus around the periphery of the plates. This phenomenon is known as capillary attraction.

Measurement of Surface Tension

Surface tension is defined as the force acting perpendicular to a section of the surface one centimeter long, and is expressed in dyne/cm units. Absolute surface tension must be measured in a vacuum, but it is more convenient to measure it in an atmosphere of its own vapor, or in air. Bikerman1 has described the various methods of measuring surface tension in detail. The

*A glossary of notation used in this chapter will be found immediately following this chapter's text.

The Surface Chemistry of Drilling Fluids Table 7-1

Temperature

Surface Tension (dynes/cm)

Water

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