101

WETTING AGENT, LB/BBL IN NO. 2 DIESEL OIL

Figure 7-11. Adsorption of wetting agent by shale particles in oil as indicated by interfacial tension of diesel oil against water. (From Simpson, etal.9 Copyright 1961 by SPE-AIME.)

Oil-wetting agents may also be used to make a mechanical invert emulsion with linely-divided chalk. Chalk particles are normally water-wet, but by adding a small quantity of a fraction of tall oil, which consists mostly of oleic and linoleic acid, the particles may be made partially oil-wet.10 Upon agitation the particles are adsorbed at the oil-water interfaces around the water droplets, resulting in a mechanical emulsion whose continuous phase is oil.

Foams

When water is encountered in air or gas drilling, foaming agents are added to facilitate its removal. Foam formation is quite a simple matter: it merely requires the injection into the air stream of a surfactant that sufficiently lowers the surface tension of the water. Foams, however, tend to collapse in a short time because the surface free energy of the system is therby reduced. In drilling foam longevity must be considered when selecting a foamer.

Foams and Mists

Foams and mists are colloidal systems in which the two phases are a gas and a liquid. Distribution of the two phases depends on the relative amounts of each present. This ratio is usually expressed as either the volume fraction of the gas (foam quality)11 or the volume fraction of the liquid (LVF).12 In the loam quality range from 0 to about 0.54 the foam consists of independent

bubbles dispersed in the gas (see Figure 7-12); in the range from 0.54 to 0.96 the system is analogous to an emulsion with gas as the internal phase and the liquid as the external phase. Above a quality of 0.96 the system consists of ultramicroscopic droplets of water dispersed in the gas, and is termed a mist or an aerosol.

The factors governing the formation and stability of foams is not well understood. Obviously, since foaming involves a huge increase in surface area, the reduction of surface tension by the addition of a surfactant is essential. However, reduction of surface tension is not the only pertinent factor: The molecular structure of the surfactant also appears to be significant. One theory is that the anions are oriented normal to the surface of the film, and the cations are distributed in the solution between the walls.13 Thus the walls carry an electrostatic charge, and repulsion between these charges hinders coalescence. Because of the lack of basic theory, foaming agents must be evaluated by empirical tests (see Chapter 3).

Foams are used for three purposes in the drilling industry: (1) to remove formation water that enters the borehole when drilling with air, (2) as a low density fluid to remove drill cuttings and other solids when completing or working over wells in depleted reservoirs,14 and (3) as an insulating medium in Arctic wells.15

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