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b. Having a filter rate greater than 115 cm3/hour

Figure 6-9. Particle size distribution of selected muds. (From Gates and Bowie.™)

In the case of asphaltic muds, filtration control is achieved only if the asphalt is in the colloidal state. Control is lost if the aromatic content of the suspending oil is too low—aniline point above about 150°F (65°C)—because the asphalt coagulates. Control is lost if the aromatic content of the suspending oil is too high—aniline point below about 90°F (32°C>—because the asphalt passes into true solution. With the other type of oil mud, filtration control is obtained by forming finely dispersed emulsions of water in oil with powerful organic emulsifiers. The tiny, highly stabilized water droplets act like deformable solids, yielding low permeability filter cakes .

Effect of Flocculation and Aggregation on Cake Permeability

As explained in Chapter 4, flocculation of muds causes the particles to associate in the form of a loose, open network. This structure persists to a limited extent in filter cakes, causing considerable increases in permeability. The higher the filtration pressure, the more this structure is flattened, so both porosity and permeability decrease with increase in pressure. The greater the degree of flocculation, the greater the interparticle attractive forces, and therefore the stronger the structure and the greater its resistance to pressure (see Fig. 6-10). The structure is even stronger if flocculation is accompanied by aggregation, because it is then built of thicker packets of clay platelets. For example, Suspension 1 in Figure 6-10 contained only 0.4 gram per liter of chloride in the filtrate, sufficient only to cause a weak floe structure. Suspension 2 was obtained by adding 35 grams per liter of sodium chloride to Suspension 1, sufficient to cause strong flocculation plus aggregation. Consequently, cake permeabilities and porosities of Suspension 2 were considerably greater than those of Suspension 1, even at high filtration pressures.

Conversely, deflocculation of a mud by the addition of a thinning agent causes a decrease in cake permeability. Moreover, most thinners are sodium salts, and the sodium ion may displace the polyvalent cations in the base exchange positions on the clay, thereby dispersing the clay aggregates, and further reducing cake permeability.

Thus, the electrochemical conditions prevailing in a mud are a major factor in determining the permeability of its filter cake. As a generalization it may be said that cake permeabilities of flocculated muds are in the order of 10 2 md, those of untreated fresh-water muds are in the order of 10"3 md, and those of muds treated with thinning agents are in the order of 10 4 md.

The Bridging Process

As already discussed, there is a mud spurt at the start of a filter test made on paper before filtration proper begins, and, thereafter, filtrate volume be-

than the pore opening cannot enter the pore, and are swept away by the mud stream: particles considerably smaller than the opening invade Lhe formation unhindered; but particles of a certain critical size stick at bottlenecks in the flow channels, and form a bridge just inside the surface pores. Once a primary bridge is established, successively smaller particles, down to the fine colloids, are trapped, and thereafter only filtrate invades the formation. The mud spurt period is very brief, a matter of a second or two at the most.1-

As a result of the process just described, three zones of mud particles are established on or in a permeable formation (see Figure 6-11).

1. An externa! filter cake on the walls of Lhe borehole.

2. An internal filter cake, extending a couple of grain diameters into the formation.

3. A zone invaded by the fine particles during the mud spurt period, which normally extends about an inch into the formation.16- 1 '■ Experimental results reported by Krueger and V'ogel-0 suggest that these fine particles do not initially cause much permeability impairment. but may do so after filtration has proceeded for some hours, presumably because of migration and consequent pore blocking.'

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