Figure 8-34. Destabilization of shale specimen by hydration of a healed fracture a. shale specimen after machining from core.

b. after placing water at base.

c. specimen fails apart.

{From Darley* Copyright 1969 by SPB-AIME.)

of the mud is equal to the in situ activity of the water in the shale formation. This requirement may be accomplished by determining the in situ water content of the shale from density logs (see the section on induced fractures, in Chapter 9) or, less accurately, from Figure 8-32. The in situ activity of the water can then be read from the adsorption isotherm of that shale at the appropriate water content (see Figure 8-27). The activity of the aqueous phase of the mud is then adjusted to the same value by the addition of sodium or calcium chloride. Field results indicated that corrections for the difference between the activity at laboratory and at downhole temperatures are usually not necessary.

pressure, and the whole of the mud pressure overbalance, (pw — pf), is applied on the walls of the hole, whereas with water base muds pressure equalization takes place, albeit slowly.

Although controlled activity oil base muds are best for preventing hydration of formation clays, their cost is high, it is sometimes difficult to obtain satisfactory formation evaluation with them, and they have various other disadvantages (discussed in Chapter 1). In recent years special water base muds for maintaining hole stability have been developed.44-61 In some areas these muds provide adequate hole stability and mud costs are lower than with oil muds, but sometimes the savings in mud costs are more than offset by higher drilling costs.

Soluble salts are used in water base muds to control swelling, and various polymers provide rheological properties and control dispersion. Salts control swelling by two mechanisms: lowering the activity of the water and cation exchange. Lowering the activity of the water can be used to stabilize shales only to a limited extent. Mud salinities below the balancing salinity will reduce osmotic swelling, but when the continuous phase is water, salinities above balancing will cause shrinkage cracks and consequent destabilization, as already discussed. Maintaining a balanced activity is not a practical proposition.

An adequate degree of shale stabilization can usually be achieved by cation exchange reactions, usually the replacement of Na+ by K+. Table 8-3 and Figure 8-36 show that KC1 is more effective at reducing linear swelling than equivalent concentrations of other salts, and Bol62 showed the same phenomenon in volumetric swelling tests on confined specimens of Pierre shale. The potassium ion is more effective because of its low hydration energy and its small size, which enables it to fit into the holes in the silica layers in the clay crystal, thus reducing interlayer swelling (see Table 4-4).

The concentration of KC1 required to suppress swelling depends on the cation exchange capacity of the shale, and the exchange constants of the ions involved. Steiger50 found that shales high in montmorillonite required up to 90 lb/bbl (256 kg/m'), whereas illitic shales required only 20 lb/bbl (57 kg/m3).

Because of its stability in high salinity brines polyanionic cellulose is commonly used to provide filtration control in KC1 muds. Starch is also used, and Steiger50 reports good results with a synergistic mixture of polyanionic cellulose and starch.

Xanthan gum or prehydrated bentonite is used to provide cutting carrying capacity, but should only be used when adequate hole cleaning cannot otherwise be obtained, because the rate of penetration will be reduced both by the increase in viscosity and by the increased difficulty of removing drilled solids at the surface. Bentonite is much less expensive than xanthan gum, and very high YP/PV ratios can be obtained with it; but it suffers from the disadvantage that Na+ on the bentonite is exchanged for K+, thus largely inactivating the bentonite, and depleting the KC1 in solution.50 This disadvantage is not significant at low concentrations of KC1; for example, it has been successfully used at KC1 concentrations of 10 lb/bbl (28.5 kg/m3).48

Figure 8-36. Effect of cation concentration and species on linear swelling. Clay mineral analysis of shale: 9.2% montmorillonite, 11.2% mixed layer, 35% illite, 5.5% chlorite, 4.4% kaolinite. {From Steiger.50 Copyright 1982 by SPE-AIME.)

21 lb/fabt KCI pH 9.0

21 Ib/fabl KQ pH 10.0

1 lb/bbl XC POLYMER 21 lb/bbl KCI pH 10.0 1 lb/bbl XC POLYMER 1 fc/fabl PHPA-C

21 lb/bbl KCI pH 10.0 1 lb/bbl XC POLYMER 1 lb/bbl PHPA-R

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