0100 0295 0505 0755 0800 0920 0960 0908

*From O'Brien and Chenevert.44 Copyright 1973 by SPE-AIME.

centage of cuttings retaining their original size after hot rolling. Note that PHPA is by far the most effective polymer, and that KC1 alone had little effect.

Densities up to 10 lb/gal (SG 1.20) may be obtained with KC1; barite must be added if higher densities are necessary. Small additions of lignosulfonate or other thinner may be necessary to control gelation at densities above 16 lb/gal (SG 1.92). KOH should always be used instead of NaOH for pH control to avoid the dispersive action of the sodium ion.

KC1 muds have been successful at maintaining hole stability in hard brittle shales, usually at considerable savings in mud and drilling costs. In soft, mont-morillonitic shales results are more questionable. Very high concentrations of KC1 are required, and maintenance costs are high because of rapid depletion of KC1 and polymers. In some cases overall well costs were as high as with oil base muds, and the KC1 mud was less effective at maintaining hole stability. However, Clark and Daniel62a report reduced overall drilling costs in over 300 Louisiana and Texas Gulf Coast wells. Benefits included higher rates of penetration, less stuck pipe, less cuttings dispersion, and better rheological properties than with other water base systems.

Lime-lignosulfonate or gyp-lignosulfonate muds are often used for drilling dispersible montmorillonitic clays and shales. Their beneficial action depends solely on cation exchange, primarily the substitution of Ca++ for Na+. Figure 8-30 shows that osmotic swelling is much less in calcium bentonite than in sodium bentonite, and that crystalline swelling is not significantly affected by the calcium ion. Thus, lime muds help stabilize shales that exhibit osmotic swelling, but have no effect on shales, such as illites, that exhibit only crystalline swell-

ing. Osmotic swelling is the cause of dispersion (see section on clay swelling mechanisms in Chapter 1); therefore, lime muds inhibit hole enlargement in dis-persible shales. Also, they inhibit dispersion of drill cuttings, and thus help maintain low viscosities and faster drilling rates.

A recently developed potassium lime mud (KLM) represents a considerable improvement over conventional lime muds.53 54 The substitution of KOH for NaOH improves the shale stabilizing action of the mud for reasons already mentioned. Also, the use of a polysaccharide deflocculant instead of lignosulfonate reduces the dispersion of the cuttings. In laboratory rolling tests there was much less dispersion of tablets of southern bentonite in muds containing KOH instead of NaOH. Wells drilled with the KLM mud experienced less hole enlargement and lower maintenance costs than did offsetting wells drilled with conventional water base muds. No tendency to high temperature gelation was noted in wells with bottom hole temperatures above 300°F (149°C). The KLM mud achieved a notable success under adverse conditions when used to drill a well in the Navarin Basin in the Bering Sea.59

Other formulations for drilling dispersible shales consist of various polymers and potassium lignite51 or KOH—lignosulfonate52-57 or KOH—calcium lignite,58 or a potassium-base derivative of humic acid.62b When low salinities are required for logging purposes, a mud containing diammonium phosphate, poly anionic cellulose, and bentonite (DAP—PAC) may be used.44 Another low salinity inhib-itive mud consists of 10 X 106 MW PHPA (compare with 3 x 106 MW PHPA used by Clark48) and as little as 1% KC1.60 A fresh water PHPA (> 15 x 106 MW) with small amounts KOH has been used to replace chrome lignosulfonate muds in south Texas. Improved hole stability was obtained due to less mechanical erosion and reduced hydroxyl concentration. Mud costs were higher but total drilling costs were lower because of higher rates of penetration and longer bit life.

Selection of Mud Type for Maintaining Borehole Stability

This chapter should have made it clear that hole instability is a complex problem, the nature of which depends on the borehole environment. The type of drilling fluid that will provide maximum hole stability therefore varies from area to area; no one fluid is best for all areas. Some investigators43,50,63,64 have attempted to base the choice of drilling mud on a classification of shales according to clay mineral composition and texture. The difficulty with this approach is that too many variables exist for shales to be placed in a few simple categories. Also, hole stability is influenced by other factors, such as tectonic stress, pore pressure, dip of the formations, and degree of compaction. For example, the Atoka shale in southeast Oklahoma is notoriously unstable in the vicinity of the overthrust Choctaw fault, yet comparatively few problems are experienced with the same shale in undisturbed areas a few miles further north.

Before formulating a mud to minimize borehole problems, the first step should be to collect as much information as possible on the geology, the stress history, and the fault patterns of the region. Temperature gradients, pore pressure gradients, and in situ water contents of shales should be obtained from logs in the nearest wells. Samples of problem shales should be obtained for laboratory testing. The best source of samples is a well preserved core, but if none is available drill cuttings must be used. Cores are to be preferred because much valuable information may be obtained from the lithology, structure, presence of fractures, degree of hydration, etc. Coring costs are high, but if cores are taken early in the life of a field, and suitable tests are made, the coring costs will be repaid many times over by savings in subsequent drilling costs. The objection to drill cuttings is that they are likely to have been altered by hydration and base exchange reactions with the mud on the way up the hole. Dust from air drilled wells avoids the contamination problem but not hydrational changes.

The following laboratory tests should be made:

1. Clay mineral analysis by X-ray diffraction, cation exchange capacity, and exchange cations (see section on ion exchange in Chapter 4). Where facilities for these tests are not available, the methylene blue test may be used instead (see section on clay mineral identification Chapter 3). This test permits a rough estimate to be made of the amount of montmorillonite present. As already mentioned, the concentration of KC1 required for the control of swelling depends largely on the amount of montmorillonite present.

2. Balancing salinity. A test to determine the salinity required to balance the in situ activity of sub-surface shales is necessary. Obviously, it is a waste of money to use an oil base mud unless the salinity of its aqueous phase is kept greater than the required balancing salinity. The test as described by Chenevert37 is made by placing chips of dried shale in desiccators containing saturated solutions of various salts covering a wide range of vapor pressures (see Table 8-6). After one day, 90% of equilibrium is reached; the chips are then removed and weighed, and the water content is calculated and plotted against relative humidity. The in situ activity of the water in the shale is then given by the intercept of its in situ water content on the isotherm. This value is an indication of the potential swelling pressure should the shale pick up water from the drilling mud—the lower the in situ activity, the greater the maximum possible swelling pressure. The salt content of an oil-base mud required to balance the shale activity may be calculated from graphs such as those shown in Figures 8-38 and 8-39. Note that a reliable estimate of the in situ water content cannot be obtained from the density of the drill cuttings, but can be obtained from a specimen cut from the center of a well preserved core, or can be estimated from density logs.

# of coclj/bbl of oil-continuous mud

Figure 8-39. CaCI2 requirements for balanced activity muds. {From Chenevert.37 Copyright 1970 by SPE-AIME.)

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