Sprague Hydraulic Pump

Figure 8-29. Compaction ceil. (From Darley.6 Copyright 1969 by SPE-AIME.)

The relationship between swelling and compacting pressure may be studied experimentally in a compaction cell such as that shown in Figure 8-29.6 Figure 8-30 plots equilibrium water content versus effective stress for samples of sodium and calcium bentonites cut from outcrops. The test specimens were cut normal to the bedding planes. Since sodium montmorillonite exhibits osmotic swelling, but calcium montmorillonite does not, the curves suggest that the high water content of the sodium clay at stresses less than about 2,000 psi (140 kg/ cm2) was due to osmotic swelling. At higher stresses both clays were being de-sorbed of crystalline water.

If the sample in the compaction cell discussed above had consisted of pure montmorillonite, and all the clay crystals had sedimented with their basal surfaces parallel to the bedding planes, the swelling pressures would have equalled the compacting pressures when equilibrium conditions were obtained. Actually, swelling pressures were less than compacting pressures, as shown by Figure 8-31, which compares bulk densities calculated from adsorption isotherms with bulk densities calculated from the compaction data. A similar plot of compaction data obtained by Chilingar and Knight38 with a sample of commercial bentonite, which had first been equilibrated with an excess of distilled water, is also shown. Evidently, in both compaction experiments the clay crystals were to some extent randomly oriented, and the compacted specimens contained pore water as well as water of hydration.

Hydration of the Borehole

Where argillaceous sediments are compacted by the weight of overlying sediments, water adsorbed by clay minerals is expressed along with pore water. The amount of water remaining in the subsurface sediments depends on the depth of burial; the species and amounts of clay minerals present in the sediment; the exchange cations thereon; and the geologic age of the formation. Average bulk densities for the various ages are shown in Figure 8-32.39 When the shale is penetrated by the bit, the horizontal earth stresses on the walls of the hole are relieved and the shale is contacted by the drilling fluid. Water is then drawn by osmosis in or out of the formation depending on the activity of the water in the shale relative to that in the mud. The activity of water in a compacted shaie is reduced by hydrogen bonding to the clay crystal surfaces, and by hydration of

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Figure 8-30. Equilibrium water content of clays versus compacting pressure. Load applied by hydraulic ram as shown in Figure 8-29. Water expelled at zero pore pressure. (From Darley.6 Copyright 1969 by SPE-AIME.)

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Figure 8-30. Equilibrium water content of clays versus compacting pressure. Load applied by hydraulic ram as shown in Figure 8-29. Water expelled at zero pore pressure. (From Darley.6 Copyright 1969 by SPE-AIME.)

Effective Stress psi Figure 8-31. Comparison of desorption and compaction data.

Curve 1 calculated from desorption isotherm.

Curve 2 calculated from compaction curve in Figure 8-30.

Curve 3 calculated from compaction data of Chilingar and Knight.

Grain density of bentonite taken as 2.8 g/cm3.

the counter ions held in the double layer by electrostatic forces (see Chapter 4). The activity of the water decreases with depth because interlayer spacing decreases with increased compaction.

Both adsorption and desorption may destabilize the hole: adsorption will cause the hole to yield if the resultant swelling pressure exceeds its yielding stress (see Figure 8-22b). Desorption causes shrinkage cracks to develop around the hole. Caving occurs when the fluid from the hole invades the cracked zone, equalizing the pressure in the cracks with that in the hole. A clear brine invades the cracks freely, and the hole rapidly enlarges (see Figure 8-33a). A mud with filtration control tends to plug the cracks with filter cake, greatly reducing the rate of pressure equalization, and allowing a large proportion of the mud pressure overbalance to be applied at the wall of the hole. Consequently, caving is greatly reduced (see Figure 8-33b).

Brittle Shales

Caving and hole enlargement are frequently experienced in the older, consolidated shales that contain no montmorillonite. It was formerly believed that swelling was not the cause of caving in these so-called brittle shales, because the cavings were hard and showed no obvious signs of swelling. Chenevert,40 however, showed that these shales can develop extremely high swelling pressures when confined and contacted with water. In a drilling well, the swelling pressure increases the hoop stresses around the hole. When the hoop stress at the wall of the hole exceeds the yield stress of the shale, hydrational spoiling occurs. In the laboratory, Chenevert observed that the swelling pressure increased with time and eventually caused an explosive enlargement of the hole. Similarly, in the field, it has often been observed that severe caving does not occur until several days after the shale is penetrated by the bit.

Many shales contain old fracture lines, or invisible microfractures. Time and high confining pressures have partially healed these fractures, so that a specimen recovered at the surface appears quite competent. When contacted with water, however, the water penetrates along the fracture lines, the resulting swelling pressures break the adhesive bonds, and the shale falls apart (see Figures 8-34, 8-35a and 8-35b). A similar process undoubtedly takes place downhole. and facilitates destabilization of the borehole.

Control of Borehole Hydration

Since borehole hydration is, in many cases, the prime cause of hole instability, and in many other cases a contributing factor, every effort must be made to control it. The introduction of silicate muds, which consisted of sodium silicate and saturated sodium chloride brine marked the first attempt to do so.41 These muds were so successful at controlling hydration and dispersion that drill cuttings of gumbo shale were recovered at the surface with bit tooth marks still visible. Unfortunately, the rheological properties of silicate muds were so difficult to control that their use was discontinued.

Since then, the muds that have been most successful at preventing hydration of shale formations have been oil muds with concentrated brines as the internal phase. As originally hypothesized by Mondshine and Kercheville,42 if the salinity of the aqueous phase was equal to the salinity of the water in the pores of the shale, hydration would be prevented. Subsequently Mondshine43 modified this method to allow for the swelling pressure, which he determined approximately from the effective stress on the shale at the depth of interest. Chenvert,37 however, pointed out that the essential factor is the activity of the water in the shale (as determined in the laboratory by the vapor pressure of preserved cores), since it determines the potential swelling pressure, as shown by Equation 8-8. Therefore, swelling will be prevented if the activity of the water in the internal phase

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