Hole Instability Caused By Interaction Between The Drilling Fluid And Shale Formations

The various forms of hole instability resulting from interaction between the drilling fluid and argillaceous formations are all related to hydration phenomena. As discussed in Chapter 4, water is adsorbed on clays by two mechanisms: adsorption of monomolecular layers of water on the planar surfaces of clay crystal lattices (commonly referred to as crystalline swelling or surface hydration), and osmotic swelling resulting from the high concentration of ions held by electrostatic forces in the vicinity of the clay surfaces. Crystalline swelling is exhibited by all clays. Swelling pressures are high, but the increase in bulk volume is comparatively small. Interlayer osmotic swelling occurs only with certain clays of the smectite group (notably sodium montmorillonite), and causes large increases in bulk volume, but swelling pressures are low.

If a dry clay is confined, but given access to free water, it develops a swelling pressure. Similarly, if a clay that has been allowed to equilibrate with free water is compacted and water is expelled, swelling pressures develop. The swelling pressure at any given water content is related to the vapor pressure of the clay at the same water content by the equation:36

where p5 is the swelling pressure in atmospheres; T is the absolute temperature (°K); V is the partial molar volume of water, liters/mole; R is the gas constant (liter atmospheres/mole °K); and p/p0 is the relative water vapor pressure at equilibrium with the shale and is approximately equal to the activity of the water in the shale. Thus, the potential swelling pressure of a compacted shale whose water content is known can be predicted from adsorption or desorption isotherms of that shale (see Figure 8-27).37 Isotherms are determined by equilibrating specimens of the shale with water vapor in atmospheres of known humidity and constant temperature. Figure 8-28 shows that the swelling pressure of the layer of crystalline water adjacent to a clay surface is extremely high, but that of succeeding layers decreases rapidly.10

Adsorption and Desorption of Clays and Shales

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