Chemo-electrical effects

Chemo-electrical effects are most relevant to small sub-micron particles, such as clays, the building-blocks of mudrocks and shales. The forces that act on clay particles include (Figure 6.26):

• Mechanical forces: solid skeletal stress and pore pressure,
• Electrical forces: van der Waals attraction, Born repulsion, short range repulsive and attractive forces induced by hydration/solvation of clay surfaces.

Figure 6.26: Schematic of clay platelet pair, forces between them, and equilibrium interparticle distance.

Shale chemical instability involves changes of the electrical forces between clay platelets due to changes of ionic concentration of the resident brine within the rock pore space. The equilibrium distance between particles is inversely proportional to salinity. Hence, decreasing salinity promotes chemo-electrical swelling of shale (see example in Fig. 6.27). During drilling, the change in ionic concentration in resident brine is caused by leaf-off of low-salinity water from drilling mud into the formation saturated with high salinity brine. Smectite clays are most sensitive to swelling upon water freshening and hydration.

Figure 6.27: Example of shale hydration and swelling due to exposure to low salinity water.

Shale swelling leads to an increase of hoop stress and sometimes rock weakening around the wellbore wall. Such changes promote shear failure and breakouts/washouts (Fig. 6.28). Prevention of shale chemical instability requires modeling of solute diffusive-advective transport between the drilling mud and the formation water in the rock. Higher mud pressures result in higher leak-off, and therefore, in more rapid ionic exchange within the shale. The process is time-dependent. Thus, the expected break out angle results from a combination of mechanical factors (stresses around the wellbore) and shale sensitivity to low salinity muds.

Figure 6.28: Shale sensitivity: variation of near wellbore stresses and time-dependent breakout angle $w_{BO}$ prediction example.

The solutions to wellbore instability problems due to shale chemo-electrical swelling are: increasing the salinity of water-based drilling muds, using oil-based muds instead of water-based muds, cooling the wellbore to counteract increases of hoop stress, and using underbalanced-drilling to minimize leak-oof of drilling mud water.