Shale Control with Balanced-Activity Oil-Continuous Muds
Authors: Chenevert M.E. 
Published: December 2007

The main cause of shale instability is water adsorption and subsequent swelling of the wellbore. By adjusting the activity of the water in the mud to that of the water in the shale, this adsorption can be prevented and gauge boreholes can be drilled through both hard and soft shales.

Introduction
Unstable shales have plagued the petroleum industry for more than 30 years. All boreholes drilled into shale formations experience some degree of washout, which may be tolerable in some wells but completely uncontrollable in others. Many causes of shale problems have been postulated: mechanical factors such as mud pressure, thermal stresses, pipe movement (both pressure, thermal stresses, pipe movement (both banging and swabbing), "rubble" shale, and plastic flow; chemical factors such as hydration of montmorillonite clay. One paper describes the dehydration of plastic "gumbo" shales with invert emulsion muds. The technique reportedly works on soft, wet shales and requires only that the salinity of the mud be greater than the salinity of the water in the shale. (The salinity of such shales seldom exceeds 40,000 ppm.)

Causes of Unstable Wellbores
Unstable wellbores occur when formation pressures exceed the pressure exerted by the drilling fluid. Two situations can produce such a condition:the wellbore fluid pressure is lower than the in-situ formation pressure andthe in-situ formation pressure pressure andthe in-situ formation pressure increases because of water adsorption.
Too low a fluid pressure occurs when mud density is too low or when pressure occurs when mud density is too low or when bottom-hole pressures are reduced by swabbing with the drill pipe. This type of instability normally can be eliminated by drilling with increased mud weight or by minimizing pressure surges with low viscosity muds and slow pipe 'movement. Another more frequent cause of degradation is the excessive formation pressure that develops when argillaceous formations pressure that develops when argillaceous formations adsorb water. Problems due to hydration usually occur some time after the formation is exposed and can be prevented by using balanced-activity oil-continuous prevented by using balanced-activity oil-continuous muds. Laboratory and field data have shown that many formations are capable of adsorbing water or ions from water-base muds and are capable of adsorbing water from oil-continuous muds. Adsorption* occurs because of differences in chemical potential between the mud and the formation, and continues until equilibrium is achieved. When considering adsorption potential for oil-continuous muds, only aqueous potential for oil-continuous muds, only aqueous chemical potentials need be considered because ion transfer does not occur. In the case of water-base muds, the problem is more complex because, of ion and clay movement as well as water movement between the mud and the formation. It has been found that the more deeply buried shales are highly compacted and contain low quantities of water. As geologic compaction occurs, water is expelled from the shale and produces a degree of dryness in the shale. The scarcity of water and the abundance of charged clay surfaces results in an increase in the adsorptive potential of the shale.

Balanced Mud Concept
JPT
P. 1309