BAKER HUGHES - Drilling Fluids Reference Manual

BOREHOLE PROBLEMS
Liquid sodium or potassium silicates are solutions of water soluble glasses, which are primarily
manufactured by fusing either soda ash (Na 2 CO 3 ) or potash (K 2 CO 3 ) with silica (SiO 2 ) sands at
high temperatures (1000 - 1200°C):
M 2 CO 3 + nSiO 2 → M 2 O . n(SiO 2 ) + CO 2
Where M is either Na + or K + and n identifies the molecular ratio (i.e. the number of SiO 2
molecules relative to one M 2 O molecule) and is typically in the range 1.5 – 3.3 for commercial;
products.
The pH of silicate solutions is a function of the molecular ratio, but is always high; pH
decreases with increasing ratio, i.e. when the silicate becomes more siliceous and less alkaline.
When pH is lowered artificially (e.g. by the addition of acid) in relatively high concentration
silicate solutions, the anionic silicate oligomers will polymerize and gel. Gelation is most rapid
at neutral pH. Gelation times are lowered by the presence of mono-valent salts. Most
importantly, silicates react almost instantaneously with dissolved polyvalent cations such as
Ca 2+ and Mg 2+ to form insoluble precipitates.
Soluble silicate starts out as monosilicate which polymerizes rapidly to form negatively charged
oligomers. At a pH of 11 – 12 (which is the pH of field drilling fluid formulations),
polymerization halts because of mutual repulsion when relatively small oligomers have been
formed. Such oligomers are still small enough to penetrate the tiny pores in the shale fabric
(typically a few nanometers in diameter), transported by diffusion or hydraulic flow. Having
entered the shale pore network, the following may happen:
1. Pore fluid pH is close to neutral (pH ≈ 7) in virtually all shales. When the oligomers are
diluted down to this neutral pH, they may overcome their mutual repulsion and coagulate,
forming 3-D gel networks.
2. In all shale pore fluids, there are free polyvalent ions present (e.g. Ca 2+ and Mg 2+ ), that
will instantaneously react with the oligomers to form insoluble precipitates.
The gelled / precipitated silicates now become a means of stabilizing shales, as they:
a) Provide a physical barrier that prevents further drilling fluid filtrate invasion and
pressure penetration. Thus, the wellbore and shale formation are effectively (pressure)
isolated. Note that the gelation / precipitation process occurs very rapidly and is
completed before significant fluid loss and pressure invasion has occurred.
b) Enhance the efficiency of the shale-drilling fluid membrane. Silicates are well known for
their membrane building potential. To prevent destabilizing osmotic water flow from the
drilling fluid to the shale the water activity of the silicate-based drilling fluid should be
at least balance the shale activity (i.e. a ≤ a ).
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The presence of solutes, in particular simple monovalent salts like NaCl and KCl, , is
synergistic in various ways:
• Monovalent cations like Na + and K + may exchange at the shale clay surfaces for Ca 2+ and
Mg 2+ , which then become available for precipitating silicate.
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BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006 7-16