BAKER HUGHES - Drilling Fluids Reference Manual

BOREHOLE PROBLEMS
• High salt concentration helps in the deposition of silicates as a gel by lowering
characteristic gel times.
df sh
• Solutes can be used to balance the drilling fluid and shale activities (i.e. a
w
= a
w
) to
prevent osmotic water flow from the drilling fluid to the shale. It may be desirable to
lower the water activity of the drilling fluid even further (i.e. a ≤ a ) using an excess
of solutes. The resulting dehydration and pore pressure decrease may benefit shale
stability.
Cloud Point Glycols
The organics typically used in high-performance water-based drilling fluid systems are to
reduce filtrate losses, provide suspension properties, improve lubrication and to stabilize watersensitive
formations such as shales. In the 1990’s polyglycols were introduced for stabilization.
Polyglycol chemistry, characteristics and benefits of their use in drilling are well documented.
Most of the applications gave been with the use of a supplemental inorganic electrolyte,
typically potassium chloride, to aid in performance. This has allowed the use of less expensive
polyglycols in many cases. The use of chloride salts is undesirable in land-based drilling due to
disposal problems and where the high conductivity from the electrolyte interferes with the
sensitivity of induction logs during exploratory drilling. Use of an alternative electrolyte, such
as potassium acetate or formate is feasible.
Polyglycol Chemistry and Application
Polyglycols, as used in the drilling industry, are oligomers of polypropylene glycol or
polyethoxylated and propoxylated short chain alcohols such as butanol. The chameleonic or
TAME (Thermally Activated Mud Emulsion) polyglycols are the most widely used. They are
c
h
a
r
a
c
t
e
r
i
s
e
d
Figure 7 - 5 Temperature Sequence of Glycol Cloud Point Formation
characterized by their inverse solubility in water with temperature. These polyglycols are
typically miscible with water at lower temperatures but, when heated, will eventually separate
into two distinct liquid phases as the polyglycol becomes partially insoluble in the aqueous
phase (See figure above). The temperature at which this occurs is defined as “Cloud Point” or
“Cloud Point Temperature” (CPT) due to the scattering of the light by droplets of the
polyglycol-rich phase which separate. This phenomenon is reversible as the separated phase redissolves
once the temperature is lowered below the CPT. The exact CPT of a polyglycol
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006 7-17
df
w
sh
w