BAKER HUGHES - Drilling Fluids Reference Manual

BOREHOLE PROBLEMS
Figure 7 - 4 Mechanics of MAX-SHIELD
The polymer is stable and
maintains its particle size
distribution even in the presence
of high salt concentrations. The
particle size of the polymer is such
that the polymer bridges fractures
on the shale surface. The
deformable nature of the polymer
allows it to mould itself along the
fracture which improves bridging
(plugging) efficiency.
Aluminum chemistry (MAX-
PLEX) is used to form an
internal bridge via precipitation
within the shale matrix. The aluminum complex is soluble at the drilling fluid pH, but
precipitates as it enters the shale matrix due to a reduction in pH, reaction with multi-valent
cations, or a combination of both.
Silicate Based Drilling Fluids
Drilling fluids based on soluble silicates for the control of “heaving shale” were first
introduced in the 1930’s. These drilling fluids, based on high concentrations of soluble silicates
(i.e. 20 – 50% v/v) were successful in drilling over 100 wells in the Gulf Coast area, but were
difficult to control because of high rheological properties, which caused their use to be
discontinued until they were re-introduced in the late 1960’s. At this time it was found that
most shales could also be stabilized with more dilute solutions (less than 20% v/v, typically 5 –
10% v/v) of sodium and potassium silicates, the latter systems being the more effective.
Adding simple salts (e.g. KCl, K 2 CO 3 , NaCl) or non-ionic solutes (e.g. glycerol, ethylene
glycol) contributed significantly to shale stability. This additional stabilizing effect was
attributed to shale dehydration through an osmotic gradient generated by the solutes. Three
wells were drilled with these fluids but they did not achieve acceptance. In the 1990’s silicates
re-emerged. The following developments enabled re-introduction of these fluids:
• Understanding of the causes of the problematic control of rheological properties of
previous silicate drilling fluids.
• The availability of compatible, high-performance polymer-based rheological property
modifiers and filtration control agents.
• Advances in solids removal equipment.
• A better understanding of wellbore stability problems in general. It is now better
appreciated that drilling success is not governed by drilling fluid chemistry alone, but
also involves such critical factors as drilling fluid density, hole cleaning capacity,
swab/surge etc.
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006 7-15