Page 250 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 226

[ ] Well Programming
2.5.6

a soluble anionic silicate network co-polymer. NaCl can be used for
saturation or KCl up to 35 ppb. The inhibition works better with the
addition of an alkaline metal oxide such as Na O. Several mechanisms
2
can be used to further improve the basic inhibitive properties:

Increase the ratio of SiO to alkali metal oxide
2
Modify the alkali metal blend
Increase the SiO concentration
2
Vary the base fluid salt content
Optimize the mud pH

Salt saturated silicate mud. A salt saturated silicate mud can be
formulated. This would be effective where salt and shale sequences
were present in the same hole section. Actual formulation (salts used)
would depend on the composition of the anticipated evaporites.

2.5.6. Formation Damage with Water-Based Muds (and Cements)

This matter should always be a key consideration in the drilling
fluids program for the reservoir sections of a well. The optimal fluid
that will minimize formation damage and allow for maximized pro-
ductivity will not necessarily have the same attributes as the mud that
was used to drill the overlying sediments.
In some cases, the mud can be designed to drill the reservoir with
minimal damage. At other times, a specially designed drill-in fluid
should be used. In other cases, it may be only sufficient to contain the
damage if it can be assured that the well can be perforated past the zone
of damage. Some of the mechanisms in which water-based muds cause
damage can also be present with cements and cement filtrates. The
commentary made below confines itself to damage caused by muds
(and cements). It is the case that in some instances formation damage
can be caused by the reservoir fluid, and the act of producing the well
can generate this damage. Refer to production operations manuals and
other literature for more detailed study of formation damage.
Just as a water-based mud can be divided into two components
(the insoluble solids and the liquid fraction containing solubles), the
causes of formation damage can also be classified as those due to the
insoluble solids and those due to the liquid fraction.

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