Page 249 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 225

2.5.5
Drilling Fluids Program [ ]

usually around 10. As the filtrate enters the shale pores, the pH drops
towards neutral. Silicate molecules form gels at neutral pH and solid
silicates precipitate out when reacting with polyvalent ions, which are
always present in shale pore fluids. These combined gels/precipitates
block the pore throats against any further invasion. In addition, this
causes a semi-permeable membrane at the shale surface so that osmot-
ic forces can be crafted to further dehydrate the shales, which causes
the shale to further harden.
The mechanism also seems to work well in microfractured shales.
These types of shales often go unrecognized and cavings from fractured
shales are made worse with increased mud density or swab pressures,
which is why the standard answers of increased mud density and wiper
trips are seriously detrimental when certain types of cavings are seen.
Shale samples left exposed to silicate formulations actually show
an increase in hardness (measured by a penetrometer), probably due to
these precipitates cementing the shale crystals together combined with
shale dehydration.
Silicate muds were tried in the 1930s for shale inhibition with some
success. The chemistry involved was not understood and the main
problem was getting stable rheology. However, these problems have
been solved and silicate muds have been successfully used in the field.
Due to the way they work, any use in reservoir rocks must be care-
fully planned; silicates in general are only suitable for the reservoir in
development wells where the wells will be cased and perforated. Open
hole completions may be a problem due to the presence of gels and pre-
cipitates giving an effective mechanical skin. Return permeability tests
on core samples will indicate whether the silicate mud is suitable for
the reservoir section.
Silicate muds are nontoxic, environmentally acceptable, noncorro-
sive, can accept all available polymers, and the constituents are readily
available at a reasonable price. Clear fluids can be used since the sili-
cates are completely soluble at high pH. Acid gas influxes (such as H S)
2
are neutralized by the alkaline mud. In addition, the anionic silicate
prevents buildup of shale cuttings on the drillstring (balling).
However, silicate depletion rates can be high; the mud needs careful
design and proper supervision to be successful. The high pH needed
may cause problems with elastomers.
NaCl or KCl/polymer/silicate mud. The mud is formulated as a
low-solids polymer using seawater, KCl, or NaCl with 20 to 40 ppb of

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