Page 210 - Well Control for Completions and Interventions

Completion, Workover, and Intervention Fluids 203

• High pH values.
High pH brines are known to release fines. Changing the salinity
of the formation brine by the introduction of completion brines can
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cause increases in pH through a process of ion exchange. Many
heavier brines have low pH values, and pH is sometimes deliberately
increased to reduce corrosion. This applies mainly to “packer fluids”,
that is, those fluids left in the annulus at the end of the completion.
• Changes in water saturation.
If brine is lost to the formation, water saturation in the near wellbore
region will increase, leading to a reduction in oil permeability, since
water occupies more of the pore space. Producing the well will usually
reverse the effects, but damage could become long lasting or permanent
if brine additives are used that change the formation wettability. If the
formation is depleted (low pressure) capillary forces can make clean-up
of the lost fluid problematic, particularly in low permeability reservoirs.
• Changing formation wettability.
Most sandstone formations are water wet. Sand grains in the for-
mation are surrounded by a thin film (coating) of formation water
held in place by surface tension. When the well is produced, oil moves
through the pore, slipping past the immobile formation water. The
use of oil wetting chemical additives (surfactants) can change forma-
tion wettability by breaking the surface tension, allowing the water to
move into the pore spaces. Permeability to oil decreases, and the
decrease can be severe. Wettability changes (water to oil wet) are usu-
ally permanent (Fig. 5.4).
• Polymer invasion.
Polymers are added to brine for a number of reasons: as friction
reducers in “slick-water,” a viscosifying agent to control losses, to
improve the carrying properties of fluids during hydraulic fracturing

Figure 5.4 Water wet formation (left) and oil wet formation (right). Water trapped in
the pore space of the oil wet formation reduces permeability to oil.