Page 211 - Well Control for Completions and Interventions

204 Well Control for Completions and Interventions

operations, and for wellbore cleaning operations. Polymers can be
adsorbed onto the surface of a formation, leading to damage. Viscous
polymers will impede flow, so must be designed to break.
Many polymers arrive at the wellsite as dry powder and need to be
mixed (hydrated) before use. Poorly hydrated (fish eye) polymers are
very damaging. If a polymer is not mixed properly, the unhydrated
powder can enter pore spaces causing significant damage (Fig. 5.5).
• Scale.
While oilfield scales are generally associated with production
related precipitation, scale can and does form during interventions and
workovers, and is the result of fluid incompatibility. The most com-
mon scales encountered during interventions are the carbonates or sul-
fates of the alkaline earth metals calcium, strontium, and barium. Scale
has the potential to damage the formation, and of more importance in
the well control context, scale can prevent the proper functioning of
safety critical completion components, such as downhole safety valves
and Christmas tree valves.
Scale can form when incompatible fluids mix. The most common
brine incompatibility occurs when carbonate ions in formation water
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and calcium ions (Ca ) in brine are allowed to mix. The result can
lead to calcium carbonate scale (CaCO 3 ). Similarly, barium ions in
formation water, if allowed to mix with sulfate ions in seawater, will
lead to the formation of a hard barium sulfate (BaSO 4 ) scale.
• Emulsion blocking.
Water based completion fluid can form emulsions with oil from
the formation. Similarly, if oil based fluids are introduced into the for-
mation emulsions can form with formation water. Emulsions form
more readily if surfactants are used with the brine. Since emulsions are
more viscous than the constituent parts, they make it more difficult
for oil to flow through the formation.

Figure 5.5 Partially hydrated polymer (fish eyes). Very damaging. Source: Image
courtesy of George King.