Page 59 - Environmental Control in Petroleum Engineering

Drilling and Production Operations 47

Hydrofluoric acid is used to stimulate wells in sandstone forma-
tions. It is normally used in a mixture of hydrochloric or formic acids,
and is used primarily to dissolve clays and muds. The reaction prod-
ucts are various forms of fluorosilicates. Like hydrochloric acid, it is
highly corrosive.
Formic acid is a weak organic acid that is used in mixtures during
stimulation. Formic acid is commonly used as a preservative. It is rela-
tively noncorrosive and can be used at temperatures as high as 400°F.
Acetic acid is used to dissolve carbonate materials, either separately
or in combination with hydrochloric or formic acid. It is a slowly
reacting acid that can penetrate deep into the formation and is useful
for high-temperature applications. Reaction products are calcium,
sodium, or aluminum acetates. Acetate salts have minimal environ-
mental impact. Like other organic acids, acetic acid has a relatively
low corrosivity.
To prevent acids from damaging or destroying tubulars from corro-
sion, corrosion inhibitors are normally used. Many commercially
available inhibitors are complex mixtures of organic compounds,
including thiophenols, nitrogen heterocyclics, substituted thioureas,
rosin amine derivatives, acetylenic alcohols, and arsenic compounds,
Most corrosion inhibitors are retained in the reservoir, so very little
is returned with the spent acid.
Highly reactive acids can react immediately with the formation.
Because the benefits of an acid are maximized if the acid is allowed
to penetrate deep into the formation before being spent, additives to
reduce the reaction rate are used. A common way to retard the reaction
rate is to emulsify the acid before injection, with the continuous phase
being the additive. Emulsions retard the reaction rate by physically
limiting the access of the acid to the formation. Commonly used
additives include salts, alcohols, aromatic hydrocarbons, and other
surfactants. Gelling agents, like xanthan gum and hydroxyethyl cellu-
lose, alcohols, acrylic polymers, aliphatic hydrocarbons, and amines,
are also used. Retarders such as alkyl sulfonates, alkyl amines, or alkyl
phosphonates are also used to reduce the reaction rate by forming
hydrophobic films on carbonate surfaces.
During production, the spent acid returning to the surface may
become emulsified with crude oil. These emulsions can be stabilized
by the fines released during acidizing. To prevent such emulsions from
forming, demulsifiers (surfactants) can be used. Common demulsifiers