Page 60 - Environmental Control in Petroleum Engineering

48 Environmental Control in Petroleum Engineering

include organic amines, salts of quaternary amines, and polyoxy-
ethylated alkylphenols. Glycol ether can be used as a mutual solvent
for both spent acid and oil.
Wettability agents are used to alter the relative permeability of
emulsions during acidizing and to change the wettability back when
acidizing is complete. The objective of such wettability changes
is to lower the injection pressure by maximizing the relative perme-
ability of the emulsion during injection and to maximize the subse-
quent production rate by maximizing the relative permeability of
oil after acidizing. Wettability is changed by the use of surfactants
such as ethylene glycol monobutyl ether, methanol, 2-butoxy ethanol,
or fluorocarbons.
To lower the pumping pressure during acidizing, friction reducers
are used with acid to reduce its viscosity. Friction reducers allow a higher
injection rate for a given pump size or allow a smaller pump for a given
injection rate. Friction reducers are normally organic polymers that convert
Newtonian acid to shear-thinning, non-Newtonian fluid.
Solvents can be used as a preflush with acid to clean oil sludges
and paraffin off of formation particles so they can be better contacted
by the acid. These solvents normally have a high alcohol content, e.g.,
methanol or isopropanol.
Because the local permeability in a formation can vary significantly,
the acid injection profile may not be uniform. To modify the injection
profile and provide a more uniform acidization, fluid loss and diverting
additives like benzoic acid flakes, naphthalene flakes (mothballs), rock
salt, silica flour, or polymers can be used.
Even when corrosion inhibitors are used, some iron compounds
will be dissolved into the acid and carried into the formation. In
some cases, this iron can precipitate in the formation, reducing its
permeability. Complexing agents, like citric, lactic, acetic, and glu-
conic acids, or derivatives like ethylene diamine tetracetic acid (EDTA)
and nitrilo triacetic acid (NTA) can be used to inhibit the precipita–
tion of iron.

Hydraulic Fracturing

Hydraulic fracturing increases the permeability around a wellbore
by creating a high permeability channel from the wellbore into the
formation. During hydraulic fracturing, fluids are injected at a rate high