American Oil and Gas Reporter - July 2020 - 42

SpecialReport: Proactive Chemistry
Ounce Of Prevention
Through its past experiences, the industry learned that the old adage "an
ounce of prevention is worth a pound of
cure" was indeed true in conventional
vertical wells, and most operators began
employing chemical conditioning treatments to protect wellbores and equipment
whenever shut-ins had to be endured for
an extended time. Unfortunately, the
knowledge gained the hard way in conventional wells has not fully migrated to
unconventional production operations,
partially because of the relatively few instances where unconventional wells have
had to be shut in for lengthy periods.
However, the sudden collapse in oil
prices in early March, followed by lockdown measures in response to COVID19, resulted in many operators having to
simply shut down wells for an unknown
period and wait for better days and improved prices. While gas wells and conventional oil wells certainly were affected,
most of the shut-in production volume
came from unconventional oil wells.
What might companies expect as they
work to restore production in these horizontal wells?
The majority of unconventional oil
wells produce some associated gas. Like

their conventional counterparts, unconventional formations yield natural gas
that almost always contains some fraction
of carbon dioxide, and occasionally also
may contain hydrogen sulfide, a potentially
poisonous gas.
Henry's Law informs us that when a
gas and a liquid are together and pressure
is applied, part of the gas diffuses or dissolves into the liquid. As such, both CO2
and H2S (if present) can be present in an
unconventional well as weak acids in the
brine. Both CO2 and H2S have been
shown to corrode downhole metals, including those used in pump systems,
tubulars and rods.
Operators protect their downhole tubing
(and rods if the well is on artificial lift
with an insert or tubing pump) from these
corrosive forces with either constant injection or periodic batch application of
corrosion inhibitor. Corrosion inhibitors
lay down a light film on the metal surface
to prevent corrosive interaction with the
produced water's weak acids. Depending
on the type of inhibitor being applied,
the films range from very weak to somewhat tenacious, but none are intended as
a permanent solution.
For conventional wells scheduled for
shut-in, the industry has developed a

Proactive application of chemical treatments prior to shutting in an
unconventional oil well can eliminate potentially costly problems
when the time comes to restart production, including the deposi-

42 THE AMERICAN OIL & GAS REPORTER

standard practice of circulating an amount
of film-forming inhibitor and also placing
a water-soluble inhibitor in the flush
fluid to protect subsurface assets during
the shut-in period. As noted, it is highly
unlikely that this practice was applied
to many unconventional wells shut in
last spring.
During shut-in, any previously applied
corrosion inhibiting film will begin to
wane and corrosion from weak acids potentially will begin. The likelihood and
severity of this attack depends on four
key factors:
· Whether the metal surface is in
contact with produced brine;
· The temperature of the brine at the
point of contact and the pressure in the
well;
· The types and levels of acid gases
present in the brine; and
· The duration of the shut-in.
Greatest Risk
The corrosion cell requires a conductive
solution (brine) for the electron transfer
necessary for an electro-chemical reaction.
Therefore, tubulars or rods that are not
in contact with brine face little corrosion
risk. Likewise, where gravimetric partitioning in the annulus and tubing have

tion of paraffin and corrosion caused by the formation of carbon
dioxide and hydrogen sulfide or bacterial growth. Shown here is
damage to tubing caused by downhole bacteria.

American Oil and Gas Reporter - July 2020

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