Summary
Pickling well tubulars to prevent pumping unwanted materials into the
formation is an issue that should be decided on an individual-job basis. Acid
pickling of tubing is a process of flow in a tube or an annulus with
heterogeneous reactions occurring at the wall of the tube. The reacting solid
species at the wall are primarily mill scale and other types of inorganic
scale. The fluids injected for pickling are usually strong acid solutions
[hydrochloric acid (HCl)] but also may include surfactants, organic solvents,
and gelled solutions to aid in lifting solids from the wellbore. Thus, the
process is a complex one involving several reactions and perhaps multiple
stages of fluids.
Tubing pickling is an essential part of well-stimulation treatments if the
main acid is to be bullheaded. Standard design parameters for a pickle
treatment depend on experience and personal judgment. It appears that the
standard pickle-treatment design is overestimating the required volume of
pickling acid. Field data indicated that excessive acid volumes were used for
tubing pickling because large returns of unreacted (live) acid were recovered
at the surface. Careful analysis of flowback samples obtained from several deep
gas wells showed that only a fraction of the acid was consumed by mill scale
and other tubing contaminants.
In this paper, proposed mechanisms are presented to explain the behavior of
acid contact with the tubing and a mathematical model is developed to predict
acid consumption and dissolution of tubular contaminants. The model applies to
the bullheading case where the acid is pumped down the tubing and then flowed
back to the surface. The model considers the reaction of acid with mill scale
(Fe3O4). The equations formulated are solved numerically
to predict the concentrations of major chemical species in the well flowback
samples.
Such a model is extremely valuable in optimizing future pickling treatments.
The acid volume needed for pickling operations can be reduced significantly,
and other improvements can be made without extensive and costly field testing.
Finally, recommendations are given to design acid-pickling treatments
better.
© 2010. Society of Petroleum Engineers
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History
- Original manuscript received:
16 May 2008
- Meeting paper published:
3 February 2005
- Revised manuscript received:
19 November 2009
- Manuscript approved:
22 October 2009
- Published online:
7 April 2010
- Version of record:
22 September 2010
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