Coiled tubing is usually used to conduct acid pickle treatments. The purpose
of the treatment is to prevent pumping damaging materials into the formation
before the main acid job. In this case, the acid is pumped down the
coiled tubing, and then flowed up through the tubing-coiled tubing annulus.
Pickling is a process of flow with heterogeneous reactions occurring with mill
scale (Fe3O4) and other contaminants at both the inner
wall of the production tubing and the outer surface of the coiled tubing.
Pickling is an essential part of well stimulation treatments if the main
treatment fluids (e.g., acidizing, fracturing, and gravel packing) are to be
bullheaded. However, it appears that traditional pickle practices are
overestimating the required acid volume and/or concentration. Field data
indicate that excessive acid volumes are used for tubing pickle because large
returns of unreacted acid are usually recovered on the surface.
In this paper, mechanisms to explain the behavior of acid contact with the
tubing are presented, and a model for predicting acid consumption and
dissolution of tubular contaminants is developed. The model includes reactions
of acid with mill scale. A system of nonlinear, partial differential equations
is developed, and the equations are solved numerically to predict the
concentrations of major species as a function of axial position along the
tubing and in the effluent from the well during flowback.
Field application included pickling of low-carbon steel tubing (11,900 ft of
5.5-in. C-95) using coiled tubing. A slug of 5,000 gals of 20 wt% HCl with
additives was used. Samples were collected from the treated well during the
flowback of the pickling treatment. The model was used to predict the
concentrations of various species in the well flowback samples. Model
predictions for acid, chloride ion, and total iron concentrations were in
agreement with field results. In addition, the model proved to be a valuable
tool in optimizing future pickling treatments.
Scientists and engineers working on chemical treatments to enhance well
performance usually focus on reservoir characteristics, fluid placement, fluid
compatibility, and thermal stability. Less attention, however, is given to
potential formation damage that might occur if the contaminants present in the
well tubulars invade the formation.
Several studies were conducted to identify the type and amount of
contaminants present in well tubulars (Holub et al. 1974; Maly 1976; McLeod et
al. 1983; McLeod 1984). Pipe dope and mill scale were identified as main
contaminants present in the production tubing (Coulter and Gougler 1984;
Gougler et al. 1985; Loewen et al. 1990; Nasr-El-Din et al. 2002). Invasion of
these materials into the target zone can cause severe formation damage.
Therefore, it is always recommended to minimize the amount of pipe dope used
and to clean well tubulars before any chemical treatments, especially if these
chemicals are to be bullheaded.
The types of fluids used in pickling treatments depend on the type of
contaminants. Xylene and similar organic solvents are used to dissolve the
organic portion of pipe dope. Hydrochloric acid is used to remove acid-soluble
material present in the pipe dope (mainly zinc) and mill scale. Selection of
acid additives depends on temperature, type of tubular, and fluids that will
come in contact with the acid. It should be noted that environmentally friendly
pickling fluids were introduced during recent years, with positive field
results (Curtis and Kalfayan 2003; Berry et al. 2003).
© 2007. Society of Petroleum Engineers
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- Original manuscript received:
21 February 2005
- Meeting paper published:
12 April 2005
- Revised manuscript received:
16 October 2006
- Manuscript approved:
19 October 2006
- Version of record:
20 August 2007