Summary
Formic and acetic acids, with and without HCl, have been used extensively
over the last 40 years as stimulation fluids and as acid additives. A
measurement method for these organic acids was not previously available for
highly contaminated acid stimulation samples that contain high acid
concentrations, dissolved oil, suspended solids, and several different acid
additives. Citric and acetic acids are also used in acid treatments as
iron control agents at temperatures less than 200 and 100°F,
respectively. An analytical method using capillary electrophoresis was
developed to simultaneously measure the concentrations of formic and acetic
acids in stimulation fluids. The developed method was used to evaluate flowback
samples from pickle treatments and stimulation treatments of several
high-temperature wells.
A method for the simultaneous measurement of formic and acetic acids was
developed, evaluated, and implemented for acid stimulation samples.
Citric acid concentration can be determined in certain cases, which are
identified in the paper.
Results of formic and acetic acid concentrations in sandstone and carbonate
corefloods were evaluated.
Flowback samples from pickle treatments and acid fracture treatments were
evaluated. Results from treatments of super chrome-13 and low-carbon steel
tubing with 15 wt% HCl/ 9 wt% formic acid show that a small decrease in formic
acid relative to chloride concentration occurred over time with carbon steel.
No significant decrease of formic acid relative to chloride concentration was
observed in the flowback of a well with a super chrome-13 completion.
Introduction
Organic acids have been used extensively in acid stimulation treatments in
the oil industry. Harris first reported the use of acetic acid for well
completion and stimulation in 1961. 1 Formic acid was used with HCl for
high-temperature stimulation by Dill and Keeney in 1978. 2 Acetic acid 3--13
and formic acid 6--9,12,14 have been used extensively in stimulation treatments
in recent years. The application of organic acids in Saudi Arabia was first
reported by Nasr-El-Din et al. and Hashem et al., 8,15 where a mixture of
acetic acid (5 wt%) and hydrochloric acid (5 wt%) was successfully used to
stimulate water supply and injection wells in a sandstone field in central
Saudi Arabia. In addition, citric and acetic acids are frequently used with
hydrochloric acid as iron control agents. 16 Acetic and formic acids are used
in field treatments at concentrations up to 13 and 9 wt%, respectively. 17
Formic acid is used as a corrosion inhibitor intensifier in wells completed
with low-carbon steel tubulars. 18 Even so, the measurement of formic acid
concentration in flowback samples has never been reported. Acetic acid
concentration in flowback samples has been reported once using a titration
method, 13 to the best of the authors' knowledge.
To effectively evaluate stimulation treatments using organic acids, it is
critical to measure their concentrations in the injected fluids and in the well
flowback samples. Analysis of these samples can be difficult because they can
contain high concentrations of chemical additives, dissolved solids, suspended
solids, and oil. Nasr-El-Din et al. 13 used a titration method to determine
acetic acid in the well flowback samples. However, this method is only
applicable for high acetic acid concentrations and only measures the
concentration of unspent acid.
Methods that have been used to determine the concentration of carboxylic
acids in aqueous (nonacid) samples were summarized by Volgger et al. 19
Chromatographic methods have usually been used, including gas/liquid
chromatography, liquid chromatography using ion exchange, ion-moderated
chromatography, reverse-phase chromatography, and ion chromatography. Although
these methods can have very low detection limits, they often require extensive
sample pretreatment, derivatization steps, and long analysis time. 20 In
contrast, capillary electrophoresis (CE) is characterized by ease of sample
preparation, low cost, and short analysis time. 21 The theory of capillary
electrophoresis has been discussed previously. 19,22--25 Evans and Beaumont 26
have compared capillary electrophoresis methods with ion chromatography (IC). A
specific disadvantage of IC is fouling of the expensive anion-exchange column
by complex samples. To summarize, no methods have been reported for the
determination of formic and acetic acids in flowback samples following
acid-stimulation treatments. Based on the literature, it appeared that
capillary electrophoresis was likely to be the most suitable method for
determination of organic acids in complex samples obtained from acid
stimulation treatments.
Several published CE methods for the determination of carboxylic acids were
considered for modification to meet the requirements of this work. These
included the methods of Volgge.et al., 19 Chen et al., 20 Wu et al., 21 Devevre
et al., 22 Oehrle, 27 Madsen and Lind, 28 and Krol et al. 29 A direct-detection
method is preferred because the dynamic range is much greater than with
indirect detection and the electrolyte contains fewer components. 25 Direct
detection methods are compared in Table 1.
For the present work, the analytical conditions of Oehrle 27 and Krol et al.
29 were investigated. This decision was based on the simplicity of the
electrolyte and the expected detection limit of the method of 1 mg/L. In
stimulation fluids, concentrations of acetic and formic acids can range up to
13 and 9 wt%, respectively. This means that field samples can be highly diluted
prior to analysis and still contain detectable concentrations of organic acids.
This dilution is extremely useful because it is expected to reduce the effect
of interferences. Use of an accurate autodiluter introduces very little
additional error into the measurements. Typically, with dilutions of 200 to 1,
a dual syringe autodiluter will have accuracy +1% and precision +0.2%. 30
© 2005. Society of Petroleum Engineers
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History
- Original manuscript received:
21 October 2003
- Manuscript approved:
22 March 2005
- Version of record:
15 June 2005
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