Summary
We propose an improved procedure for measuring acid numbers. Major changes
include spiking crude oil samples and blank solutions with a known amount of
stearic acid to force a clear titration endpoint, replacing potassium hydroxide
with tetrabutyl ammonium hydroxide in the alcoholic titratant, and correctly
accounting for changes in electrode response that occur upon exposure of the
electrode to crude oil.
Introduction
Chemical methods of improved oil recovery are not equally effective in all
reservoirs. An important factor that can influence a project’s success is crude
oil composition. Because crude oils are complex mixtures, evaluation of oil
composition in a way that is meaningful with respect to specific chemical
recovery processes can present many problems. In particular, there is a need
for improvements in acid number (AN) measurements, also known as total acid
number (TAN).
AN is important in evaluating crude oils for alkaline and surfactant
processes, but in order to be useful, measurements must be comparable from one
laboratory to another and must also capture chemically meaningful information
about the crude oil. Standardization (e.g., the current ASTM recommended
procedure) should assist with the first requirement: that different labs be
able to reproduce the AN value within some reasonable tolerance.
Standardization does not, however, ensure that the measurement captures
information about a crude oil that can be used to predict its interactions in
chemical recovery processes.
AN measurements are used to characterize an oil with respect to total
concentration of strong and weak acids by means of nonaqueous potentiometric
titration. The standard procedure (ASTM 2001) is designed to measure ANs in the
range of 0.05 to 250 mg KOH/g oil. Stock-tank samples of crude oil usually have
ANs that are at the low end of this range; strong acids are not encountered.
Thus the sensitivity of the ASTM method is barely adequate for many samples of
interest. According to the ASTM procedure, 20 g of oil should be used if AN is
less than 1 mg KOH/g oil. Unfortunately, high-quality samples of crude oil are
expensive to obtain and the quantity is very limited. Using 20 g for AN
measurement would often preclude making any other measurements. The usefulness
of AN data is greatly increased if it forms part of a matrix of information
that includes, at a minimum, base number (BN), SARA fraction data, and
information about asphaltene stability. There are few, if any, interfacial
phenomena that correlate exclusively to AN.
Basic constituents of an oil can also be assessed by nonaqueous
potentiometric titration, but endpoints are often more difficult to detect
because the organic bases that occur in crude oils can have a wide range of
dissociation constants. More than a decade ago, Dubey and Doe (1993) published
recommendations for improved base number measurements by adding a known amount
of quinoline to force a readily detectible titration endpoint. Base numbers
measured using spiked oil samples were significantly higher than those measured
by the ASTM method and the higher base numbers were shown to correlate,
together with AN for the same oils, with observations of wetting reversal on
silica surfaces. A similar procedure was shown to improve the precision of AN
titrations using stearic acid as the spiking agent for routine AN measurements
(Monsterleet and Buckley 1996). Precipitated material was observed for some
crude oils in the standard solvent (50% toluene, 49.5% isopropanol or IPA, and
0.5% water). Stearic acid and o-nitrophenol were used as spiking agents by
Zheng and Powers (2003).
© 2007. Society of Petroleum Engineers
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History
- Original manuscript received:
16 February 2006
- Meeting paper published:
22 April 2006
- Revised manuscript received:
2 February 2007
- Manuscript approved:
10 February 2007
- Version of record:
20 December 2007
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