Summary
Alkalinity is needed in many water-treatment calculations such as scale,
corrosion, precipitation, and oxidation, yet the concept is often
misunderstood. In natural waters, alkalinity often is not equal to bicarbonate
concentration, because natural waters contain base-contributing anions that can
significantly affect alkalinity. However, alkalinity is commonly assumed to be
equal to the bicarbonate concentration in many scale- and corrosion-prediction
algorithms. When other anions (e.g., carboxylates) are present, bicarbonate
concentration in production tubing is not a conservative quantity; it varies
with CO2 partial pressure, temperature, and carboxylate concentrations in a
complicated manner up and down a well.
Reliable methods to accurately measure true alkalinity are scarce,
especially when multiple weak acids are present and the effects of TDS on
electrode and color endpoint are significant. Oilfield brines contain aliphatic
carboxylic acids of one to six carbons (e.g., acetate) up to 5000 mg/L. The
highest concentrations of carboxylates tend to be in waters from reservoirs at
temperatures of 80 to 100°C. In this paper, a new analytical procedure and
computation routine to determine alkalinity and carboxylic acids simultaneously
will be discussed. The procedure was recently debugged and simplified by the
Rice U. Brine Chemistry Consortium (Rice BCC). The new titration method is
based upon simultaneous analysis of the titration curve determined at fixed
PCO2 and emphasizes the titration shape (profile) instead of the endpoint
inflection as is done presently.
A wide range of natural and synthetic waters has been tested. Excellent
agreement was observed between the true and calculated carboxylic acid
concentration with a correlation coefficient squared of 0.9986. Once the total
alkalinity and acetic acid concentrations are determined, the theoretically
correct bicarbonate concentration and/or pH at any given operation temperature
or pressure can be calculated. The intricate interrelationship of total
alkalinity, carboxylic acids, and pH on scale and corrosion will be discussed
by using case studies.
© 2006. Society of Petroleum Engineers
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History
- Original manuscript received:
30 June 2004
- Revised manuscript received:
31 May 2005
- Manuscript approved:
15 August 2005
- Version of record:
20 March 2006
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