Measurement of Total Alkalinity and Carboxylic Acid and Their Relation to Scaling and Corrosion
- Mason B. Tomson (Rice U.) | Amy T. Kan (Rice U.) | Gongmin Fu (Rice U.) | Lili Cong (Rice U.)
- Document ID
- Society of Petroleum Engineers
- SPE Journal
- Publication Date
- March 2006
- Document Type
- Journal Paper
- 103 - 110
- 2006. Society of Petroleum Engineers
- 4.2.3 Materials and Corrosion, 4.3.4 Scale, 5.4.10 Microbial Methods, 4.1.2 Separation and Treating
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- 535 since 2007
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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.
|File Size||1007 KB||Number of Pages||8|
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