Field Method for Determination of Bicarbonate Alkalinity

Abstract

Alkalinity is used to calculate the bicarbonate concentrations under various production conditions and the bicarbonate concentration is needed to calculate saturation indices for the prediction of carbonate and sulfide scale formation and also to predict the corrosivity of the produced water during production. Since the saturation index is related to the square of the bicarbonate concentration, the accuracy of alkalinity is vitally important for the prediction of corrosion and scaling indices of all oil and gas production systems. Unfortunately, the total alkalinity remains one of the most difficult parameters to be measured accurately in produced water due to both interferences and the difficulty in preserving the sample during storage. When collected samples de-gas during transport, the dissolved CO 2 concentration decreases, pH increases, and brine components start to react with O 2 and these reactions cause a reduction in bicarbonate alkalinity.In this study, an experimental method was developed to determine bicarbonate alkalinity in minimal time and operating difficulty. The apparatus consists of four parts: a low pressure sealing vessel, high accuracy pressure gauge, a syringe for acid addition and a thermometer. The changed pressure in this closed vessel, which indicates the amount of CO 2 evolved after the addition of a strong acid, is used to determine the bicarbonate alkalinity of the sample. A wide range of synthetic waters and two production water samples were tested. Excellent agreement has been observed between true and calculated concentration. The results enable a more accurate measurement of produced water composition without the error caused by collection and preservation of samples. The field method combined with our automatic titration method enables more accurate prediction of pH, saturation index and corrosion risk at typical conditions of deep water production.This paper provides a reliable and detailed approach for field test of bicarbonate alkalinity with high accuracy and precision and a set of recommendations for field use.