Conductometric Measurement of Carbonate Scale Deposition and Scale Inhibitor Effectiveness
Authors: Englander H.E. 
Published: December 2007

Most diagnostic scaling tests currently used are basically qualitative, indicating trends in scaling and corrosion. Although these methods are widely used, their applicability is limited. Electrolytic conductometry can be used successfully for determining quantitatively the capacity of a water to precipitate CaCO3, and for evaluating the effectiveness of different carbonate scale inhibitors.

Introduction

This paper discusses the successful use of electrolytic conductometry for determining quantitatively an oilfield brine's capacity to precipitate calcium carbonate (CaCO3) under given conditions, and for determining the optimum dosages of scale inhibitors for specific brines. Changes in a sample's dissolved CaCO3 concentration cause small, reproducible changes in specific solution resistivity. These changes can be converted by a precalibrated graph to parts per million of CaCO3 precalibrated graph to parts per million of CaCO3 precipitated. precipitated.The method described is sensitive to traces of inhibitor in the water. Errors introduced by chemical changes between sampling and testing are prevented by using a special sampling method.

The new conductometric method has shown that inhibitor effectiveness is a function of brine composition. Using synthetic brines rather than natural brines to test inhibitors may introduce errors.

Correlations between laboratory tests and field data have shown that the conductometric scale-potential test is a valuable diagnostic tool for predicting scaling quantitatively.

Removing CaCO3 deposes that plug producing equipment and reservoirs is time consuming and costly. Reducing recurring scale deposition requires diagnostic methods that closely estimate the degree of scaling anticipated.

Diagnostic Scaling Tests

The Langelier saturation index and the Ryznar stability index are used frequently to predict a water's stability with respect to carbonate scale formation and corrosivity. These and similar methods are basically qualitative. They indicate trends in scaling and corrosion. The indices are calculated from geochemical analyses of the water, as well as from pH and temperature.

Although these methods are widely used, their applicability is limited. Produced waters may contain natural organic surface-active agents or residual traces of scale inhibitors. Methods now used to calculate the stability of a water neglect these important factors and give no information on the stability changes caused by adding inhibitors. They cannot be used to evaluate the effectiveness of inhibitor treatment in the field? The accuracy of pH measurement at the time of brine sampling is often doubtful, but it is a predominant factor in estimating scaling tendency and treatment. Predictions based only on geochemical water analyses and solubility data should be viewed with limited confidence.

The analytical determination of residual inhibitor concentration in produced brines is an inadequate indicator of inhibitor effectiveness. However, such tests can be referenced to the conductometric method to assure maintenance of the dosage shown to be effective by the conductometric method.

JPT

P. 827