Rapid And Accurate Electrochemical Determination Of Water Content From Core Flood Experiments
Authors: Ritz Charles L. 
Published: December 2007

This paper will outline a procedure for the rapid determination of water content in the effluents obtained from laboratory core floods. The procedure makes use of the Karl Fischer method, an established procedure for the quantitative analysis of water. It procedure for the quantitative analysis of water. It differs from distillation and centrifugation in that the water content is determined chemically. The main advantages of the Karl-Fischer method are:it is five to six times more rapid andit is more accurate by an order of magnitude compared to the usual routine methods for determining water content.

Introduction

The determination of water content in core flood effluents is an important routine operation in the proper evaluation of enhanced oil recovery techniques. proper evaluation of enhanced oil recovery techniques. It has been found in practice, however, that the conventional methods such as distillation and centrifugation leave something to be desired in terms of accuracy and time required.

For this reason, the electrochemical determination of water content with the aid of Karl Fischer's reagent and an automated titrator made by Brinkmann Instruments was evaluated and found very useful. The titration end-point is found by the dead-stop method. This is done by applying a constant DC voltage to a double platinum electrode immersed in the titration mixture. The change in current through the solution is measured as the titration proceeds (amperometry).

The titrating reagent is a commercially available Karl Fischer reagent consisting of a mixture of iodine, sulfur dioxide, methyl alcohol (or methyl Cellosolve) and pyridine. In the presence of water, the sulfur dioxide is oxidized by the iodine to the trioxide and then to sulfuric acid according to the following equation:

SO +I +2H O —&gt;2HI+H SO2 2 2 2 4

The acid reaction products (HI and H2SO4) are bound by the pyridine. When no more water is left in the reaction mixture, the free iodine acts as a depolarizer and causes the current to change.

Average consumption of titrant is 1 ml per 6 mg water. This raises a problem in determining water content of effluents from core floods. Such samples are often multiphased and may contain up to 10 ml water. This amount of water would require 1.6 liters of Karl Fischer reagent. Since Karl Fischer reagent sells for $6/quart, titrating the entire sample is not economical.

Dilution of the effluent sample would overcome this difficulty because a small fraction of it could be titrated with only a few ml of titrant. It would then be a simple matter to calculate the mass of water in the original, undiluted sample.

A solvent suitable for dilution of water and crude oil mixture is a 2:1 (by volume) chloroform and methanol solution. Both oil and water are soluble in this solvent. Furthermore, the chloroform-methanol mixture does not present problems in endpoint sensing. The amount of solvent needed depends on the size of the sample. For example, a 10 gram sample requires 300 ml of solvent for dilution. Using a lesser amount may result in phase separation.

After dilution, a 1–5 cc sample is titrated. Experience has shown that this sample size will consume about 2–5 ml of Karl Fischer reagent.

PROCEDURES PROCEDURES The following sections outline the steps necessary to carry out the water content determination of effluents from care flood experiments. The appendix provides a detailed format for gathering data and provides a detailed format for gathering data and calculating water content. These steps should be used as a guide and not a cookbook.