Acid Numbers Of Saskatchewan Heavy Oills
Authors: Parker R.J. , Chung E.S.N. 
Published: March 2010

A modified ASTM D664 procedure was used to measure acid numbers for several Saskatchewan crude oils. Acid numbers were low, ranging from 0.3 to 2.2 mg KOH/g oil for heavy oils. Acidic components were quantified as strong (carboxylic acids) and weak (phenols) acids, with the former always predominant.

High concentrations of emulsified water and water soluble salts interfered with the procedure. These interferences were removed by dilution of the oil with toluene, followed by azeotropic distillation and centrifugation.

Crude oils produced by in-situ combustion had increased acid numbers. Acid numbers of wellhead oils could be correlated with the firefront location and emulsion treating difficulties.

Introduction

The presence of naturally occurring acidic components in crude oils is well established(1). These components can cause treating problems during production by stabilizing emulsions(2), and have been linked to corrosion problems during refining(3). Carboxylic acids, phenols, indoles(4,5) and sulphur containing functional groups(6) have been reported in oils, asphaltenes and coal liquids. Acidic components can also be generated in the oil by the recovery technique. Carboxylic acids and phenols are potential products of low temperature oxidation during in-situ combustion(7) or steam/air injection(8) processes. Sulphuric acid and water soluble carboxylic acids have been reported in produced waters from laboratory(9,10) and field combustion(11) tests. These acids could hydrolyze esters and amides (if present) in the oil to carboxylic acids(12).

A titration procedure (ASTM D664) can be used to determine the total acid content of an oil. The reported total acid number is then a composite of all the acidic functionalities in the oil. The titration curve can be resolved into two or more breaks by high resolution automatic titration techniques; these breaks represent strong (carboxylic acids), weak (phenols) and very weak (indoles) acids. The purpose of this study was to quantify the acidic components of various Saskatchewan crude oils produced by primary and thermal recovery processes and to correlate changes in acid content with production characteristics.

Experimental Procedures

Pipeline quality oil, containing less than 0.5% BSW, was used without further treatment. Wellhead emulsion samples were separated into oil and water by centrifugation in Nalgence bottles (250 ml) using a Beckman J2-21 centrifuge filled with a JA-10 rotor. Some oils still contained up to 2% water, as measured by Karl Fisher titration using a Photovolt Aquatest model IV instrument, even after 10 hours of centrifugation at 6000 – 8000 rpm. A variety of techniques, which are described later, were used to remove this interfering water.

Acid numbers were determined by an adaptation of the ASTM D664 procedure. The recommended solvennt, toluene/water/isopropanol (50.0:0.5:49.5), caused precipitation of asphaltenes, and was therefore replaced by a mixture of toluene/ water/ isopropanol/ tetrahydrof uran (50.0:0. 5 :24. 5:25.0). The titrant remained 0.1 M potassium hydroxide in isopropanol, which was standardized daily with potassium hydrogen phthlate. A Metrohm E636 autotitrator employing a glass (EAI09) and saturated calomel (EA404) elecu ode pair was used to perform the titration. The reported acid numbers are averages of at least three determinations.