Abstract

Several disturbing results have been observed during recent studies into the compatibility of iron-contaminated acid with conventional additives and formation crudes. Along with the traditional problems of gellatinous iron hydroxide precipitation on acid spending, several additional problems with iron in acid have been discovered. These include additive precipitation or separation, asphaltic sludge precipitation, and formation of sludge with normally non-sludging oils. Parameters which affect these reactions include temperature, acid strength, degree of ethoxylation of surfactants, concentration of iron and the nature of formation oils. Since conventional iron sequesterants are inactive in live acid, other chemical approaches and alterations to field treatment techniques will be discussed.

Introduction

As a result of the gradually increasing use of acid stimulation treatments in Western Canada, Nowsco has conducted considerable development work to provide state-of-the-art, cost-effective acid additives. These additives are highly dispersible with a broad range of mutual compatibility. They are designed to control acid corrosion, prevent emulsion and sludge formation with the formation oil, and provide water-wetting and fines suspension properties to the acid as well as control the precipitation or iron during acid spending. Considerable pre-treatment and on-site testing has also been carried out. As well, the use of accurate additive dispensing systems at our field stations and effective field mixing equipment is being used to ensure that the correct concentrations of additives are present throughout the acid.

In spite of these precautions, two case histories are described (Appendix A)which show that dissolved iron even in the unspent acid was responsible for emulsion and sludge formation. Pre-treatment tests carried out in the absence of iron showed no such incompatibility. These findings were very disturbing and a great urgency was attached to studying the effect of dissolved iron on additive behavior.

The studies to be described will clearly show that dissolved iron, particularly ferric iron in 15%-28% hydrochloric acid has a very pronounced effect on acid additives containing organically bound oxygen. Some chemical terms describing such compounds are alcohols, polyglycols, sugars, glycol esters, ethers, oxyalkylates (ethylene oxide and propylene oxide treated compounds), and organic acids to name just a few. These compounds are widely used because they provide a means of obtaining water solubility without imparting polarity to the surfactants. This enables these compounds then to be compatible with other cationic or anionic additives. Even though products may be described as cationic or anionic, they will quite often contain non ionic components.

In addition to the effect of dissolved iron on surfactants, preliminary studies also showed that iron played a key role in the sludge forming tendencies of oil. The studies to be reported will also address this aspect of acidizing.

A preliminary survey of oilfield acidizing literature reveal that no studies on the above subjects have been reported. A review 1,2 of the chemistry of aqueous solutions of ferric iron however, showed that ionic complexes do form with many oxygen-containing compounds. Also, several foreign references3,4 to iron complexes of non-ionic surfactants were noted.

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