Methods of Sludge Control During Acid Stimulation Treatments

Abstract

Abstract The formation of asphaltic sludge during acid stimulation has been a seriousproblem in many areas for several years. Such sludges have had adverse effectson the results of treatments in several Canadian reservoirs. Recent studieshave shown that sludge may also affect results in many areas where its effectshave not yet been recognized. These studies indicate that:Sludge is a precipitate of colloidal materials present in crude oils.The precipitates occur due to changes in the environmental conditions ofthe crude by the addition of materials such as acid.Once formed, sludge is insoluble in most treating chemicals.Sludge can be prevented or controlled by the use of stabilizing agents intreating fluid or by the use of certain solvents as the outer phase ofacid-in-oil emulsions. The purpose of this paper will be to show how and why sludge is formed andhow it can be prevented or controlled. Simple laboratory tests to determine theprobability of sludge formation prior to treatment are discussed. Actual fieldresults point out some of the problem areas in Canada and show that sludgecontrol during acidizing treatments can increase productivity in theseareas. Introduction The formation of sludge by crude oil on contact with acid has beenrecognized as a serious problem in isolated areas for some time. The problemwas apparently first observed in certain California wells. In these wells, itwas noted that following acidizing treatments the wells were very slow to cleanup, and often a great deal of asphalt-like material was returned with thetreating fluids. In some cases, complete or partial plugging of the wellresulted from the treatment. A study of this problem revealed that the crudeoils produced from these wells actually formed solid precipitates upon contactwith acid. An analysis of these sludge-like precipitates showed them to be madeup mainly of asphaltenes, resins, paraffin waxes and otherhigh-molecular-weight hydrocarbons. These materials were apparentlyprecipitated from the crude by the reduction in pH as a result of acidcontact.