Introduction

The phenomenon of scale formation has been experienced by generations of different races whenever water has been used. It was formed in Roman aqueducts and canals and is also found in the present day in the most modern oil production operations. The most commonly found scales in this industry are the carbonate and sulphate salts of calcium, barium and strontium which can be encountered from the reservoir rock itself all the way through the complete process train to the oil exporting system. It can therefore be appreciated that considerable decreases in efficiency of equipment and reservoir production can result from this.

The need to maintain high rates of oil production makes this a serious problem and remedial action is desirable. This can be in the form of problem and remedial action is desirable. This can be in the form of inhibition to prevent scale formation, or, where scale is already present, removal is required.

This paper outlines the mechanisms of scaling and the principles of scale dissolution using sequestrants. Details of treatment techniques, especially in downhole situations, are also presented.

SCALE DEPOSITION

Although we are dealing mainly with scale dissolution some comments on scale deposition are pertinent to our understanding of the techniques involved.

Scale deposition results from the crystallisation of material from solution. It is generally accepted that for crystallisation to occur three factors are required (1):

SUPERSATURATION NUCLEATION PRECIPITATION PRECIPITATION Of these three factors, SUPERSATURATION is the primary cause of scale formation and occurs when a solution contains dissolved materials which are at higher concentrations than their equilibrium concentration. Solutions can become supersaturated by changes in conditions, such as temperature, pressure and pH.

NUCLEATION is the initial formation of a precipitate and is commonly caused by the presence of a foreign crystalline material on sites e.g. walls of pipes, vessels, welds or downhole formation rock. PRECIPITATION and adherence can then occur to produce the scale deposit

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