An Evaluation of Scale Inhibitors

Abstract

A rating of inhibitors in the order of their effectiveness is a relative matter and depends upon test conditions. The most important conditions are the degree of supersaturation and the temperature; the higher the supersaturation and the higher the temperature the less effective a scale inhibitor becomes. Scale Inhibition Is Still an Art Scale inhibition is still closer to being an art than a science. The literature deals only to a very small extent with the problems of scale inhibition. Most of the published data do not consider the basic facts of crystallization and inhibition. such as reaction mechanisms and kinetics. Because these fundamental considerations are often neglected, there are many recorded failures of scale treating procedures in oil and gas fields. This neglect of fundamentals and lack of data is surprising because the formation of scale is a rather costly problem for some industries, above all the petroleum industry. We lose many millions of dollars petroleum industry. We lose many millions of dollars in revenue every year because scale creates flow restrictions in our fields. Accurate evaluation of scale inhibitors is the first and most important step toward effective scale control. How Are Scale Inhibitors Presently Tested? Presently Tested? Test Machines Several investigators have been concerned with the development of test machines designed to simulate the conditions of scale deposition in the field. The degree of scale buildup in these test devices is used to screen and rank commercial products and experimental chemicals. Scale Coupons The use of scale coupons in the field is a test somewhat related to the use of test machines in the laboratory. Steel coupons are inserted in flow lines, and the amount of scale they accumulate is used to evaluate an inhibitor that has been injected into the system upstream of the coupon. Precipitation Test Precipitation Test In the "chelation test," which should be called the precipitation test, two chemically incompatible precipitation test, two chemically incompatible solutions are combined. The precipitation of scale-forming solids - for example, calcium carbonate (CaCO ), calcium sulfate (CaSO,) or barium sulfate (BaSO ) is measured in the presence of an inhibitor of varying concentrations by means of chemical analyses. The amounts of precipitate obtained are then compared with the amount of precipitate formed by mixing solutions containing no scale inhibitor. The more solids that are kept in solution by a given amount of inhibitor. or the less inhibitor that is needed to keep a given amount of solids in solution, the more effective and desirable the inhibitor is thought to be. This test is widely used for laboratory evaluation of scale inhibitors. The test is very quick and convenient to run, gives fast results, does not require costly equipment, and can be run by relatively untrained personnel. Unfortunately, the reproducibility of the result is sometimes very poor, and data reported by different testers is often contradictory. P. 997