Dissolver technology has been developed and applied with varying degrees of success over the past few years to clean carbonate and the more challenging sulphate/sulphide scales from production tubing and process equipment. A common question that is often raised but which has no published experimental work to provide an answer is "under what liquid to solid ratio would a sulphate dissolver not be effective to clean production tubing?"

A set of experiments were conducted at a range of temperatures to review the impact of surface area/mass to fluid volume ratio for barium sulphate scale. The surface area/mass to fluid volume ratio were measured from 13 examples of scaled or partially scaled production tubing in an attempt to understand why in some laboratory tests dissolvers used to remove sulphate scale appear to show better performance than is observed in the field.

This paper shows that simple dissolver tests, when performed with high volume/volume ratio (1 part scale to 45 chemical) which equates to a mass to liquid volume ratio of 1:10 which is most commonly used in screening studies, can give misleadingly high performance information as they show enhanced dissolution rates and mass of scale dissolved. In fact, from the 13 scaled pipe sections examined in this work, a much higher ratio of solid mass to liquid volume (10:1 to 1:1) (equating to a volume/volume ratio of 1:4.5 to 2:1) is more typical and with these ratios dissolver performance is greatly reduced.

The results from these experiments are used to give a guide as to the measured liquid to solid ratio where sulphate dissolvers have the potential to work for barium and calcium sulphate scales and to where mechanical removal of the deposit would be the better treatment option.

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