Identification of Scale

There are times when the engineer in the field is called upon to identity scale samples. An operator may need to take immediate steps to remove scale from production tubing, flow lines, or other pieces of equipment, and time does not permit submitting a sample of the scale to a laboratory for analysis. The engineer must be able to determine whether the scale is calcium carbonate, iron carbonate, calcium sulfate, barium sulfate, or a combination of scales. The following procedures outline various methods that the field engineer may use to determine the type of scale in question. 

Prior to subjecting any scale sample to an analytical procedure, the sample should be rinsed in a suitable solution of water and a surfactant to water-wet any preferentially oU-wet sample. 

Place a sample of the scale in a beaker and add enough 15% or 37% hydrochloric acid to cover the scale sample. If there is a rapid effervescence (bubbling effect) and the sample dissolves, the scale is calcium carbonate (CaCOj). If the effervescence is very slow, heat the acid to approximately 65.5 °C (150 °F). If the rate of effervescence increases with the addition of heat, and the acid solution turns yellow, the scale is iron carbonate (FeCOj). 

If a reaction does not take place in the hydrochloric acid solution, proceed to Step II. 

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Table 6.8 offers a guide to the field identification of scales and probable causes. The use of this abbreviated chart is often complicated by appreciable amounts of oil and corrosion products being in the deposit. It is seldom possible to make a satisfactory examination of a scale which is oil soaked. After removal of most of the oil from compact scales, the criteria for tentative identification are usually made apparent by viewing with a hand lens and testing acid solnbiUty. 

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