This paper reports a challenging and unique scale forming environment in Northern Alberta, Canada. Calcite was forming on the fire tubes in a series of treaters. Bulk fluid temperature was 110°C but skin temperature of the fire tubes was in excess of 450°C. This created an incredibly high scaling potential.
The paper provides background on the field case history with a detailed description of the process and the scale forming environment. An in-depth scale modeling study characterizing the scaling up to 200°C is provided.
Further laboratory studies are reported including performance testing on candidate scale inhibitors and differential scanning calorimetry to determine their molecular stability. The testing looked at scale inhibitors and investigated the direct inhibition properties as well as any dispersion properties to prevent surface adsorption of scale on the fire tubes.
The testing found that the incumbent scale inhibitor (phosphonate based) was a good product for the bulk scale inhibition but was not stable at the temperature of the fire tubes. The most thermally stable scale inhibitors tested were polymaleic acid copolymers and polymeric acrylic acids of low to medium molecular weight. No scale inhibitors were tested to be fully stable up to 450°C but it was found in field application that the best product identified through laboratory testing was sufficient enough to delay the scale formation on the fire tubes. It is hypothesized that the product was stable at high temperature for the residence time of fluid in the treaters or imparted some degree of scale inhibition even after thermal molecular scission.
The paper concludes with data of the fire tube change out history highlighting the step change in performance when applying the polymeric inhibitor quantified by change out frequency and presence of hot spots.