Metal sulfide mineral scaling, fouling and deposition are frequently encountered problems in geothermal systems. Their formation, crystallization and deposition occur principally because of their extremely low solubility, based on the low solubility product (Ksp). Among the metal sulfides that cause problematic issues, the most common ones are iron sulfide (FeS), zinc sulfide (ZnS), lead sulfide (PbS), and, less frequently, antimony sulfide(s) (Sb2S3 and Sb2S5). Zinc sulfide, for example, has a Ksp of 2·10-25 mol²/L², whereas for PbS, it is 1·10-28 mol²/L² (~ three orders of magnitude less soluble). ZnS can precipitate either as Sphalerite or Zinc Blende, and PbS commonly crystallizes as Galena. Mitigation of such ZnS and PbS precipitates and deposits can be achieved by chemical interventions, by the addition of organic chemical additives to the water. Herein, we report the inhibitory effects of phosphonate-based chemical additives for ZnS and PbS scales. These additives can inhibit formation of sulfide scale, and, significantly, prevent its deposition on metal surfaces. The efficiency of these additives is dosage-dependent, and relatively high inhibitor concentrations are needed for their inhibitory activity to take place. Possible mechanisms will be discussed focusing on inhibition and dispersion.
The accurate and precise analysis of scale inhibitors plays an important role in making key decisions on the efficiency of scale squeeze and continuous-chemical injection treatments. At present, several techniques exist for scale inhibitor analysis, but each method has its own limitations and tedious analysis process. In addition, these methods often give results of either total chemical content or elemental analysis without details of chemical speciation. Especially for phosphonate scale inhibitors, it is well known that there is no analytical methods available on the market to differentiate different species of phosphonate inhibitors, which impedes the applications of different types of phosphonate inhibitors on the scale treatment. There was therefore a need for a next-generation method for phosphonate analysis. An experimental methodology has been developed based upon the use of gold nanoparticles to enhance chemical signatures of scale inhibitors in brines using Surface Enhanced Raman Spectroscopy (SERS). This methodology enables speciation and measurement at low concentrations in the range of 1 to 100 mg/L (ppm). This study used two different phosphonate-type scale inhibitors, and initial laboratory results prove that this novel technology can help to differentiate between two different phosphonate-based chemicals.