Scale is often defined as deposits of insoluble inorganic minerals. Common oilfield scales include calcium carbonate, barium sulfate, and metal sulfides.
While calcium carbonate scales deposition depends partially on pH and pressure, scale deposits generally occur when waters from different sources with different ion contents are mixed. The resulting deposits can quickly block tubulars and stop production.
In many cases, scale deposits can be dissolved, but for some scales (calcium fluoride in particular) mechanical scale removal is the only remedy.
Scale inhibitors work by sequestering metal ions, making them unavailable for ion pairing with anions and hence preventing precipitation of insoluble scale. Ion sequestration is usually accomplished with functionalized polymers. Negatively-charged carbonate and sulfonate groups bind to metal cations, keeping them in solution and removing them from the scale solubility equilibrium.