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Scale inhibitor

Also known as
antiscalant, scale inhibition

A scale inhibitor is a chemical that prevents or slows the formation of mineral scale from water, usually at very low concentrations, by interfering with how scale crystals form and grow. Phosphonates and polymers such as polyacrylates are common types.

What is a scale inhibitor?

Water that is supersaturated with minerals, such as calcium carbonate or barium sulfate, will eventually deposit them as mineral scale. A scale inhibitor, also called an antiscalant, keeps that from happening, or slows it enough to matter, even though the water remains supersaturated.

Most scale inhibitors are threshold inhibitors: they work at concentrations far too low to bind all the scale-forming ions, often just a few parts per million, which distinguishes them from chelating agents.

How scale inhibitors work

  • Nucleation inhibition. They interfere with the formation of the first tiny crystals.
  • Crystal growth retardation. They adsorb on growing crystals and block the sites where new material would attach, distorting the crystals so they stay small and weak.
  • Dispersion. Some keep small crystals suspended so they do not settle and stick to surfaces.

Common chemistries

  • Phosphonates, such as HEDP, ATMP and DTPMP.
  • Polymers, such as polyacrylates, polymaleates and sulfonated copolymers.
  • Phosphate esters.

The choice depends on the scale type, temperature, water chemistry, compatibility with other chemicals, and environmental requirements.

How scale inhibitors are applied

  • Continuous injection into wells, flowlines, cooling systems and boilers.
  • Squeeze treatments, in which inhibitor is pumped into the rock around a well, adsorbs there, and is released slowly into produced water over months; see scale squeeze.
  • In fracturing fluids, to protect newly completed wells.

Operators monitor residual inhibitor concentrations to make sure they stay above the minimum effective level.

Frequently asked questions

A chelating agent binds scale-forming ions in roughly equal proportion, so it must be used in large amounts. A threshold scale inhibitor works at very low concentrations by disrupting crystal formation and growth.

Generally no. They prevent new scale. Existing scale is removed with acid, chelating dissolvers or mechanical methods.

  • Mineral scale

    Mineral scale is a hard, inorganic deposit that forms when dissolved minerals in water, such as calcium carbonate or barium sulfate, crystallize on surfaces. It builds up in wells, pipes, heat exchangers and equipment and restricts flow and heat transfer.

  • Scale squeeze

    A scale squeeze is a treatment in which scale inhibitor is pumped down a producing well and into the surrounding formation, where it is retained on the rock and then released slowly into the produced water, protecting the well and near-wellbore area from mineral scale for months.

  • Corrosion inhibitor

    A corrosion inhibitor is a chemical added in small amounts to a fluid to slow the corrosion of metal it contacts, usually by forming a protective film on the metal surface or by interfering with the electrochemical reactions that cause corrosion.

  • Chelating agent

    A chelating agent, or chelant, is a molecule that binds a metal ion at two or more points, wrapping it in a ring-shaped complex called a chelate. Chelants keep metal ions such as calcium, magnesium and iron in solution and prevent them from reacting, precipitating or forming scale.

Sources

  1. scale inhibitor, SLB Energy Glossary
  2. Scale inhibition, SPE PetroWiki