Search
Contact Us

t: (440) 561 0510

Linkedin Facebook-f X-twitter

Mineral scale

Also known as
scale, scaling, oilfield scale, carbonate 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.

What is mineral scale?

Scale is the hard, crusty deposit left when minerals come out of solution and crystallize on a surface. The white crust inside a kettle is a household example. In oil and gas production and industrial water systems, scale builds up inside tubing, pumps, flowlines, valves, heat exchangers and boilers, and in the rock around wells, where it narrows flow paths and insulates heat-transfer surfaces.

Common types of scale

ScaleChemical formulaTypical cause
Calcium carbonate (calcite)CaCO3Pressure drop releasing carbon dioxide; temperature increase
Barium sulfate (barite)BaSO4Mixing sulfate-rich water with barium-rich water
Strontium sulfate (celestite)SrSO4Mixing incompatible waters
Calcium sulfate (gypsum, anhydrite)CaSO4Temperature change, evaporation, mixing
Iron sulfide and iron compoundsFeS and othersCorrosion and hydrogen sulfide
Silica and silicatesSiO2 and othersEvaporation, temperature change

Scales often occur mixed with each other and with organic deposits.

Why scale forms

Water can hold only a limited amount of each mineral in solution, and that limit depends on temperature, pressure, pH and the other substances present. Scale forms when water becomes supersaturated:

  • Pressure drop. As produced water rises and pressure falls, dissolved carbon dioxide escapes, pH rises, and calcium carbonate precipitates. This is why carbonate scale is common near the wellbore and in tubing.
  • Temperature change. Calcium carbonate and calcium sulfate become less soluble as temperature rises, which causes scaling on hot surfaces such as heat exchangers.
  • Mixing incompatible waters. When injected water containing sulfate, such as seawater, meets formation water containing barium or strontium, sulfate scales precipitate.
  • Evaporation. In cooling towers and boilers, water evaporates and minerals concentrate.

Waters with high total dissolved solids or hardness are generally more prone to scaling.

Effects of scale

  • Restricted flow and lost production in wells and pipelines.
  • Plugged perforations and reduced permeability in the rock, a form of formation damage.
  • Reduced heat transfer and efficiency in heat exchangers and boilers.
  • Stuck valves, damaged pumps, and corrosion under deposits.

Scale is one of the main threats addressed by flow assurance and industrial water treatment.

How scale is prevented and removed

  • Scale inhibitors, such as phosphonates and polymers, used in small concentrations to stop crystals from forming or growing. In wells they are often placed by a scale squeeze treatment.
  • Acid treatment. Hydrochloric acid dissolves carbonate scales; see acidizing. Sulfate scales such as barium sulfate are not dissolved by common acids.
  • Chelating agents, which bind scale-forming ions and can dissolve some deposits.
  • Mechanical removal, by milling, jetting or scraping, especially for acid-resistant scales.
  • Water management, such as avoiding the mixing of incompatible waters, and controlling concentration in cooling systems.

Scale and Locus biosurfactants

Locus’s AcidBoost is a biosurfactant microemulsion for acid treatments that Locus describes as improving how acid contacts carbonate scale near the wellbore. Locus also supplies its Amphi biosurfactants to water treatment formulators as components of deposit control packages.

Frequently asked questions

Mainly changes in pressure and temperature as water flows from the reservoir to the surface, which push dissolved minerals past their solubility limits, and the mixing of incompatible waters, such as injected seawater with barium-rich formation water.

No. Acid dissolves carbonate scales well, but sulfate scales such as barium sulfate are essentially insoluble in common acids and usually need chelating dissolvers or mechanical removal.

Scale is a deposit that forms on a surface from minerals in the water. Corrosion is the loss of metal by chemical reaction. They are related, because corrosion products can form deposits and corrosion can occur under scale, but they are different processes.

  • Scale inhibitor

    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.

  • 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.

  • Acidizing

    Acidizing is a well treatment in which acid is pumped into a well to dissolve rock, scale or other damage near the wellbore and restore or improve the flow of oil, gas or water. The main forms are acid washing, matrix acidizing and acid fracturing.

  • Fouling

    Fouling is the unwanted buildup of deposits on surfaces in contact with water or process fluids, such as heat exchangers, pipes, membranes and cooling towers. Deposits can be mineral scale, suspended solids, corrosion products, organic material or biological growth.

  • Water hardness

    Water hardness is the amount of dissolved calcium and magnesium in water, usually expressed as milligrams per liter of calcium carbonate. Hard water reduces the performance of soaps, surfactants and some pesticides, and causes scale in pipes and equipment.

  • Total dissolved solids TDS

    Total dissolved solids (TDS) is the total concentration of minerals, salts and other substances dissolved in water, usually reported in milligrams per liter. It is the standard measure of how salty or mineralized a water is, from drinking water to oilfield brine.

Sources

  1. scale, SLB Energy Glossary
  2. Scale problems in production, SPE PetroWiki