Oil & Gas

Potassium chloride inhibits reactive shales. Potassium ions exchange with sodium and other hydrated cations on clay surfaces and, because they fit closely into the clay structure, reduce how much water the shale takes up, so it swells, softens and sloughs less. KCl muds are usually combined with polymers such as xanthan gum. The right concentration depends on the shale's clay mineralogy and is set by mud-engineering tests, since more is not always better.

They remove different things. Monoethanolamine (MEA) is an amine that chemically absorbs the acid gases hydrogen sulphide and carbon dioxide to sweeten sour gas, and the rich amine is regenerated by heating in a stripper. Triethylene glycol (TEG) is a hygroscopic liquid that absorbs water vapour in a contactor to dry gas and prevent hydrates and corrosion; it is regenerated in a reboiler at around 200 °C. A plant treating sour, wet gas often needs both.

Formic acid is chosen when hydrochloric acid would react too fast or corrode too much. As a weaker organic acid it reacts more slowly with carbonate and is easier to inhibit at high temperature, so it suits hot wells, deeper live-acid penetration and long acid contact with tubulars. Its strength is limited because calcium formate can precipitate at higher concentrations, so formic acid is used at lower strengths than HCl, sometimes blended with it.

Hydrochloric acid dissolves carbonates but not the clays, feldspars and fine silica that block pore throats in sandstone; only hydrofluoric acid does that. Ammonium bifluoride dissolved in HCl generates HF within the acid, avoiding the handling of concentrated HF, to make mud acid such as 12% HCl with 3% HF. An HCl preflush removes carbonates first, because calcium reacting with HF forms insoluble calcium fluoride that can damage the formation.