Hydrochloric acid, also known as muriatic acid, is an aqueous solution of hydrogen chloride gas. Hydrochloric acid is used both to remove rust, scale and undesirable carbonate deposits in oil wells to encourage the flow of crude oil or gas to the well. This use is called “stimulation”. Acidizing is generally done in carbonate or limestone formations by stimulation. An acid solution is injected into the formation, which dissolves a portion of the rock and creates a large pore structure in the formation, increasing its effective permeability and the flow of oil.