A mixture of hydrofluoric acid (HF) and hydrochloric acid (HCl) or organic acid used as the main fluid in a sandstone matrix treatment. Hydrochloric acid or organic acid is mixed with HF to keep the pH low when it spends, thereby preventing detrimental precipitates. The name 鈥渕ud acid鈥?was given to these mixtures because they were originally developed to treat damage from siliceous drilling muds. Mud acid is also called hydrofluoric-hydrochloric acid.