About 50% of all oil reservoirs worldwide are based on carbonate minerals ( limestone/chalk/dolomite) and about 50% are sandstone (quartz, feldspar etc.), although they can contain a small percentage carbonate minerals also. Acid stimulation is used to increase permeability both in production and injector wells, carbonate or sandstone, by dissolving various acid-soluble solids naturally present in the rock matrix or as formation damage. There are many types of formation damage only some of which can be treated with acids, for example, organic deposits such as wax and asphaltenes cannot be treated. This section on acidizing should also be read together with the section on scale dissolvers in Chapter 3 on scale control since low-pH acidizing will also remove carbonate and sulfide scale deposits. To chemically remove sulfate scale, high pH chelates such as salts of polyaminocarboxylic acids are used.