Introduction

Acidizing treatments are used to remove wellbore damage, enhance matrix permeability or both. The acid reacts with the rock matrix and, as a result, the permeability of the formation will increase. Reagents commonly used to stimulate carbonate reservoirs are hydrochloric (15 or 28 wt%), formic (9 wt%), and acetic (10 wt%) acids. Recently, Fredd and Fogler introduced ethylenediamine tetraacetic acid (EDTA) as an alternative fluid to acidize carbonate formations.

Acids commonly used in sandstone formations include hydrofluoricbased acids. Full strength mud acid (12 wt% HCl + 3 wt% HF) has been used to stimulate sandstone reservoirs for several decades. Because of the fast reaction of hydrofluoric acid with clay minerals, various retarded mud acids were introduced.