TABLE OF CONTENTS Sandstone acidizing is performed for two primary purposes:
Perforation breakdown
Near-wellbore formation damage removal
It is sometimes necessary to break down perforations by temporarily pumping acid above fracturing pressure to initiate production or injection of a subsequent treatment, such as hydraulic fracturing. Typically HCl is used in concentrations ranging from 5% to 20%, with 15% HCl being standard. As this is a routine procedure in certain areas, it will not be discussed in further detail here.
The primary purpose of matrix acidizing in sandstones is to remove formation damage caused by clay and other siliceous fine particles plugging near-wellbore permeability. Particles may be naturally occurring or may have been introduced into the formation during well operations. Damage can occur during drilling, completion, production, and stimulation operations.
HF is the only common acid that dissolves siliceous minerals appreciably. Therefore, sandstone acidizing formulations include HF or one of one of a variety of HF precursors, or compounds that generate HF. The most commonly used formulations are mixtures of HCl and HF. These are referred to as mud acid, from the early days of sandstone acidizing. Acid concentrations can vary from the low end (e.g., 3% HCl; 0.5% HF) to the high end (12% ?15% HCl; 3% ?5% HF or more).
For sandstones with high carbonate mineral content (>15% ?20%), HCl-HF mixtures should be avoided. HCl alone should be used in such cases. However, carbonates are often present in sandstones as grain...
