Understanding Sandstone Acidizing Leads to Improved Field Practices
Authors: Shaughnessy Christopher M. , Kunze Kenneth R. 
Published: December 2007

Summary
A design criterion based on type and depth of damage is proposed to replace laboratory design curves that may have been distorted by CO2 evolution. Furthermore, sandstone acidizing experiments have provided evidence that reprecipitation of dissolved silica from spent HF can be a real limitation to acidizing success if operational precautions are not taken. Finally, the potential for iron precipitation from spent HCl acidizing solutions is seen to be dependent on certain conditions such as iron oxidation state and the presence of carbonates.

Introduction
Over the years, many field practices have been devised to maximize the success of conventional sandstone acidizing techniques. Much of this technology has been developed by field experimentation supported by only a limited amount of scientific data. A comprehensive examination of sandstone acidizing chemistry has revealed sound technical reasons for many of the common field acidizing operations. Other practices have been shown by experiment to be groundless. By emphasizing this technical basis, we hope to reinforce good acidizing practices and discourage poor ones.Through experimental laboratory work, three important concepts of sandstone acidizing have enserged:1. Hydrofluoric acid (HF) begins to increase rock permeability as soon as it is injected, provided the rock has been preflushed properly with HCl to remove carbonates.2. Silica precipitation from spent hydrofluoric acid can be very damaging to rock permeability.3. Sandstone formations containing iron-bearing minerals can be acidized successfully.These three concepts are important to the successful design of hydrofluoric acid stimulation treatments in the field.
Permeability Increase Caused by HF The phenomenon of initial permeability decline during laboratory acidization of cores is of particular interest because of its influence on many acidizing procedures. One of the earliest reports of this phenomenon appeared almost 15 years ago, and numerous others have followed. These observations have been incorporated into general field guidelines that frequently call for large acid volumes just to overcome the damage caused by initial acid injection.Various theories have attributed this initial damage to lines plugging, precipitation of insoluble fluosilicates, and precipitation of silica. However, recent laboratory experiments have convincingly demonstrated that the initial damage in laboratory cores is caused primarily by CO2 gas saturation.
CO2 Damage in Cores Most sandstones contain various amounts of carbonate minerals. When contacted with acid, carbonate minerals will react to generate CO2 and a corresponding salt. If this CO2 is liberated as a gaseous phase within the pore structure, the rock will appear damaged. However, this phenomenon is not true damage because the absolute permeability of the rock is not decreased. Instead, the development of a CO2 gas saturation temporarily lowers the relative permeability of the aqueous phase.
Solubility Of CO2 in Spent Acid When 15% hydrochloric acid is reacted totally with carbonate minerals, the spent acid is very similar to a 20% calcium chloride (CaCl2) solution. Likewise, the spending of 50% HCl produces a solution similar to 7.5 % CaCl2. Fig. 1 plots the solubility of CO2 in 7.5% and 20% CaCl2 vs. pressure.
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