A Contemporary Approach To Matrix Acidizing
Authors: Coulter G.R. , Jennings A.R. 
Published: December 2007

Abstract
This paper presents an updated look at the many variables associated with successful matrix acidizing. Damage types and formation characteristics significant when considering formation damage and damage removal are presented. Emphasis is placed on knowledge of formation mineralogy and temperature. The requirement for and function of the many acid additives are discussed. General information and guidelines for matrix stimulation of carbonates and sandstones are provided.

Introduction
The oil and gas industry has been acidizing oil and/or gas bearing formations since the 1890s. The primary acid used in the early days was hydrochloric acid. It is interesting to note that hydrochloric acid is still the primary acid used today for oil and gas well damage removal and stimulation. Since the early days, various other acid types have been developed along with additives to help make acidizing more effective. Acid types in use today include inorganic (hydrochloric and hydrochloric/hydrofluoric acid blends), organic (formic and acetic acids) and mixtures of inorganic - organic acids. Other acids such as citric, sulfamic, chloroacetic, EDTA (ethylene diamine tetracetic acid), NTA (nitrilo triacetic acid), erythorbic acid and benzoic acid may be used in acidizing systems for specific purposes, such as pH control, iron sequestering, diverting, etc. In addition to these, many other additives are often used in acid treatments. These include corrosion inhibitors, surfactants, alcohols, friction reducers, fluid loss control additives, diverting agents, acid reaction rate retarders, anti-sludging agents, suspending agents, mutual solvents and carbon dioxide and nitrogen gases. Determining the acid type, concentration, additives required and their concentrations for various temperature and mineralogical environments can be very perplexing to the practicing engineer. The objective of this paper is to discuss the important factors in selecting the correct acid for the proper well situation. General information is presented and points out the need to know formation mineralogy and as much about the type of formation damage existing as possible. It is always best to conduct laboratory tests on core and fluid samples from the well and interval of interest prior to conducting any damage removal treatment.
A number of prior authors have contributed greatly to the industry's understanding of matrix acidizing and damage removal. Many of these are referenced at the end of this paper. One of the widely referenced papers was presented by McLeod in 1984. This work made an excellent contribution to the industry. With the many new theories, additives and acid blends in use today, there is a need to update some of the acid selection criteria from earlier work. Additionally, the amount of interval now being treated in many horizontal wells invites a review of the industry's treating and diverting methods, as well as additives and acid types.
A premise of this paper is that the wellbore tubulars have been thoroughly cleaned with solvent and acid or with other efficient cleanout methods prior to the acidizing treatment. Pumping acids and other fluids down the wellbore prior to obtaining knowledge of the wellbore condition is discouraged. Even new tubing can have significant amounts of mill scale as well as pipe dope, etc, which can be detrimental to a matrix acidizing treatment. Older producing (or injection) wells may have deposits which may only be partially dissolved by the reactive acid and may act to divert the acid away from portions of the producing interval where it is most needed. Circulation of such deposits out of the well may provide information pertinent to the acidizing treatment (that is, confirm the need of organic solvents with the acid, provide scale samples for further analysis, etc.).

Formation Damage Types
Formation damage, or restriction to flow through natural flow paths in the formation, can be of three basic types. P. 297^