High-Concentration Hydrochloric Acid Aids Stimulation Results in Carbonate Formations
Authors: Harris O.E. , Hendrickson A.R. , Coulter A.W. 
Published: December 2007

Abstract
Use of high concentrations of hydrochloric acid in fracture acidizing treatments has solved the stimulation problems in many stubborn reservoirs and has improved results in many others. Treatments of this type were first designed for injection wells in the Aneth field in Utah about a year ago. Their use has now spread to many areas throughout North America. Success of the treatments has led to a re-evaluation of the role of acid concentration in stimulation by acidizing. Extensive laboratory studies and wide-spread field experience have uncovered many new facts concerning acid concentration and have shown that some of the older assumptions are not true at all. This paper presents the technology of acid concentration and factors involved in using high concentrations. Advantages of the strong acid are discussed, as well as the problems (such as inhibition and additive compatibility) encountered in its use. Actual field examples are used to substantiate the work done to the present time.

Introduction
Use of high-strength hydrochloric acid for fracture acidizing of carbonate-type formations has become extremely popular within the past few months. This popularity is probably an outgrowth of the use of 28 per cent HCl for stimulation of injection wells in the Aneth field in Utah. The strong acid was so successful there that use of higher concentrations of acid spread rapidly to many other areas where more conventional treatment had failed or had shown only limited success. Actually, use of high-strength acid is not new, but lack of technical information and knowledge of how best to use it has limited its past use to occasional isolated cases. Only recently has renewed laboratory and field investigation provided information necessary to properly engineer a treatment using this material.
Laboratory studies indicate that at HCl concentrations above 15 per cent, acid properties change significantly with increasing concentration. With a knowledge of what these changes are and where and how they occur, high-strength acid may be used to attain these advantages:
- greater fracture conductivity resulting in sustained production and injection increases (this is particularly true in reservoirs of high water saturation which make weaker solutions of acid ineffective because of rapid dilution),
- greater penetration because of more rapid initial decrease in area-to- volume ratio,
- greater volume of CO2 gas per barrel of reacted acid (this has proved extremely effective in aiding well clean-up), and

Technical Discussion
Fracture acidizing-hydraulic fracturing with acid-is an effective method of improving the production or injection characteristics of many carbonate-type oil- and gas-bearing formations. The success of such fracture treatment depends on two factors: fracture conductivity and depth of fracture penetration. In fracture acidizing, the amount of conductivity depends on both the amount of material removed from the formation and where it is removed along the fracture faces. While many factors control the action of acid in removing the formation material, concentration of the acid can play an important role in increasing conductivity during a fracture acidizing treatment. Of the various types of acid available, 15 per cent hydrochloric acid has long been recognized as a very effective chemical for removing portions of the formation and providing this conductivity. Since the conductivity obtained is proportionately related to the amount of material removed, it is a logical conclusion that if more of the formation is removed the resultant fracture conductivity will be greater. Mathematical calculations show that, in fact, conductivity varies as the cube of the fracture width according to the following equation.