Paper: A New Concept In Hydrochloric-Hydrofluoric Acid Mixtures For Acidizing Low Permeability Sandstone Formations

Hydrochloric-Hydrofluoric acid mixtures for sandstone acidizing have been the topic of many investigations with respect to optimum hydrofluoric (HF) concentrations necessary for effective treatment. The hydrochloric acid (HCl) concentration needed is only at a concentration necessary to inhibit the formation of calcium fluoride (CaF2), and to provide a conversion media when utilizing ammonium bifluoride for HF production. All indications show that HCl-HF acids in concentration ranges between 3% HCl-0.6% HFto 6% HCl-1.2% HF are in many instances more effective acidizing systems with regard to eliminating the initial damaging effect caused when more conventional HCl-HF treatments are used. These conventional systems tend to chemically and physically dislodge or remove more than they can effectively "clean up", and depend on additional amounts of following acid volume to "eventually" remove these dispersed formation particles, or force them within smaller spaces in the formation. This fives the indication on conventional HCl-HF acids causeing initial "damage", which is followed by restoration of lost permeability, with some improvement, but seldom in proportion to the volumes and concentrations used. Lower concentration HCl-HF acid systems tend to initiate slower and less damaging reactions, thereby "cleaning up" after themselves and offering improved permeability without the initial probable sloughing threat and subsequent "damage" of the higher concentration conventional HCl-HF combinations. Thus an innovation of utilizing weaker HCl systems with intensification via the use of weak HF concentrations has provided an effective means to successfully acidize or acid-frac low permeability sandstone reservoirs.