Predicting Results of Sandstone Acidizing
Authors: Gatewood J.R. , Hall B.E. , Roberts L.D. , Lasater R.M. 
Published: December 2007

This work proposes to investigate the reaction rates of hydrofluoric- hydrochloric acids on silica and various clays and mixtures thereof, and then mathematically to determine the amounts of both sand and clay removed from formations at various depths. Also calculated are theoretical increases in the productivity of undamaged and mud-damaged formations and of formations naturally damaged by clay.

Introduction
Treatment of sandstone formations by mixed hydrofluoric-hydrochloric acids has been used as a means of removing damage caused by the presence of days. The removal of such damage presence of days. The removal of such damage results from the dissolution of clay by reaction with the hydrofluoric acid:
36 HF + Al2Si4O10(OH)2-&gt;4H2SiF6 + 12 H2O+ 2H3A1F6.
The acid will also react with sand and other siliceous minerals:
6 HF + SiO2 -&gt;H2SiF6 + 2 H2O.
These reactions, though they appear simple, are really very complex. Further reactions may take place that produce insoluble reaction products. It is for this reason that excess hydrochloric acid should be maintained in the mixture and dilute hydrochloric acid solutions are used as preflushes ahead of the mixed acids. A discussion of these reactions and their effect on productivity has been presented by Smith and Hendrickson. An awareness of these possible problems allows us to approach such treatments with problems allows us to approach such treatments with proper design to circumvent undesirable results. proper design to circumvent undesirable results. Another equally important aspect of successful acid application is that of adequately describing the effect of hydrofluoric acid reaction on depth of acid penetration. It is this aspect that ultimately penetration. It is this aspect that ultimately determines the extent of damage removal and subsequent productivity increase. productivity increase. Relatively little has been done in the area of investigating the reaction rates of hydrofluoric acid, or the mixed acids, on silica and silicates. It has been generally state that the reaction is faster on clays than it is on sand, but without quantitative work the desired calculations have not been possible. It has long been assumed by some that these acid systems could be injected into a sandstone formation to dissolve clay at almost any depth from the wellbore, and that the reaction on sand is so slow that little reaction takes place. It has been our purpose to investigate the reaction rates of hydrofluoric-hydrochloric acids on silica, various clays and mixtures thereof, and then to determine mathematically the amounts of both sand and clay removed from formations at various depths, as a function of acid reaction. Also, theoretical increases in productivity have been calculated, on the basis of the penetration determinations, for the cases of undamaged and mud-damaged formations and for formations naturally damaged by clay.

Reaction Rate Studies Experimental
The determination of reactant concentrations has been a problem in following the HF-silica reaction and probably accounts for the meager data available in the literature. Development of a specific ion electrode for fluoride ions by the Orion Corp. has enabled us to obtain much information accurately and rapidly. The analytical procedure and equipment have been previously described by Gatewood. previously described by Gatewood.
JPT
P. 693