The Chemical Retardation of Acid and How it can be Utilized

Five years of research and development went into the technique for evaluating the reaction of acid on calcareous materials. With this technique, improvements in several of the standard acidizing solutions were discovered aswell as a new method for chemically retarding acid. The new chemically retarded acid has properties which allow it to beutilized for a wide variety of well problems which no other single acid systemis versatile enough to handle. Actual field results are presented to support its utility. Introduction Acidizing has been known as a method of well stimulation for almost seventyyears. (1). It has been widely used for thirty years, but there has been, andstill is, a great deal of mystery associated with its use. One of thefactors obscuring the technique of acidizing is that little is actually knownabout what goes on downhole during a treatment. It has been the general belief that regardless of the type of formation or permeability of that formation, any kind of acid solution, viscous or thin, slow reading or fast, can be pumped into the existing permeability or down afracture at will and do what is desired of it. This, of course, is not true. With the advent of fracturing about fifteen years ago, a method for stimulating sandstone as well as calcareous formations became available. Along with the technique also came a more scientific and engineered approach to stimulation. Soon, sand and water or oil had almost replaced acid, even incalcareous formations, and acidizing, as such, went into a severely depressed period. There were still many things which acid could do that were not possible witha straight fracturing treatment, and therefore acid began a comeback withbetter retarded systems and other innovations (2) but still perhaps with less science than art. One of the greatest problems facing the science of acidizing has been the development of reasonably simple testing procedures which would allow evaluation, on a comparative basis, of the reaction rates of different types of acid on actual formation cores.