The evaluation of a present acid injection profile used in acid flowback analysis is an important tool for optimizing acid treatments, as it affects the acidizing process by improved results of hydrocarbon production rather than subjecting the well to yet another optimized acid treatment in the future. In the standard method of acid flowback analysis, the engineers interpret the plots of the return fluid ionic content versus return volumes at certain increments i.e. every 10-20 bbl. In our work, we have found that this method may pose more questions than it answers.

In order to simplify the acid flowback analysis method and provide a more definitive interpretation, we introduced a simple derivative technique in the analysis of the dissolved formation or the scale dissolved ionic content. In this technique the ratio of incremental ionic concentration per incremental produced volume (the derivative) was plotted against the cumulative produced volume. The significance of using this technique is that the derivative of ionic concentration with respect to the produced volume is very sensitive to the minute changes in the ion concentration per unit volume or cumulative acid flowback volume.

We applied our method to two wells, which we shall call them wells #1 and #2. In the case study of well #1, first we found that the acid stimulation treatment dissolved significant amounts of material containing Potassium, Sodium, Magnesium, Iron, Aluminum, Silicon, Barium, and Calcium. But the treatment did not remove the Zinc completely. The source of Zinc could have been either the formation of zinc hydroxide from heavy brines or the 60% Zinc content of some pipe dopes. Incomplete dissolution of material containing Zinc resulted in an inefficient treatment. Consequently, the treatment was able to restore only the original production rate but failed to result in any increase in incremental production. Secondly, from the disordered dissolution profiles of Magnesium, Calcium and Barium, we found that these three elements existed in the formation as individual mineral rather than scale.

In the case of well #2. we noticed that all materials containing the above mentioned elements including Zinc were dissolved. A complete dissolution of material containing Zinc from well #2 by acid treatment not only resulted in restoration of the production rate to the original one but also showed a significant increase in incremental production rate. From a near ideal, well-ordered, correspondence of dissolution profiles of Magnesium, Calcium and Barium, constructed with derivative technique we found that these three elements existed in the formation as scale rather than individual formation minerals. This is simply because, unlike the rock mineral, the scales, due to the well established thermodynamic principles, follow a well-ordered phase behavior and ionic concentration upon dissolution in acids.

On the basis of our finding, we concluded that (a) our derivative method provides a more efficient tool for interpretation of the acid flowback results and (b) the method is simple, easy, quick, and practical enough to be used immediately by the personnel in the field or in the lab.

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