Abstract

It has been observed that several oilwells, after submitted to acidizing operations, exhibited zonal intercommunication problems. This is attributed to the occurrence of reactions between the hardened cement slurry in the annulus and the acid. The indication of cement deterioration was the motivation of an extensive experimental program to study the involved mechanisms.

The work consists on the analysis of samples of hardened cement slurries, aiming the verification of changes in chemical compositions due to acid attack, as well as on the analysis of the acid solutions, aiming the identification and quantification of the elements liberated from the cement into the acid solutions. The techniques of X-ray diffraction and fluorescence were used for this purpose.

Based on the results, the depth of the acid attack and the influence of the acid solution composition and of the cement chemical composition on the degree of acid attack were evaluated. The acetic/hydrofluoric acid mixture proved to be the less aggressive solution among the ones analyzed, such as hydrochloric/hydrofluoric, acetic and hydrofluoric/acetic acids. The reactivity differences among the cement samples analyzed, due to small differences in their chemical compositions, showed to affect enormously the degree of acid attack.

This study enabled a better comprehension of the process of acid attack on cement, and consequently a reduction of this problem in the field was obtained.

Introduction

While there are a number of papers in the literature concerning the attack of acid on cement utilized in civil construction, there has been less work reported on the attack on cement under the conditions applied during the acidization of oilwells.

Fattuhi has studied the deterioration of concrete caused by sulfuric acid. By observing the variation in mass with time of a number of samples, it was verified that the resistance of concrete to attack by sulfuric acid is increased in concrete samples which contain styrene-butadiene latex. Chandra studied the resistance of mortars to hydrochloric acid by means of chemical analysis of the samples and Mehta evaluated the resistance of concretes to a number of aggressive solutions including hydrochloric, sulfuric, lactic, acetic acids, ammonium sulfate and sodium sulfate based on the loss of mass of the samples upon exposure to these solutions after various times.

Blount specifically treats the problem of resistance of cement utilized in oilwells with respect to attack by hydrochloric and hydrofluoric acids. The author, utilizing the methods proposed by Brady, compared the solubilities of different formulations of cement slurries in various acid solutions. The present work evaluates the resistance of cement cubes to acid based on various approaches. This includes the study of the loss of mass of the samples (utilizing the methods developed by the Research Center of PETROBRAS) and the study of alterations of chemical composition of both the cement cubes and the attacking acid solutions. The studies included both pure cement slurries (cement and water) and slurries containing additives in contact with a number of acids and acid mixtures including, acetic acid (HAc), mixtures of hydrochloric and hydrofluoric acid (HCl/HF) and mixtures of hydrofluoric and acetic acid (HAc/HF) of various relative concentrations.

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