Abstract

Cement attack by acid during matrix acidizing operations has created severezonal isolation problems in wells with multiple adjacent permeable zones, operated by PETROBRAS. This effect was observed in several wells treated with conventional HCl-HF acid mixtures, even when cement bond logs prior to the acidjob were excellent. This paper presents a series of lab experiments showing that acetic acid, alone or mixed with HF, dissolves much less cement than HCl and HCl-HF mixtures with the same carbonate dissolving power. The reaction of acetic acid with cement forms a protective skin that inhibits further acid attack. The organic formulations were tested in an acidizing simulator, before being used in the field. The paper also presents field test results which confirmed that the acetic acid mixtures did not break zonal isolation (cement consumption was restrained to the perforated interval) and were able to generate 6- to 40-fold increases in the injectivity and productivity of the wells.

Introduction

Cement solubility in acids has been shown to be a problem, both in the field and by laboratory testing. Ref. 1 shows that HCl, HCl-HF, and citric acid have a strong detrimental effect on the cement, resulting in weight loss and compressive strength loss. Traditional views held that due to the formation of a protective skin, acid-cement reaction would be limited. However, severe zonal isolation problems have been observed, following HCl-HF treatments. The first approach to solve the problem was the development of acid-resistant cement. These blends use liquid latex that inhibits acid attack by coating cement particles and by reducing the permeability of the cement. Refs. 2 and 3 address the use of acid-resistant cement blends for primary cement and squeeze operations. This solution may be easily applied to new wells, in which the primary cement and squeeze jobs in front of the pay zones are composed of acid-resistant slurries.

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