Acidizing Wells with Acetic/HF Acid Mixtures to Minimize Cement Dissolution

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.

New Fluosilicic Acid System Removes Deep Clay Damage

“…6 Well CAM-710, however, was treated with H2SiF6/HCl because it does not have any adjacent zones. Both wells were injecting below their quota, and the treatments were able to restore their injectivity and sustain it for several months, as shown in the injection-history curves in Fig.…”

New Fluosilicic Acid System Removes Deep Clay Damage

“…Due to the mishap in well AP-008, the designer of the acid job for well CAM-707 decided to substitute acetic acid for HCl to minimize the risk of acid attack to cement (6) . Well CAM-710, however, was treated with H 2 SiF 6 /HCl, since it does not have any adjacent zones.…”

Effect of Mud Acid on Portland-Aqueous Polyurethane Composites to Oil Well Cementing

“…Acidic solutions are pumped down under pressure through the casing towards the formation during oilwell acidizing operations [2], and come in contact with the cement sheath. There they react and damage the cement [3][4][5][6]. Traditional industry views hold that acid reacts with cement for a short period of time to a point where a protective acid-inhibiting skin is formed [4,5,7].…”

“…There they react and damage the cement [3][4][5][6]. Traditional industry views hold that acid reacts with cement for a short period of time to a point where a protective acid-inhibiting skin is formed [4,5,7]. However, reports show that significant numbers of cement squeeze jobs were found to break down or develop zonal isolation problems after HCl-HF acidizing [5,6].…”