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Materials and Corrosion Risk Mitigation Associated with Flowback of Acid Stimulation Fluids

Acid stimulation is a growing practice in the deep water subsea environment. Two acids were tested, one of hydrofluoric and acetic acids and another of hydrofluoric, hydrochloric, and acetic acids. Results and outlines recommendations for different alloys are presented.

Product Number: 51317--8917-SG
ISBN: 8917 2017 CP
Author: Sandra Hernandez
Publication Date: 2017
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Acid stimulation is a growing practice to improve well productivity in the deep water subsea environment. Spent acid “flowback”, in which the acid returns are transported through the subsea system and topside processing facilities, is not a routine activity and poses significant materials, corrosion and degradation risks. Live acid contains corrosion inhibitor to protect the metallurgy during treatment operations, however most, if not all, of the corrosion inhibitor is spent shortly after entering the reservoir. When the well is opened to production, the spent acid is flowed back containing little or no corrosion inhibitor to protect the wellbore equipment, flowlines/pipelines, risers, tapered stress joints, or topsides piping/equipment. In addition to corrosion, environmental cracking is a major threat in acid fluids especially for Titanium alloys. This presents a challenge for the diligent operator where mitigation processes must be in place.
A multi-year testing program was undertaken to assess the compatibility of acid stimulation chemicals with the materials comprising downhole, subsea, and topsides equipment of deepwater projects in the Gulf of Mexico. Two different acids were tested, one composed of hydrofluoric and acetic acids (HF/organic), and another composed of hydrofluoric, hydrochloric, and acetic acids (HF/HCl/organic). This paper summarizes the results of this testing and outlines recommendations for different alloys so that the operation can be performed in a safe manner without compromising the integrity of the production system.
Keywords: acid stimulation, flowback, hydrofluoric acid, hydrochloric acid, carbon steels, low alloy steel, stainless steels, titanium

Acid stimulation is a growing practice to improve well productivity in the deep water subsea environment. Spent acid “flowback”, in which the acid returns are transported through the subsea system and topside processing facilities, is not a routine activity and poses significant materials, corrosion and degradation risks. Live acid contains corrosion inhibitor to protect the metallurgy during treatment operations, however most, if not all, of the corrosion inhibitor is spent shortly after entering the reservoir. When the well is opened to production, the spent acid is flowed back containing little or no corrosion inhibitor to protect the wellbore equipment, flowlines/pipelines, risers, tapered stress joints, or topsides piping/equipment. In addition to corrosion, environmental cracking is a major threat in acid fluids especially for Titanium alloys. This presents a challenge for the diligent operator where mitigation processes must be in place.
A multi-year testing program was undertaken to assess the compatibility of acid stimulation chemicals with the materials comprising downhole, subsea, and topsides equipment of deepwater projects in the Gulf of Mexico. Two different acids were tested, one composed of hydrofluoric and acetic acids (HF/organic), and another composed of hydrofluoric, hydrochloric, and acetic acids (HF/HCl/organic). This paper summarizes the results of this testing and outlines recommendations for different alloys so that the operation can be performed in a safe manner without compromising the integrity of the production system.
Keywords: acid stimulation, flowback, hydrofluoric acid, hydrochloric acid, carbon steels, low alloy steel, stainless steels, titanium

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