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.
Matrix acidizing, also called acid stimulations or acid jobs, are increasingly utilized to improve oil production from deep water reservoirs. The typical primary objective of an acid stimulation is to dissolve precipitated scale, sand, clay, and fine particles from produced water or oil, or remove damage caused by drilling, completion, or workover fluids, all of which can cause formation damage/plugging and drastically reduce productivity. Acidizing is not a routine activity. The live or ‘fresh’ acid is typically bullheaded into the well via the production tubing. The return of acid, during which the acid is transported through the subsea system and topside processing facilities, is called ‘flowback.’ Flowback is not a routine activity and poses corrosion and materials degradation risks.
The acid composition used in the field is chosen dependent upon the mineralogy of the well and the type of damage. For example, acidic mixtures containing hydrofluoric (HF) acid are commonly used where sandstone is prevalent; hydrochloric acid (HCl)-containing acids work well in carbonate formations. Combinations of HF and HCl, along with organic acids like acetic and formic acid are also used, depending upon mineralogy and scale composition. A common acidizing procedure consists of several steps, including injection of a solvent, an acidic pre-flush, injection of the live, or ‘fresh’ acid into the well, injection of an overflush, and then return of the ‘spent’ acid mixture.