Acid preflush

The main purpose of the preflush is to dissolve carbonate minerals in the formation prior to injection of the main HF mixture. HF reacts with carbonates, such as calcium carbonate and magnesium carbonate, to form insoluble calcium and magnesium fluorides. If a separate water displacement stage is not employed, the acid preflush serves the additional purpose of displacing formation water from the main HF stage. If given the opportunity, spent HF will react with sodium, potassium, and calcium ions in formation brine to form insoluble precipitates that can cause severe plugging in the formation. 

The standard preflush acid is HCl, usually at a concentration of 5%-15%. Organic acids, such as acetic and formic, can also be used by themselves, in combination with each other, or in combination with HCl. Organic acids are espedally useful in high-temperature applications, because they are less corrosive than HCl. Formic-acetic acid blends are popular for this reason. Acetic add is also useful by itself or in combination with HCl in formations with a high iron content (see the preceding subsection). 

because of very severe damage, the preflush cannot be injected, it may be necessary to break down the formation by pumping above fracturing pressure. This is acceptable as long as injection is returned to matrix rate (below fracturing pressure) as soon as possible. An alternative is to forgo the preflush and break down the severely damaged formation with the main HF phase. 

The purpose of the main acid stage is to dissolve siliceous particles that are restricting near-wellbore permeability, plugging perforations or gravel packs. Conventional solutions for use during the main acid phase are mixtures of HCl and HF. Common HCl-HF mixtures include the following  

Risks associated with acidizing—such as fines migration, precipitation of reaction products, and rock deconsoHdation—normally can be controlled (if not minimized or eliminated) by use of proper volumes and concentrations of acids. 

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A water displacement stage ahead of the standard acid preflush is also optional. The recommended fluid is a 2%-8% ammonium chloride (NH Cl) solution, with 5% NH Cl preferable for most applications. The recommended volume is 15-75 gal/ft, and a typical volume is 25-50 gal/ft. The purpose of this stage is to displace formation water containing appreciable bicarbonate and sulfate ions. A concentration greater than 1,000 parts per million (ppm) of these ions is considered appreciable. 

The HCl (or organic acid) preflush should be 50%-100% of the HCl-HF volume. If the solubility of the formation in HCl is less than 5%, then the preflush should be 50% of the HCl-HF volume. If the solubility in HCl is between 5% and 10%, then the preflush should be 100% of the HCl-HF volume. In formations with HCl solubility greater than 10%, larger preflush volume should be considered. 

A corrosion inhibitor is always necessary. It must be added to all acid stages (pickling treatment, acid preflush, main acid, and acid overflushes). Figure 6-1 shows the inside of a coiled-tubing string after exposure to insufficiently inhibited 15% HCl. 

The simplicity of a single-step treatment is possible because buffered HF solutions (with pH as high as 3.2-4.8) are essentially nonreactive with carbonate minerals. Calcium fluoride precipitation is thus prevented. Therefore, a conventional acid preflush is not required, even in carbonate-containing sandstones. HF, in the absence of HCl, experiences minimal reaction with iron. Hence, with the elimination of the mineral acid (HCl) preflush and a low concentration of HCl in the HF acid solution, reaction with iron (tubulars) is largely mitigated. HF reacts primarily as a compound. 

Examples of treatment procedures using such formulations are given in tables 7-3 and 7-4. Table 7-4 presents the more recently employed, simplified procedure, which eliminates an acid preflush ahead of HF injection. 

Acetic acid preflush 10% acetic acid + 5% NH CI 25gal/ft... 

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