Acid Use as Part of Well Maintenance

Not all acids are created equal. It’s important to know which ones to use.

By Michael Schnieders, PG, PH-GW

Well fouling is a broad term that typically reflects impacts on the operation of a well or the produced water quality.

Well fouling can be a result of bacteria, scale accumulation, corrosion, or sediment influence. Well fouling occurs differently in each well and reflects the contributing formations; the well design, use, and operation; and the maintenance conducted (or lack of).

Analysis of the fouling is important to direct maintenance efforts by identifying the actual mechanism as well as the degree of impact on the well.

What Are Acids?

From a chemical scale standpoint, wells can be impacted by oxides, carbonates, sulfates, and also in some rare cases, hydroxides. Mineral scale accumulation is often combined with biofilm development and may have some formation sediment embedded. To address mineral scale accumulations, acids are commonly employed as part of a well rehabilitation effort.

The pH scale is used to identify whether a solution is an acid or alkali. Acids fall on the lower end of the scale, between 0 and 7 (neutral), with the more reactive being between 0 and 2.

Acids are a chemical substance that contains hydrogen and can give off hydrogen in water. Acids can be strong or weak depending on how many hydrogen ions they have to release. Acids vary greatly by their composition, their reactivity, their strength, and even the physical state available (liquid or powder).

The most common acids used in well maintenance include hydrochloric, sulfamic, phosphoric, oxalic, and hydroxyacetic (glycolic).

Hydrochloric, sulfamic, and phosphoric acids are mineral acids. Mineral acids are used to dissolve mineral precipitates such as calcium carbonate, calcium sulfate, iron oxide, and manganese oxide, or combinations of these compounds. Carbonates are the most easily dissolved and will result in the release of carbon dioxide.

The following example shows the action of hydrochloric
acid (HCl) on calcite (CaCO₃):

2 HCl + CaCO₃ → CaCl₂ + H₂CO₃
H₂CO₃ → H₂O + 2 CO₂↑

In this reaction, the hydrochloric acid (HCl) breaks apart the calcite (CaCO₃) into calcium chloride salt (CaCl₂) and carbonic acid (H₂CO₃), which further breaks down into water (H₂O) and carbon dioxide (CO₂), which bubbles off.

This reaction illustrates what happens as acids are introduced into a well with mineral scale. There is a reaction, heat is generated, gas is released, and the water becomes chemically congested. These are important factors to consider from a health and safety standpoint, but also in understanding what is occurring downhole.

Oxalic and hydroxyacetic acids are organic acids. In general, organic acids are weak acids, and unlike the strong mineral acids discussed already, do not dissociate completely in water.

Organic acids are commonly used for oil and gas well stimulation treatments or environmental remediation systems. When scale accumulations contain a high degree of biological activity and are still “soft,” organic acids can be used with benefit.

The challenge with organic acids is that as weak acids, they can neutralize and act as a carbon source for resident bacterial populations. These acids are much less reactive when compared to mineral acids and commonly need to be employed with or followed by a more aggressive mineral acid treatment.

Which Acids to Use?

Not all acids are created equal. The choice of acid used for well cleaning should be based on the identified fouling, the construction and integrity of the well, and the means in which it is employed downhole—not availability or price.

Similarly, the strength used and the time applied should also be balanced with the well and the fouling present.

Whether the acid is mineral or organic, concentrated or diluted, the grade and reactivity are all important factors to consider during well maintenance. Each chemical has a different effective strength, weight, and other properties that define its use and compatibility with other products.

Always use a chemically clean form of acid, and when necessary, use an approved inhibitor to reduce the harmful effects of chemical corrosion. An inhibitor is a chemical compound that when added to an acid decreases the corrosion rate of a material, typically a metal or an alloy.

Corrosion inhibitors are typically used in conjunction with mineral acids to reduce the acid’s attack on the metal structures downhole. During use, inhibitors typically have a short lifespan (typically less than six hours) and typically need to be reapplied, especially during larger well cleaning projects.

When choosing an acid for well rehabilitation:

• Select a chemical that targets the fouling mechanism.
• Make sure the products are NSF Standard 60 certified for potable well use.
• Ensure the products are compatible with the well construction.

Calculate the proper dosage based on the well design and the manufacturer’s recommendations for the type of problem and degree of impaction identified.

One mistake often made is the under-treatment of wells to target one specific zone, thereby discounting the influence of an entire column of water on the selected products.

A second common mistake is using an acid when there’s little for it to dissolve. If a well is suffering from severe biofouling, formation influence, or acute corrosion—a heavy acid treatment may in fact be detrimental to the well. In these cases, polymers are often used with only minor amounts of acid, mainly for pH adjustment.

From a health and safety standpoint, locate current safety data sheets (SDS) for each product and review prior to arriving at the well site. The SDS discloses reactivity, necessary safety equipment, health concerns, and disposal guidelines, all of which are important information to review ahead of time with the entire team and to keep on site during treatment.

Mechanical cleaning efforts are generally used as a means of both pre-treatment to remove bulk material before chemical cleaning, and agitation during treatment to disperse chemical solutions throughout the target zone.

As with chemical selection, identifying the right tools for pre-treatment and agitation should reflect the well’s design and structural integrity, and the identified fouling mechanisms. Brushes, swabs, jetting, and gas injection are just a few of the methods employed during rehabilitation.

During chemical cleaning efforts, pH, conductivity, and visible turbidity should be monitored and recorded. Testing should begin at the wellhead prior to the introduction of any chemicals and continue throughout the entire treatment process. Testing during rehabilitation allows for the monitoring of reactions occurring downhole. Upon evacuation, monitoring these parameters will assist in tracking the removal of both the chemical solution as well as the disrupted material.

If a well is experiencing greater than 50% loss of capacity and testing has identified a mixture of fouling mechanisms present, consider breaking the treatment up into two separate events.

Acid use during rehabilitation should be conducted in a safe manner with careful attention placed on the handling and administration of the chemicals into and out of the well. The investigation efforts conducted prior to treatment will help and act to focus the treatment into areas needing more aggressive efforts or longer contact times.

Proper prior planning regarding materials handling, neutralization procedures, containment equipment, and safety precautions will help assure a safe work site during treatment.

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Michael Schnieders, PG, PH-GW, is the president and principal hydrogeologist at Water Systems Engineering Inc. He has an extensive background in groundwater geochemistry, geomicrobiology, and water resource investigation and management. He specializes in the diagnosis and treatment of fouled wells and potable water systems. Schnieders is the 2017 McEllhiney Distinguished Lecturer in Water Well Technology. He can be reached at mschnieders@h2osystems.com.