Understanding the role of chemicals during treatment will help you select the right ones for your job.
By Michael J. Schnieders, PG, PH-GW
Science has allowed us a much better understanding of the conditions occurring downhole in well systems.
Water and scale analysis, coupled with pump tests and video surveys, can give us better insight into the condition of the well structure and the location of various fouling mechanisms. These advances have also impacted the way we respond to different problems.
In our work in potable well systems, we have found combined chemical and mechanical treatment strategies work best. Much like cleaning pots and pans in the sink following dinner, the combined efforts of soap and a scrub brush are more effective than soaking or brushing alone.
Before we go any further, let’s break down the three main aspects of well rehabilitation. First, there is disinfection. Disinfection occurs at the completion of construction of the well following modification or pump work, at the end of cleaning efforts, and periodically in an effort to target bacterial issues. Disinfection commonly uses chlorine or other oxidative chemistries in an effort to reduce the active microbial population downhole, not to sterilize the well environment.
Chemical cleaning is typically a combined chemical and mechanical effort that targets mineral scale, biomass, or combinations of both. Chemical cleaning is generally acid based, but may also include dispersants, surfactants, inhibitors, and in some rare cases, may use a caustic solution.
Development or redevelopment is a method of treatment that specifically targets drilling fluids and fine sediment following construction or after many years of migration. Development relies heavily on mechanical efforts, but can also utilize chlorine and specifically designed dispersants to target clay and silt.
When choosing chemicals to be used in well systems, remember to use NSF-approved products designed for use in potable well systems. The National Sanitation Foundation, known by its acronym NSF, is an independent public health and environmental organization providing standards development, product certification, testing, auditing, education, and risk management services for public health and the environment. NSF certifies products to verify they meet public health and safety standards, and as such, have become the preferred means of certification for many states and countries.
As NSF certifies a wide variety of chemical products, ensuring the chemical chosen has been approved for use downhole, in the unique environment present in well systems, is important. As part of the NSF Standard 60 approval, chemicals introduced into a potable well are to be flushed out prior to the well system being returned to active use as a public water supply.
When selecting and employing chemicals for use in the well environment, there are four main questions to ask. These questions guide the selection and use of the variety of chemicals available.
- Do we know what the problem is and to what extent is the well impacted?
- Is this chemical the right product for the identified problem?
- Are we using the chemical correctly and monitoring the right field conditions during treatment?
- Are we limiting harmful impacts to the well and environment?
Traditionally, chlorine and acid have been the products of choice when combating fouled well systems. However, as we learn more about the nature of fouling and the impacts of rushed treatment practices, more effort has gone into fine-tuning the cleaning process.
Understanding what role each chemical plays during treatment is important. Misapplication can result in damage to the well and the environment, compounding existing fouling mechanisms, as well as pose a health and safety challenge to those working around the well.
Acids, including mineral and organic acids, are used to dissolve mineral precipitates (scale) in the well environment. Mineral scale generally reflects the background water chemistry and includes common compounds such as calcium carbonate, iron oxide, and calcium sulfate. During its reaction with mineral scale, acid usage typically results in heat generation, a release of gas, and an increase in the conductivity of the water.
Acids vary significantly by their composition, weight, reaction time, and relative strength or activity. For example, hydrochloric (HCl) acid is a fast-reacting mineral acid commonly used in a liquid form of 31% strength at a rate of 3% to 12% of the targeted treatment volume. At 9.7 lbs/gallon, HCl is slightly heavier than water.
Sulfamic acid is a slower reacting compound commercially available in a powder or granular form. Sulfamic acid is available in a 99% active form typically used at a rate of 2% to 5% of the targeted treatment volume.
Each acid is different physically and chemically, providing different benefits or challenges. For example, hydrochloric acid is effective at combating calcium carbonate scale, but can be harmful to iron and stainless steel well components.
Oxalic acid is useful against iron and manganese scale, but is hindered by high hardness or significant biology presence. Selection of the acid and its rate of use should be dependent on an understanding of the problems occurring downhole as well as the well design.
An inhibitor is a chemical compound that, when added to a cleaning solution or acid, decreases the corrosion rate of a material, typically a metal or an alloy. The effectiveness of a corrosion inhibitor depends on fluid composition, quantity of water, and flow regime.
Corrosion inhibitors are typically used in conjunction with mineral and organic acids in an effort 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 need to be reapplied, especially during larger well cleaning projects.
A variety of products have been introduced recently to the industrial cleaning field as inhibitors. Our research has shown many of these so-called “safe” acids limit the aggressiveness toward organic material, reducing the risk associated with handling of the acid, while failing to reduce the acid’s attack on the metal.
As with any chemical selected, find out the extent of the protection offered before application. Reducing risk to the crew is certainly important, but do not jeopardize the well structure during treatment.
Acid enhancers and dispersants are products used alone and in conjunction with acids to improve well rehabilitation procedures. Dispersion chemistry is the use of specific polymers to block the attraction of positive and negative forces which account for the formation of salts (compounds), crystals, crystals, and colloidal masses.
When used with acids, these products are designed to aid in the dissolution of mineral scale, prevent re-precipitation, and improve the evacuation of both small particles and dissolved solids (ions). In biofouling situations, these chemicals aid in the penetration and dispersal of biofilm and biomass.
As most fouled wells consist of a mix of mineral scale encrustation and biomass, biodispersants and acid enhancers can be very helpful in improving rehabilitation efforts.
Chelating agents are chemical compounds that react with a metal to form an aggregate, often in a liquid form used to trap and remove heavy metal ions. Chelating agents are especially useful in wells experiencing heavy iron or iron bacteria fouling where removal of iron deposition is important to improving flow and operation. Chelating agents are often added to dispersants and used in conjunction with acids in an effort to improve cleaning efforts.
Surface active agents, also known as surfactants or wetting agents, are used to reduce surface tension and increase penetration of scale and biomass deposits, gravel pack, and formation. Surfactants are generally concentrated and typically only a small quantity is used. Surfactants are especially useful in tight formations and areas of heavy fouling. Surfactants are most commonly used in conjunction with other chemicals during cleaning and disinfection efforts.
An emulsifier is a chemical additive which encourages the mixing of one substance into another, as in the mixture of oil and water. The oil is said to be solubilized removing its adhesive properties, much like dish detergents allow for the easy cleaning of a fry pan. Generally a more specialized chemical, emulsifiers are used when oil fouling has occurred as in the cleaning of some environmental wells. Closely related to emulsifiers are stabilizers, substances that maintain the emulsified state.
Caustic or alkaline solutions are chemicals that have a pH greater than 8. Caustics are occasionally used in water well cleaning as the base chemistry to remove oil contamination and some heavily biological fouling. The use of caustics is limited though as they have almost no effect on mineral compounds, and in some cases can alter the water chemistry to form scale during treatment. There are special dispersants designed to improve caustic cleaning applications, but it remains a complicated method of cleaning.
Caustic solutions are more commonly employed as a means of neutralizing acid-based solutions following downhole use and evacuation from the well environment. Caustic chemicals are reactive to human skin and require a different response as compared to acid spills.
Chlorine is a strong oxidative chemistry that remains a commonly used chemical in well treatment efforts. Chlorine is available in gas, liquid, and solid forms and is typically used as a means of disinfection. Due to the difficulty and dangers associated with gas chlorine, the liquid and solid forms are more commonly used in water well treatment.
Calcium hypochlorite is the more common solid form of chlorine and is available as a tablet or powder. Similar to other forms of chlorine, calcium hypochlorite is available in several different strengths.
Due to the calcium component, calcium hypochlorite can be difficult to dissolve downhole. As such, dissolving it at the surface before applying it to the well is recommended to assure the chemical is active. Additionally, the introduction of additional calcium is often discouraged to reduce the introduction of additional scale-forming ions.
Sodium hypochlorite is the more common liquid form of chlorine used. It is available in a variety of strengths ranging from 4% to 15%. Sodium hypochlorite solutions naturally degrade over time and at a quicker rate with exposure to air, sunlight, and temperature fluctuations. It is recommended to always use a new or fresh batch of sodium hypochlorite for disinfection applications.
The use of chlorine solutions, while common, can still be complicated. The dosage rate varies depending on the application. Typically, for disinfection of new or recently cleaned wells, the rate falls within the range of 50 to 350 parts per million. “Shock chlorination” using chlorine concentrations above 500 ppm for disinfection is not recommended. For development purposes, when targeting enhanced drilling fluids, a range of 1200 to 1500 ppm is typically desired.
Additionally, when using chlorine for disinfection purposes, pH adjustment may be required. Depending on the natural buffering capability of the water, the development of hypochlorous acid, that biocidal form of chlorine, may be limited. Neutral, alkaline, or groundwater containing high carbonate hardness or elevated total alkalinity, may require buffering of the solution to improve the biocidal effectiveness. In some cases, a subtle pH change may increase the effectiveness of treatment by up to 300%.
Chlorine enhancers are employed to increase the effectiveness of disinfection treatments. At a minimum, the product should aid in buffering the pH of the treatment solution to allow for effective production of hypochlorous acid to maximize the biocidal effects of chlorine. This allows for less chlorine to be used and a more efficient treatment effort.
The best products buffer these solutions based on the background alkalinity and amount of chlorine used. In addition, chlorine enhancers typically include biodispersants and surfactants to improve the treatment process.
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When choosing chemicals to use in well cleaning, redevelopment, or disinfection activities, we generally recommend concentrated products, reducing the volumes required for transportation and mixing. It is strongly encouraged each chemical be reviewed and the compatibility verified with other chemicals being used and with the materials present within the well.
When conducting any form of mechanical or chemical maintenance on a well, it is important that any disrupted material as well as any introduced materials be fully evacuated from the well.
Why is it important we remove this material? Simply put, leaving the disrupted materials or introduced chemicals behind leaves the building blocks for future fouling. By leaving the dissolved material behind, we are continuing the chemical congestion that led to the formation of solids, enhancing the process, and reducing the time for the return of fouling.
Each chemical is different, targeting specific problems or mechanisms with specific guidelines for use. Follow the provided directions or specifications to limit harmful or counterproductive reactions. In addition to selecting the right product and rate of application, monitoring of the process and documenting the results is strongly recommended.
Monitoring during treatment is an important yet often overlooked step. Each process and every chemical will have specific parameters that should be monitored to ensure the process is working in a safe manner.
Conductivity and total dissolved solids are two easy means of evaluating the baseline water chemistry for monitoring of effective removal of the disrupted material or introduced chemicals. Visual turbidity, measured using a clear glass or plastic container against an opaque or white background, is an easy visual means of monitoring changes. When using most chemicals downhole, pH is a valuable parameter to track to gauge reactivity and the reaction.
Test strips and hand-held meters are available for just about every field parameter. Depending on your ability, confidence, or work environment, they make monitoring an easy process.
It is also important that the monitoring be recorded. Simple documentation on a notepad or smartphone image in sequence can help you evaluate the procedure and record more detail when documenting the treatment effort.
The measured use of chemicals has enhanced the success rate of well treatment efforts, improving disinfection, rehabilitation, and development. Although the market has increased significantly, there remains no silver bullet for use downhole. Identify the problem and the extent at which it is impacting production, quality, or the life of the well, before grabbing a product off the shelf.
By taking the time to understand the nature of well fouling, you can better select the right product and method of application, reducing the risk of harmful impacts on the well and crew, while extending the operational life of the well.
Michael Schnieders, PG, PH-GW, is a hydrogeologist and senior consultant for Water Systems Engineering in Ottawa, Kansas. Schnieders is the 2017 NGWA McEllhiney Lecturer in Water Well Technology. He has an extensive background in groundwater geochemistry, geomicrobiology, and water resource investigation and management. He can be reached at firstname.lastname@example.org.