Restoring yield and reducing sand pumping.

Many wells experience continuing reduction in yield throughout their service lives. Sand pumping often creates expensive operation and maintenance problems. In the past, loss of well yield and/or sand pumping was tolerated until continued operation of the well no longer was economical, and the well then was redrilled.

Advances in well treatment and rehabilitation technologies offer an alternative to redrilling problem wells. These technologies include sonar-jetting, treatment using specialized tools for placing and removing chemicals, and installation of suction flow control devices within the existing well.

This article will discuss the factors that contribute to well failure and identify alternative methods for well treatment and rehabilitation.

Types of Well Problems

The performance of a well often declines during the life of the well. The decline may be due to one or more of the following factors:

1. Lowering of the static water level
2. Inefficient pumping caused by worn, corroded or plugged pumps
3. Blockage of the well screen and gravel pack by mud, sand and silt
4. Deposits of encrustation, corrosion products and microorganism growths at the well screen, at the aquifer face and/or in the pump
5. Failure of the well casing and/or screen

Static water levels should be monitored over time to detect trends, so that loss of yield caused by regional static water level declines are not mistaken for well maintenance problems. Pump problems can be identified by inspection and/or a properly conducted step-drawdown test. Factors 3, 4 and 5 can be limited by proper well design, construction and operation, and are the primary topics of this paper. In the past, loss of well yield and/or sand pumping often was tolerated until continued operation of the well was no longer economical, and the well then was redrilled. Redrilling of a problem well sometimes is the most cost-effective approach. However, the performance of many wells experiencing these problems can be improved by well rehabilitation techniques.

Whatever the solution, it is important to identify and understand the cause of well failure. If redrilling is chosen, the previous design and construction methods should be evaluated and improved upon, if possible. If well rehabilitation is attempted, the proper technique must be selected. The most frequently encountered problems are discussed below.

Mechanical Blockage

There are two types of mechanical blockage, which can act to restrict the movement of ground water into a well. The first type involves the movement of fine-grained soil materials from the natural formation to the borehole face or the face of the screen. The second type is caused by corrosion by-products of the metal portions of the well, which act to cover the openings of a well screen.

The movement of fine particles typically is caused by improper well design or by overpumping a well. An improper well design may include the selection of a gravel pack, which is too large to effectively filter fine formation materials, or selection of an inappropriate screen slot size. Also, a well screen could be placed opposite layers of sand that are significantly smaller or more graded in particle size than the other aquifer materials.

Overpumping a well will cause turbulent flow in the formation near the well screen and promote the movement of fine-grained materials. These same materials may not migrate at pumping rates that maintain laminar flow throughout the formation. The migration of fine materials acts to reduce the effective porosity and restrict water flow, increases head loss in the immediate vicinity of the well, and often results in sand pumping.

The corrosion of well casings or screens can cause holes to develop in a casing and cause the screen slot size to increase, allowing sand and/or gravel pack to enter the well. Corrosion by-products also can cover portions of the screen and cause higher entrance velocities through the remaining open area, thus increasing head loss across the screen.

Chemical Encrustation

Chemical encrustation is the deposition of minerals on the well screen or gravel pack which act to restrict the movement of water into a well. Chemical encrustation is caused by the precipitation of minerals dissolved in the ground water due to changes in flow and/or pressure conditions at the well. Well encrustation typically consists of iron and manganese oxides or of calcium and magnesium carbonates or sulfates.

Chemical encrustation can be reduced by designing the well with a minimum of head loss through the screen, by correct placement of well seals, by proportioning the flow vertically in the well and by maintaining proper pumping rates.

Bacteriological Plugging

For the purpose of this article, the term “nuisance bacteria” is used to describe a variety of microorganisms that can cause clogging problems in wells, pipelines and treatment facilities. This term includes the types of iron-related bacteria that utilize dissolved iron as an energy source and other nuisance bacteria that cause iron precipitation in a secondary manner.

Nuisance bacteria are not believed to cause health concerns but are a nuisance in the production and transmission of ground waters. Nuisance bacteria have been characterized by their unusual capacity for accumulating ferric (iron) hydrate around their cells. A relatively small number of bacteria are able to clog a well because they can accumulate many times more ferric hydrate than the actual bacterial cell material.

There are two methods by which nuisance bacteria infest a well. The bacteria may either be native to the aquifer or they may be introduced directly by man. Bacteria are known to exist in the ground as either active organisms or as inactive spores. It also is possible for the operations of man to introduce new or additional bacteria into an aquifer during drilling or when a pump or other equipment is serviced or operated.

Bacteriological plugging problems exist in wells throughout the United States and the world. For many years, the approach to cleaning wells plugged by bacteria has been one of trial and error. There has been a limited understanding of the dynamics associated with bacteria kill in a well. Today, more and more research is being undertaken by the water well industry and by microbiologists. This expanding awareness has led to several innovations and other improvements to existing well cleaning procedures will certainly follow.

Well Rehabilitation Techniques

A decline in well performance can often be counteracted by appropriate well cleaning and rehabilitation procedures. Many techniques have been used to rehabilitate wells. Most techniques have provided some beneficial results and all have failed at some time to achieve substantial well improvement.

An important obstacle to successful water well treatment is a lack of understanding of the cause of a problem at a specific well. In order to develop a successful treatment program for a specific well, it is necessary to review detailed operating data collected throughout the well's life, and any data regarding similar successful and/or unsuccessful treatment procedures conducted previously.

Well treatment typically includes some use of chemicals to break down encrusting materials. Mechanical techniques such as wire brushing, swabbing, jetting and surging can be used to assist the chemical treatment. The best well improvement results typically are achieved by the use of a properly selected combination of treatment methods. This section describes various mechanical and chemical rehabilitation techniques.

Physical Methods

Swabbing, by wire brushes or other methods, can be used to remove encrusted materials on the inside of a well screen. The material can be removed from the well by pumping or bailing. Swabbing often is used prior to chemical treatment. Care must be taken in using swabs to ensure that the well screen is not damaged. Improper technique may transfer loosened material and cause further plugging of perforations in the casing or screen.

Jetting sometimes is used in well treatment for problems due to mechanical blockage. When used in conjunction with polyphosphates, jetting can loosen and wash out fine-grained materials in the vicinity of the well screen. Sonar jetting is a well stimulation technique designed to reopen plugged perforations and wash deep into water-bearing formations. This technique consists of two physical actions working simultaneously. First, a mild harmonic frequency of shock waves loosens mineral, bacterial or other type deposits adhering to the screen and casing. Second, pulsating, horizontally directed gas pressure jets fluid at high velocity back and forth through the perforations to deep-clean the aquifer. Light “shooting” with prima-cord also can physically break hardened scale from the casing or screen.

Chemical Methods

The use of polyphosphates, surfactants and physical agitation has been successful in some cases for improving well performance due to mechanical blockage. Fine-grained soil materials can be dispersed by polyphosphates and surfactants. Surfactants are a group of chemicals called “surface active agents” that have the ability to reduce the surface tension of water and allow particulates to move more freely in the formation pore space. These compounds cause the fine particles to repel each other and move through the gravel pack and screen and out of the well. Physical agitation can act to accelerate this process. Detergents and surfactants also may be used to improve the performance of wells in which drilling fluids were not fully developed out of the well during construction.

Acids are used to chemically dissolve encrustation formed in a well. Acids can dissolve calcium and magnesium mineral deposits. Although acids typically are not effective at killing bacteria, they are used to dissolve iron and manganese oxides formed because of bacteriological growth. Physical cleaning methods, such as brushing, swabbing or sonar-jetting can be used to supplement the acid treatment.

Disinfectants are chemicals that are used to kill bacteria present in the immediate vicinity of a well. Chlorine compounds are the most widely used disinfectants because they are inexpensive, readily available and proven effective against many types of bacteria. Bacteriological plugging typically is treated by a combination of methods to destroy bacteria and to remove iron encrustation. This treatment often consists of alternating chlorine and acid solutions, combined with physical treatment methods.

Chemicals may be placed in a well in several manners. One typical procedure involves the introduction of the chemical into the well through the well pump. This procedure does not require the expense of pulling and resetting the pump. However, this procedure may allow the pump to be damaged by concentrated chemical solutions. Also, this procedure does not ensure thorough mixing of the chemical throughout the entire length of screen. Often, the chemicals dissipate through the unplugged portions of the well screen and do not penetrate the area of greatest need.

Alternative procedures for placing chemicals in a well screen include the use of surge blocks to accurately place the chemicals in specific zones of the screen, and the use of a bailer. These procedures require that the well pump be pulled from the well.

All chemicals used during a well cleaning process must be carefully removed from a well and properly disposed. Water should be pumped from the well until the water quality is essentially the same as prior to treatment.

Suction Flow Control

Overpumping and excessive entrance velocities are the cause of many well problems, including sand pumping. For the flow of particulate matter to occur in a well, flow must be in the turbulent state. If flow were in the laminar state, there would be insufficient energy to move sand particles. When a pump suction is set above the top of a well screen, it is believed that the velocity distribution through the height of the screen often is highly variable, with the highest velocities occurring near the top of the screen. A suction flow control device is designed to more uniformly distribute flow into a well throughout the vertical well screen. The device is constructed of slotted PVC pipe coated with a reinforced silica sand pack and treated with an epoxy compound. The device can act as a gravel filter pack as well as redistributing energy in the screen. Installation of these suction flow control units has proven effective in controlling sand pumping in many wells.

A Rehabilitation Program

Well and pump performance should be evaluated on an on-going basis. Data to be collected should include static water level, pumping water level, flow rate, discharge pressure, sand production and energy consumption. Data should be collected in a consistent manner at specified periods of time (usually monthly or quarterly). A significant change in operating conditions indicates that a problem may exist.

Once a problem has been identified, the potential sources of the problem should be evaluated. The operation history of the well should be reviewed, along with the results of any previous treatment efforts. The existing data may need to be supplemented by conducting performance tests to determine the current condition of the well and pump. If it is necessary to remove the well pump, a down hole video survey also should be performed. A proper treatment procedure then should be developed to address the specific well problems.

Implementation of rehabilitation procedures for an individual well should generally include some or all of the following steps:

Pull and inspect the pump

Perform a video survey

Perform mechanical cleaning of the well screen

Apply the proper quantity and type of chemical treatments

Allow sufficient chemical reaction time

Remove spent chemicals from the well

Reinstall the well pump

Conduct a performance pumping test

In summary, there are water well rehabilitation procedures and techniques that are proving that well rehabilitation and treatment is effective in restoring well yield in an economical manner.
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