Conventional well theory holds that water enters the well evenly across the length of screened or open borehole. It has always been assumed that a well with 100 feet of producing length, and a 1,000-gpm pump will. uniformly produce 10 gallons of water per foot of producing zone. However, it has been clearly demonstrated that this is not what really occurs.
What actually happens is that pump suction energy is distributed very unevenly in the well. This results in water entering the well at very high velocities near the pump, and being virtually stagnant at the farthest points from the pump. Water rushes into the well near the pump to satisfy the pump demand. Once this water reaches turbulent velocity, no additional water flow occurs at that point, and the suction energy is pushed farther down in the well. This process will continue until all of the pump suction energy is dissipated or the pump breaks suction and begins to pump air. Thus it can be seen that most of the water is produced in only part of the well.
In many cases, 100% of the water is pulled from a third or even less of the available producing zone. The result of this uneven energy distribution is mineral encrustation and sand production as described above. These preventable problems will certainly cause deterioration in well performance, and can often lead to premature failure of the well.
Typical approaches to these problems can fairly be described as "treating the symptom" rather than correcting the underlying cause of the problem. These treatments include periodic removal and replacement of the pump, repeated attempts to rehabilitate the well through various chemical and mechanical methods, re-screening or lining the well, the use of settling tanks, sand separators and/or filtering systems. In the worst cases, wells are replaced and then the cycle repeats itself.
Common Causes of Well Deterioration
Well deterioration can be caused by: 1) Mineral encrustation, 2) Sand production and/or 3) Biofouling. In all three cases, well deterioration occurs both to the well screen and casing as well as to the surrounding formation itself.
1) Mineral Encrustation
Ground water is in constant motion in an alluvial aquifer. Velocities of a few inches to several feet per day are common and, obviously, do not result in plugging of the natural sand and gravel. It is only when a production well is installed and the ground water velocity is increased several orders of magnitude, that the dissolved minerals in the ground water become unstable and become deposited in the well system. This is a function of both velocity and reduced pressure due to drawdown in the well. Mineral deposits accumulate in the pipelines, discharge column pipe and well pumps, resulting in decreased pumping capacity. Mineral encrustation also plugs the well screen, gravel pack and native sand and gravel. These effects reduce well efficiency, resulting in a greater drawdown in the well than in the surrounding formation and thus reduce the pumping capacity of the well.
The conventional operation of water production wells results in the deposition of unstable dissolved mineral constituents at points of high velocity in and near the well screen. Because water velocities are typically greatest near the pump, the phenomenon begins in the area immediately surrounding the pump intake. As the plugging continues, higher velocities occur at a greater radius out into the formation and vertically away from the pump, which result in plugging in the
screen, gravel pack and aquifer. Evidence of this can usually be seen in down-hole video recordings of the well. Typically, visible encrustation will occur on the well screen perforations closest to the pump setting, and diminish with distance away from the pump. If the plugging is distant enough and severe enough, it will resist virtually all conventional well-rehabilitation techniques. At best, these techniques are time consuming, expensive, sometimes hazardous, often involve toxic materials and are rarely totally effective. Once the well is put back in service, the process of encrustation begins again, usually at an accelerated pace. Symptoms can become severe enough to require abandonment and replacement of the well.
2) Sand Production
In a properly constructed well with appropriate selections of gravel pack material, even though no sand is produced, induced turbulent flow causes migration of very fine sand particles to move toward the well. When restrained by the gravel pack, mechanical blockage occurs that resists normal chemical and mechanical cleaning methods. This results in the permanent loss of capacity and increased pumping costs.
As a well ages, sand pumping can increase above the original "normal" amounts. When sand is removed from the formation there is damage to the pump, as well as gradual erosion of the well screen, which then allows more sand to enter the well. Even relatively small concentrations of sand production can eventually add up to significant volumes of sand excavation. For example, "acceptable" levels of sand, even 5 ppm or less, can add up to several cubic meters of material within a few months. This can accumulate in the well, thereby blocking off part of the producing zone, which increases the demand on the remaining sections. Sand pumping can also cause cavern formation behind the screen, ground subsidence, gravel pack settling and can eventually result in catastrophic collapse of the well itself.
Iron bacteria are naturally occurring and are present in virtually every well everywhere in the world. If conditions are right, these bacteria can flourish, and are often a primary or contributing cause of loss in specific capacity. Also, these bacteria can lead to pitting and premature failure of the well and pumping system. Since most alluvial wells have significant amounts of iron, the bacteria have a readily available source of food. Though not harmful to humans, the bacteria have a very detrimental effect on well and pumping systems.
As the biomass grows and spreads in a well, it begins to plug off the perforations, thus restricting water inflow to the well. Well production is further reduced by decreased efficiency and deposits in the pump and discharge lines. If unchecked, an iron bacterial slime can eventually cause complete and sudden loss of specific capacity.
Attempts have been made to disinfect wells with concentrated calcium or sodium hypochlorite solutions. This treatment can be somewhat effective in the short term. However, it has been clearly demonstrated that this method of disinfection never results in complete sterilization. Furthermore, there is compelling evidence that the bacteria eventually come back more rapidly and with stronger resistance to later disinfection attempts. Protective slimy biofilms are virtually impervious to even very high concentrations of chlorine and mineral acid/chlorine treatments.