Well maintenance has historically been ignored until a significant amount of deposition and plugging of the pore volume is experienced in and around a well. When wells experience loss of specific capacity and water quality changes, it is often necessary to pull pumping equipment in order to perform more aggressive rehabilitation treatments to clean the water well and pore volume of deposited material. The most effective method of cleaning the well and surrounding aquifer is with the use of gaseous and liquid carbon dioxide.
A group of wells throughout the U.S. have been maintained effectively without pulling pumping equipment from the well since the pilot installation of a preventative maintenance system on Sept. 11, 2001. These wells were experiencing iron bacterial growth so rapidly that they required well rehabilitation on a monthly basis, and the facility required five micron prefilters be changed twice a day at a considerable expense. With the installation of the patented Aqua Gard well maintenance system, the specific capacity could easily and cost effectively be maintained and the water quality improved.
In order to remove deposits from surfaces, adequate energy must be delivered into the well and aquifer. This energy can be described as energy to disrupt dissolve, detach, mobilize and fluidize plugging and encrustations. The energy needs to operate on the surfaces of sand, gravel and the well screen, allowing the deposits to be removed more effectively during the pumping-off phase. The effective removal of the detached sediments requires keeping the sediments in suspension, which is accomplished by producing energy of agitation during the pumping-off phase.
Shock chlorination as a preventative maintenance chemistry is too limited in its ability to remove deposited material, and its effectiveness is lost once the microbes have developed their protective mechanisms by producing copious amounts of extracellular polysaccharide (slime) and accumulation of minerals. Once they have formed these protective mechanisms using disinfectants, other chemicals are often not going to be able to impact the biofilms.
Even with the best blended chemistries for preventative maintenance, it is difficult to achieve adequate removal of material in large part because of the inability to get enough energy into the bottom of the well and surrounding aquifer. Carbon dioxide is the most effective method of getting energy into the well and the surrounding formation. Wells can be more effectively maintained by removing the deposits at an early stage when the deposited material is soft in nature instead of waiting for the surfaces to become completely fouled and encrusted.
A New Concept
The application of liquid and gaseous CO2 to well screens and formations brings about the desired result. This new concept relies on the permanent, strategic placement of energy injection equipment at various points in the well. The concept is to keep surfaces clean in a well and the surrounding aquifer instead of waiting for the surfaces to become fouled and encrusted.
Preventative well maintenance programs can be effective in reducing well problems and can help maintain the well’s production. It is a common practice to operate wells until they experience a significant loss of specific capacity before rehabilitation efforts are performed. At this point, it can be more difficult, if not impossible, to restore the capacity to its original condition because of the amount of plugging of the well screen. Preventative maintenance treatments offer the advantage of removing deposited material at the early stages.
The periodic removal of the deposits at the early stages of formation may also help to slow the rate of corrosion experienced in wells. Much of the corrosion in wells is caused by tuberculation resulting from a symbiotic growth of iron-related and slime-forming bacteria with the sulfate-reducing bacteria. The sulfate-reducing bacteria are growing in the anaerobic zone created by the aerobic slime-forming bacteria. Most of the corrosion is occurring under the tubercles. Because surface fouling is necessary for the corrosive environment to occur, using a CO2 well maintenance system will prevent much of the under-deposit corrosion.
The CO2-based preventative maintenance system, with energy injection equipment installed along with the existing pump, eliminates the need to remove the pump while achieving energy necessary to remove deposits from surfaces and move them out of the well.
The use of gaseous and liquid CO2 offers one of the best methods of delivering energy into every part of the well and the surrounding formation without removing the pump. This periodic cleaning of the surfaces can be performed on a scheduled interval, determined by the historic fouling rate and the current data readings. The cost of the periodic cleanings is significantly less than the cost of a major well rehabilitation. CO2 offers the advantage of not having to neutralize or dispose of spent chemicals. Some applications may need the addition of chemical energy along with the use of liquid CO2.
In some instances, this chemical energy may help to more completely detach the biomass and associated minerals. Once the material has been detached from the surfaces, it needs to be removed from the bottom part of the well and the surrounding formation. This can be achieved with the simultaneous pumping and occasional fluidization of the sediments and deposits. This will allow more complete cleaning of surfaces and allow the original pore volume that exists around wells to be maintained more effectively. The maintenance of the pore volume and maintaining clean surfaces extends the time frame between rehabilitation efforts.
Barrier wells and injection wells that are not equipped with pumps can be maintained using the CO2 to airlift the material from the well and surrounding formation. Once the material has been detached from the surfaces, it needs to be removed from the bottom part of the well and the surrounding formation. This can be achieved with the simultaneous pumping and occasional fluidization of the sediments and deposits.
Prevention in Action
A successful example of preventative well maintenance occurred at a facility used for breeding laboratory mice primarily for the pharmaceutical industry that shipped 100,000 mice per week. Water at the site was used for facility cleaning, stock feed and domestic purposes, excluding consumption. Prior to the installation of the well treatment systems, all of the wells at the site tested positive for total coliform, rendering it nonpotable.
The site was chosen based on the following criteria:
- Multiple wells (nine);
- High rate of well fouling;
- Extreme biological activity (iron-related bacteria);
- Poor water quality, multiple levels of filtration, nonpotable finished water; and
- Supplemental water required to meet the needs caused by rapid yield decline.
All wells are installed in bedrock with ±50 ft of steel casing, average dimensions equaling 6 in. by 200 ft and producing an average yield of 15 gpm. Specific capacity and well yields would decline rapidly from one to six months after well maintenance efforts. Severe plugging and fouling occurred not only within the well but also inside the pump, drop line and offset lines to the facility. Maintenance of the nine wells located at the site was previously performed on each well using non-CO2 chemical cleaning and CO2-based cleaning (Aqua Freed) every six months in an effort to maintain yields and capacities. After rehabilitation treatments, the production would drop rapidly.
Poor water quality caused by high levels of biological activity required multiple levels of filtration, which cost an average of $5,000 per month for filters. Labor to clean and change filters at the facility cost approximately $12,000 per month, calculated at 20 man-hours per month for each of the 38 buildings at a rate of $15 an hour for a total of $11,400. Filtration cost totaled $16,400 or $1,822 per well per month. Energy costs for the pumping wells are difficult to calculate because all wells are on the same meter. The site project superintendent stated that prior to the preventative maintenance program, all nine wells were pumped constantly during 24-hour periods to meet the demand of the plant.
Preventative maintenance installation allowed the site to pump five wells for 10 minutes every half hour, or 20 minutes per hour to meet demand because of the maintained pumping level and well efficiencies compared to pumping nine wells for 24-hour periods prior to installation and scheduled service events.
The wells now provide all of the water needed to meet site demand with improved quality. Energy costs have been reduced by approximately 82% per month during the three-year analytical period (40 minutes per hour of energy savings), further reduced by pumping only five of the nine wells as a result of the well’s ability to produce more water with less drawdown.
The demand on water treatment was greatly reduced because of the improved water quality. The required labor to clean and change filters after the well treatment and installations was reduced from 760 man-hours per month to 160 man-hours per month for the 38 buildings, representing a labor cost reduction of 80%. Filter costs were reduced to $111 per month. The improved water quality is a result of effective well cleaning and the ability to keep well and formation surfaces clean through the early and preventative scheduled service events.
In addition to stable increases in water production, the client has reduced finished water costs at the site by $456,105 over a three-year period, excluding the cost savings associated with supplemental water and actual energy costs, as a result of installing preventative maintenance.
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