The key to success is focusing on removal of plugging material and reversing formation damage.
By Neil Mansuy
Well rehabilitation and maintenance are well known to most working professionals in the groundwater industry.
However, while most people are familiar with these terms and familiar with problems such as lost capacity, red water, water quality changes, and taste and odor issues that wells experience, many people are not familiar with the causes and the effective solutions.
I think it is important to establish that my educational background is groundwater microbiology, and the focus of my graduate degree was on the factors that cause well plugging, with a primary focus on iron bacteria. The groundwater industry has been focused on iron bacteria as the primary cause of water well problems.
But after working the past 40 years with a primary focus on well rehabilitation, I have come to recognize we have focused too much on the bacterial component of incrustation.
Numerous bacteria that are naturally present and will never be eliminated from subsurface environments are growing mostly attached to surfaces in biofilms and are involved in the oxidation and filtering of dissolved minerals in the groundwater. The fact that most microorganisms in water well environments are attached to surfaces is one of the reasons I have zero confidence in bacterial numbers that come from pumped water samples.
We are also not able to satisfy the nutritional requirements of most microorganisms when relying on growth to enumerate the bacterial numbers. There are other procedures that are more accurate in enumerating total bacterial numbers but are still dependent upon detachment.
Investigations of microbiology of the deep subsurface have demonstrated that naturally occurring microorganisms are present at a billion bacteria per gram of soil. One study undertaken by the Department of Energy in the mid-1980s, where four wells were drilled in South Carolina to almost 2000 feet below surface, demonstrated that the billion bacteria per gram were present independent of depth.
Samples taken during drilling in the oil industry have shown similar numbers much deeper. One of the findings that surprised the researchers at the time was the occurrence of total coliforms in abundance coming from the deepest samples.
Causes of Well Problems
Once incrustation becomes extensive enough, it can cause loss of capacity, increased electric cost of pumping water, and a variety of water quality changes. When the pumping water level gets much lower in the well, the increased cost of pumping water can be significant as electric costs are increasing significantly.
The cost of rehabilitation can often be recovered from restoring production, resulting in a higher pumping water level inside the well and decreased electric cost to pump the same amount of water. The focus on the bacterial component when wells are cleaned or rehabilitated leads us astray from the more important component of plugging material.
Approximately 90% of deposited material are the minerals that have come out of solution from the groundwater. The mineral loading or dissolved minerals of groundwater is determined by the formation geology from a weathering process.
With a focus on the bacterial component, approaches that are taken for rehabilitation are often not effective at removing the mineral component. If chemical rehabilitation is chosen for the bacterial component, such as disinfectants and organic acids, they are often too weak to dissolve, disrupt, and remove the 90% component of mineral scale.
It has long been believed that the iron bacteria or the biofilms are difficult to penetrate and remove due to the extracellular polysaccharide often referred to as bacterial slime. This belief originated from the focus on bacteria and that bacteria were responsible for most lost capacity in wells.
Therefore, disinfectants were often chosen and when the deposited material was not removed, and capacity was not restored, it was believed that the bacteria (biofilms) were difficult to remove. The biofilms are easily disrupted and removed, and the reason that results of restoring capacity was not achieved is there was nothing to remove the mineral scale.
I no longer believe it is important to kill bacteria or focus on disinfection when rehabilitating wells because of the understanding that we will never sterilize an aquifer or kill all the microorganisms. The key to success is focusing on the removal of plugging material (mineral scale) and reversing the formation damage in the formation and the filter pack.
Formation damage caused by invasion of the pore space in the near-well environment by silt, clay, and fine sand is often underestimated because we cannot see it with video inspection of wells. And we tend to blame everything on what we can see, such as biofouling and the associated minerals.
Well Hydraulics Related to Rehabilitation and Maintenance
The most important factor in understanding well problems and well solutions is well hydraulics.
Well hydraulics refers to how water flows towards an operating well. A water well works as a blending unit from variation of flow from different zones within the well. Many thousands of production profiling have been performed on wells and demonstrated there is significant variation in where water is being produced in an operating well.
With pumping equipment most often in the upper casing, most of the water is being produced from the upper part of a well (screened or open hole) and often little water is being produced from lower zones in a well. This can result in low flow zones or stagnant zones existing in the well.
These stagnant zones can result in water quality issues and the inability to flush the bottom part of a well. Variation in production with depth and the change in where water is coming from in an operating well can result in water quality changes if there is variation in quality with depth. This change in the production profile is a result of plugging of the most productive zones of the well.
Time-Based Preventive Well Maintenance
The most significant thing learned during the past 40 years is that we have historically waited too long before we undertake a cleaning event (rehabilitation or preventive maintenance cleaning) on a well. The most significant reason we wait too long is our reliance on monitoring specific capacity (Q/s). We have historically used loss of 15%-20% specific capacity that would trigger a rehabilitation or preventive maintenance cleaning. We do not detect the early loss of pore volume by monitoring specific capacity.
There is a lot of extra pore space that exists when wells are newly constructed. There is also extra production capacity in other zones within a well that can make up losses that occur in productive zones without any loss in specific capacity.
If specific capacity is being monitored, a measured loss in specific capacity occurs at a single moment in time. The single moment in time occurs when the excess pore space and the extra production from mostly deeper zones is plugged and the remaining pore transitions from laminar to turbulent flow.
We then experience turbulent flow losses that can be measured during a flow test. When we wait for loss of specific capacity, the plugging material can become more extensive and can also become more mineralized, thus making it more difficult to remove.
I have come to recognize we must establish time-based rehabilitation and maintenance. As previously discussed, plugging material is mostly biological filtration of minerals and therefore the early stages of accumulated material feature softer slimy material with the beginning of mineral content and can generally be easily removed. The frequency of preventive maintenance cleaning is mostly annual and sometimes more frequent, depending upon the rate of plugging.
From the graph above, we see the typical operate to failure mentality versus annual preventive maintenance. The cost of an annual cleaning generally ranges from 10%-20% of a full rehabilitation. My confidence is with the use of gaseous and liquid carbon dioxide with the use of the Aqua Gard® Preventive Maintenance Technology. This allows the well to be cleaned effectively with pumping equipment in place.
Injection of carbon dioxide into the deepest point in a well allows the limitation of well hydraulics to be overcome. Injection of carbon dioxide in a sealed well allows the cleaning energy inside the well to penetrate the filter pack or fractures of rock wells and the surrounding formation.
The downtime for a well with a preventive maintenance cleaning is generally less than 24 hours. Another advantage of gaseous and liquid carbon dioxide makes it easy to comply with discharge permits without the need to neutralize chemicals.
Steps for Effective Rehabilitation
There are many different technologies and procedures that are used to rehabilitate wells. The steps taken in the cleaning process are targeting the removal of biological and associated minerals as well as removing the invasion of fines and the formation damage.
Well rehabilitation has become well known and somewhat mature in the groundwater industry. Most professionals in the industry have recognized the importance of a multiple-step approach to cleaning wells. Each step is important in better cleaning of wells. A common stepwise process is:
- Pre-rehabilitation flow test.
- Pulling pumping or injection equipment.
- Pre-rehabilitation video inspection.
- Pre-treatment of the inside of the well with wire brushing, swabbing, jetting, or percussive energy. Pre-treatment is the easiest part of a well system to clean.
- The next step is the main part of the well rehabilitation or cleaning process that must penetrate the surrounding formation with effective energy.
There are many different things that can be used for this part of the rehabilitation process. Some of these can include chemicals or chemical blends, jetting procedures of various pressure ranges, hydrofracturing, percussive explosives and compressed gases, the use of gaseous and liquid carbon dioxide (Aqua Freed®) mostly without chemicals. Being in the industry a long time and experience with all the technologies, my confidence is with the use of carbon dioxide that has been used on many thousands of wells around the world.
- The development step follows the major cleaning process and is one of the most important steps because all of the material that has been loosened up has to be removed.
It is often not as simple as pumping a well even above the design rate since the limitation of achieving high velocity conditions over the entire producing interval of the well. The most effective method of the development step is simultaneous swabbing with a double-disk swab and pumping with airlifting or pump, depending upon static water level.
The double-disk swab consists of a perforated section between an upper swab and a lower swab. The distance between the swabs can range from a few feet to 20 feet. One important component of the simultaneous swabbing and airlifting is the movement of the swabbing tool up and down inside the well while pumping water. The speed at which this tool moves is important, and the effectiveness is achieved by the displacement of the swabs moving up and down as a plunger at the same time achieving higher velocity of water within the isolated section.
- Post-rehabilitation video inspection.
- Installation of pumping or injection equipment. Wells are often equipped with Aqua Gard while the pumping equipment is being installed.
- Post-rehabilitation flow test.
The groundwater industry has finally made a paradigm shift from operate to failure to more effective asset management. The source is the most important asset in a groundwater system. More demand on groundwater supplies, the increasing cost of pumping water, and decreasing budgets make water well rehabilitation and maintenance much more important.
Removal of plugging material when it is soft in nature and not extensive allows water quality to be more consistent in production, and to be maintained at a higher discharge rate, a higher pumping water level, and lower electric cost.
The goal of good well asset management is lowering cost and extending the life expectancy of wells.
Neil Mansuy is vice president of technical services for Subsurface Technologies Inc. in Rock Tavern, New York. Mansuy has more than 40 years of experience in well rehabilitation and has volunteered on numerous NGWA committees. He is also a regular workshop presenter at NGWA’s Groundwater Week. He can be reached at firstname.lastname@example.org.