There are a variety of methods and the best one varies with each job.
By Michael Schnieders, PG, PH-GW
A modified double disc surge block that allows for chemical injection during swabbing. Photo courtesy Hydro Resources.
Well rehabilitation is a catch-all term that can indicate a variety of methods and products used to restore efficiency, target water quality issues, or relieve an aesthetic problem. As a result, well rehabilitation efforts vary widely across the country depending on the contractor, types of wells in the region, and nature of the problems encountered.
It can generally be said that a method or tool had to work somewhere, otherwise it wouldn’t still be in use. However, just because it worked somewhere doesn’t mean it will work everywhere.
This article will focus primarily on the mechanical side of rehabilitation efforts. While chemicals do play an important role, their topic is quite broad and sufficient for an article all by itself.
As mentioned above, well rehabilitation is a broad term used to indicate a wide variety of methods targeting everything from bacterial infestations and hard mineral scale to migrating sediments and remnant drilling fluids. Rehabilitation is typically broken down into these three distinct categories: disinfection, cleaning, and redevelopment.
The Basic Tenants
A multi-use tool that allows for simultaneous swabbing, brushing, and chemical application as needed.
But before we dive into particular methods that we like to recommend, let’s review some basic tenants of rehabilitation efforts.
First, do no harm. Much like the Hippocratic Oath for physicians, we need to make sure our work is not worsening the situation downhole. Taking the time to collect a water sample, run a video survey, and inspect the wellhead will aid in improving the level of understanding and concern needed prior to initiating any physical contact.
Furthermore, this investigation phase should help us identify what the problem or problems impacting the well most likely are.
Second, separate the treatment steps. Regardless of the chemicals being utilized, rapid neutralization or consumption occurs as the products come into contact with the bulk material.
A multi-use tool that allows for brushing and injection during
treatment. Photo courtesy Layne Christensen, A Granite Company.
Pre-treating the well and evacuating the disrupted material prior to chemical dosage helps to preserve the chemicals for the harder-to-reach areas where their activity is intended and most beneficial. This adds time to a project, but given the cost of chemicals, helps to focus their use where they can be the most effective. Common mechanical pre-treatment methods include brushing, swabbing, and low-pressure jetting.
Third, recognize third- and fourth-order effects. Just like the first tenant, we do not want any of the methods used to aggravate existing damage or create issues downhole.
This may go beyond our normal evaluation phase but can be very important. Two examples that come to mind are wire brushes and high-pressure injection.
Most wire brushes in well cleaning are made by using high-tensile strength remnant cable and a piece of pipe. The same properties that make the cable good for use on the rig do not translate into benefits downhole. The splayed cable ends can cut into softer steel, leaving reactive surfaces that can aggravate corrosion rates, attract iron bacteria, and mobilize iron from the well structure. As a result, use of stiff, nylon brushes are typically preferred to limit the development of damage to the well structure.
A stiff nylon, angled brush assembled on a rotatable mandrel allowing for effective brushing of wire wrapped screen.
A second example, pressure jetting, should be a bit easier to visualize. Whether we are looking at pressure jetting or gas injection, we need to respect the tolerances or limits of the well structure. Excessive pressures generated and applied downhole can damage or destabilize the well and have the potential to do permanent damage to the well structure. Water, under pressure, can cut steel, so its use and the means by which it’s employed, should be weighed with the strength and capability of the structure.
Another aspect of jetting that is often overlooked is the potential to push the softer material back into the filter pack. Jetting can denature biomass and force the denser material into the annulus where it can be difficult to remove. This does not mean jetting doesn’t have a place in rehabilitation, just that the nature of downhole conditions should be understood before attempting treatment.
Variety of Means
Surging or swabbing is the use of raising and lowering a single- or double-disc tool through the well column. To provide benefits, the discs should be flexible but ridged and large enough to displace water but not too large that the fit is too snug and causes additional challenges.
Typically, the pipe is weighted to provide additional pressure in the raising and lowering of the device. The downward stroke and the upward pull help to cause a “surge” similar to a wave going out and coming back in.
This particular method may be slower but has shown to be effective in a variety of treatment applications. Surging is beneficial in the application and agitation of chemicals, including acid-based solutions and chlorine. Surging provides agitation to help disperse the chemicals downhole while providing agitation at the point of reaction.
Gas injection is another means of mechanical cleaning. Just as with other methods, the nature of fouling, the degree of impact, and the structural integrity of the well must be known.
Gases are employed in one of two manners. They can be used to create a bubble that, when collapsed, creates a percussive wave causing a mechanical “shock” to the well. A second means is to use the gas to create secondary reactions including weak acid generation and freeing.
Due to the pressures involved with gas usage, extreme caution with regards to crew safety and well stability should be exercised. Additionally, you should verify that the gas being put to use will not cause additional challenges with the natural aquifer geochemistry.
Jetting can be useful as either a means of development, pre-treatment, or agitation during chemical application. As was noted, when bulk biomass is present, it is more beneficial to use jetting as a secondary means of treatment to limit the pushing of biomass into the filter pack.
Important aspects of jetting include the pressures applied, the direction of focus, and the cleanliness of the injected water. Focused, high-pressure water can be an effective cutting tool. As such, the injection nozzles should be angled and rotated during treatment to limit damage to the well structure.
Use of a clean water source free of biomass and sediment is important to limit introduction of particulate and reduce the potential for abrasion corrosion to occur. In alluvial wells, jetting has proven to be a beneficial means of agitation during development and cleaning efforts.
So Which One?
What’s the best method? It all depends on the well and the nature of fouling.
Time spent collecting data prior to beginning treatment typically pays greater dividends than rushing the job with only a limited amount of knowledge. The chosen method should depend on an understanding of the problems impacting the well and the well’s structural integrity.
Michael Schnieders, PG, PH-GW, is the president and principal hydrogeologist at Water Systems Engineering Inc. in Ottawa, Kansas. Schnieders was 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 email@example.com.