The goal of all drilling programs should be to minimize the amount of impact the drilling process has on the geology outside the borehole. From the beginning of the hole to bottom, the minimal impact will create stable borehole conditions that allow easy product installation and efficient extraction of water, mineral, or energy out of the ground.
Water well production is 100-percent dependent on minimizing impact to the production zone. We use particular well development methods to repair our negative impact to a production zone by improving the zone’s water passage. In fact, the term “porosity” comes from the Latin term “poros,” meaning passage. Good-producing water wells are drilled and developed properly to ensure the most open pore spaces to effectively allow water into the screened interval.
The goal is to let water in. However, just as easily as these pore spaces allow for water to enter a borehole, drilling fluids and solids can quickly go out into the production zone. The path of flow in or out of the borehole is always on the path of least resistance. If that path is up the stable borehole, then water comes in. But if the weight and force in the bore is greater than the water coming in, then the drilling fluid (which is full of solids) goes out into the formation filling the pore spaces. Returning a production zone to its highest capacity is only possible when we maximize our drilling capabilities while knowing the factors that prevent the well development process from being successful.
A Balanced Borehole
Maximizing our drilling capabilities is all about energy and how it is directed. Does the equipment on site have the power required to create proper velocity in the drilling phase? The larger the borehole, the more pumping energy is required to drill and clean the borehole effectively. Remember, best drilling practices need 60 to 150 feet per minute of uphole velocity for drilling mud, and a minimum of 3,500 feet per minute for dry air. These uphole velocity minimums create drilling conditions where the path of least resistance is favorable for drill solids to come up and out of the borehole, as opposed to out into the production zone. A good drilling program is designed around minimizing impact to the surrounding geology, especially the water production zone. When the velocity falls below 60 feet per minute for mud or 3,500 feet per minute for dry air, drill cuttings build up downhole and are forced out into the surrounding formation.
Now let us consider low uphole velocity, with drill solids loading up downhole, while drilling into a production zone. Once drill solids start to push into a porosity zone, every minute that the drilling process continues out of balance, we lower the capacity of the well to produce. In the situation of mud rotary, solids-laden drilling fluids are the second-worst contaminate to plug the production zone. The worst types of contaminants to try to produce out of a production zone? Unyielded bentonites and polymers. The first reaction to poor uphole velocity and hole cleaning is often to quickly mix more mud and circulate it downhole. But that haste can mean using mud without allowing it to fully yield. That unyielded mud continues to yield in the porosity zone, creating new blockages that require a significant amount of energy to remove.
To develop a well properly, we must maximize the energy available to clean and open up the production zone. We deploy this energy in the form of agitation, scrubbing, jetting and surging, depending on the method and tooling available to develop the well. That is why energy is a major factor in proper well development on large holes. It all starts with the diameter of the borehole, and the distance from the screen to the actual borehole wall. The greater the distance, the more energy required to develop blocked passages in the production zone.
The distance from the screen to the borehole wall is only one variable. The next is the amount of filter material filling the annulus. The larger the annulus, the more energy required to push through the screen and filter pack, to the filter cake, and into the porosity zone. Hole size consideration is paramount in the success of well development. I have had many discussions with drillers who say, if a 2.5-inch oversized hole is good, creating a 4- or 6-inch oversized hole must be even better for easier casing installation and grouting. Sadly, the larger the hole, the more energy required to develop the well, and the larger the annulus the faster that energy loses momentum or dissipates completely.
Many water well engineers are concerned with the limitations of well development and often specify reverse circulation drilling as a better solution when designing large production wells. Reverse circulation is thought to be a better drilling process because the method creates a very low impact on surrounding geology while effectively cleaning the hole. However, conventional rotary applications can be just as successful in the same conditions if we maximizing the rig’s hole-cleaning capacity by engineering a proper fluid and optimizing uphole velocity.
It Comes Down to Development
Regardless of the drilling method, every water well finishes with a well development program. That program can be a few hours to clear up and determine the capacity of the well, or it can be several weeks of attempting to undo the damage done during drilling. A great development plan starts with a suitable drilling program to minimize that damage. However, the next step is to understand to what extent the production zone is damaged, and apply the best development method to get the porosity zone back to 100 percent.
An evaluation of the damage to a production zone starts with knowing your final drilling fluid properties. Next, assess the size and amount of cuttings that came out of the hole. Follow that by measuring the amount of filter media that was required to fill the screened annulus. All of these questions will tell you how much damage has been done. Once we understand the damage, we can utilize the best development method to repair the production zone.
Air development is often the first tool of choice, and an easy tool to measure how much damage has been done to the production zone. On shallow wells, it’s effortless to use an open-ended tool to surge a short-screened section. Air becomes an excellent tool when the energy is directed out into formation with a perforate development tool. Some of the best air tools I have worked with are 2- to 3-feet long, and were one-of-a-kind designs created out of a need to better direct their air compressor energy for agitation. The larger the screen interval, the more critical it is to localize the power of the tool to a few feet at a time.
Beyond air development, jetting and scrubbing tools work great at breaking up thick filter packs. A good jetting tool should also maximize its energy by working no more than two feet of a screen interval at a time. Ideally, when using a jetting method, a development fluid is utilized to help break down the filter pack. Development fluids are designed to work by getting out into the formation through the filter pack and wall cake. Very few development fluids were intended to be dumped from the surface and expected to go out into the production zone. If the passage is blocked, there is no way a development fluid is going to get out.
Yes, Old-School Still Works
The best energy efficient development method will always be surging and pumping. There are many ways to creating a pumping tool that can create surging energy. The goal is to understand that direct and constant surge agitation is one of the best ways to remove unwanted solids from a porosity zone. Yes, all of the cable-toolers and old-school drillers who kept their machine with a walking beam, it is still one of the most efficient ways to guarantee the best capacity of a well.
One crucial step that is often skipped or forgotten about is allowing the aquifer time to rest between agitation, surging and pumping. The best way to remove more undesirable solids from a porosity zone is by giving the zone time to settle. Many of my drilling mentors have said that the longer a well takes to clear up, the longer the life of the well.
Well development starts in the drilling program and continues through taking the proper steps to open the passageways. When a company plans to minimize the impact to the production zone and maximize their development method and tools, it becomes easy to create a good, clean water well. Efficient well development creates a productive source of water for decades.