Drilling fluids are important to providing the well you and your customer want.
By Jeff Blinn
I’ve had many discussions about why we pay so much attention to drilling fluids and why they are such an important part of the water well drilling process.
The focus of the Clear as Mud columns has mainly been on the science of drilling fluids because I believe it is necessary for an in-depth understanding of just how drilling fluids are part of the bigger picture of the drilling process—or as I like to call it, one part of the total drilling system.
Another approach to understanding a drilling fluid’s importance is to look backwards and ask this question: “What do we want to have when we are done drilling, the well construction is complete, and all associated equipment has been installed?”
The simple answer of course is plenty of clean water! But the reality of the answer is much more complex. For example:
- We want a sustainable supply of clean water that doesn’t exceed the ability of the aquifer to produce the volume we need.
- We don’t want to restrict the flow paths within the aquifer.
- We do not want to damage the aquifer by contaminating the water source during construction.
- The whole process needs to be economical for the well owner, not only during construction but during the life of the well.
I am sure you can think of several more. So, with that, I want us to look at how our drilling fluid helps us answer the question, “What do we want to end up with?”
One of the biggest culprits of reduced productivity is flow path blockage. This can be inherent in the formation with fine-grained material plugging the space between larger grains or filling fractures. The blockage can also be self-induced by plugging the flow paths with drilled material.
Both are common, and part of the well construction process is to clear the flow paths of these obstructions by developing the well before production. Proper drilling fluid selection can help minimize blockages and make development time shorter and more efficient.
One of the functions of a drilling fluid is to transport drilled cuttings to the surface where they can be removed by the solids control equipment—either gravity settling them in the mud pits or by solids removal equipment.
If we maintain sufficient suspension properties of viscosity and yield point in the drilling fluid and adequate annular velocity of the fluid in the annulus, the cuttings will be transported to the surface in as close to the original size of the cutting as possible, making it easier to be removed from the fluid. Of course, no solids control system is 100% effective at removing all unwanted particles from the drilling fluid, so some undesirable solids will be recirculated downhole.
A second drilling fluid function is to wall the hole with a thin, slick, impermeable wall cake to stabilize the wellbore and coat the drilled cuttings with a protective film to minimize their reaction to the drilling fluid. The drilling fluid properties of filtration, filtrate volume and filter cake thickness, and inhibition provide these functions.
The wall cake also seals the borehole wall so drilled material does not enter any porous and permeable water-producing zones, causing flow path blockages.
The protective film coating on the cuttings helps in different ways as well, depending on the type of material the cutting is made of. Inert material that does not react in the presence of water is coated by the drilling fluid and gives the particle a net negative electrical charge, thereby slightly repelling particles from each other.
Drilling fluids … contribute to an easier and more effective well development program resulting in plenty of cool, clear water.
Reactive material, such as clay, is also coated with drilling fluid additives that seal the surface so water cannot be absorbed, preventing these particles from either swelling and becoming sticky or in some cases disintegrating into ultra-fine material that cannot be removed.
Note: As a quick review, the primary drilling fluid additives of bentonite, PAC filtration control polymers, and PHPA inhibitive polymers are used to give the drilling fluid viscosity, suspension, slick impermeable wall cake-building, and water uptake inhibition. They also contribute to a net negative charge on the wall cake and drilled solids. This is important when it comes time to develop the well.
The well has been drilled, casing set, filter pack installed, and now it’s time to clean up the well. This cleanup process is called well development and generally means following systematic procedures to ensure maximum output at the highest specific capacity with minimum production of particulate matter.
That is a fancy way to say we want to efficiently produce clean water.
As we indicated earlier, the use of a well-designed drilling fluid that addresses the characteristics of the formations drilled and is maintained with efficient solids control is the first step. Call it pre-development as we are taking steps during drilling to make primary development easier and more effective.
We need to flush out the drilling fluid and any obstructions blocking the flow paths in the aquifer. Just circulating water through the well or airlifting or pumping the well does not provide enough energy to remove the filter cake from the wellbore or remove particulate matter lodged within the formation, but it can remove excess drilling fluid and any easily removed material.
Since water flow from the formation follows the path of least resistance, we will only clean these areas of the aquifer. Depending on the purpose of the well, this production may be sufficient to meet design needs but does not make the best use of the aquifer.
Additional energy must be supplied after this first stage to reach and break down any residual filter cake in these initial sections and then target the remaining sections of the aquifer. This can be achieved through swabbing and surging, and jetting and pumping.
These methods focus energy on short sections of the aquifer at a time. We also can introduce well development chemicals designed to react with the filter cake and particulates in the formation.
Do you believe in magic? Breaking down the filter cake requires the development chemicals to interact with the filter cake. They do so by reacting with the negatively charged sites on the drilling fluid additives surfaces that I mentioned earlier. In practice, it is easier to remove the filter cake developed by using engineered drilling fluid products than it is to break down a filter cake made up of native drill cuttings.
So put to rest a long-time assumption that you can’t drill a water well with drilling fluid additives. The reality is when done correctly it is easier and more effective to clean up a well drilled with an engineered and maintained drilling fluid.
To answer my initial question of why engineered drilling fluids are important—aside from controlling the formations and minimizing drilling problems—they contribute to an easier and more effective well development program resulting in plenty of cool, clear water.
Jeff Blinn has had a 40-plus year career as a professional drilling fluids engineer. Beginning with mud school in 1978, he has worked in many drilling disciplines including minerals exploration, water well, oil and gas, geothermal, geotechnical, and horizontal directional drilling. He has held positions as field sales engineer, engineering supervisor, account representative, technical services representative, and training manager. He can be reached at firstname.lastname@example.org.