It’s important to understand the process of properly mixing drilling fluids.
By Jeff Blinn
Welcome to the new year y’all! I hope all of you survived 2021 in good shape and are looking forward to 2022.
I was looking through some of my old photographs—yes, these were printed photos taken way before the digital era—and found one circa 1979 of a rig hand mixing mud.
That got me to thinking about the process of mixing mud. Is it just an everyday dirty task to you to be gotten out of the way as quickly as possible? Have mixing methods and equipment changed? Do we think about mixing any differently today?
On the one hand, an old cliche comes to mind: “The more things change, the more they stay the same.” But on the other hand, with advances in new drilling fluid products, their requirements for proper mixing have changed.
Let’s take some time to explore mud mixing.
Learning by Observation
All drilling fluid products, whether they come as a powder in a bag or as a liquid in a bucket, need to be mixed in water so we can get those products’ properties (see my previous Clear as Mud columns from 2021) available for our use.
It can be a dusty, dirty, boringly slow process. This is where mixing can feel like a necessary evil, and it has since drilling fluid products were first invented. As I’m sure all of you are aware, nobody in the drilling business wants to think about slow and everything needs to be faster. This may be good for your schedule, but is it good for drilling fluid?
I didn’t think much about what had to happen during mixing early in my career; it was just dump a bag of bentonite gel into a hopper over a moving stream of water and the gel would mix into the water, viscosify the water, make mud, and we were ready to drill.
I also didn’t think too much about the pile of unyielded bentonite gel on the bottom of the mixing pit that would come to light as we pumped the pit down, other than to think perhaps our mixing system was lacking.
I remember jetting that pile up with a water hose, and after pumping that lumpy material downhole, it came back smooth. I learned the slower you added the bentonite, the smaller the pile on the bottom of the pit.
I also noticed when we mixed quickly when the mixing pump was turned off, a layer of water would form on top of the bentonite mixture; the bentonite was settling out and falling to the bottom of the mixing pit. The pump would have to be turned back on to remix the pit.
From learning by trial and error, I understood that the rate of addition of material made a difference in the final drilling fluid; and the longer the mixing pump was on recirculating the fluid in the mixing pit made a difference in the final drilling fluid.
Learning the Process
It wasn’t until years later that a colleague of mine, Ed Anderson of Baroid Industrial Drilling Products, found a great way to explain what many of us learned by observation: mixing is a three-step process consisting of 1) dispersion, 2) hydration, and 3) homogenization.
Step 1: Dispersion
This is getting the drilling fluid product into the water to begin with. It requires proper mixing equipment to supply sufficient energy to physically force the particles of product away from each other and to keep them from sticking back together.
I have witnessed several inefficient ways to do this, including dumping the product into the water and hoping it will mix on its own, stirring the water with a shovel while adding the product, and jetting the mixing pit with a hose to mix product. I have even seen a video of a horizontal directional drilling crew dumping bentonite gel into their mixing pit and breaking up the clumps and stirring by hand!
None of these methods created what we would call a good drilling fluid. Proper dispersion requires high-fluid volume and pressure supplied by a dedicated mixing pump to an engineered venturi with attached hopper.
This system assures the highest liquid volume to solids being added. This helps to keep the size of the added solids as small as possible, keeps the particles as far away from their neighbors as possible, and minimizes clumping of solids. This maximizes the surface area of the added solids coming in contact with water.
It should go without saying that the size of the mixing system should be able to outpace the rate at which fluid is used. A 150-gallon tank and small 2-inch trash pump will not be able to keep up with the fluid requirement of a 12¼-inch hole drilling 100 feet per hour. This leads to the next step.
Step 2: Hydration
Most all drilling fluid products need water to maximize their properties. Let’s use bentonite gel as an example because bentonite gel needs more water for hydration than any other product.
After dispersing bentonite into water, allowing for a large surface area to come in contact with water, water is adsorbed into the bentonite structure, forcing the bentonite aggregates—the powder that is in the bag—to break down to their smallest individual size, a bentonite platelet.
If we continue circulating our bentonite/water slurry, the additional energy supplied by the pump and venturi aid in breaking down the aggregates, accelerating hydration. The hydration process does not happen instantly but takes time.
Have you ever had a broken bag of bentonite get wet? You have a gooey slimy mass of bentonite as it is hydrating on the outside. But if you try to move it, there is still a lot of dry bentonite powder on the inside. This illustrates hydration without additional energy tearing apart an aggregate structure.
Now say you have bentonite dust all around your mixing area and it starts to rain. Everywhere around you is slippery and slimy with no dry bentonite to be found. This is dispersed and hydrated bentonite—exactly what we are doing during mixing dispersion and hydration.
Step 3: Homogenization
While steps one and two are in progress, gravity is still in effect. Larger and heavier clumps and particles will try to settle towards the bottom of the mixing pit. This is especially true when the mixing pump is turned off.
This can result in a layering effect in the pit, with fluid with larger suspended particles towards the bottom and fluid with finer suspended particles towards the top. Before use, the mixing pump should be turned back on to circulate the fluid in the mixing pit to even out or homogenize the different layers to give us a nice even fluid.
Mixing a Complex System
Drilling fluids are certainly more complex than the simple example of bentonite gel and perfect water as I referred to earlier. Our mixing water is not always up to par in the real world, and we may be mixing several other additives along with or in place of bentonite gel.
Is there a right way to mix a complex system? Most of our drilling fluids are water-based, and therefore the quality of the mix water contributes directly to the quality of drilling fluid produced.
Since we don’t drill with distilled water, our mix water usually has many other dissolved minerals and compounds in it and some of these are detrimental to the hydration of drilling fluid products.
The most common of these are salt and the ions that cause water hardness. Most drilling fluid additives do not hydrate in saltwater at all, so salty or brackish water should be avoided for mixing. Hard water is defined as having large concentrations of minerals, predominately calcium and magnesium.
Most fluid additives hydrate most effectively when the water hardness is less than 100 parts per million (ppm). Hence, you need to know the hardness of your mix water. Hardness indicator strips are available for this purpose.
According to the U.S. Geological Survey, the average water hardness in the United States is 250 ppm. Bad for mixing mud. Luckily, there is a simple solution—just treat the water with a small amount of soda ash.
Soda ash is sodium carbonate, and it combines with the calcium ion in the water to form calcium carbonate which is insoluble in water, effectively removing the calcium from the water. Often less than one-half pound of soda ash per 100 gallons of mix water is needed.
Soda ash has a secondary benefit for mixing efficiency as well—it raises the pH of the water. Most fluid additives mix most effectively in a slightly alkaline solution, or when the pH of the water is between 9 and 10 on the pH scale. As soda ash lowers hardness to less than 100 ppm, it raises the pH to 9 to 10.
Soda ash is the most cost-effective additive you can use as 50¢ of soda ash can save $10 or more of other additives.
Adding Products in Order
Once we have the mix water treated, it’s ready to receive the other drilling fluid additives in our fluid mix. And now that we’re at this point, we need to understand the order of addition of additives.
I’ve mentioned bentonite gel requires a lot of water for hydration, so it is mixed before any other additives. During the process of hydration, much of the water attaches to the individual bentonite particles, removing this water from the free water available for hydration of other additives.
It takes time for hydration to occur and allowing time for the bentonite to hydrate before adding any other additives is preferred. How much time is required? Okay, 10 to 15 minutes of circulating will allow approximately 90% to 95% of bentonite to hydrate.
Next, add dry polymer products, as they also need some water for hydration. Then, liquid polymers come after the dry polymers. If surfactants, lubricants, or thinners are required, they go in last.
We tend to think mixing drilling fluids is a simple process, but how the drilling fluid was mixed is often overlooked when troubleshooting drilling problems.
Sometimes when all other factors seem correct—you have the right products in the right concentrations, the equipment meets the needs of the project, and all best practices and drilling fundamentals are being followed—and you still can’t find the root of the problem, it might be in how the products were incorrectly mixed and therefore what we assumed should be controlled by the drilling fluid just isn’t.
When you take a close look at mixing drilling fluids, it turns out to be more complex than one might think. It’s a three-part process combining dispersion, hydration, and homogenization. It takes time. It takes water treatment. And you need to follow the order of addition of products.
Is understanding what needs to happen and taking the time to mix additives correctly worth it? In my opinion, yes. Circulating drilling fluid is the lifeblood of the drilling system. It helps us control the subsurface conditions presented by the local geology. It allows us to effectively create a hole that will provide that priceless commodity—potable water.
Thanks for reading and always contact your drilling fluid supplier for proper use of its products. Feel free to reach out and let me know if you would like any more information.
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.