Understanding engineered drilling fluids can benefit your drilling operation.
By Jeff Blinn
I know what you’re thinking: “Not another column about drilling mud!”
Well, I’m here to tell you drilling mud doesn’t get the respect or attention it deserves. It can even be exciting! I hope to provide new insight on drilling mud, or more correctly, engineered drilling fluid, by way of this column.
So here we go with my bio. My name is Jeff Blinn and I have been a professional mud engineer for most of the past 43 years. I attended the Magcobar Drilling Fluids mud school in the first quarter of 1978 after graduating from the University of Arizona with a degree in geology.
My first posting as a new mud engineer was in Grants, New Mexico, as a field sales engineer for Magcobar’s mining and water well division supporting uranium exploration drilling. As the exploration programs wound down, I was transferred to Long Beach, California, as a mud engineer in the oil well drilling fluids group. I then had a transfer to Elko, Nevada, to support oil and gas exploration for Magcobar.
A layoff came, some consulting work, and then I was hired by International Drilling Fluids for minerals and water well mud engineering in Nevada. I was laid off again, and then hired by Baroid Industrial Drilling Products for minerals and water well work in Nevada.
I was transferred to Fort Worth, Texas, as an account representative for water well and horizontal directional drilling applications, transferred to Houston, Texas, to work as an IDP technical representative, and finally as the training manager for Baroid IDP.
The next question I’m sure you have is “Why did I become a mud man?” It dates to a part-time job I was fortunate to have for the city of Tucson, Arizona, water department during my junior and senior years at the University of Arizona.
One of my duties was to collect geologic samples during the drilling production of water wells and exploration wells for new water sources. I witnessed—especially on the exploration side—many of the things that can go wrong during drilling.
One day a drilling contractor was using the dual tube, reverse circulation method and sticky clay was giving him fits with slow penetration rates, balled bits, and plugged inner tube.
The local mud man came to the location with a bag of a drilling additive and added it to the injection water. Magic happened! The sticky-clay problems vanished, and the rate of penetration took off.
I was amazed at what I saw—and I was also hooked. I wanted to understand the magic I witnessed and have spent the next 43 years believing in drilling fluid magic.
I arrived at my first posting after mud school, and of course as a newly minted mud engineer, I thought I knew everything. Well, I didn’t.
We were led to believe in mud school that the mud man had a pretty important job on location. However, I found out the mud man could be pretty low on the importance scale, especially one that is a newbie. Fortunately, with time, experience, and asking a lot of questions, I was accepted and my opinions were considered.
Something else I learned early on from this experience: No single part of the drilling operation is independent of any of the other parts. Drilling is a complex system where all the parts—the rig, tools, pumps, fluids, and personnel—must work together to control the subsurface environment the wellbore intersects.
This concept of drilling as a system is something everyone needs to understand. All the parts need to be balanced to optimize performance.
When there are drilling problems, I often hear, “It’s the mud’s fault.” It might be, but it could also be another part in the system.
This column will be a journey to understanding the importance of drilling mud, engineered drilling fluids, what they are, how they work, where to use them, and how they can be economically viable to the success of a drilling operation. Essentially, it will be the who, what, where, when, why, and how.
What Exactly Does It Do?
We are in the business of constructing water wells. The National Ground Water Association’s definition of a “water well” from its Lexicon of Groundwater and Water Well System Terms is quite lengthy but basically states it is “an excavation that is drilled . . . or otherwise constructed for the purpose of extracting groundwater . . . .”
Of course, our tools for excavating include a drilling rig, drill bits, and a circulating system. Without the circulating system to flush the hole, the drill rig and bit could not progress far.
The flushing fluid is commonly called drilling mud, and early in the history of rotary drilling, mud is exactly what was used—water mixed with the drill cuttings. This mixture had its limitations as all mud is not created equal.
Water mixed with sand cuttings did not work well as the sand would not stay in suspension. Water mixed with clay cuttings worked a little bit better. It created a mud that could suspend cuttings and remove them from the hole. The drilling fluids industry was born from these early realizations.
Let’s take time to understand what a drilling fluid is asked to do. As an excavation tool, it needs to be able to remove the cuttings created by the bit and transport them to the surface. Once at the surface, the fluid needs to drop the cuttings so as not to carry them back downhole. And when the circulation stops, the fluid needs to suspend cuttings in the borehole in place and not let them fall back to the bit.
These three functions of the drilling fluid are like the three points of a triangle all pulling away from each other. This is where the magic of drilling fluids begins. These functions of the fluid can be described by measurable properties such as viscosity, yield point, and gel strengths.
The drilling fluid also needs to stabilize and support the wellbore to keep the hole open long enough to complete well construction. Notice I didn’t say keep the hole stable forever. We are only buying time from Mother Nature to get the job done. The drilling fluid needs to protect the formation and the fluids within the formations by creating a thin impermeable wall cake.
The drilling fluid also helps provide reliable geologic information. The returned cuttings indicate the formations being intersected. The drilling fluid is our only means of controlling subsurface pressures. The fluid can be adjusted to control positive pressure such as an artesian condition or adjusted to control low pressure that we see as lost returns.
And finally, the fluid is our means to cool and lubricate the bit and drill string. All of these functions are not an exhaustive list but cover the main purposes of a drilling fluid.
Who knew that old drill mud was so smart?
The Introduction on Bentonite
Could original drilling mud, that mixture of water and drill cuttings, perform the functions of a drilling fluid as indicated above? No, not really. Yes, today some wells are drilled just using water and native material, but in many areas this method is asking for disaster.
The worst possible outcome is these native solids are the right size and shape to plug the water-producing zones we want to use, and they cannot be sufficiently removed during well development.
As I mentioned above, it was recognized that water and native clay helps remove cuttings and keeps the borehole open, but native material has drawbacks.
Bentonite clay was used as a suspending agent for pigments in paints in the early 1900s. Bentonite was adapted from the paint industry to the drilling industry to be mixed with water to be a basic drilling fluid capable of suspending drill cuttings and creating a gelled structure for suspension when circulation was stopped.
The structure of the bentonite allowed for tremendous swelling characteristics to give carrying ability and known properties with a minimum amount of material, created a gelled structure capable of suspending cuttings when circulation stopped, and walled the exposed formations with a thin slippery and impermeable wall cake.
Bentonite was the first commercial drilling fluids product that solved the problems associated with using native materials. Bentonite is so good at creating a basic drilling fluid with known properties that it is still the primary additive used today.
Drilling is a complex system where all the parts—the rig, tools, pumps, fluids, and personnel—must work together to control the subsurface environment the wellbore intersects.
Water plus bentonite clay makes a basic drilling fluid capable of performing the drilling fluid functions we talked about earlier. How do we know if this is true? In practice, we know we have fewer drilling problems when we use an engineered fluid, so it must make a difference.
But to prove it works, and to understand why it works, each of the drilling fluid functions has a property or characteristic that can be measured. When a mud man is on location, he will take out his mud kit and run a series of tests to measure the drilling fluid’s properties to be sure the fluid is able to control the formations the drill bit intersects. The proper drilling fluid and requisite properties are governed by the geology of the drilling location.
Talking About Viscosity
The first property to talk about is viscosity, the thickness of the fluid. This property is the simplest to understand because it is the only property we can see—our eyes can easily tell if the fluid is thick or thin.
Viscosity is measured in the field using a Marsh funnel and cup and reported as the time it takes for one quart or liter of fluid to flow from the funnel into the cup. For reference, water has a Marsh funnel viscosity of 26 seconds per quart or 27 seconds per liter.
This simplicity can lead to a misunderstanding of the role viscosity plays in the drilling fluid’s ability to perform its functions. Viscosity relates to the hole-cleaning ability, suspension characteristics, and borehole-stabilizing ability of the drilling fluid. Thicker fluids tend to have better suspension and carrying capacity and be more stabilizing to the borehole.
Viscosity also directly affects the pressure needed to pump the fluid through the circulating system. The literal definition of viscosity is “the fluid’s resistance to flowing,” as a thin fluid offers little resistance to flow, and a thick fluid has more resistance to flow.
Higher pumping pressures also place more pressure against the borehole wall, which can lead to breaking down the formation or inducing loss of fluid to the formation. This is where our eyes can lie to us. We think a thick fluid is helping us, but it may actually be hurting us. The proper viscosity for any drilling project is a balance between suspension and stabilizing needs versus the pressures required to pump the fluid without creating problems.
Adding more bentonite to increase viscosity to help carry cuttings out of the hole or to stabilize caving formations also raises the viscosity and pumping pressures and can worsen our problems. A colleague of mine put it this way to me: “We need a skinny fluid that acts fat.”
Again, viscosity is not the only property of a drilling fluid. There is fluid density or weight, the filtration characteristics of filtrate volume and filter cake thickness, the rheology of the fluid (the scientific aspects of viscosity), sand content, and pH and calcium content of the fluid.
These all play a role in how well the drilling fluid performs its functions. Other drilling fluid additives contribute to the properties of bentonite to help create a relatively thin fluid that behaves more like a thick fluid without the pressure concerns.
There is more to come down the road with real world examples and how all of this will be a benefit to your drilling operation. Thanks for reading!
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.