Why drilling fluid is essential to HDD and the mixing process
Drilling fluids are the secret sauce on horizontal directional drilling (HDD) jobs. Drilling fluids can also be one of the most misunderstood aspects of drilling operations. Every member of your crew working around drilling operations should understand what drilling fluid is, how to mix it, the type of additives they should use for various soil conditions, how to determine pumping and pullback rates, as well as how to make adjustments on the job.
This series of short articles will help take some of the mystery out of drilling fluids, beginning with why it is essential to HDD operations and ways to mix it.
Why drilling fluid is important
Here are a few of the reasons drilling fluids are used on HDD projects:
- Helps extract cuttings: Failure to remove cuttings from the bore path can result in inadvertent returns (frac-out), limit distance length the drill can pilot bore, heave ground surfaces above the drill head and lead to product pullback failures.
- Creates downhole stability: Drilling fluid helps the drill path keep its shape during the drilling process. Using the right type of fluid additive is also important to help a drilled hole maintain its shape when working in different soil types (drilling in sand for example).
- Cools and lubricates tooling: Downhole tooling temperatures can rise quickly without the use of drilling fluids. Drill head temperatures rising above 220° Fahrenheit will permanently damage the transmitter that gives critical depth and pitch data. Drilling fluids also help keep tooling clean and reduce pullback/rotational torque pressures required to pilot out and pull product back through.
Drilling fluid viscosity
You’ve now learned why using drilling fluid is essential, but it’s also necessary to understand having the right viscosity (thickness) of drilling fluid.
Drilling fluid that is too thin or light can limit the amount of cutting returns. You may still get the majority of returns, but heavier cuttings can sink to the bottom of the bore (creating a cuttings bed) and not be flushed out. After a while, this cuttings bed can create return flow issues during the rest of the drilling process (which can result in excessive product stress and inadvertent returns).
Drilling fluid that is too thick can result in additional downhole pressure to get fluid to flow and eventually lead to inadvertent returns. Fluids follow the path of least resistance, and if the fluid is too thick — that direction is typically towards the surface.
To check that you have the right viscosity, it’s important to measure it every time you mix up a batch of drilling fluid. You will need a Marsh funnel and cup, following these steps.
- Start by filling your cup up to the fill line with drilling fluid.
- Next, plug the hole in the bottom of your funnel with your finger and pour the fluid through the funnel’s screen (ensure you are wearing proper safety gloves).
- There shouldn’t be any clumps of material left behind on the screen. If there is, your drilling fluid is likely not mixed correctly or fully hydrated (more on that next). Repeat until drilling fluid crests the top fill line (just under the screen) of your Marsh funnel.
- Now, place the empty cup below the funnel and start a stopwatch as you remove your finger from the bottom of the funnel. You want to stop counting when the drilling fluid crests the top fill line (32 oz [.9 kg]) of your cup. This number is your drilling fluid viscosity measurement.
You should aim for a viscosity rate of between 45 and 60 seconds for most drilling applications.
Mixing process
Always make sure the person assigned to drilling fluid mixing is fully trained on the proper steps.
On every new job, you should check pH levels from your water source. Drilling fluid additives (bentonite and polymers) mix better with a pH level range between 8 to 9. Municipal water will typically measure around that range. However, if you’re using water from a pond or well, the pH level will likely fall below that range. Adding soda ash will help raise the pH levels and help you use less drilling fluid products.
When you’re ready to mix your additives with the water, powders like bentonite must be added to the hopper and liquids should be poured directly into the tank.
Every Vermeer fluid mixing system is equipped with a bag cutter next to the hopper. Use it to split and empty the whole bag into the hopper at once. Doing it this way, instead of cutting it with a knife and slowly sifting material into the hopper, will help reduce dust levels and help prevent air from getting into the system.
When adding bentonite, make sure it’s sent through the venturi and hopper at full throttle. You want to get the maximum implosion through the venturi to mix the product well. After enough bags of bentonite are added, keep product mixing through the venturi for approximately 10 minutes until it’s thoroughly broken up and hydrated.
If you plan to add a polymer, you’ll need to wait until the bentonite is completely mixed (10 minutes). Adding a polymer too soon or out of sequence can cause the bentonite to ball up in the tank rendering it ineffective.
The final step is to check the drilling fluid viscosity. If it’s too thick, add additional water or thinners; if it is too thin, add more drilling fluid additives.
A few housekeeping notes
Mixing drilling fluid doesn’t have to be a messy job. Make sure you’re cleaning your mixing system’s hopper and venturi regularly to help prevent clogs. Also, make sure you’re washing your tank often. If there is polymer residue left in the tank when you go to mix up your next batch of drilling fluid, you could end up with small balled-up clumps of bentonite in your tank.
Future articles of “Making sense of HDD drilling fluids” will cover the steps to calculate reamer pullback drill rates and explain the different types of drilling fluid additives available. For more information about drilling fluid mixing or the MUD hub, reach out to your local dealer, or visit Vermeer.com.
