Mixing the Mud: The Science of Drilling Fluid in HDD

By Tabitha Mishra
Published: August 7, 2020 | Last updated: July 5, 2023
Key Takeaways

Most people think of mud as something to be avoided, but mud (aka “drilling fluid”) is actually a critical component in drilling operations.

Drilling mud, or drilling fluid, is a critical but often overlooked contributor to the overall success of a trenchless construction project. The wrong drilling mud has the potential to cause severe problems like frac-outs or borehole collapse. The right drilling fluid can make a horizontal directional drilling (HDD) project run smoothly and efficiently.


Understanding the science of drilling fluid is essential for choosing the right additives and a successful project.

What is the function of drilling mud?

Drilling mud is the fluid pumped into an HDD borehole. It has several functions, all of which contribute to the success of the drill:

  • Cooling and lubricating the drilling head.
  • Suspending the cuttings and removing them from the borehole.
  • Stabilizing the borehole.

Drilling heads generate large amounts of heat as they cut through the ground. The more friction between the drill bit and the rocks, clay or shale in its path, the more heat gets generated. The presence of drilling fluid reduces friction and removes heat from the cutting edge.

Cuttings removal is part of the HDD process. This is only possible if the drilling fluid holds the cuttings in suspension as it flows out of the borehole. (Read also: Understanding Drilling Fluids Solids Control with Horizontal Directional Drilling Rigs.)

Each type of soil requires specific drilling fluid properties that match the characteristics of the ground. The science of drilling fluid, therefore, plays a significant role in choosing the right mud composition.

Drilling mud stabilizes the borehole by laying down deposits on the sidewalls. The wrong mud composition can cause the fluid to break through the sidewall, causing a frac-out. This shuts down the HDD project until the borehole's integrity can be re-established, thus wasting time and adding to the cost of the project. The wrong composition could also cause a collapse of the borehole with similar consequences.

A joint study by the University of Illinois and the Illinois Department of Transportation explored the approval of HDD as a construction method in specific applications. The study highlights the importance of drilling fluid for the success of an HDD project.


What is in drilling mud?

The primary constituent of drilling mud is either oil (called synthetic-based or low toxicity oil-based mud) or water. Other components are added to this base in order to achieve the desired characteristic. The main secondary ingredient is bentonite, which is a mineral found in clay beds. It is the bentonite which forms a cake-like film on the inside walls of the borehole to stabilize it. (Read also: Bentonite and the Use of Drilling Mud in Trenchless Projects.)

When using water-based drilling mud (WBM), the quality of water also influences its performance. pH should be kept between 8.5 and 9.5, while calcium content should be below 100 parts per million (ppm).

The final constituent of drilling mud is an additive. Several additives are available, and each is suited for different conditions. Polyanionic cellulose (PAC) is common in sand and cobbles. Partially hydrolysed polyacrylamide (PHPA) polymers work well in reactive clays instead of bentonite. Polymers with larger molecular weights are most suitable for rocky soil.

The science behind making the best mix

Finding the right drilling mud mix for any project starts with an understanding of the soil conditions. The more thorough the geotechnical investigation of the proposed HDD path, the more precisely the ideal drilling mud formula can be determined.

While it is possible to complete an HDD project with a generic drilling mud mix, it is often not the most efficient method, and it can lead to major problems like frac-out. Even if frac-out or borehole collapse are avoided, the project may take longer and cost more than necessary.

Inadequate lubrication can cause drill head damage and inefficiency. In some cases, it can also cause the pipe to stick in the borehole. A poor suspension characteristic can consume more energy to circulate the drilling mud through the system, thus slowing down the drill's progress.

The order of mixing also plays a role in the science of drilling mud. Adding polymers before the bentonite has thoroughly mixed can cause balling. PAC polymers go before PHPA polymers and dry polymers before liquid polymers.

The science of drilling fluids goes beyond chemistry and includes the physical characteristics of the soil. Shear modulus determines the maximum drilling fluid pressure in the borehole before raising the risk of frac-out. This idea is explored in a paper available from the International Society for Soil Mechanics and Geotechnical Engineering. (Read also: Job Role: Mud Engineer.)

How much drilling mud do you need?

Having the right volume of drilling mud in the system is also a key parameter for HDD success. The following process describes how to calculate the right amount of drilling fluid:

1. Determine the actual borehole size: D (not the pipe diameter)

  • D (inches)2 / 25 = gallons/foot

2. Determine fluid-to-soil ratio in gallons/foot

  • For sand, gravel, cobble or rock: 1-to-1
  • For clay: 2-to-1 up to 6-to-1
  • For reactive shale: 10-to-1 or 20-to-1

3. Determine rod length and pump output

4. Amount of drilling mud required = (gallons/foot × fluid ratio × rod length) ÷ pump output

Making this simple calculation before the start of a project can help in estimating and planning.

Recycling and disposing of drilling mud

Drilling mud systems on an HDD rig consist of a mixing hopper and fluid tank. Pumps take suction from the tanks and pump the drilling mud down the borehole. The return fluid passes through screens and hydro cyclones to remove cuttings before being recycled.

The used drilling fluid may need to be transported to hazardous waste dump sites, often some distance away from the HDD installation. Disposing of hazardous waste is expensive and adds to the cost of an HDD project. (Read also: Why Proper Disposal of Used Drilling Fluid Should Be a Crucial Trenchless Planning Step.)

Studies investigate the effect of drilling mud on vegetated land. These studies yield promising results and propose a philosophy that could minimize transport and disposal requirements.


Drilling mud is a critical element of the success of an HDD project. Getting the science of drilling mud wrong can lead to inefficiencies, delays, and higher costs. It can even result in frac-outs or collapses, resulting in major problems. On the other hand, getting the science of drilling mud right can significantly improve the HDD rig's performance and save both time and money.

A thorough geotechnical survey is the starting point for developing drilling mud recipes. This information enables engineers to choose the right base and the best combination of additives for the job.

Share This Article

  • Facebook
  • LinkedIn
  • Twitter

Written by Tabitha Mishra | Civil Engineer, Technical Content Writer

Tabitha Mishra

Tabitha has a Bachelors Degree in Civil Engineering from Mumbai University, India, and is currently freelancing as a technical content writer. Prior to writing, she has worked as a site engineer and site manager for various building construction, building rehabilitation, and real estate evaluation projects.

Tabitha is also certified as a Primavera project management professional and is well versed with Auto CAD. In her spare time, she does private consultation for small-sized home builders and assists with plans and permissions.

Related Articles

Go back to top