Drilling fluid is the life fluid of a trenchless project; it can literally make or break a borehole. Drilling fluid serves many purposes in a drilling operation such as lubricating the drill bit, stabilizing the borehole, suspending the cuttings during trips, and conveying cuttings out of the borehole.

This in turn protects the drill rod and other downhole equipment from damage. Managing and selecting the right drilling fluid is critical to a successful bore and installation. Drilling fluid is customized to the prevalent ground conditions in the path of the bore, and requires extensive geotechnical and soil investigation before arriving at the correct mix of additives.

This means that drilling fluid is not just a random mix of water and additives, rather there’s a science behind the mud mix used in a horizontal directional drilling (HDD) project.

What Is Drilling Mud?

Drilling mud is a mixture of natural and synthetic chemical compounds that function as lubricant and carrier. Drilling muds are either water based or oil based, and mixed with appropriate additives. The type to be used depends on the formation through which the bore will pass and also on fluid disposal post-use. Environmentally sensitive locations have strict laws when it comes to drilling fluid disposal, and carelessness can attract heavy penalties to contractors and owners.

The additives in drilling fluid provide essential characteristics such as filtration and suspension, and prevent frac-out, heaving or settlement of ground surface and failure during pullback.

Constituents for Mud Mix

Though there’s no perfect formula for making the right mix, the mix has to be relevant to the soil through which drilling will take place. The main constituent of drilling fluid is bentonite which is a mineral found in clay beds. When combined with water it forms a cake-like substance that helps seal the borehole walls, preventing the drilling fluid from escaping through crevices in the borehole wall, which could lead to frac-out.

The support provided by the slurry also keeps the borehole from collapsing. (For more on bentonite, check out Bentonite and the Use of Drilling Mud in Trenchless Projects.)

The primary constituent of drilling fluid is water, and additives constitute only about 3% to 4%. The quality of water will also affect the final drilling mud because water has to be sourced from creeks, rivers or the city. High concentration of sand or clay can weaken the binders in the mud mix and reduce the quality and efficacy of the drilling fluid, while contaminants in the water can shorten the life span of the mud pump, according to Jon Heinen, pipeline segment manager for Vermeer Corporation.

Additives will be more effective when the pH of the water is between 8.5 and 9.5, while calcium content should be below 100 parts per million (ppm).

Drilling Fluid Additives

Additives depend largely on the soil conditions prevailing in the drill path. Since extensive geotechnical investigation is carried out prior to designing the bore path, contractors should take advantage of the report to design the right mud mix. In non-reactive clay, bentonite mixture and a lubricant are usually sufficient to carry cuttings, flush the borehole and lubricate the drill bit to prevent sticking. In reactive clays, sand or cobble, a polymer additive will be required depending upon the soil condition.

In sand and cobbles, polyaluminum chloride (PAC) polymers are added to the bentonite mixture to provide secondary filtration control. Where there is high concentration of reactive clays, partially hydrolyzed polyacrylamide (PHPA) polymers are used instead of bentonite because it prevents the clay from swelling by wrapping itself around the clay. For cobble and rocky soil conditions, a polymer with larger molecular weight is used because it supports suspension, stabilizes the bore path and helps extract larger cuttings from the annulus.

Assessing Drilling Fluid Requirement

The amount of drilling fluid required depends on the borehole diameter and soil condition. Since drilling fluid disposal has become very difficult and costly, making the right amount of drilling fluid and recycling it can help control cost to some extent. However, not having enough drilling fluid to pump downhole by compromising on additives can prove to be a more costly affair. Here are some steps to calculate the amount of mud needed for a particular bore.

  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.

Mixing the Mud

Check the pH level of the water source and if it falls below 8, mix with soda ash to increase pH because it helps bentonite and polymers to mix better. Dry powder additives should be added to the hopper while liquids are poured directly in the tank. The bentonite should exit the venturi and hopper at full throttle to get a good mix. Polymers, if any, should only be added once the bentonite has completely mixed to prevent balling. PAC polymers should be added before PHPA polymers and dry polymers before liquid polymers.

Check the plastic viscosity, gel strength and yield point, and add additional water, additives and thinners as required. (In order to complete a successful bore, your equipment must be in good shape. Learn more in Proper Maintenance for Drill Rig Equipment.)

Making a good mix is not child’s play – it’s mud science – and due attention should be given to it. Appropriate calculations should be made to ensure the right amount of drilling mud is available to successfully complete the project. Outrunning the mud can be costly and so can skipping the right additives as per soil conditions.

A high fluid-to-soil ratio due to mud outrun can cause the mud to become too thick to pump out of the exit or launch pit. It can also cause inadvertent return, ground heave, and stuck or damaged downhole tools.