In trenchless engineering methods, bentonite is widely used for drilling mud as a lubricating and supporting agent in subsurface tunneling operations such as horizontal directional drilling (HDD) and pipe jacking. Drilling Mud is also known as drilling mud, mud mix and slurry.

What is Bentonite?

Bentonite belongs to the smectite mineral family of non-metallic clays containing mainly montmorillonite consisting of individual crystallites, most of which have a maximum size of fewer than 2-micrometers.

Bentonite got its name from where the clay was first found around 1890 near Fort Benton in Wyoming. The clay is often generated from altering volcanic ash, and the origin of the clay may explain the presence of a variety of other minerals such as quartz, feldspar, gypsum and calcite.

Depending on the application, these additional minerals can impact the value of the deposit. A predominant property of bentonite is its ability to absorb water and increase its own volume by several times, forming a gelatinous and viscous fluid. This makes it valuable for a wide range of industrial applications.

Trenchless Drilling Mud

Drilling fluid consists of water, clay and chemicals that are pumped through the borehole while the drilling is in progress. This is done in order to lubricate and cool the drill bit and string, suspend and carry away the drill cuttings, to make the drill string buoyant and to stabilize the borehole.

For successful completion of a drilling project, it is necessary that the borehole remain open during the entire drilling operation. The drilling mud ensures that the formation through which the borehole is drilled is minimally affected during the drilling process.

The most important properties of drilling mud are its viscosity, density, chemical composition and fluid loss control. To determine the correct density of drilling fluid, the subsurface formation pressure should be known. Formations that are strong can be drilled with the fluid density less than 1 while high-pressure formations and pressurized shales may require densities up to 2.4, which can be adjusted with the addition of weight materials such as solids or soluble salts.

The viscosity of drilling fluid can be increased by adding polymers or clay material. It can be decreased by adding water or chemical thinners. As the depth of the borehole increases, more viscous drilling fluid may be required. Directional drilling requires paying close attention to the flow properties of the drilling fluid.

Types of Drilling Mud

During the drilling process, it is possible that some fluid is lost into the formation, causing damage, especially in porous formations. Additives such as bentonite can be used to control fluid loss. Drilling fluids are mainly of three types namely, water-based muds, oil-based muds and gas.

Water-Based Muds

Water-based muds consist of solid particles suspended in water or brine. Bentonite clay and organic colloids are added to water-based muds to increase its viscosity and prevent fluid loss while a heavy mineral such as barite is added to increase the density. This fluid gets mixed up with the cuttings from the drilling which are suspended in the solution and are circulated back to the surface.

On the surface, the mud is processed to settle heavy particles and the useable mud is pulled into a reclaimer and circulated back into the borehole.

Oil-Based Muds

Oil-based muds have solid particles suspended in oil.

Gas Muds

Gas or air used at high velocity is used to remove drill cuttings, but this method is restricted to use under certain conditions.

The Importance of Drilling Fluid

Whether the trenchless project is for laying of pipelines, fiber-optic, gas or electric cables, using the right drilling fluid is very crucial for successful completion.

The addition of additives with suspension and filtration properties is essential to ensure that frac-out, ground heaving, borehole collapse and pullback failure does not occur. (See also "How to Avoid Tunnel Collapse When Boring Big.")

Geotechnical Investigations Before Drilling Mud Mixing

Geotechnical investigation prior to the commencement of project will help the contractor to understand the kind of additives that will be needed for the types of soil conditions that will be encountered during the process. This geo information is compiled into a geo-report that is referenced when mixing drilling fluid.

Bentonite for Sandy, Cobbled or Reactive Clay

While working in sand, cobble or reactive clay, polymer additives may be needed, while for non-reactive clay, bentonite may be enough to lubricate the bit, stabilize the bore and carry the suspended particles.

Where there is a high concentration of reactive clays in the formation, partially hydrolyzed polyacrylamide (PHPA) polymers are used in place of bentonite. PHPA polymers wrap themselves around clay, preventing it from swelling.

Polyaluminum Chloride for Cobbled Soil

In sand and cobbles, polyaluminum chloride (PAC) polymers are added to the bentonite mixture to provide secondary filtration control.

Rocky Soil

For rocky soil conditions, a polymer with a larger molecular weight should be used because it helps in drilling a stable borehole by acting as a suspension for removing large drill cuttings.

Drilling Mud Formulas

The correct amount of drilling fluid is critical for a successful project. To calculate the amount of fluid needed, it is essential to know the soil type, diameter and length of the borehole. Depending on the soil types, different volumes of drilling fluid will be required. The general formula to calculate the required volume of the borehole is given below.

Take the reamer size squared divided by 25 to get the gallons per foot of the hole i.e.

Diameter of hole (inches)² / 25 = Volume of borehole gal. / ft. (A)

A x Length of bore = gallons (B)

To get the fluid to soil ratio (C), determine the type of soil.

Sand, Gravel, Rock or Cobble

When working with sand, gravel, rock or cobble, 1 gallon of fluid will be required to carry 1 gallon of cuttings out of the bore.

Sand, Gravel, Rock or Cobble Mixed with Clay

For sand, gravel, rock or cobble mixed with clay, it will take 2 gallons of fluid to carry 1 gallon of cuttings out of the bore.

Clay or Reactive Shale

For clay or reactive shale, it will take 3 to 5 gallons of fluid to carry 1 gallon of cuttings out of the bore.

B x C = Total amount of drilling fluid required to clean the hole.

Calculating the amount of drilling fluid required before commencing a project will help in estimation and planning of a successful project.

Outrunning mud on site while the project is in progress happens when the team tries to get the work done with less fluid. This can cause an inadvertent return due to lack of sufficient flow underground. Inadvertent returns can also be caused when the wrong type of fluid or wrong amount of additives are mixed. Excess use of fluid can lead to an escalation in cost and wastage of drilling fluid.