Drilling mud is a standard component of many horizontal directional drilling (HDD) construction projects. It plays a vital role in stabilizing the bore, removing the cuttings, as well as cooling and lubricating equipment.

The growth in HDD as a construction method has generated more drilling mud and raised the profile of disposal methods.

Drilling mud disposal can contribute a significant portion to the total cost of a project. In addition, misperceptions about drilling mud toxicity affect the public perception of trenchless construction and its impact on the environment. (Read also: An In-Depth Look at the Role of Drilling Fluid Systems in Trenchless Construction.)

How Drilling Mud Is Created

Drilling mud is 97% water, but specific additives give the product its desired properties. However, water quality plays a significant role in the performance of the additives. Therefore, it is vital to treat the water first. Soda ash helps control pH and hardness levels.

Bentonite, available commercially from Wyo-Ben, contributes to the viscosity of the fluid. Getting this property right is critical for carrying the cuttings out of the bore. If the viscosity is too low, cuttings will drop out of the solutions and settle in the bore, restricting its size. A narrow bore makes it challenging to pull the product into position. Bentonite also helps stabilize the bore by creating a cake on the tunnel wall. This cake prevents an inadvertent return and also restricts the ingress of groundwater.

Many drilling mud applications also use a polymer additive, which could be 0.005% of the mix. Polymer additives help with stabilization, enhance suspension and aid filtration.

Is Drilling Mud Toxic?

Both trenchless construction and oil-field drilling projects use a form of drilling mud. However, there is a difference between the mud used for each application. Toxic chemicals associated with oil drilling are not found in trenchless construction unless the bore passes through a section of contaminated soil. In this case, care should be taken to dispose of those contaminated cuttings in an environmentally safe manner. (Read also: Understanding the Importance of Solids Control.

An interesting study by the Oklahoma State University shed some light on the toxicity of HDD drilling mud. Researchers took 56 samples of HDD mud from 28 states in the USA. Tests showed that 49 samples had typical concentrations of heavy metals and similar properties to standard soil. All the samples met the EPA standards for land application of exceptional quality biosolids.

The university followed up these tests with practical experiments on agricultural land. They found that plots receiving up to 50 tons per acre of mud showed no negative impact on grass production. Bare plots showed some impediments to grass growth at 50 tons and above.

How Much Mud Does an HDD Project Make to Dispose?

An HDD project will generate at least the equivalent volume of mud as the product they installed. The product displaces the soil from the bore. The larger the diameter and the longer the bore, the higher the volume of mud generated.

However, there may be additional mud volumes on longer projects. Mud recycling systems use shakers to remove large particles like rocks and hydro cyclones to remove the remaining sand. After recycling several times, the mud quality will deteriorate. A rise in annular pressure is a clear sign of drilling mud problems, and some operators use a cut off pressure limit to determine the point where mud should be changed out.

One advantage of recycling systems is that you can move the recycled mud with you to the following job site. That way, you don't have to make up a fresh batch of drilling mud at the start of each job, saving yourself time, chemicals, and money. (Read also: The Importance of Mud Recycling in HDD.)

Drilling Mud Additives

One problem with mud disposal is that cuttings still contain water. When loaded into a dump truck, dripping water can result in fines and may trigger a negative community response.

Wyo-Ben supplies an absorbent/polymer blend for solidifying aqueous slurries. Solidibond 2.2 is a fast-acting product that turns slurry into a solid that passes the paint filter test for solid waste disposal. Addition rates of 0.5% to 3% will usually be sufficient to solidify the slurry. Actual dosing quantities depend on the density of the slurry and the time required for setting. The product is easy to apply by spreading over the surface of the slurry and mixing using an excavator or front-end loader.

Drilling Mud Disposal Options

The simplest and cheapest methods for drilling mud disposal are on land. While these options are not available everywhere, some sites allow the dumping of drilling mud at the construction site on the Right of Way. Where this is not possible, it may still be feasible to dispose of drilling mud on farmland. As confirmed by the OSU study, an appropriate amount of drilling fluid is not harmful to agricultural activity and may even be helpful in some cases.

Disposing of drilling fluid at landfill or state sites is costly. These costs vary significantly from place to place but can be in the order of $600 for every 1,000 gallons of drilling fluid. Haulage is another significant contributor to disposal cost. Dumping drilling mud at landfill sites far from the construction activity uses a lot of fuel. Many disposal loads quickly build up to a significant expense.

In addition, each load must pass the paint filter test before acceptance. This simple test is designed to prevent liquid dumping at a landfill. No fluid or portion of the sample should pass through a filter within five minutes to pass this test.

However, additives like Solidibond 2.2 can minimize the quantity of drilling mud for disposal. This product transforms slurry into a solid mass that will pass the paint filter test.

Conclusion

Disposal of drilling fluid is becoming a more and more significant issue for HDD contractors. It affects the cost of a project and can damage a company's reputation if not handled correctly.

Studies show that HDD drilling mud is not harmful to the environment as long as the drilling path was free from contamination. Contractors can dispose of limited quantities on ROWs and agricultural land in areas that permit this practice.