You may have heard the phrases “horizontal directional drilling,” “inadvertent return” and “drilling mud” as they relate to Mariner East pipeline construction and wondered exactly what they all mean. Our goal is to provide factual, accurate information — as well as a list of resources — to clear up any mystery.

Horizontal directional drilling

All of these phrases are relevant to a certain method of pipeline installation called horizontal directional drilling (HDD). We are utilizing HDD to install our Mariner East pipelines at specific locations throughout Pennsylvania, such as where the pipe is installed underneath a waterway, wetland or road crossing. The use of this method greatly reduces the environmental impact and disruption to roadways and other existing infrastructure during construction. Additionally, a pipeline installed with HDD is buried deeper than by open trench, resulting in enhanced safety and protection from potential third-party damage.

Learn more about the methods of pipeline installation utilized for Mariner East here.

HDDs are also used to install many other underground utilities, including water and sewer pipes, telecommunication and fiber optics and electric conduits. HDD is a safe, commonly used technology.

Drilling mud

“Drilling mud” is commonly used during HDD operations. This natural, nontoxic mixture consists predominantly of water, which must adhere to safe drinking standards, and a naturally occurring clay called bentonite. In most cases, the drilling mud composition ranges between 2% and 15% bentonite and between 85% and 98% water, depending on the specifics of the particular drill. The mixture is used during the HDD process to keep drilling tools cool, to remove drilled material, to support the drilling hole, and to lubricate the drill bit and the pipe during the final stages of pulling the pipe.

Bentonite clay is also a common ingredient in many applications in our everyday lives — from cat litter and fabric softener to many skin care and beauty products. It is also used in the clarifying process in winemaking and home brewing, and as a drinkable ingredient in some body-cleanse juices. Bentonite is listed by the U.S. Food and Drug Administration (FDA) as a “Generally Recognized Safe Food Substance” when used as a processing aid, and it serves a number of other practical uses, including for medicinal purposes.

Inadvertent returns — and how we respond

During an HDD, varying geology along the project footprint can cause drilling mud to find its way to the surface through natural crevices in the geology and soil — commonly referred to as an inadvertent return. This is well known by state and federal environmental regulators and is accounted for within environmental permits.

While IRs are not uncommon in the HDD process, we try to take all precautions to prevent them from happening and to minimize the impacts when they do. In accordance with the Department of Environmental (DEP) regulations, Energy Transfer is required to have response plans in place at all designated HDD locations to contain the drilling mud that flows to the surface in the event of an inadvertent return. 

If at any time during the drilling process an inadvertent return is suspected, the construction team immediately works to contain and remove any drilling mud that is released during HDD activities. We then notify the DEP and work toward complete remediation of the area.



When talking about technical pipeline construction methods and environmental impacts and remediation, it is important to fact-check with the experts who are doing the work. Below, please find additional resources from Energy Transfer, as well as third parties including regulators and others in the industry.