How Do You Drill Through a Mountain?

Dominion now says it would run the ACP underground to avoid the habitat of the rare Cow Knob salamander. And they have talked about drilling under Reed’s Gap, under 664, and under the James. How do you drill through a mountainor under a river? The process is called Horizontal Directional Drilling (HDD).

From the Pipeline and Gas Journal comes this article about drilling in Colorado – except the Colorado job was only 2,000 feet with a 16-inch pipe compared to nearly 5,000 feet with a 42-inch pipe. Based on this article, the opening for a 42-inch pipeline would need to be at least 50 inches or larger to avoid drag during pullback.

Here’s a more detailed article about the geotechnical requirements for HDD. The pertinent info begins on page 6. The drilling rigs are mounted on “multiple trailers, self-propelled track vehicles, or trucks.” (Imagine the staging area needed!) Drill hole stability “is generally achieved by pumping large volumes of drill mud into the hole at the cutting tool to support the hole and to remove the cuttings. The most commonly used drill fluids are bentonite-based, and sometimes they are combined with polymers to achieve certain performance requirements. The drill fluid is pumped down the drill hole and circulated back to the surface and collected in pits excavated at the drill entry and exit locations. Sufficient pumping capacity is critical for large crossings. The fluid is pumped from these pits to settling/containment pits and then passed through mud-system machinery that separates the cuttings from the slurry. This process typically involves a series of shaking sieves and cyclones.”

So in addition to the drill rigs, other large equipment would be required: pumps, mud-system machinery, sieves and cyclones – not to mention the cachment basins and settlement/containment pits. This article also notes that “Upon completion, the pilot drill hole is enlarged to about 1.5 times the diameter of the product pipeline to be installed.” For a 42 inch pipeline that would mean enlarging the bore hole to 68 inches. Since the pipe is ultimately installed by pulling it backwards through the hole, large equipment would be needed to do that part of the job as well. There is no mention of what happens to the significant amount of material displaced from the bore hole. Haul it away in many, many big truckloads?

The article section on HDD ends with this cautionary comment: “One of the key concerns in HDD crossings is the potential for drill fluid to escape into watercourses or environmentally sensitive habitat due to hydraulic fracturing (“frac-outs”), through pre-existing hydraulic pathways, or spills at the entry/exit points. As such, the drill path must be designed so that it is located at a sufficient depth to limit the risk of drill fluid fracturing through the soil formation.”

No wonder Dominion has no engineering plan – or perhaps they just don’t want to make it public!