Horizontal Environmental Drilling – Not Just for Well Installations Anymore
September 29, 2015Horizontal directional drilling (HDD) methods have been used in the environmental market since the late 1980s. Most of the work has been for the installation of monitor or remediation wells at sites ranging from dry cleaner spills to U.S. EPA Superfund sites. HDD methods are used to complete wells under obstructions or to mimic the contaminant plume geometry. But did you know that soil samples can be obtained using HDD drilling methods?
In the past, when an environmental consulting engineer or geologist wanted a soil sample from under an obstruction, he only had two options: unguided angled drilling from outside the footprint of the impediment or vertical drilling through the obstruction. In many cases neither of the two traditional options is viable. Let’s say the sample point is 6 ft below the floor of a manufacturing plant and 100 ft from the edge of the building. An unguided angle drilling operation will not be able to reach that location. The other option, drilling through the floor of the building won’t work if the building is an occupied apartment or the operations inside the building preclude access; silicone chip manufacturing or pharmaceutical development are locations where drill rigs are not allowed.
Buildings aside, think about all the other places where traditional soil sampling methods can’t be used. Ever try and drill through 100 ft of waste in an unlined landfill to get a sample below the trash? How about a pit of liquid waste? We once had a consultant ask us to get a soil sample from under the Mississippi River. Try getting pricing for a barge and permission from the U.S. Army Corps of Engineers to anchor the boat in middle of the shipping channel.
All of these challenges have been addressed using HDD methods to obtain soil samples in areas inaccessible to traditional vertical drill rigs and methods.
But first things first. What’s the process to get a soil sample from a horizontal borehole? Isn’t it impossible due to the locating tools and the mud in the borehole? Those two things complicate the process. The tools we use to locate the drill bit in the borehole are locked into the drillstring and not removable. This means we can’t drill/steer to a location and run a sampling tool inside the drill pipe as the normal practice with a hollow stem auger sampling system. So we have to drill and steer to a precise location and then remove the drill pipe with the locating tool.
Now the only thing keeping the borehole open is the drilling fluid — more on that in a minute. After the steering string is removed, the sampling tool is placed on the drill pipe and then run back to the bottom of the fluid filled hole to the sampling point. That process creates two complications:
1. How do we keep the drilling fluid inside the borehole from entering the sampler, and how do we confirm the soil in the sampler was not “scooped up” from the bottom/side of the borehole as we push the tool though the curve to the end of the hole? Directed Technologies Drilling (DTD) has modified a piston sampling tool such that a spring loaded “bullet nose” end cap is held in place at the front end of the sampler. The bullet cap prevents drilling fluid and soil from the bottom and sides of the hole from entering the sampler. Once the tool reaches the sampling location at the end of the borehole, the force of the formation causes the spring to collapse and the bullet nose is able to retract as the soil is driven into the sampler tool.
2. How do we know if the sample is taken from the correct location? What if the drill string and tooling leave the borehole? Our experience in most cases has been that the drill string stays in the curved section of the hole. We are able to monitor push pressure along with the amount of pipe in the hole to verify that the sample is taken from the correct location. However, once the soil sample is taken, the entire string needs to be pulled back to the surface for retrieval. Then the process needs to be repeated for the next sample:
a. Drill/steer to the sample point
b. Remove drill string with locating tool
c. Add sampler to the drill pipe
d. Run back through curve to the end of the hole
e. Push sample
f. Remove drill pipe with sample string
As you can imagine, this can be a long drawn out process if multiple samples are taken in a single borehole.
Now for a reality check — Does this really work?
• Case Study 1 — Sampling under occupied residential buildings: The objective of the project was to collect soil samples for chemical analysis immediately below homes without disrupting the residents. Complicating the issue was the depth of the samples, only 4 ft below the ground surface up to 10 ft from the edge of the building. The soil conditions at the site were unconsolidated sand, clay and fill material. Based on the required set back from the buildings, due to utilities and other concerns, the total bore lengths were about 50 ft. Bentonite drilling fluid was used due to the soft, unconsolidated formation and worked well to keep the hole open and stable during the drilling and tripping of the drill pipe and sampling string. This was a very high profile project and we completed a test bore under an unoccupied building to verify all of the equipment and proposed methodology functioned properly. Multiple phone calls and onsite meetings were held to determine the final drilling, sampling and waste handling protocols. A total of 12 samples were obtained from multiple buildings without any impact to the residents or neighborhood.
• Case Study 2 — Sampling under a landfill: The scope of this work was to obtain soils samples from under a capped landfill at a site in the western United States. Initially, several horizontal bores were planned with lengths up to 300 ft. Soil samples were to be taken at 30-ft intervals along the entire length of the holes. By the conclusion of the project, more than 91,000 ft of drill pipe had been tripped in and out of multiple boreholes. But as things normally go in drilling projects, one of the locations did not go perfect. As we were drilling, we encountered what appeared to be landfill debris at an unexpected depth/location. Working closely with the client, the DTD field manager and crews, revised the bore profile and were able to pull back several rods and redirect the bore to a new elevation, missing the potential waste. This allowed soil samples to be taken at two depths at the same X-Y location. Based on the success of redirecting the bore once, we were able to steer to a third location from the same initial bore, collecting viable soil samples in all three forks of the boring.
So there you have it, HDD used for environmental soil sampling operations. Is it a new, viable technology? Yes. Is this something we do on a day-in-day-out basis? Absolutely not. The costs of this method can be enormous. In one case, each sample cost about $10,000 in rig and crew time. A cost many environmental consulting engineers and geologists will be hesitant to bring to the site owner. However, in certain instances as detailed in the two case studies — HDD soil sampling solved the specific access problems.
David Bardsley, PG, is business development manager at Directed Technologies Drilling.