Nitrogen gas escapes from a borehole created by the cryogenic drill. Photo print available upon request.
By Mike Wooldridge, MAWooldridge@lbl.gov
BERKELEY -- Researchers at Lawrence Berkeley Laboratory have put a cool twist on the underground drilling used to clean up toxic waste sites. They have invented a drilling method that blasts super-cold nitrogen gas as it bores, creating frozen holes that won't collapse even in the sandiest of soils.
Nitrogen is injected down the drill's center pipe and exits through nozzles near the spinning drill bit. At -196 degrees C, the gas freezes difficult soils rich in sand, gravel or ash long enough for workers to insert stabilizing metal casings into the holes before the ground thaws.
The cryogenic method should be valuable to researchers trying to clean up contaminated grounds at many industrial facilities, closing military bases, and Department of Energy weapons laboratories, where loose dirt has often made it difficult to drill holes that do not cave in.
George Cooper, a researcher in the Earth Sciences Division at LBL and professor of petroleum engineering at the University of California at Berkeley, developed the technology with Rafael Simon, a UC Berkeley graduate student.
Boring holes in the earth is usually the first step in assessing contaminated areas. Soil samples taken from the holes tell scientists the types of pollutants that are present as well as their distribution. Drilling is also critical for removing contaminants from the ground, which can involve pumping pollutants out with groundwater or boiling them out with injected steam.
Unfortunately, cleanup experts at many facilities often find themselves faced with crumbling soils that will not hold their holes. "It's also no coincidence that you find difficult soils at weapons labs -- they were built on land of little commercial value," Cooper says. "In the west this means sandy deserts. In the east this means agriculturally poor land, which often has sandy soil."
Drillers at contaminated sites are at a particular disadvantage since they can't use substances such as "drilling muds," the clay-based goo that is used to help stabilize bore holes. Drilling muds can complicate the cleanup effort by changing the permeability of the soil or spreading the contaminants further.
One location where soil collapse has been an issue is the nuclear cleanup site in Hanford, Washington, which sits on layers of loose gravel and sand. Cave-ins have hampered attempts to analyze the soil beneath storage tanks that were used to store radioactive waste.
"People have had difficulty drilling vertical holes at Hanford, and those are the easy ones," Cooper says. "To get to the soil beneath the tanks, you need to drill holes at an angle. These are even less stable."
The cryogenic drilling should allow scientists to drill the difficult holes at Hanford and other problem sites such as the Savannah River site in South Carolina, Idaho National Engineering Laboratory, and the Oak Ridge Reservation in Tennessee.
The method performs well in other problem soils as well, such as the clay-rich dirts that clog up drill bits. Clay soil is frozen hard by the nitrogen gas and is crushed by the drill bit into rock-hard chips, which are blown out of the hole by the gas pressure.
"The nitrogen gas also has an advantage when it comes to sampling soil for monitoring since the freezing locks the pollutants in," Cooper says. "Methods that use water may wash some of the pollutants completely away. You may seriously underestimate the level of pollution."
Engineers have occasionally used cryogenic techniques in the past when working with loose soil. Construction crews may pipe in liquid nitrogen to freeze and stabilize the surrounding ground when working underground near buildings. This is the first time such a super-cold idea has been put into practice with drilling.
Lawrence Berkeley Laboratory is a U.S. Department of Energy national laboratory located in Berkeley, California. It conducts unclassified scientific research and is managed by the University of California.