Fracturing is an enhancement technology designed to increase the efficiency of other in situ technologies in difficult soil conditions. The fracturing extends and enlarges existing fissures and introduces new fractures, primarily in the horizontal direction. When fracturing has been completed, the formation is then subjected to vapor extraction, either by applying a vacuum to all wells or by extracting from selected wells, while other wells are capped or used for passive air inlet or forced air injection. Technologies commonly used in soil fracturing include pneumatic fracturing (PF), blast-enhanced fracturing and LasagnaTM process.
Blast-enhanced fracturing is a process used at sites with fractured bedrock formations. The increased well yields, hydraulic conductivity values, and capture zones occur as a result of the highly fractured area created by detonation of explosives in boreholes.
LasagnaTM is an integrated, in situ remedial technology, which combines electroosmosis with treatment zones that are installed directly in the contaminated soil. In LasagnaTM process, hydraulic fracturing is used to create sorption/degradation zones horizontally in the subsurface soil.
Pneumatic Fracturing (PF)
In the PF process, fracture wells are drilled in the contaminated vadose zone and left open (uncased) for most of their depth. A packer system is used to isolate small (0.6-meter or 2-foot) intervals so that short bursts (~20 seconds) of compressed air (less than 10,300 mmHg or 200 pounds per square inch) can be injected into the interval to fracture the formation. The process is repeated for each interval within the contaminated depth.
Fracturing is applicable to the complete range of contaminant groups with no particular target group. The technology is used primarily to fracture silts, clays, shale, and bedrock.
Factors that may limit the applicability and effectiveness of the process include:
- The technology should not be used in areas of high seismic activity.
- Fractures will close in non-clayey soils.
- Investigation of possible underground utilities, structures, or trapped free product is required.
- The potential exists to open new pathways for the unwanted spread of contaminants (e.g., dense nonaqueous phase liquids).
Soil characteristics that need to be determined include the depth and areal extent of contamination, the concentration of the contaminants, and soil type and properties (e.g., structure, organic content, texture, permeability, water-holding capacity, and moisture content).
The technology is currently available from only one vendor. PF was tested with hot gas injection and extraction in EPA's SITE demonstration program in 1992. Results indicate that PF increased the effective vacuum radius of influence nearly threefold and increased the rate of mass removal up to 25 times over the rates measured using conventional extraction technologies. A Phase II demonstration is planned for 1994. The technology has been demonstrated in the field, including the one under EPA's SITE program. In addition, numerous bench-scale and theoretical studies have been published.
During the summer of 1993, a pilot demonstration of pneumatic fracturing was sponsored by DOE at Tinker AFB to enhance remediation of the fine-grained silts, clays, and sedimentary rock that underlie the site. At one test area, where No. 2 fuel oil was being pumped from existing recovery wells, pneumatic fracturing increased the average monthly removal rate by 15 times. Tests conducted in the unsaturated zone also showed enhanced air permeability as a result of fracturing, ranging from 5 to 30 times greater than prefracture values.
Normal operation employs a two-person crew, making 15 to 25 fractures per day with a fracture radius of 4 to 6 meters (15 to 20 feet) to a depth of 15 to 30 meters (50 to 100 feet). For longer remediation programs, refracturing efforts may be required at 6- to 12-month intervals.
The approximate cost range for pneumatic fracturing is $9 to $13 per metric ton ($8 to $12 per ton).
Cost for LasagnaTM is estimated at $180 to $200 per metric ton ($160 to $180 per ton) for remediation in 1 year, $110 to $130 per metric ton ($100 to $120 per ton) if 3 years are allowed for remediation.