Hydraulic and Pneumatic Fracturing Fosters Soil Remediation

Hydraulic and pneumatic fracturing at geologically appropriate sites has improved the recovery of contaminating fluids from soil by ~10 to greater than 1000 times. For vertical recovery wells, typically this beneficence occurs by increasing the radius of influence ten-fold.

Hydraulic and pneumatic fracturing was adapted in the early ‘90s from the petroleum industry for use in the remediation field. The New Jersey Institute of Technology (NJIT) patented pneumatic fracturing for environmental applications in 1992 and licensed the technology to Accutech Remedial Systems (ARS). Terra Vac, Malcolm Pirnie, and others have participated in pneumatic-fracturing projects. Hydraulic fracturing is commercially available from several companies: FRX, Inc., Golder Associates Ltd., Hayward Baker Environmental, Inc., and others.. Research has been ongoing to improve the technology to enhance removal of contaminants both above and below the water table

The technology works by creating new, or enlarging existing, fractures in the subsurface. This improves fluid flow either into or from such low-permeability ground strata as clays, shales, and tight sandstones. (However fracturing need not be limited to low-permeability sites.)

Fractures typically generate in a horizontal or subhorizontal plane at specific horizons (less than 2 feet) by injecting either liquid or gas into a sealed borehole until the pressure exceeds a critical value and nucleates a fracture. Fractures propagate according to the state of stress in the subsurface. Sites with horizontal stress greater than vertical stress--typically overconsolidated fine-grained deposits of silts or clays--will produce horizontal or subhorizontal fractures. Pneumatic fracturing can use a nozzle to control the direction of fracture propagation.

Induced fractures increase the

• permeability of the soil,
• effective radius of recovery or injection wells,
• potential contact area with contaminated soils, and
• the availability of intersecting natural fractures.

After injection is complete, fractures hold open naturally or with an injected "proppant" (material to prop the opened fractures).

Hydraulically developed fractures have been demonstrated to be effective for more than one year. Vapor flow rates have been increased by 15 to 30 times that of unfractured wells; water flow rates, by 25 to 40 times.

Fracturing adds only a small percentage to the up-front costs of an overall remediation system. On the other hand, it may provide significant reduction in the life-cycle costs to remediate a site, since the technology may reduce the number of wells required and the time of cleanup.

Hydraulic and pneumatic fracturing has been used with vapor extraction, pump and treat, bioremediation, free product recovery, and in-situ devitiation at contaminated sites. Fracturing has helped introduce the materials used for other innovative technologies to improve remediation system at difficult sites. Examples include:

• nutrients or slowly dissolving oxygen sources to improve bioremediation processes;
• electrically conductive compounds (e.g., graphite) to improve electrokinetic processes; and
• reactant materials such as zero-valent iron or permanganate.

Demonstrations of other applications, such as passive chemical barriers or electrokinetics, are underway.

The foregoing article is the first of a two-part presentation. It is adapted from one of a collection of "Remediation Case Studies: Soil Vapor Extraction and Other In Situ Technologies," contained in volume 6, EPA 542-R-97-009. The concluding article, covering equipment and techniques will appear prior to 1998.