DEN001 – Pneumatic Fracturing
Pilot Study: Former Waste Chemicals Handling Station in Hedehusene, Denmark
Figure 1: Soil Coring Showing Rhodamine WT Dye Tracer
The work performed by ARS and NIRAS of Denmark was the first field-scale application of Pneumatic Fracturing (PF) in Denmark. The site was a former station for handling of waste chemicals in Hedehusene near Copenhagen. The pilot test was located outside the contaminated area; its primary objective was to demonstrate the effectiveness of PF in the typical Danish basal till found in Denmark. In conjunction with the fracturing, a mixture of different tracers was injected in order to compare an array of methods for documentation of tracer distribution in the till as the secondary objective.
The geology at the site consisted of 14-165 m of the clayey till underlain by 2-3 m of fine sand.
Pneumatic Fracturing was conducted between the depths of 3 and 8 m bgs in five discrete 1 m intervals. The mixture of tracers (consisting of sodium bromide, brilliant blue, Fluorescein, Rhodamine WT and Uvitex) was injected into each interval by Atomized Liquid Injection to facilitate the identification of the fracture network.
Gas pressures between 40 and 125 psi were required to initiate and propagate fractures within the subsurface. In all cases, fracture dilation and/or maintenance pressures were significantly lower than the initiation pressures ranging between 20 and 85 psi. Nitrogen flow rates during the fracturing ranged between 240 and 940 standard cubic feet per minute (SCFM). The highest flow rates were observed within the deeper intervals and decreased during the more shallow intervals. 250 liters of the tracer solution mixture were injected throughout the fracture intervals.
Twelve (12) nearby monitoring wells were used to measure pressure influence during the PF/ALI injections. Several of these wells showed significant pressure response during the PF injections, indicating direct pneumatic connection and lateral propagation of fractures in excess of 5 m. Pressure response was also observed around an abandoned Geoprobe boring located approximately 7 m from the PF borehole.
Figure 2: The American-Danish Team Working Under Limited Daylight in November
Three (3) locations surrounding the PF borehole were monitored for heave during the fracturing injections. In general, the magnitude of surface heave was observed to be inversely proportional to depth with heave magnitude diminishing as the injections became shallower. The pattern of observed heave during the PF injections indicated a relatively uniform surface heave in different directions, thus confirming that the fracturing propagation was a relatively uniform 360˚ around the borehole.
The injected tracers were detected in soil cores taken after the PF injection. Mass balance indicated 90% of the tracer mass was distributed within 2 m from the fracturing borehole. Given that the injected gas would travel a greater distance than the liquid tracer, fracture propagation can be assumed to be greater in distance.
The results of this pilot test were presented in Session B3 of the Battelle Fifth International Conference on Remediation of Chlorinated and Recalcitrant Compounds in Monterey, May 2006.
Figure 3: Pilot Test
NIRAS Monterey Presentation (PDF)