WY003 – Pneumatic Fracturing
Hydrocarbon Plume, Glendo, Wyoming
ARS was contracted by a national consulting firm to apply Pneumatic Fracturing (PF) at several sites located along the route 80 corridor in southeastern Wyoming to facilitate the recovery of petroleum hydrocarbon from the subsurface. The work was performed under the State of Wyoming UST Program. Since the ‘30s, leaks from USTs at service stations in the state had resulted in extensive free and dissolved-phase hydrocarbon plumes impacting groundwater.
The application process consisted of drilling borings for fracture wells, and then applying PF in both the saturated and unsaturated zones.
Site 1 consisted of a dissolved phase hydrocarbon plume covering nearly 50 acres. A free product plume was characterized as covering an area of 5 acres. Shallow groundwater is used by local residents.
Pre-fracture SVE testing at 4 locations of the site revealed the Brule clay formation in the unsaturated zone was not permeable enough to generate adequate air flow or vacuum ROI for technology to be effective. Aquifer testing showed drawdown influence was minimal.
Pneumatic Fracturing was conducted at 9 locations between a 30-40 ft. bgs depth interval. The formation was fractured using pressures between 150 and 380 psig in 27 discrete intervals. Up to a 70 ft. fracture radius was observed during the field effort.
Post fracture testing of the vadose zone resulted in significant increases in vacuum (ROI) and extraction air flow rate. These increases were observable both in fracture boreholes and a pre-existing monitoring well 15 ft. from the nearest fracture well. In the saturated zone, pumping tests demonstrated increased draw-down as far as 100 ft. from the pumping well.
Site 2 consisted of a hydrocarbon plume with the presence of free product. PF was applied at 3 locations between 10-30 ft. bgs. A total of 21 PF intervals were conducted at the site. Fracture pressures ranged from 150–270 psig. In the vadose zone, PF resulted in significant increases in extraction air flow rate, vacuum ROI and contaminant concentrations in the extracted air stream. PID measurements changed form 194 ppm pre-fracture to 1140 ppm after PF was conducted. Vacuum influence was observed to change from non-detect pre-fracture to 37 in. of water at 48 ft. bgs.
Site 3 also consisted of a hydrocarbon plume with free product. A total of three pneumatic fracture points were installed in a 15 ft. fracture zone between 35-50 ft. bgs. Fifteen PF intervals were conducted in this vertical treatment zone. In the saturated zone, air sparge testing showed significant air flow rate increases at locations where pre-fracture testing showed no flow rate. Pressure influence of sparged air was observed at 17 ft. from the PF location.