GA001 – Pneumatic Fracturing, Hydrogen Sparging
Marine Corps Logistics Base, Albany, Georgia
ARS Technologies, Inc. (ARS) implemented an integrated treatment process consisting of Pneumatic Fracturing (PF) and hydrogen gas sparging into the saturated zone at Marine Corps Logistics Base in Albany, Georgia. This pilot-scale system sparges hydrogen gas into the fractured saturated zone for enhanced bioremediation of chlorinated volatile organic compounds (CVOCs), which have been detected at elevated concentrations in the groundwater beneath the site. The installation, testing, and start-up of the sparge system were completed during 2002.
It is well understood that hydrogen can be an important electron donor for biologically-mediated dechlorination of CVOCs. Instead of adding an indirect electron donor to the impacted zone (such as benzoate, mulch, edible oils, lactate, etc.) the direct addition of hydrogen gas via sparging facilitates the dechlorination process.
Figure 1: Pneumatic Fracturing Wellhead Assembly
The lithology within the targeted sparge interval (60-80 ft. bgs) was alluvium consisting of medium-reddish brown sandy clay. The water table occurred at a depth of approximately 40-50 ft. bgs. The hydraulic conductivity of the alluvium was relatively low, in the range of 5-10 cm/s. The predominant CVOCs detected at the target depths consisted of TCE, PCE, DCE, TCA, and VC.
A key factor to the success of hydrogen sparging was to increase the bulk permeability of the geologic formation so that the sparge gas could more effectively be distributed throughout the treatment zone. Therefore, to enhance bulk permeability of the formation and to reduce geologic heterogeneities, ARS first pneumatically fractured the targeted interval. The fracturing effort significantly enhanced the primary and secondary fracture network through which the hydrogen gas could be better distributed.
Pressure-history curves collected during the fracturing events indicated that fracture initiation occurred at pressures from 80-140 psig, with the highest pressures occurring in the deepest intervals. Maintenance pressures ranged from 80-120 psig. Pressure influence was observed in multiple monitoring wells located around the study area. Pressure readings, physical observations of air flow and O2 readings all measured at wells surrounding the injection point indicated that influence occurred as far as 33 ft. from the injection point.
Subsequent to Pneumatic Fracturing, an open-hole sparging test was conducted to evaluate flow rates and minimum initiation pressures required for the pilot scale system. Following the sparging test, the borehole was then converted to a nested sparge well with three screened portions from 60-65, 68-73 and 76-81 ft. bgs.
Hydrogen Sparge Skid at Federal Facility in Georgia
Figure 2: Hydrogen Sparge Skid at Federal Facility in Georgia
The sparging test conducted after each Pneumatic Fracturing event provided quantitative data on flow rates at various injection pressures, as well as evidence that the Pneumatic Fracturing was successful at creating an interconnected fracture network. At 100 psig, flow rates were less than 800 scfm. These flow rates were tested again once the nested sparge wells were installed. The reduced pipe diameter (2-inch using the Pneumatic Fracturing assembly to the 1-inch sparge wells) and the use of a normal gas regulator (rather than a specialized high flow regulator) reduced average flow rates to approximately 26 scfm at an injection pressure of 90 psig.
The Pneumatic Fracturing and subsequent sparging test concluded that flow rates of approximately 9 scfm per sparge well could be achieved when simultaneously injecting into all three sparge wells at a pressure of approximately 90 psig. These flow rates confirmed a sparge radius of influence of at least 10-15 ft.
During the initial sparging event, sodium hexaflouride was added to the hydrogen as a tracer gas and pressure was recorded at surrounding wells to evaluate the injection radius of influence. Based on the results of the tracer analysis and pressure readings, an injection radius of influence of at least 20 ft. was obtained.