Environmental Engineering Reference
In-Depth Information
can be used multiple times as the CPT is able to withdraw the complete sys-
tem. There are a number of sensing, additional testing, and sampling tools
available for both percussion sounding and CPT, which give the advantage
of flexible multifunctional applications (Dietrich and Leven, 2006). Lances of
DIL-type B allow for pressure-controlled groundwater sampling, permeabil-
ity testing, and in situ groundwater screening of dissolved gases.
High-quality direct push lances can be installed up to 80 m at a moderate
cost using the Sonic
®
sounding technology.
Vibration Injection Lances
(VIL)
are good alternatives to classical dry drilling in sediment environments in
terms of depth and core probing, and both multilevel and coupled sensing
installations are available. Another advantage is grouting and sealing of the
lances or filters is done by sonic withdrawal of the casings, which results in
autocompaction and consolidation (Engelmann et al., 2009). VIL lances are
preferred, even for HDI gas injections.
10.3.2.2 Gas Supply, Gas Mixing, and Distribution
Injection gases used in RGBZ include pure gases (e.g., oxygen), or gas mix-
tures. Air is typically used as a carrier gas to achieve high ROI, and partial
pressure can be controlled with a few lances and oxygen. Inert trace gases
(e.g., He, Ar and Ne or reactive gases such as methane and carbon dioxide)
can be mixed with the injection gas.
Pure gases are economically stored in pressurized tanks, and additional
gas compression is not required for injection. Oil-free compressors are used
for the injection of atmospheric air mixtures, and a postdrying step for com-
pressed air is necessary.
Mass flow controllers and flow meters are recommended for the mixing
and distribution of injection gases as they allow the balancing of injected gas
amount for each lance. These devices require calibration to the specific gas
mixtures (Figure 10.7). Pressure meters and magnetic valves enable effective
gas distribution and dynamic injection intervals.
Gas injection is performed as either low-pressure (NDI) or pulsed high-
pressure injection (HDI). Continuous NDI injection is applied in the initial
formation period for a gas PRB when there is a high demand for reactants
(e.g., oxygen), and it results in full ROI formation, preconditioning, and pre-
oxidation of the rock matrix. It can also lead to some emission of gas into the
unsaturated zone. Constant injection pulses over a few hours are used dur-
ing a regular RGBZ operation, and these pulses are interrupted by periodic
break periods. HDI injection consists of high frequency, high-flow rate gas
pulses in the range of seconds to minutes. Gas breakthrough to the unsatu-
rated zone is avoided by the time limitation of coherent gas flow periods.
HDI can be used for formation of local gas storage zones with higher gas
saturations, and for repairing of clogged gas lances. While a gas supply sys-
tem for NDI has to resist a total pressure of approximately 500 kPa, an HDI
supply system (including lances) needs to be operated at >1000 kPa.
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