Environmental Engineering Reference
In-Depth Information
A natural groundwater domain is characterized by multiple gas transport
barriers caused by the horizontal layering and compaction of sediments.
The transport of gas clusters is highly sensitive to these structures and
heterogeneities. Gas accumulation occurs, and regions of coherent mobile
gas saturations can result. These structures must be explored during a gas-
hydrogeological surveying.
There is a weak interaction between gas and water flow during NDI; per-
vasive and bubbly gas propagations facilitate the simultaneous use of mac-
ropore structures for water and gas flow. There is some rearrangement of
the path of water flow during gas injection due to local gas accumulation
in capture zones. Subsequent conductivity changes are limited to the local
scale and a degree of homogenization of the water flow can be achieved by
temporary clogging of coarser high-permeability zones.
An effective displacement of mobile water by mobile gas in a near flow
region is induced using HDI. The displacement results from high-gradient,
high-frequency pulses with injection periods in the range of seconds to min-
utes. HDI is applied when source zone or soil matrix decontamination is
required and it has been used in combination with NDI (NDI-HDI) for local
gas storage homogenization in the large scale BIOXWAND application. HDI
has reported to cause more significant changes to groundwater flow in terms
of flow direction, velocity, and dispersivity (Selker et al., 2007; Geistlinger
et al., 2006). Applications in bedrock and other low permeable environments
(e.g., sandstone structures or silt barriers) may generate gas accessible pore
networks.
There is evidence from field-scale gas tracer applications that the mutual
displacement of gas flow networks can occur during simultaneous injection
at locally distributed lances (Uhlig, 2010 and Schinke, 2008). The effect can
be explained by applying the pervasive gas flow concept of moving incoher-
ent clusters where effective mixing of cluster flow paths is not possible. The
practical outcome is that the determination of the ROI of an array of gas
lances must be performed by complex lance array testing.
10.2.4 Gas Propagation and Storage
Gas storage in aquifers mainly appears as either mobile gas capturing or
accumulation below geological barriers or the residual pore-trapping of gas
clusters. Gas saturation (volume of gas per volume of pore space) is used to
characterize storage.
During NDI in sandy sediments, typical gas saturations are 1%-5% for
residual gas, 5%-10% for mobile gas (during injection periods), and greater
than 15% for mobile gas capture zones (Weber, 2007 and Engelmann et al.,
2010). Texture and mechanical stress only exert minor influences on these
means of gas saturation. It has been reported that pervasive incoherent
cluster flow can occupy a denser pore channel network than coherent flow
over large distances, and can be maintained for hours after gas injection has
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