Without limiting the scope of the invention, its background is described in connection with detecting gas in the vadose zone.
Measurement, monitoring and verification (MMV) will be required at geologic carbon storage (GCS) sites to document that storage effectively retains CO2 in the subsurface [European Commission, 2009: US EPA, 2010a, b]. MMV can utilize many techniques deployed at a range of depths from the storage reservoir to the atmosphere, however techniques that monitor leakage through the near surface vadose zone are valuable because: (1) the vadose zone is the interface between subsurface storage and release to atmosphere, (2) gases moving through the shallow subsurface are easily and cheaply monitored, and (3) vadose zone monitoring can directly address concerns of landowners living above GCS sites [Shenk et al., 2011].
The most studied and currently widely accepted approach for vadose zone gas monitoring above GCS sites directly measures CO2 concentrations either by extracting vadose zone gas through hollow push probes or by measuring CO2 surface flux with accumulation chambers. Measurements are made in a grid pattern or in areas of concern, such as faults, fractures, or plugged and abandoned wells [Riding and Rochelle, 2009: Strazisar et al., 2009: Furche et al., 2010]. A minimum of 1 year of background concentration measurements is required prior to CO2 injection to document natural seasonal ranges in vadose zone CO2 apart from leakage. If CO2 concentrations statistically exceed the background range during the lifetime of a GCS project, a storage formation release may be indicated. This approach is herein referred to as a “CO2 concentration-based” approach.
A CO2 concentration-based approach has several drawbacks: (1) high variability of CO2 generated in situ could mask a moderate leakage signal; (2) 1 year of background characterization cannot account for CO2 variability from climatic, land use, and ecosystem variations over the lifetime (tens to hundreds of years) of a storage project; (3) background measurements require a long lead time potentially hindering a project's progress; and (4) background CO2 cannot be measured across all potential small diameter leak points within the area of review: therefore, if concerns arise in an area lacking local background measurements, no baseline data exist with which to compare monitored CO2 concentrations.