St. Gallen Deep Geothermal Well: Implications for Gas Monitoring in the Shallow Subsurface
The exploitation of unconventional reservoirs for natural gas production has rapidly increased in the last decades, and policy makers are planning and implementing its development worldwide.
Public concerns regarding the potential environmental impacts induced by unconventional gas production operations have also risen, with a specific attention to surface and subsurface water resources contamination. In particular, the major risks of contamination of shallow aquifers are associated with: i) infiltration of flowback water (fracturing fluid and/or formation brine) from spills at the ground surface, and ii) leakage and upward migration of stray gases and formation brine through preferential pathways connected to deep geological formations.
Typical leakage pathways include hydraulically induced fractures, which may reach the shallow groundwater resource by interception of natural faults, abandoned wells, or communicating permeable and shallower formations, and failure of the wellbore annulus, due to a faulty installation, abandonment, poor cement quality, casing and tubing corrosion, formation damage around the wellbore, or mechanical and thermal stresses, among others.
Well integrity failure is probably among the most common causes of leakages, and the presence in shallow groundwater of methane (CH4) concentrations higher than baseline conditions has recently been attributed to this in areas of intense shale gas exploitation and in proximity to decommissioned oil and gas wells.
Dissolved methane in drinking water is not considered a public health hazard, and it may also occur naturally in groundwater as the result of thermogenic and microbial processes. Its presence in groundwater may change pH and redox conditions, causing either the release or the depletion of some trace metals depending on site conditions. However, elevated aqueous methane concentrations may also induce the separation of a gas phase, with risk of asphyxiation and explosions. Furthermore, unburned methane is a strong greenhouse gas if released to the atmosphere, although it acts over relatively short time scales in comparison to carbon dioxide (CO2).
Among the sites of the S4CE Consortium, the St. Gallen field site required further investigations with respect to possible fugitive emissions of methane. Here, a deep well (St. Gallen GT-1), originally meant for geothermal use, was developed up to a depth of about 4250 m bgl, crossing a thick sequence of molasse deposits. In 2013 the deep well was shut-in due to both insufficient water productivity and induced seismicity resulting from injection operations. The maximum seismic event (3.5 magnitude) was registered with the occurrence of a gas kick after the interception of an unexpected natural gas reservoir. Production tests were performed to assess whether the natural gas resource (94.1% methane by volume) could be exploited. However, despite the estimated high gas volumes, the project was stopped.
The site is located in a groundwater protection area, next to a river and a densely urbanized area. Therefore, assessing the vulnerability of the shallow aquifer underneath the site with respect to methane contamination was a pressing need. To this aim, numerical simulations of a hypothetical methane leakage were performed.
Predicted maximum methane fluxes are comparable to those recently estimated in the unsaturated zone in the close vicinity of a cut and buried abandoned gas well, and to surface casing vent flow fluxes reported for a large dataset of conventional and unconventional wells with leakage issues in Canada. Therefore, if a leakage occurs at the bottom of the aquifer (Figure 1), a risk of explosion may exist since non-negligible amounts of gaseous methane could quickly reach and accumulate in the unsaturated zone. As most of the top soil is covered by a thick concrete/asphalt layer (well pad), gaseous methane release in the atmosphere could only occur through preferential pathways, like the edges of the well pad or cracks (if present) in its concrete/asphalt layer. Therefore, these specific locations should be taken into account for monitoring operations together with the deep well. Subsurface measurements of methane fluxes in the unsaturated zone might also be useful, as measurements at the ground surface may fail to detect leaking gas.
However, since shut-in (2013), weekly measurements proved a pressure of zero at the well head, suggesting a good integrity of the deep well. Nevertheless, depending on cement quality of the casing annulus, methane could take several years before reaching the shallow aquifer. Therefore, in order to assess the complete safety of the site, further analysis on the well integrity and additional gas monitoring at the shallow subsurface might be valid actions to undertake before decommissioning of the deep well.
The presence of dissolved methane in groundwater might alter pH, redox conditions, as well as the microbial community, potentially affecting groundwater quality. However, according to the numerical simulations performed, the migration of dissolved methane is not deemed as a significant threat for local water resources. Indeed, over a long time scale (20 years) the extent of the plumes is quite limited (Figures 2 and 3). Therefore, it is unlikely that any other contaminant potentially released due to the presence of dissolved methane may reach the nearby river. Nevertheless, a localized risk may exist since methane concentrations may reach and overcome in a few years the risk mitigation thresholds of 7 and 10 mg/L within a range of 20 m from the deep well. However, it is unlikely that a gas leakage may persist for such a long time, since pressure readings and quality checks are performed weekly at the well head. Of course, in case of site decommissioning, with no monitoring, the risk of leakage persistence would be more pronounced.
Overall, the analysis showed that the risk of contamination for local water resources might be limited in this site. Up to now, pressure readings and quality checks at the well head suggested no leakage issues. However, methane could still migrate through the casing annulus and reach the shallow aquifer, depending on cement and casing degradation over time. Therefore, monitoring at the well head, analysis on the well integrity, and surface/subsurface measurements over time of methane fluxes at the well pad are strongly advised before decommissioning to ensure the safety of the site.
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Authors: Andrea D’Aniello, firstname.lastname@example.org, University of Naples Federico II, Department of Civil, Architectural and Environmental Engineering, via Claudio 21, 80125 Napoli, Italy
*To find out more regarding the work performed on the St. Gallen field site, please refer to: D’Aniello, A., Fabbricino, M., Ducci, D., & Pianese, D. (2019). Numerical Investigation of a Methane Leakage from a Geothermal Well into a Shallow Aquifer. Groundwater. DOI: 10.1111/gwat.12943. https://doi.org/10.1111/gwat.12943.