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GMEG - Geology, Minerals, Energy, & Geophysics Science Center

Geothermal Resource Investigations

 

 

 

 

Impacts of Geothermal Energy Development

Issue: Although geothermal power production is relatively limited in its environmental impacts, particularly with respect to greenhouse gas emissions, there are important issues to be addressed as production extends over decades. Specifically there have been problems related to reservoir depletion, and this task represents the start of an effort to characterize the impacts of development at one well-documented and heavily studied location. Geothermal power production within the Long Valley caldera began in 1985 and increased to ~40 MWe in 1991. A large new expansion (to ~100 MWe) is planned during the next few years, with the drilling of new production wells to start in 2011. The binary power plant uses isobutane as a working fluid, and all produced water is injected after cooling in the heat exchangers. However, pressure support has been imperfect, and rising surface temperatures at the plant have caused problems (e.g., forced abandonment of a storage/shop building). Rising soil temperatures have also caused the death of vegetation at and around the plant and extending to several patches of mature forest as far as 3 km away. Vegetation-kill areas total some tens of hectares at present, and kill at a few of these areas continues to expand. The kill areas occur on public lands (USFS, BLM) that are open to recreational use, and although direct hazards to the visiting public are probably minor, continued monitoring of these areas is warranted. Geothermal gas emissions are elevated in these hot areas, and above-ground concentrations of gases such as CO2 or H2S could become objectionable or even hazardous in the future as the fluid production rate increases (e.g., as in recent abandonment of well RDO-8 due to H2S hazard). These hot areas are thought to mark zones where water table drawdown due to pumping has permitted some boiling in the geothermal aquifer; i.e., pressure support from injection is insufficient. Such a condition, if allowed to become severe, can lead to more significant problems such as ground instability or hydrothermal explosion. As the production well-field moves further away from the generating plant at Casa Diablo and closer to inhabited areas (Town of Mammoth Lakes), the importance of monitoring the effects of pumping becomes greater.

Objectives: We plan to track the impacts of existing and expanded geothermal development within the Long Valley caldera through a program of locating and measuring anomalous heat and gas emissions that might be linked to production well pumping. Methods for quantifying heat and gas discharge have been largely developed in previous studies – heat discharge is evaluated by point measurements of shallow temperature gradients, and gas discharge is evaluated by grid measurements using accumulation chambers. Annual or semi-annual campaign-style measurements are envisioned, but some continuous monitoring capabilities exist and would be deployed if necessary. Analyses of gas chemistry and isotopic ratios will be performed as necessary. Locations investigated will be determined based on reports of anomalous vegetation death (e.g., from USFS or the public) and possibly from airborne reconnaissance. Well water levels, currently being monitored by the USGS will also help identify areas of pronounced drawdown. We will also regularly attend the semi-annual Long Valley Hydrologic Advisory Committee meetings to get confidential production data from the geothermal plant and to serve as advisors to the local community.

   

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