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Title: Near-Surface CO2 Monitoring And Analysis To Detect Hidden Geothermal Systems

Abstract

''Hidden'' geothermal systems are systems devoid of obvious surface hydrothermal manifestations. Emissions of moderate-to-low solubility gases may be one of the primary near-surface signals from these systems. We investigate the potential for CO2 detection and monitoring below and above ground in the near-surface environment as an approach to exploration targeting hidden geothermal systems. We focus on CO2 because it is the dominant noncondensible gas species in most geothermal systems and has moderate solubility in water. We carried out numerical simulations of a CO2 migration scenario to calculate the magnitude of expected fluxes and concentrations. Our results show that CO2 concentrations can reach high levels in the shallow subsurface even for relatively low geothermal source CO2 fluxes. However, once CO2 seeps out of the ground into the atmospheric surface layer, winds are effective at dispersing CO2 seepage. In natural ecological systems in the absence of geothermal gas emissions, near-surface CO2 fluxes and concentrations are predominantly controlled by CO2 uptake by photosynthesis, production by root respiration, microbial decomposition of soil/subsoil organic matter, groundwater degassing, and exchange with the atmosphere. Available technologies for monitoring CO2 in the near-surface environment include the infrared gas analyzer, the accumulation chamber method, the eddy covariance method, hyperspectralmore » imaging, and light detection and ranging. To meet the challenge of detecting potentially small-magnitude geothermal CO2 emissions within the natural background variability of CO2, we propose an approach that integrates available detection and monitoring techniques with statistical analysis and modeling strategies. The proposed monitoring plan initially focuses on rapid, economical, reliable measurements of CO2 subsurface concentrations and surface fluxes and statistical analysis of the collected data. Based on this analysis, are as with a high probability of containing geothermal CO2 anomalies can be further sampled and analyzed using more expensive chemical and isotopic methods. Integrated analysis of all measurements will determine definitively if CO2 derived from a deep geothermal source is present, and if so, the spatial extent of the anomaly. The suitability of further geophysical measurements, installation of deep wells, and geochemical analyses of deep fluids can then be determined based on the results of the near surface CO2 monitoring program.« less

Authors:
;
Publication Date:
Research Org.:
Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
Sponsoring Org.:
USDOE. Assistant Secretary for Energy Efficiency and Renewable Energy. Office of the Geothermal Technologies Program (US)
OSTI Identifier:
841068
Report Number(s):
LBNL-56900
R&D Project: G31915; TRN: US200513%%291
DOE Contract Number:
AC03-76SF00098
Resource Type:
Conference
Resource Relation:
Conference: Thirtieth Workshop on Geothermal Reservoir Engineering, Stanford, CA (US), 01/31/2005--02/02/2005; Other Information: PBD: 19 Jan 2005
Country of Publication:
United States
Language:
English
Subject:
02 PETROLEUM; 15 GEOTHERMAL ENERGY; 58 GEOSCIENCES; DEGASSING; DETECTION; EXPLORATION; GASES; GEOTHERMAL SYSTEMS; MONITORING; ORGANIC MATTER; PHOTOSYNTHESIS; PROBABILITY; PRODUCTION; RESERVOIR ENGINEERING; RESPIRATION; SIMULATION; SOLUBILITY; WATER; Geothermal Legacy

Citation Formats

Lewicki, Jennifer L., and Oldenburg, Curtis M.. Near-Surface CO2 Monitoring And Analysis To Detect Hidden Geothermal Systems. United States: N. p., 2005. Web.
Lewicki, Jennifer L., & Oldenburg, Curtis M.. Near-Surface CO2 Monitoring And Analysis To Detect Hidden Geothermal Systems. United States.
Lewicki, Jennifer L., and Oldenburg, Curtis M.. Wed . "Near-Surface CO2 Monitoring And Analysis To Detect Hidden Geothermal Systems". United States. doi:. https://www.osti.gov/servlets/purl/841068.
@article{osti_841068,
title = {Near-Surface CO2 Monitoring And Analysis To Detect Hidden Geothermal Systems},
author = {Lewicki, Jennifer L. and Oldenburg, Curtis M.},
abstractNote = {''Hidden'' geothermal systems are systems devoid of obvious surface hydrothermal manifestations. Emissions of moderate-to-low solubility gases may be one of the primary near-surface signals from these systems. We investigate the potential for CO2 detection and monitoring below and above ground in the near-surface environment as an approach to exploration targeting hidden geothermal systems. We focus on CO2 because it is the dominant noncondensible gas species in most geothermal systems and has moderate solubility in water. We carried out numerical simulations of a CO2 migration scenario to calculate the magnitude of expected fluxes and concentrations. Our results show that CO2 concentrations can reach high levels in the shallow subsurface even for relatively low geothermal source CO2 fluxes. However, once CO2 seeps out of the ground into the atmospheric surface layer, winds are effective at dispersing CO2 seepage. In natural ecological systems in the absence of geothermal gas emissions, near-surface CO2 fluxes and concentrations are predominantly controlled by CO2 uptake by photosynthesis, production by root respiration, microbial decomposition of soil/subsoil organic matter, groundwater degassing, and exchange with the atmosphere. Available technologies for monitoring CO2 in the near-surface environment include the infrared gas analyzer, the accumulation chamber method, the eddy covariance method, hyperspectral imaging, and light detection and ranging. To meet the challenge of detecting potentially small-magnitude geothermal CO2 emissions within the natural background variability of CO2, we propose an approach that integrates available detection and monitoring techniques with statistical analysis and modeling strategies. The proposed monitoring plan initially focuses on rapid, economical, reliable measurements of CO2 subsurface concentrations and surface fluxes and statistical analysis of the collected data. Based on this analysis, are as with a high probability of containing geothermal CO2 anomalies can be further sampled and analyzed using more expensive chemical and isotopic methods. Integrated analysis of all measurements will determine definitively if CO2 derived from a deep geothermal source is present, and if so, the spatial extent of the anomaly. The suitability of further geophysical measurements, installation of deep wells, and geochemical analyses of deep fluids can then be determined based on the results of the near surface CO2 monitoring program.},
doi = {},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Wed Jan 19 00:00:00 EST 2005},
month = {Wed Jan 19 00:00:00 EST 2005}
}

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  • We investigate the potential for CO2 monitoring in thenear-surface environment as an approach to exploration for hiddengeothermal systems. Numerical simulations of CO2 migration from a modelhidden geothermal system show that CO2 concentrations can reach highlevels in the shallow subsurface even for relatively low CO2 fluxes.Therefore, subsurface measurements offer an advantage over above-groundmeasurements which are affected by winds that rapidly disperse CO2. Tomeet the challenge of detecting geothermal CO2 emissions within thenatural background variability of CO2, we propose an approach thatintegrates available detection and monitoring techniques with statisticalanalysis and modeling.
  • ''Hidden'' geothermal systems are those systems above which hydrothermal surface features (e.g., hot springs, fumaroles, elevated ground temperatures, hydrothermal alteration) are lacking. Emissions of moderate to low solubility gases (e.g., CO2, CH4, He) may be one of the primary near-surface signals from these systems. Detection of anomalous gas emissions related to hidden geothermal systems may therefore be an important tool to discover new geothermal resources. This study investigates the potential for CO2 detection and monitoring in the subsurface and above ground in the near-surface environment to serve as a tool to discover hidden geothermal systems. We focus the investigation onmore » CO2 due to (1) its abundance in geothermal systems, (2) its moderate solubility in water, and (3) the wide range of technologies available to monitor CO2 in the near-surface environment. However, monitoring in the near-surface environment for CO2 derived from hidden geothermal reservoirs is complicated by the large variation in CO2 fluxes and concentrations arising from natural biological and hydrologic processes. In the near-surface environment, the flow and transport of CO2 at high concentrations will be controlled by its high density, low viscosity, and high solubility in water relative to air. Numerical simulations of CO2 migration show that CO2 concentrations can reach very high levels in the shallow subsurface even for relatively low geothermal source CO2 fluxes. However, once CO2 seeps out of the ground into the atmospheric surface layer, surface winds are effective at dispersing CO2 seepage. In natural ecological systems in the absence of geothermal gas emissions, near-surface CO2 fluxes and concentrations are primarily controlled by CO2 uptake by photosynthesis, production by root respiration, and microbial decomposition of soil/subsoil organic matter, groundwater degassing, and exchange with the atmosphere. Available technologies for monitoring CO2 in the near-surface environment include (1) the infrared gas analyzer (IRGA) for measurement of concentrations at point locations, (2) the accumulation chamber (AC) method for measuring soil CO2 fluxes at point locations, (3) the eddy covariance (EC) method for measuring net CO2 flux over a given area, (4) hyperspectral imaging of vegetative stress resulting from elevated CO2 concentrations, and (5) light detection and ranging (LIDAR) that can measure CO2 concentrations over an integrated path. Technologies currently in developmental stages that have the potential to be used for CO2 monitoring include tunable lasers for long distance integrated concentration measurements and micro-electronic mechanical systems (MEMS) that can make widespread point measurements. To address the challenge of detecting potentially small-magnitude geothermal CO2 emissions within the natural background variability of CO2, we propose an approach that integrates available detection and monitoring methodologies with statistical analysis and modeling strategies. Within the area targeted for geothermal exploration, point measurements of soil CO2 fluxes and concentrations using the AC method and a portable IRGA, respectively, and measurements of net surface flux using EC should be made. Also, the natural spatial and temporal variability of surface CO2 fluxes and subsurface CO2 concentrations should be quantified within a background area with similar geologic, climatic, and ecosystem characteristics to the area targeted for geothermal exploration. Statistical analyses of data collected from both areas should be used to guide sampling strategy, discern spatial patterns that may be indicative of geothermal CO2 emissions, and assess the presence (or absence) of geothermal CO2 within the natural background variability with a desired confidence level. Once measured CO2 concentrations and fluxes have been determined to be of anomalous geothermal origin with high confidence, more expensive vertical subsurface gas sampling and chemical and isotopic analyses can be undertaken. Integrated analysis of all measurements will determine definitively if CO2 derived from a deep geothermal source is present, and if so, the spatial extent of the anomaly. The appropriateness of further geophysical measurements, installation of deep wells, and geochemical analyses of deep fluids can then be decided based on the results of the near surface CO2 monitoring program.« less
  • A variety of hydrocarbons, C{sub 1} - C{sub 12}, have been found in volcanic gases (fumarolic) and in geothermal waters and gases. The hydrocarbons are thought to have come from products of pyrolysis of kerogen in sedimentary rocks or they could be fed into the geothermal system by the recharging waters which may contain dissolved hydrocarbons or hydrocarbons extracted by the waters from the rocks. In the hot geothermal zone, 300{sup 0}+ C, many of these hydrocarbons are in their critical state. It is thought that they move upwards due to buoyancy and flux up with the upflowing Geothermal fluidsmore » in the upflow zones together with the magmatic gases. Permeability which could be provided by faults, fissures, mini and micro fractures are thought to provide pathways for the upward flux. A sensitive technique (Petrex) utilizing passive integrative adsorption of the hydrocarbons in soil gas on activated charcoal followed by desorption and analysis of the hydrocarbons by direct introduction mass spectrometry allows mapping of the anomalous areas. Surveys for geothermal resources conducted in Japan and in Indonesia show that the hydrocarbon anomaly occur over known fields and over areas strongly suspected of geothermal potential. The hydrocarbons found and identified were n-paraffins (C{sub 7}-C{sub 9}) and aromatics (C{sub 7}-C{sub 8}). Detection of permeable, i.e. active or open faults, parts of older faults which have been reactivated, e.g. by younger intersecting faults, and the area surrounding these faulted and permeable region is possible. The mechanism leading to the appearance of the hydrocarbon in the soil gas over upflow zones of the Geothermal reservoir is proposed. The paraffins seems to be better pathfinders for the location of upflows than the aromatics. However the aromatics may, under certain circumstances, give better indications of the direction of the outflow of the geothermal system.« less
  • A variety of hydrocarbons, C 1 - C 12, have been found in volcanic gases (fumarolic) and in geothermal waters and gases. The hydrocarbons are thought to have come from products of pyrolysis of kerogen in sedimentary rocks or they could be fed into the geothermal system by the recharging waters which may contain dissolved hydrocarbons or hydrocarbons extracted by the waters from the rocks. In the hot geothermal zone, 300┬░+ C, many of these hydrocarbons are in their critical state. It is thought that they move upwards due to buoyancy and flux up with the upflowing geothermal fluids inmore » the upflow zones together with the magmatic gases. Permeability which could be provided by faults, fissures, mini and micro fractures are thought to provide pathways for the upward flux. A sensitive technique (Petrex) utilizing passive integrative adsorption of the hydrocarbons in soil gas on activated charcoal followed by desorption and analysis of the hydrocarbons by direct introduction mass spectrometry allows mapping of the anomalous areas. Surveys for geothermal resources conducted in Japan and in Indonesia show that the hydrocarbon anomaly occur over known fields and over areas strongly suspected of geothermal potential. The hydrocarbons found and identified were n-paraffins (C 7-C 9) and aromatics (C 7-C 8). Detection of permeable, i.e. active or open faults, parts of older faults which have been reactivated, e.g. by younger intersecting faults, and the area surrounding these faulted and permeable region is possible. The mechanism leading to the appearance of the hydrocarbon in the soil gas over upflow zones of the geothermal reservoir is proposed. The paraffins seems to be better pathfinders for the location of upflows than the aromatics. However the aromatics may, under certain circumstances, give better indications of the direction of the outflow of the geothermal system. It is thought that an upflow zone can be defined when conditions exist where the recharging waters containing the hydrocarbons feed into the geothermal kitchen. The existence of open and active faults, fissures, mini and micro fractures allow sufficient permeability for the gases to flux up and express themselves at the surface as hydrocarbon anomaly in the soil gas. When any of the requirements is absent, i.e. in the absence of the recharging waters, hydrocarbons, temperature, or permeability, no anomaly can be expected. It assumes a dynamic convective system, i.e. recharging waters, upflow and outflow. The anomalies however can define to a certain extent, regions of geothermal upflow, buoyant transport of gases, and frequently down-gradient of cooling waters.« less
  • Long Valley caldera is a tectonically active area in east-central California that contains a high-temperature geothermal system currently being explored and developed for electric power production. Concerns expressed by public agencies and private citizens over the potential impacts of geothermal development on thermal springs at the Hot Creek Fish Hatchery and Hot Creek gorge have resulted in the establishment of a Hydrologic Advisory Committee. The committee includes representatives from development projects and regulatory agencies. Its role is to formulate and oversee a hydrologic monitoring program capable of detecting such impacts before they become significant. The advisory committee and the monitoringmore » program provide effective mechanisms with which to proceed with geothermal development in areas such as Long Valley, where the level of environmental risk cannot be determined a-priori because the relevant geo-hydrologic parameters are not adequately delineated.« less