Surface Gas Sampling

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Exploration Technique: Surface Gas Sampling

Exploration Technique Information
Exploration Group: Field Techniques
Exploration Sub Group: Field Sampling
Parent Exploration Technique: Gas Sampling
Information Provided by Technique
Hydrological: Gas composition and source of fluids.
Thermal: Distinguish magmatic/mantle heat inputs. Can be used to estimate reservoir fluid temperatures.
Surface Gas Sampling:
Gas sampling is done to characterize the chemical, thermal, or hydrological properties of a surface or subsurface hydrothermal system.
Other definitions:Wikipedia Reegle

Gas samples are often collected at the surface in addition to water samples from springs, fumaroles, and wellheads to facilitate detailed characterization of the geothermal system. Gas samples may also be collected from surface discharges to facilitate ongoing monitoring of the hydrothermal system during production. Samples are used to analyze the volatile gas contents emanating from a hydrothermal system. Typical geothermal resources expel anomalously high concentrations of gases (CO2, CH4, N2O, He, etc.) indicating the presence of enhanced permeability and potentially of high temperature resources at depth. Geothermal fluids develop a unique chemical signature through a series of complex processes relating to reservoir rock interactions, fluid mixing ratios, and phase transitions. All of these factors ultimately influence the chemistry of surface gas discharges derived from the original fluids that recharge the dynamic hydrothermal system. Data obtained through analysis of sampled fluids can also be integrated into regional-scale hydrologic and thermal gradient models that can assist in identifying the source of thermal fluids in the geothermal system.

U.S. Geological Survey scientist sampling a fumarole at Mount Baker, WA. Photo taken by W. Chadwick in 1981, featured in the Wikipedia public domain image cache.

Use in Geothermal Exploration
Gas sampling is sometimes used in geothermal exploration and monitoring to better characterize the bulk chemical composition of the thermal fluids, measure the temperature and pressure of surface gas discharges, or conduct isotope studies to determine the source of heat sustaining the system. Similar to water sampling, gas sampling is an important tool for characterizing a geothermal system because the gas chemistry, temperature, pressure, and source can assist in determining the quality of the resource. The chemistry of gas discharges relates to reservoir temperature, pressure, water-rock interactions at depth, residence time, and contributions from other fluids (mixing) prior to phase separation, such as cold groundwater, seawater, magmatic fluids, etc.[1] Some gases that reach the surface in geothermal systems form acid-sulfate springs, generated from rising steam and volatile compounds that condense and mix with an overlying freshwater aquifer, whereupon the H2S in the steam oxidizes to form sulfuric acid.[2][1][3] These acidic discharges form distinct alteration footprints that may consist of a silica leach capping (consisting predominantly of remnant silica) and/or advanced argillic alteration, both of which may be recognized at the surface using remote sensing or basic field mapping techniques.

A geochemical study of some 80 gas samples collected from fumaroles, springs, and wells confirmed that gases at the Valles Caldera, NM are chemically and isotopically similar to those in other volcanic-hosted geothermal systems, and that the gases are in apparent equilibrium at temperatures >200°C .[4] Relative proportions of Ar, He, and N2 were similar to those measured at hot spot locations such as the Yellowstone and Kilauea calderas. He isotope R/Ra values of 4-6 within the caldera were suggestive of mantle/magmatic degassing, whereas R/Ra values of < 0.7 outside the caldera reflect an He input dominated by U/Th decay in crustal rocks. Major gas components of the caldera surface discharges have remained relatively constant over the course of the nearly 20 year sampling program, and generally resemble gas compositions of the geothermal wells. The study also revealed that Valles caldera gases contained relatively little CH4 and N2 compared to other geothermal systems hosted within sedimentary rocks, suggesting that organic carbon and nitrogen in Paleozoic and Miocene strata were depleted during 13 million years of magmatism in the Jemez volcanic field.

Field Procedures
Gases from fumaroles, springs, and wellheads are collected into double-port evacuated glass bottles filled with ~100 mL of high-purity 4 N NaOH using a tygon tube connected to various sampling apparatuses attached to the gas sources.[5] Collection pressure and temperature data are measured as the gas samples are fed into sampling bottles. Hot spring gas samples are typically collected by submerging a plastic funnel into the pool over the bubble stream. Fumarole gas samples are typically collected by either burying a similar plastic funnel aparatus or a wide-diameter pipe into the fumarole vent. Gases sampled from sub-boiling, non-flowing wells are collected by attaching metal and/or plastic fittings to the wellheads, which are then attached to the tygon tubing. Gas samples from flowing geothermal wells can be collected using a Webre separator attached to the flow line downstream of the wellhead. In-situ fluids can also be sampled downhole using a variety of different sampling techniques, which are described in detail on the downhole fluid sampling page.[6]

Fluid samples to be used for dissolved noble gas and tritium analyses can also be collected using a simple copper tube sampling apparatus attached to the tygon feed tube.[7] The copper tube is suspended in an aluminum channel with steel pinch-off clamps. The liquid sample is fed through the copper tube, taking care to remove any bubbles that may have become entrained in the flow. The pinch-off clamps are then closed tightly to seal the copper tube with a cold weld at both ends.

Copper tube gas sampling device used to collect thermal waters at Poncha Hot Springs, CO for dissolved noble gas analysis.

More detailed descriptions of gas sampling procedures commonly used to evaluate geothermal systems are offered by Fahlquist & Janik (1999) [5] and online at the U.S. Geological Survey Reston Chlorofluorocarbon Laboratory website.[7]

Data Access and Acquisition
Temperature and pressure data are typically measured in the field at the time of sampling, and are recorded in conjunction with the sampling coordinates. Gas samples are typically subjected to chemical and isotopic analyses in order to characterize the hydrothermal system, interpret concentrations of different tracer isotopes, and allow for estimation of reservoir temperatures through the application of various chemical and isotopic geothermometers. Data from these analyses can also provide useful information regarding the source of thermal fluids and help to constrain the age of the hydrothermal system.
Best Practices
Surface gas sampling is best carried out by a qualified hydrologist, geologist, or geochemist familiar with current sampling standards. A practical understanding of how different surface features relate to hydrothermal processes within the geothermal system is also ideal for the purposes of data interpretation, application of various chemical and isotopic geothermometers, and geochemical modeling of the reservoir.
Potential Pitfalls
Geothermal fluid sampling techniques are designed to prevent concentrations of dissolved species from changing via reactions that occur as samples cool, or through exposure of samples to the atmosphere.[8] Failure to adhere to proper sampling procedures and treatment practices can result in re-equilibration of the sample at surface conditions, which disturb the chemical composition of the fluid. These processes shift the fluid chemistry of the sample away from that of fluids present in the geothermal reservoir at depth, which impacts the results of the fluid analyses and will ultimately affect the results of geothermometric calculations and geochemical modeling.

Page Area Activity Start Date Activity End Date Reference Material
Surface Gas Sampling (Klein, 2007) Unspecified

Surface Gas Sampling At Fenton Hill HDR Geothermal Area (Goff & Janik, 2002) Fenton Hill HDR Geothermal Area

Surface Gas Sampling At Fenton Hill HDR Geothermal Area (Grigsby, Et Al., 1983) Fenton Hill HDR Geothermal Area

Surface Gas Sampling At International Geothermal Area Mexico (Norman, Et Al., 2002) International Geothermal Area Mexico

Surface Gas Sampling At Jemez Springs Area (Goff & Janik, 2002) Jemez Springs Area

Surface Gas Sampling At Lassen Volcanic National Park Area (Janik & Mclaren, 2010) Lassen Volcanic National Park Area

Surface Gas Sampling At Lightning Dock Area (Norman & Moore, 2004) Lightning Dock Area

Surface Gas Sampling At Lightning Dock Area (Norman, Et Al., 2002) Lightning Dock Area

Surface Gas Sampling At Valles Caldera - Redondo Area (Goff & Janik, 2002) Valles Caldera - Redondo Area

Surface Gas Sampling At Valles Caldera - Sulphur Springs Area (Goff & Janik, 2002) Valles Caldera - Sulphur Springs Area

Surface Gas Sampling At Yellowstone Region (Goff & Janik, 2002) Yellowstone Caldera Geothermal Region

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