Downhole Fluid Sampling
Exploration Technique: Downhole Fluid Sampling
|Exploration Technique Information|
|Exploration Group:||Downhole Techniques|
|Exploration Sub Group:||Well Testing Techniques|
|Parent Exploration Technique:||Well Testing Techniques|
|Information Provided by Technique|
|Hydrological:||Water composition and source of fluids. Gas composition and source of fluids.|
|Thermal:||Water temperature. Distinguish magmatic/mantle heat inputs. Can be used to estimate reservoir fluid temperatures.|
Geothermal waters typically range in total dissolved solids (TDS) from a few hundred to > 350,000 parts per million (ppm). Liquid dominated reservoirs usually have a composition dominated by Na, K and Cl, but in very saline systems reservoir waters can be Na, K, Ca, and Cl rich. Silica and trace element (As, B, Br, and Li) concentrations tend to be high compared to the average meteoric waters, and pH is generally between 6 and 9, although acidic saline liquids can also be associated with geothermal systems (as described above).
Downhole sampling methods and controlled wellhead sampling techniques can be used to minimize (or at least account for) the effects of cooling and depressurization on fluid chemistry. For example, samples of single-phase liquid were collected from the production wellhead at Casa Diablo, Long Valley Caldera, CA in 1985 and 1986 using a cooling coil to prevent flashing of thermal waters during ascent. Samples were also collected from a mini-separator that allowed flashing of the thermal water to liquid and gas under known conditions, allowing for back-calculation of the true reservoir fluid chemistry. 
- Well Testing Techniques
Relatively simple methods for sampling groundwater from a well utilize bailers or airlifting techniques, however these methods result in aeration of the water sample and sometimes lead to flashing of thermal waters as they encounter lower pressure conditions during ascent. Several different techniques can be used to control or account for phase transitions of thermal fluids as they ascend the wellbore during sampling, thereby ensuring that the results of subsequent lab analyses are representative of fluid properties in the geothermal reservoir. Reservoir fluid characteristics can be approximated by using a cooling coil to prevent flashing of thermal waters when they are sampled from the wellhead at surface conditions. These samples are used in conjunction with fluid samples collected using a separator that flashes the fluid to liquid and gas under known conditions, allowing the gas fraction to be subsampled in the field using a gas extraction system that enables precise measurement of gas volume and pressure. These accommodations allowing for calculation of the bulk composition and phase state of the in-situ reservoir fluid.
Waters sampled for chemical analysis are stored in brimful polyethylene bottles with Polyseal caps following filtration from a large syringe attached to a filter holder containing 0.8 um filter paper. Each individual sample consists of 10-500 mL of filtered water, depending on the requirements of lab analytical techniques to be applied later. Duplicates are taken at each sample point and then treated in the field in preparation for chemical analyses. A set of duplicates sampled from a single surface discharge might included a bottle of unacidified (untreated) water for anion analysis, a bottle of water acidified dropwise with dilute HCl to pH <2 for cation analysis, a bottle containing sampled water diluted with deionized water (between 1:5 and 1:10 ratio) for measurement of silica content.
Samples to be used for isotopic analysis are collected in glass bottles filled to the brim with raw (unfiltered) water and sealed with a Polyseal cap. As with standard compound and major/trace elemental analyses, analysis for isotopes of different elements requires specialized treatment of the sample in the field. For example, tritium analysis requires a significant volume of water (up to 500 mL), whereas analysis for stable isotopes that are present in greater abundance in natural samples requires less water to be sampled by a full order of magnitude (approximately 30 mL). In order to analyze the 13C content of dissolved HCO3, the water sample must be treated with NH4OH and then saturated with SrCl2. For analysis of the 18O content of dissolved SO4, the water sample is treated with formaldehyde.
For a detailed description of modern water sampling techniques, methods, and instrumentation, consult Chapter A4 of the National Field Manual for the Collection of Water-Quality Data, published online by the U.S. Geological Survey. A synopsis of geochemical sampling and analysis techniques used in geothermal exploration is also provided by Arnorsson et al. (2006). Detailed methodologies for downhole sampling of geothermal fluids are also described by Klyen (1982), Brown & Simmons (2003), and Arnórsson & Stefánsson (2005).
- theGrio. Kenya Becoming a Geothermal Powerhouse [Internet]. 05/21/2012. theGrio. [updated 2012/05/21;cited 2013/10/10]. Available from: http://thegrio.com/2012/05/21/kenya-becoming-a-geothermal-powerhouse/
- Encyclopedia of Volcanoes
- Chapter 4: Geochemistry
- Fraser E. Goff,Dennis L. Nielson,Jamie N. Gardner,Jeffrey B. Hulen,Peter Lysne,Lisa Shevenell,John C. Rowley. 1987. Scientific Drilling at Sulphur Springs, Valles Caldera, New Mexico- Core Hole VC-2A. EOS, Transactions American Geophysical Union. 68(30):649-662.
- Masakatsu Sasada,Fraser E. Goff. 1995. Fluid Inclusion Evidence for Rapid Formation of the Vapor-Dominated Zone at Sulphur Springs, Valles Caldera, New Mexico, USA. Journal of Volcanology and Geothermal Research. 67(1-3):161-169.
- Geothermal Waters: A Source of Energy and Metals
- Michael L. Sorey,Gene A. Suemnicht,Neil C. Sturchio,Gregg A. Nordquist. 12/1991. New Evidence On The Hydrothermal System In Long Valley Caldera, California, From Wells, Fluid Sampling, Electrical Geophysics, And Age Determinations Of Hot-Spring Deposits. Journal of Volcanology and Geothermal Research. 48(3-4):229-263.
- Christopher D. Farrar,Michael L. Sorey,S.A. Rojstaczer,Cathy J. Janik,T.L. Winnett,M.D. Clark. 1987. Hydrologic and Geochemical Monitoring in Long Valley Caldera, Mono County, California, 1985. Sacramento, CA: U.S. Geological Survey. Report No.: Water-Resources Investigations Report 87-4090.
- Christopher D. Farrar,M.L. Sorey,S.A. Rojstaczer,A.C. Steinemann,M.D. Clark. 1989. Hydrologic and Geochemical Monitoring in Long Valley Caldera, Mono County, California, 1986. Sacramento, CA: U.S. Geological Survey. Report No.: Water-Resources Investigations Report 89-4033.
- Fraser E. Goff,Jamie N. Gardner. 1994. Evolution of a Mineralized Geothermal System, Valles Caldera, New Mexico. Economic Geology. 89(8):1803-1832.
- Fraser E. Goff,Tamsin McCormick,Pat E. Trujillo Jr,Dale A. Counce,Charles O. Grigsby. 1982. Geochemical Data for 95 Thermal and Nonthermal Waters of the Valles Caldera - Southern Jemez Mountains Region, New Mexico. Los Alamos, NM: Los Alamos National Laboratory, NM. Report No.: LA-9367-OBES.
- John A. Musgrave,Fraser E. Goff,Lisa Shevenell,Patricio E. Trujillo Jr,Dale Counce,Gary Luedemann,Sammy Garcia,Bert Dennis,Jeffrey B. Hulen,Cathy Janik,Francisco A. Tomei. 1989. Selected Data from Continental Scientific Drilling Core Holes VC-1 and VC-2A, Valles Caldera, New Mexico. Los Alamos, NM: Los Alamos National Laboratory, NM. Report No.: Report No. unavailable.
- Chapter A4: Collection of Water Samples (ver. 2.0)
- Sampling and Analysis of Geothermal Fluids
- Sampling Techniques for Geothermal Fluids
- Precious Metals in High-Temperature Geothermal Systems in New Zealand
- Wet-Steam Well Discharges. I. Sampling and Calculation of Total Discharge Compositions
- Estimates of Geothermal Reservoir Fluid Characteristics: Geosys. Chem and WATCH
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