Downhole Fluid Sampling

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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.
Downhole Fluid Sampling:
Downhole fluid sampling is done to characterize the chemical, thermal, or hydrological properties of a surface or subsurface aqueous system. Downhole fluid sampling is typically performed to monitor water quality, study recharge and flow in groundwater systems, and evaluate resource potential of geothermal reservoirs. Analysis of both the liquid and gas fractions of the reservoir fluid allows for detailed characterize the chemical, thermal, or hydrological properties of the subsurface hydrothermal system.
Other definitions:Wikipedia Reegle

Reservoir fluid sampling is routinely used in geothermal exploration to provide accurate evaluations of the geothermal system under investigation. Geothermal waters develop a unique chemical signature through a series of potentially complex processes relating to reservoir rock interactions, groundwater recharge, fluid mixing ratios, and phase transitions. All of these factors influence the chemistry of thermal fluids derived from the original fluids that recharge the dynamic hydrothermal system. Fluid samples collected from wellbores are used to evaluate these processing and thereby assist in siting additional exploratory and temperature-gradient boreholes during the early stages of prospect evaluation. Data obtained through analysis of sampled well 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.

Analyst testing a sample of condensed steam at the Olkaria geothermal power plant in Kenya’s East African Rift Valley. Photo taken by Roberto Schmidt of Getty Images, featured on theGrio Inspiration website.[1]

Use in Geothermal Exploration
Water sampling is routinely used in geothermal exploration and monitoring to characterize the chemical composition of the fluid, measure the temperature, or conduct isotope studies to derive the provenance of thermal waters. Water sampling is a critical aspect of characterizing a geothermal system because the water chemistry, temperature, and source can reveal the quality of the resource. Water chemistry is largely controlled by temperature, water-rock interactions, volume of water vs rock, residence time, and contributions from other fluids (mixing), such as cold groundwater, seawater, magmatic fluids, etc.[2] Interaction of fluids with reservoir rocks may cause waters to become oversaturated with silica or carbonate, leading to precipitation of sinter or travertine at surface conditions, respectively.[3][2] Some systems feature a vapor-dominated portion of the geothermal reservoir, from which steam and volatile compounds ascend, condense, and mix with an overlying freshwater aquifer, as at the Valles Caldera, NM.[4][5] Oxidation of H2S in the steam forms sulfuric acid, producing acid-sulfate springs and advanced argillic alteration at the surface.[3][2][6]

Geothermal waters typically range in total dissolved solids (TDS) from a few hundred to > 350,000 parts per million (ppm).[3][2] 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.[6][2] 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).[3][2]

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.[7] 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.[8] [9]

Field Procedures
Waters can be collected from geothermal reservoirs using a variety of downhole and wellhead sampling techniques. There is some variation in the methods and instruments used for taking geothermal water samples that depend primarily on the type of fluid being sampled and on the analytical techniques to be applied to the sample. Water samples collected at the surface experience increasing disequilibrium from reservoir conditions as a result of cooling, decompression, boiling/flashing, and water-rock interactions during ascent. To minimize the effects of these processes, specialized field treatments and downhole sampling techniques are used to obtain representative reservoir fluid samples.

Various types and sizes of commercially available reusable bailers commonly used for well water sampling. Image featured on the Geotechnical Services website product page.

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.[7] 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.[10] These accommodations allowing for calculation of the bulk composition and phase state of the in-situ reservoir fluid.[8][9]

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.[11] 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).[12] 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.[13] A synopsis of geochemical sampling and analysis techniques used in geothermal exploration is also provided by Arnorsson et al. (2006).[14] Detailed methodologies for downhole sampling of geothermal fluids are also described by Klyen (1982),[15] Brown & Simmons (2003),[16] and Arnórsson & Stefánsson (2005).[17]

Schematic diagram of the well fluid sampling apparatus for a geothermal system. Geothermal fluids are extracted from the reservoir and are split into liquid and vapor fractions at the wellhead using a separator. The vapor fraction is collected at the separator and condensed, whereas the liquid sample is collected at the weir box. Data from these samples is used in geochemical modeling to calculate the composition the original reservoir fluids. Figure 1 from Torres-Alvarado et al., 2012 (after Verma, 2012).[18]

Data Access and Acquisition
Temperature, pressure, and pH data are typically measured in the field at the time of sampling, and are recorded in conjunction with the sampling coordinates. The flow rate is also measured at the wellhead gauge. Sampled waters are typically subjected to chemical and isotopic analyses in order to characterize hydrothermal systems and allow for estimation of reservoir temperatures through the application of various chemical 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
Sampling of geothermal reservoir fluids is best carried out by a qualified hydrologist, geologist, or geochemist familiar with current sampling standards. A practical understanding of how various processes affect the bulk chemistry of the reservoir fluids as they are brought to the surface ensures that appropriate sampling procedures are used to obtain a sample that is representative of reservoir conditions. A working understanding of geothermal systems 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.[14] Failure to adhere to proper sampling procedures and treatment practices can result in mineral precipitation in sample vials and/or re-equilibration of the sample at surface conditions, both of 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.

Additional References

  1. theGrio. Kenya Becoming a Geothermal Powerhouse [Internet]. 05/21/2012. theGrio. [updated 2012/05/21;cited 2013/10/10]. Available from:
  2. 2.0 2.1 2.2 2.3 2.4 2.5 Encyclopedia of Volcanoes
  3. 3.0 3.1 3.2 3.3 Chapter 4: Geochemistry
  4. 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.
  5. 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.
  6. 6.0 6.1 Geothermal Waters: A Source of Energy and Metals
  7. 7.0 7.1 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.
  8. 8.0 8.1 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.
  9. 9.0 9.1 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.
  10. Fraser E. Goff,Jamie N. Gardner. 1994. Evolution of a Mineralized Geothermal System, Valles Caldera, New Mexico. Economic Geology. 89(8):1803-1832.
  11. 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.
  12. 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.
  13. Chapter A4: Collection of Water Samples (ver. 2.0)
  14. 14.0 14.1 Sampling and Analysis of Geothermal Fluids
  15. Sampling Techniques for Geothermal Fluids
  16. Precious Metals in High-Temperature Geothermal Systems in New Zealand
  17. Wet-Steam Well Discharges. I. Sampling and Calculation of Total Discharge Compositions
  18. Estimates of Geothermal Reservoir Fluid Characteristics: Geosys. Chem and WATCH

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