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 | |
| Lithology: | |
| Stratigraphic/Structural: | |
| 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. |
- "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. [[File:Kenya_geothermal_sampling.jpg|thumb|500px|center|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.'"`UNIQ--ref-00000000-QINU`"'" cannot be used as a page name in this wiki.
- The given value was not understood.
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]
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