POSIVA groundwater flow measuring techniques
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Résumé
Posiva Oy has carried out site characterisation for the final disposal of spent nuclear fuel in Finland since 1987. To meet the demanding needs to measure the hydraulic parameters in bedrock Posiva launched development of new flowmeter techniques including measuring methods and equipment in co-operation with PRG-Tec Oy. The techniques have been tested and used in the ongoing site investigations in Finland, in the underground Hard Rock Laboratory (HRL) at Aespoe in Sweden and in URL in Canada. The new methods are called difference flow and transverse flow methods. The difference flow method includes two modes, normal and detailed flow logging methods. In the normal mode the flow rate measurement is based on thermal pulse and thermal dilution methods, in the detailed logging mode only on thermal dilution method. The measuring ranges for flow rate with thermal pulse and dilution methods are 0.1-10 ml/min and 2-5000 ml/min, respectively. The difference flow method(normal mode) for small flows (0.1-10 ml/min) is based on measuring the pulse transit time and direction of a thermal pulse in the sensor. For high flows (2-5000 ml/min) the method is based on thermal dilution rate of a sensor. Direction is measured with monitoring thermistors. Inflow or outflow in the test interval is created due to natural or by pumping induced differences between heads in the borehole water and groundwater around the borehole. The single point resistance (and the temperature of borehole water) measurement is carried out simultaneously with the difference flow measurements, both in normal and detailed flow logging modes, while the tool is moving. The result is utilised for checking the exact depth of the tool. As the result a continuous log is obtained from which single fractures can be detected. The transverse flowmeter is able to measure the groundwater flow across a borehole. A special packer system guides the flow through the flow sensors. Four inflatable seals between conventional packers divide the test section into four sectors. The length of the test section between the inflatable packers is two metres. Flow guides are available at the moment for boreholes with diameters 56 mm and 76 mm. The flow sensors operate using a thermal pulse principle. The flow sensors must be calibrated for the acquisition of quantitative information. The sensitivity of the instrument is better than 1 ml/in (millilitre per hour) for the flow across a borehole which corresponds to a flux value of about 2 10-9 m/s. In addition to the flow rate determination across the borehole, the system also makes it possible to determine the approximate direction of flow across the borehole. Both methods have been used to determine hydraulic connections between adjacent boreholes by measuring flow responses in a borehole caused by pumping in another borehole. The suite offered by the Posiva Flow Log tools includes also Electric Conductivity (EC) measurements from the fracture-specific water in the borehole test section. It has been found convenient to conduct EC measurements in connection with the detailed flow logging. In this way hydraulically conductive fractures can be located during the same logging phase as EC values are attained from the most conductive fractures. The results of both the EC and the detailed flow logging measurements give valuable information for the determination of groundwater sampling points. The objective of EC measurement is to determine the distribution of the content of Total Dissolved Solids (TDS) in the groundwater. The detailed flow logging makes it possible to stop on a fracture and to measure there as long as the water volume within the test section is flushed well enough to get a reliable EC reading. EC readings are measured from fractures with higher flow rates than the pre-set limit. In this report all groundwater flow techniques developed by Posiva are presented including the methods and different logging tools. Some background on the interpretation as well as case measurements and results are also given. (orig.)
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