Formation Factor Determinations by In-Situ Resistivity Logging
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areas where the ionic strength of the pore fluid is low, surface conduction can contribute considerably to the total conduction and strongly influence the wet rock resistivity. It is essential that this effect can be accounted for as it influences rock matrix diffusivity determinations. During 1930s electrical resistivity measurements became a standard procedure in geophysical well logging. The first tools were primarily developed to detect anomalies in rock and not for quantitative measurements of the rock resistivity. The approach unifying these methods was to wire an electrode down a borehole and connect it, through a power supply, to an electrode at the surface. This would create a circuit where the current had to go through the rock. The tool was winched up or down, logging the borehole. By using Ohm's law the resistance of the rock could then be calculated. If the resistance suddenly changed it could be interpreted as for example a water bearing fracture, an ore body, a mineral change or, if the geophysics were lucky, oil. Soon there was a need to also be able to perform quantitative measurements of the electrical resistivity of rock. A breakthrough in achieving measurements with good accuracy and vertical resolution was the focused resistivity logs using guard electrodes forcing the current emitted by the current electrode to leave in a dish co-axial with the borehole. After the introduction of focused technique in 1950 the earlier logs where more and more abandoned and are rarely used today. The most important area where resistivity logging is used is in oil prospecting. Therefore most tools are developed for sedimentary rock. Only recently slimhole focused tools have been developed for measurements in igneous and metamorphic rock. When measuring the formation factor in-situ the conductivity of the pore fluid is needed. Today there are no methods of measuring this parameter directly in-situ in igneous/metamorphic rock. In a recent work [2] it has been suggested that as a first approximation the conductivity of the borehole fluid could be used as the pore fluid conductivity at a corresponding depth. New measurements of the electrical conductivity of the borehole fluid and also of the groundwater flowing from water-bearing fractures in the rock shows that this approximation is not valid in many cases. Using the conductivity values from the measurements of the fracture water as an approximation of the pore fluid conductivity in the area surrounding the fracture seems more appropriate. Still investigations are needed to obtain knowledge on how extended pumping disturbs the natural water flow situation. If too much time elapses from the drilling to the measurements, the natural conductivity of fracture water may have been altered. It is also needed to investigate if the conductivity of the fracture water could be used as the conductivity of the pore fluid even if the water flow situation is not disturbed. First scooping comparisons of formation factors obtained by in-situ methods with laboratory measured data on
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