The A638 (HL-382) Guard Conductivity tool is designed for measurement of electrical conductivity of rocks in boreholes. This tool requires the presence of conductive fluid and the absence of casing. The tool is capable of providing measurements under adverse conditions, such as where high borehole fluid salinity and resistive formations cause normal resistivity logs to show poor results. Good vertical resolution and depth of investigation facilitate the detection of fractures, permeable zones, formation water quality and strata correlation between
boreholes. The response of the probe is practically instantaneous (<0.5 s).Applications
The purpose of the Guard Log is to measure resistivity or its inverse, conductivity, in order to calculate porosity and to interpret lithology. The Guard Log is useful in several types of ground investigation and is more suitable than other resistivity measurements in high conductivity fluids and when vertical resolution is important.
Mineral Investigation
The resistivity of rocks depends on several factors. Rocks are usually poor conductors. Rock resistivity is determined, primarily, by porosity and the salinity of the pore fluid.
Clay minerals reduce the resistivity of the formation. Some alteration processes result in the formation of clay minerals, reducing resistivity, while others reduce rock porosity and increase formation resistivity. Decreases in resistivity may be a result of either significant concentration of conductive metal sulphides or oxides. Fracturing increases effective porosity and can, therefore, decrease resistivity.
The complexity of resistivity interpretation means that complementary information from other geophysical measurements or other geological logs is usually required. However, massive sulphide deposits generally consist of conductive ore zones and resistivity logs are often used to delineate these ore zones.
Coal Investigation
Coal usually has high resistivity. This may be reduced according to its porosity, clay, and water content. The porosity of coal varies with rank and so its resistivity increases from lignite to semi-anthracite and then decreases slightly with anthracite. Clay inclusions reduce the resistivity of coal.
Clay-rich lithologies, such as shales, have low resistivity. Clean, relatively dry sandstone has high resistivity. This may be reduced according to porosity, salinity, and clay content.
Resistivity (or conductivity) logs may also indicate oxidation of a coal seam or alteration by an intrusion.
Ground-water Investigation
The Archie equation defines the relationship between porosity and resistivity in reservoir rocks:
F = Ro / Rw where;
Ro = resistivity of a clean formation
Rw = resistivity of the water saturating the rock
F = a constant of proportionality called the formation factor
Specifications
Tool Parameters Diameter 38 mm Length 83 cm Weight 3 kg Max. Working Temperature 70 degrees C Max. Working Pressure 15 MPa Min. no. of Conductors 1 Supply Voltage Range 20 - 45 VDC Max. Current Consumption 160 mA Nominal Current Consumption 45 mA Communication Digital RS232 Measuring Parameters Sense electrode size 10cm Guard electrode size 60cm Conductivity Range 0.1 - 200 mS/m (10000 - 5 ohm metres)
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