Use Of Packer Test Results In Hydrogeological Characterization: A Comparison Of Calculation Methods For A Representative Value
Tesema, Fethangest Weldemariyam
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Packer test is one of the commonly applied methods in determination of hydraulic conductivity particularly in low permeability fractured rock mass. Originally, hydraulic conductivity is defined in Lugeon units which correspond to 1 litre per minute per meter of intake section less than 10 atmospheres of pressure. Since the test is applied under different pressures, which gives the opportunity to observe the hydraulic behaviour of the rock under different stresses. Calculations of hydraulic conductivity for each step of pressure usually give different value of hydraulic conductivity. Selection of a representative hydraulic conductivity value therefore becomes an issue. The arithmetic mean is the commonly accepted value, without justifying its representativeness. In this paper, about 75 packer tests conducted at 17 core wells at different depth with 3.5 m long test sections using double packer were evaluated. The tests are conducted at five pressures that run in three increasing and two decreasing pressures according to the recommendation of Houlsby. Hydraulic conductivity was calculated for every pressure step and evaluated to find a representative value using different methods. The representative values found by different methods is then compared. Results indicate that the arithmetic mean has good correlation with all the methods in particular it is nicely correlated with the modified Lugeon analysis for hydrogeological investigations with a correlation value of R-2 = 0.91. Therefore, it is concluded that the uncertainty associated with the use of the simple arithmetic mean is within an acceptable range of error.