This topic intends to compare permeability data obtained from nitrogen gas flowing to water flowing testing methods. Various confining stresses were applied to simulate the pressure at greater depth. The Nam Phong Formation sandstones, collected from west margin of the Khorat Plateau were tested in order to reveal its cap rock characteristic. Their anisotropy in the directions perpendicular and parallel to cross lamination of the rock specimens was also observed. The rock mineral compositions and fabric were examined under optical microscope. Other physical properties as grain size, porosity and density were also determined. Consequently, the rock specimens could be classified into two units as ferruginous arkosic arenite, siliceous and calcareous arkosic arenite, according to their petrofabric and cementing materials. Confirmation of their cap rock characteristic, experimental results indicate both units have very low permeability as lower than 1 μDarcy. A linear relationship between infinite gas permeability and water permeability of the rocks has been established. This relation made the prediction of water permeability from nitrogen gas flowing testing possible. Most permeability values from nitrogen gas flowing test provide lower value than water flowing ones. The results also reveal the drastically drop in permeability as confining changing from 4 to 8 MPa, then turn steady at higher pressure. Permeability anisotropy ratios evaluated from water flowing method shows a greater variation than from nitrogen gas. The percentage of quartz, porosity and frequency of cross lamination are significantly affected the permeability values.

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