Permeability of the Neocene Sedimentary Rocks and Its Interpretation by Using the Equivalent Channel Model

Abstract

A series of permeability tests were performed on Shirahama sandstone and Horonobe mudstone of the Neocene under triaxial stress and pore pressure conditions. Main results are as follows:
Firstly, for loading condtions, effect of pore pressure on the permeability is less than that of hydrostatic pressure. At the same time, permeability-pressure curve bends at a certain pressure, at which volumetric strainpressure also bends accordingly. For unloading condition, contrary to the loading one, permeability is not altered by the pressure change.
Secondly, the permeability abruptly increases accompanying to the fracture. But the permeability-pressure curve is much steeper than the one for the intact condition, at the onset of the pressurization, This means that the effect of the fracture planes on the permeability is quickly diminished, by virture of the increasing pressure.
These characteristics of the permeability should have close relationship with the microstructure of the rock. Stress analysis was conducted for the equivalent channel model, which can be interpreted to give a simplification of the real structure, assuming several material properties for the model medium. Predicted variations of the permeability in relation to the hydrostatic pressure and pore pressure are in good coincidence with the observations. Particularly, the case that the permeability-pressure and/or volumetric strain-pressure curves bend can well be explained when the medium is assumed to be composed of elastic material which changes its stiffness at certain pressure due the closure of microcracks.