Anisotropy of permeability and complex resistivity of tight sandstones subjected to hydrostatic pressure

Abstract

The dependence of permeability and complex electrical resistivity on direction was measured for low-permeable sandstone samples from a tight gas reservoir. Both properties were measured in three directions at hydrostatic pressures up to 100 MPa. The decrease of permeability as a function of effective pressure (measured with a modified pressure-transient method) can be described by a power function. The pressure dependence is more controlled by the closure of thin aspect ratio pores and cracks than by the minor reduction of porosity. The anisotropy of permeability is also a function of pressure. For some samples the preferred direction of flow changes with increasing pressure. The Cole–Cole response-function can be fitted well to the complex resistivity spectra (kHz–MHz). Interfacial polarization is the dominant polarization effect in this frequency range. The relaxation time of the Cole–Cole model increases with increasing effective pressure, whereas the frequency exponent does not show any continuous behavior. According to the model of Lysne (1983) the geometrical distribution of pore shapes and their orientation can be derived from these quantities. The formation resistivity factor, taken from the real part of resistivity at 10 kHz, also increases with pressure. As the porosity does not change significantly, this increase means an increase of Archie's cementation exponent. Both, relaxation time and formation factor are also a function of the considered direction. But an overall relationship between these quantities and permeability could not be observed; neither for absolute values nor for their anisotropy.