Pressure dependence of permeability and spectral induced polarisation (SIP)

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An essential precondition for a derivation of hydraulic transport parameters of sandstones from electric data (spectral induced polarization, SIP) are a basic understanding of the transport of charge carriers in the pore space of sandstones. The mobility of ions in the pore fluid, will be hampered by polarizations caused by interaction with the rock matrix. The degree of interaction is controlled by numerous parameters like salinity, chemistry of the pore fluid, the mineralogical composition, the inner surface of the pore space and the cation exchange capacity of the clay minerals. The transport of charge carriers in the electrochemical double layer (DL) is thus in the Stern layer and the diffuse part of the double layer significantly different, whereas the contributions of the two conduction processes to the total conductivity are determined by chemical parameters and pore-specific petrophysical properties such as porosity, permeability, inner surface, pore radius distribution, etc.. A detailed knowledge of the interactions of these parameters with each other are essential to establish common model concepts, which would allow to assign the electrical transport parameters to measured SIP signatures. The problem to interpret the previously performed SIP measurements that were performed on different types of sandstones, is that the fundamental electrochemical and petrophysical parameters were determined very precise, but due to the different pore geometries of the sandstones a general correlation with the permeability could not be created. Thus various groups interpreted the SIP spectra using different model approaches. These were tested with respect to a general informational value, however, a general relationship could not be established so far. But measurement of the pressure dependence of the SIP can contribute to solve this fundamental problem, since the total chemistry of the system rock/pore-fluid will be constant in these measurements, while variables are the pressure induced changes in porosity and permeability, especially the degree of interconnection of the pore system. Consequently only the contributions of the charge transport in the Stern- and the diffuse part of the electrochemical double layer (EDL) will be changed. Thus the change in the imaginary part of the complex conductivity (quadrature component) of the SIP measurement can then be correlated with the independently measured change of the pressure-dependent permeability.