Laboratory Methods For Relating Electrical And Hydraulic Properties Of Shaly Sandstones: Example Of The Sherwood Sandstone Aquifer (Triassic, England)

We highlight recent developments in laboratory electrical<br>properties methods in the domains of time and<br>frequency, with particular attention to quality control.<br>We present data from Sherwood Sandstone (Triassic,<br>England), an important shaly sand aquifer.<br>Electrical Impedance Spectra (EIS) were<br>collected over the range 1 mHz to 1 MHz, for nominally<br>fresh water (conductivity 0.02 S/m), 0.1 M<br>NaCl brine (0.98 S/m) and 1.0 M brine (7.6 S/m).<br>Dispersion of real and imaginary parts of the rock<br>conductivity reveal significant low frequency polarizations.<br>Induced polarization data in the time domain<br>using stepped pulses in current and in potential yield<br>d.c. resistivity, capacitance and integrated chargeability.<br>Polarization / chargeability is confirmed to be<br>much stronger in fresh water, decreasing markedly<br>with salinity. We compare the response of 2 electrode<br>and 4 electrode methods, and identify pitfalls that can<br>arise from incorrect sample and electrode preparation.<br>Permeability measurements made at constant<br>flow rate and with a constant head show that hydraulic<br>conductivity decreases over time, by up to one<br>order of magnitude. We ascribe this to structural<br>changes in the clays. These time-dependent effects,<br>which are quite typical of shaly sands, obscure any<br>simple relationship between permeability and salinity.<br>Finally, we report some preliminary data on streaming<br>potential measurements acquired while the 0.1 M<br>NaCl brine was passed though the sample.