Flow in fractured and permeable media: impedance spectroscopy as a probe for flow characterisation and saline breakthrough mechanisms

Recent experiments have been undertaken using AC impedance spectroscopy as a probe for changes in rock properties and saline breakthrough curves. In impedance spectroscopy a fixed frequency sinusoidal drive voltage is applied across a sample and the amplitude and phase-lag of the resultant sinusoidal current is measured. Using measurements made across a range of frequencies enables the electrical characterisation of a sample in terms of a simple RCL network model. Thus if a sample is flooded with a solution of known ionic strength, the connectivity and apparent fluid cross-sectional area can be derived from the equivalent DC resistance value, R. We have conducted steady-state flow experiments with simultaneous impedance measurements on 50-90mm long cylindrical samples (38mm diameter) of chalk and micro-fractured granite. In these flow experiments we have introduced of a step concentration change in saline strength at the inflow end of the sample and examined the impedance response during the expected evolution of this initial step function into a classical advection-dispersion profile during flow down the sample. We present a simple mathematical model for the deconvolution of the impedance evolution with time in terms of the fundamental parameters of aperture, flow rate, and advection-dispersion .We apply this model to data for flow through porous chalk and microfractured granite media. Results for the fractured medium, when compared to existing studies of flow in porous sandstones, reveal that at low Peclet numbers even a very fine-scale isotropic microfracture network is significantly less dispersive than the equivalent porous medium.