Experimental clay characterization using spectral induced polarization

Abstract:

Clay formations are ubiquitous in the Earth’s subsurface and their study is of importance for many geo-engineering applications such as CO₂ and nuclear waste geological storage. Clay characterization is traditionally done through invasive and costly geochemical and hydrogeological methods that give rather local information and hence may lack the heterogeneity and complexity of clay formations in the subsurface. In our study, we use the spectral induced polarization (SIP) geophysical method to non-intrusively characterize clay muds at the laboratory by measuring their complex conductivity in the mHz to 20 kHz frequency range. We measured the SIP response of 4 different clay samples (red and green montmorillonite, kaolinite, and illite) for five different salinities (initially de-ionized water, 10^-3, 10^-2, 10^-1, and 1 mol/L NaCl). For all the clay samples, we observe an increase of the real part of the complex conductivity with increasing salinity. However, for the imaginary part and at a frequency of 1.46 Hz, we see a maximum at a particular salinity, and then a decrease with higher salinity values. We observe a peak of the polarization for the montmorillonites at a salinity in the 10^-1 mol/L NaCl range, but for both kaolinite and illite, we observe this peak in the 10^-2 mol/L NaCl salinity range. We interpret this as a salinity saturation of the electrical double layer polarization mechanism. We used a double-Pelton model to fit our data with: one describes a mid-frequency (~1 Hz) polarization peak, and the other describes a high frequency (>1 kHz) polarization peak (that rather describes capacitive noise and clay polarization). We also observe an overall decrease of the chargeability for the mid-frequency imaginary conductivity peak with salinity because conduction increases more quickly than polarization with salinity. Further petrophysical modeling needs to be done to better interpret our dataset with respect to individual polarization mechanisms.