# Laboratory Measurements of Zeta Potential in Fractured
###### Abstract
More recently, alternative, low-cost surface geophysical methods, such as self-potential (SP), have shown promise to better quantify fracture flow, including anisotropy, in crystalline rock aquifers (e.g., [19; 20; 21]). SP arises in response to pressure concentration or temperature gradients associated with the motion of water or aqueous solutions [22]; therefore, any signal acquired from electrode arrays reflects the dynamic behavior of the effective aquifer pore space, i.e., hydraulically active fractures in fractured rock aquifers. The method is passive and can be applied using surface arrays installed into the overlying soil layer (e.g., [23]) or discrete observation boreholes equipped with electrodes (e.g., [24]) and, therefore, provide information at both high spatial and temporal resolution. A numerical study [25] investigated the potential application of SP to characterize fractured systems, and the authors demonstrated that the simulated SP signal not only accurately predicted preferential orientation of hydraulically active fractures but also identified fracture-matrix fluid exchange. The reported results indicated that the SP signal was primarily sensitive to hydraulically active fractures having matrix-fracture fluid exchange, thus implying that a non-zero porosity and near-zero permeability should be assigned to the matrix, which accommodated a significant volume of water but did not contribute significantly to the flow, and would be required to characterize the flow using SP. Assuming a strictly impervious matrix (zero porosity and zero permeability), resulted in a negligibly small electrokinetic signal. However, the authors assumed their fracture apertures to range between 0.2 mm and 2 mm, thus implying near-zero pressure gradients along the fractures and, therefore, a negligible SP response. In real fractured crystalline rocks, the aperture is expected to be a few orders of magnitude smaller (as discussed below); hence, the SP signal in such systems would be measurable even without matrix-fracture fluid exchange, thus, making the SP method applicable for such systems. In support of our hypothesis, a field SP survey on a crystalline bedrock [20] clearly demonstrated that azimuthal SP gradients tightly correlated with the investigated fracture strike orientation even though the matrix of the studied field site was impermeable and non-porous, consistent with the nature of crystalline bedrocks. Moreover, azimuthal resistivity surveys conducted on the same site showed the expected orientation of high water content, but were completely misaligned with the measured fracture network, thus making the SP method superior for identifying hydraulically active fractures, essential for accurate characterization of groundwater storage and flow. Other published studies used the SP method to characterize and predict temporal variation of head gradients in a fractured aquifer as the results of diurnal tide cycles [26], and also to characterize saltwater intrusion into freshwater chalk aquifers (e.g., [24, 27]). These studies demonstrated that the flow of freshwater and/or seawater through fractures was responsible for observed temporal and spatial variation of the SP measured in an observation borehole, even though the matrix was nearly impermeable. Moreover, the observed SP precursor that preceded seawater breakthrough was demonstrated to result from spatial variability of the so-called exclusion efficiency that was primarily associated with existing anisotropy in mineralogy and permeability of the aquifer. Therefore, not only was the SP method proven to be applicable to fractured systems, but it was also shown to rely on the system's anisotropies to characterize dynamic behavior of flows. Another modelling study on heterogeneous hydrocarbon reservoirs ([PERSON] et al., 2012a) during oil production showed that the SP measured in the production well would successfully predict the shape of the waterfront encroaching on the well, even when it was hundreds of meters away from the well.