The wetting condition of reservoir rocks is a crucial parameter for the estimation of reservoir characteristics like permeability or residual saturation.Since standard methods are often costly,especially in terms of time,we aim at assessing wettability of reservoir rocks using impedance spectroscopy(IS),a frequency dependent measurement of complex electric resistivity. IS is sensitive to the electrochemical properties of the inner surface of rocks.These electrochemical properties on the other hand,are decisively influencing wettability. Unfortunately,besides wettability there are other parameters(e.g.pore/grain size) influencing the impedance spectra of rocks.To be able to quantify the relative importance of parameters influencing rock IS response,we study model systems which consist of sintered porous silica beads of different sizes leading to samples with different pore sizes. The main advantage of using model system compared to natural rocks is its well-defined and uniform mineralogical composition and thus uniform electrochemical surface properties. In order to distinguish pore geometry,fluid electrochemistry and wettability(contact angle) effects on the IS properties we measured the IS response of the fully water saturated model systems in the frequency range from 1 mHz to 1kHz.The influence of wettability was studied by modifying the originally hydrophilic silica beads surface into a hydrophobic state.The wettability change was verified by contact angle measurements. As results,we find pore size dependent relaxation times and salinity dependent chargeabilities for the hydrophilic samples,whereas for the hydrophobic samples chargeabilities are close to zero and independent of pore size and water salinity.
展开▼
