Laboratory study of effect of changing water salinity on spectral induced polarization response of sandstone and sand samples

Summary: In this study, the influence of pore fluid salinity on the complex electrical conductivity responses of sandstone and sand samples is analyzed. The investigated samples in this study include clean sandstone and sand samples provided by an Iranian offshore oil company and sandstone samples containing clay minerals that have taken from a sandstone aquifer located in northwest of England. All samples have been saturated with sodium chloride solution. The fluid electrical conductivity ( ) of sodium chloride solution has gradually been increased from 57 mS/m to 6000 mS/m. The expected linear relation between and the real component of electrical conductivity ( ) of the saturated samples has been observed. It is also observed that the imaginary component ( ) increases with the increase of salinity, and consequently, the increase of fluid conductivity. To determine the relaxation time ( ) of spectral induced polarization (SIP) data, ColeCole model has been fitted on the measurements carried out at four different salinities of sandstone aquifer samples. The Cole-Cole relaxation time increases with the increase of fluid conductivity for all samples except two samples. The behavior of is also comparable to as both parameters measure the polarizability. In other words, normalized chargeability also increases with the increase of fluid salinity. In this study, the dependence of polarizability on fluid salinity is also demonstrated. Maximum values of polarizability are observed at high salinity for quartz dominated siliceous material. Introduction: The induced polarization (IP) geophysical method measures the low-frequency electrical properties of rocks and soil materials. This method has appeared as a potentially powerful tool for subsurface imaging due to the dependence of the measurements on rock or soil internal surface area. Given the increasing interest in using IP measurements to deduce rock textural properties such as permeability, further studies of the dependence of IP measurements on pore fluid composition are needed. In SIP, a phase lag between the current and the electrical field provides complementary information to electrical conductivity measurements. The conductivity and the phase can be rewritten into a complex conductivity (or a complex resistivity) that can be measured over a broad range of frequencies, typically from 1 mHz to few tens of kHz in the laboratory and from 10 mHz to 10– 100 Hz in the field. Methodology and Approaches: SIP measurements were collected in a fully saturated state for each sample at four different salinities. All samples were saturated with a NaCl solution starting with the lowest salinity and then fluid electrical conductivity ( ) of sodium chloride solution was gradually increased from 57 mS/m to 6000 mS/m. The electrical impedance was measured using a four-electrode array. Two silver coils were used as current electrodes to inject an alternating sinusoidal current. The voltage and the phase shift between the applied current and measured voltage was determined using Ag-AgCl nonpolarized potential electrodes. Electrical measurements were taken using the ZEL-SIP04-V02 impedance meter in a frequency range of 2 mHz to 45 kHz (Forschungszentrum Julich GmbH). Results and Conclusions: Our investigation has shown that the fluid chemistry is an important parameter that controls the polarizability of the mineral-fluid interface. Real and imaginary components of conductivity increase with the increase of saturating fluid conductivity. It has also been demonstrated that normalized chargeability show a positive power law relationship with fluid conductivity. The results of Cole-Cole model fitted on data have indicated that relaxation time increases with salinity for all samples except two samples. We have also shown that polarizability parameters are controlled by fluid conductivity.