Experimental and theoretical study of the electrical conductivity, NMR properties under different salinities and the membrane potential of shaly sandstone

Abstract

Although the salinity of formation water determines a list of important petrophysical parameters in reservoir rocks, little work on the salinity effects on the bound water saturation and the consequent effects on the electrical and nuclear magnetic resonance (NMR) characteristics has been done. In this study, the measurements of the bound water saturation, electrical conductivity and NMR responses of rocks with different salinities have been conducted. The experimental results indicate that the bound water saturation of shaly sandstone decreases with an increase in the salinity. The electrical conductivity of rock with low salinity formation water may be high because of high bound water saturation and, additionally, clay content. The transverse relaxation time (T2) spectrum distribution changes with the salinity of solution. The cutoff value of the T2 decreases with the increase of the solution salinity. The property of membrane potential in shaly sands is critical in determining the resistivity of formation water (Rw) using a spontaneous potential (SP) log. In order to understand the influences of cation exchange capacity, hydrocarbon saturation and salinity on membrane potential, a systematical study on the membrane potential of oil-bearing shaly sands has been conducted based on electrochemical theory and the electrical conductivity properties of shaly sands. Laboratory experiments were designed and carried out. The experimental results show that the membrane potential of shaly sands increases with the increase of cation exchange capacity, the hydrocarbon saturation and the salinity difference of the solutions. The developed membrane theory in shaly sands should improve SP log interpretation. If using SP to predict Rw, we suggest that the influences of hydrocarbon saturation and shale content should be taken into account.