Time Dependent Electrical Properties of Rock-Solution Systems

Abstract

Abstract The electrical conductivity of 25.4-mm diameter cylindrical cores of sedimentary and igneous rocks was measured at a frequency of 1 kHz under ambient conditions after saturation in tap water and in 0.1 M NaCl solution. Between measurements, each rock was stored in contact with its own separate solution that at the start of the experiment was either tap water or O. 1 M NaCl solution. Eighteen rocks having porosities between 0.07 and 42 percent were investigated; ten were sedimentary, eight were igneous. All solution conductivities were measured employing a dip cell. The time dependence of the conductivity was investigated as well as the range of validity of Archie's law. The formation factor and conductivity of rock specimens and their reacting solutions display time-dependent behavior up to 8,000 hours. The formation factor and electrical conductivity depend on the salinity of the pore fluid, the rock porosity and permeability, and the clay minerals present in the rock. INTRODUCTION We are currently investigating rock-solution interaction and its effect on electrical properties of rocks as a function of time, porosity, pore solution concentration, frequency, specimen aspect ratio, and Soxhlet extraction procedures. This work is part of a program to construct a laboratory data-base as an aid in the interpretation of field geoelectrical data. We1 previously observed that the formation factor and the electrical conductivity of the Pictured Cliffs sandstone (permeability range, 7Ã?10â?'4â?'4Ã?10â?'1 md; porosity range, 3.8 to 4.9 percent) display time-dependent behavior when the sandstone is saturated in tap water. Pretreatment of the rock by Soxhlet extraction lowers the formation factor, F, but does not significantly change the time dependence. Also, Duba et al.2 reported on the Ï? of cylindrical cores of Westerly granite; Nugget, St. Peter, and Kayenta sandstones; and Indiana limestone having aspect ratios (length/diameter) from 2.00 to 0.25. Before measurement, the cores were saturated in distilled water, tap water, and in 0.1 M NaCl solution. Aspect ratio affects least the conductivity of rocks saturated in 0.1 M NaCl; for these rocks a linear relationship exists between log Ï? and log Ï?, commonly termed 'Archie's law.' However, no simple correlation was found between log Ï? and log Ï? for rocks saturated in tap or distilled water. This paper is a progress report on the Ï? and F as a function of time up to 104 hours for 18 rocks saturated in tap water or in 0.1 M NaCl solution at 100 kPa pressure.