Electrical Resistivity of Geothermal Brines

Abstract

Introduction The electrical resistivity of dilute aqueous salt solutions has been studied for a number of years, but very few data exist on concentrations above 0.1 molar. Normal groundwaters commonly are near 0.1 molar, while most geothermal and oilfield fluids are at least several molar (Table 1). Thus, the interpretation of electrical measurements in geothermal areas at present is based mainly on extrapolation of lower temperature and lower concentration data. Such extrapolation may introduce serious errors into the interpretation of geothermal reservoir characteristics determined from electrical measurements. This paper presents new experimental data and an improved descriptive model of the electrical properties of brines as a function of temperature properties of brines as a function of temperature from 22 to 375 deg. C and concentration from roughly 3 to 26 wt% while under 31 MPa hydrostatic pressure. Data and models are given for brines composed of the chlorides of sodium, calcium, and potassium, and their mixtures. Comparison of the older log interpretation formulas to the new models illustrates an order of magnitude improvement in accuracy with an overall fit to within 2%. Resistivity Dependence Upon Temperature Some researchers have postulated that the electrical resistivity of fluid saturated rocks follows the temperature dependence of the saturating fluid in the absence of conducting minerals or significant surface conduction along altered pore walls. This assumption resulted from the success of a simple empirical formula relating the resistivity of a rock to the resistivity of the fluid filling the pores of the rock: Pr=FPw, where Pr= resistivity of clay-free, nonshale material that is 100% saturated, Pw= resistivity of saturating solution, and F= formation resistivity factor. A number of investigators have derived formulas that add the temperature of the saturating fluids. Experimental observations have shown that some rocks obey these formulas while others do not. 17–19 Part of the problem is the inadequate knowledge of Part of the problem is the inadequate knowledge of the resistivity dependence on temperature for the solution that fills the rock pores. We have found empirically that the best fit of the resistivity data to temperature is pw=bo+b1T-1 +b2T+b3T2+b4T3, where T is temperature and coefficients b are found empirically.