Estimation of Geothermal Brine Viscosity

Abstract

Summary Viscosity of synthetic brines consisting of sodium chloride, potassium chloride. and calcium chloride were measured at concentrations ranging from 0.99 to 16.667 wt% and at temperatures up to 275 degrees C. From the use of laboratory-derived data, a method is presented whereby the viscosity of a geothermal brine may be estimated from a knowledge of its composition. Introduction Many theoretical studies published on reservoir and production characteristics of liquid-dominated geothermal systems have used fluid properties representative of distilled water. Presence of dissolved ions in water under the combined effect of temperature and pressure causes the reservoir flow properties to deviate from those of distilled water. These differences are because of water/rock interaction and fluid mobility. The aspect under consideration in this paper is the fluid mobility as affected by its viscosity. Because of the gradation of fluid temperature from the bulk of the formation to lower temperatures around producing or injection wells and to even the lowest at the wellhead, no uniform temperature or fluid viscosity should be used in wellbore productivity or flowline computations. Data on temperature dependence of the dynamic viscosity for sodium chloride solutions have been published by several authors. A very comprehensive literature review on this subject was published by Ozbek et al. in 1977. They developed an empirical correlation to relate the dynamic viscosity of sodium chloride solution with temperature and molal concentration. Data used were mainly from Korosi and Fabuss, and Kestin et al. The correlation is valid to 150 degrees C and cannot be extrapolated to higher temperatures. Numbere et al. developed a correlation for viscosity of sodium chloride solutions using the Fabuss and Korosi data. The correlation is based on the ratio of brine viscosity to water viscosity as a function of temperature and molal concentration. Potter used the concept of equivalent water temperature to relate the viscosity of NaCl solutions to water. He indicated that the correlation may be used for extrapolation purposes up to 325 degrees C. Potter and Haas indicated that geothermal fluids may be represented by the properties of NaCl solution. Studies conducted on the composition of dissolved ions in geothermal fluids indicate considerable variations from one area to the other. In general, geothermal brines are chlorine rich, with clements Na, K, and Ca being the dominant ions. The purpose of this study was to measure the dynamic viscosities of NaC1, KC1, and CaC12 solutions and to study the effect of elevated temperatures on viscosity at various levels of dissolved ion concentrations. Furthermore, we intended to develop a procedure for estimating the dynamic viscosity of a given geothermal fluid once the chemical composition has been determined. Finally. we compared the measured values of viscosities for mixed brines with estimated values using the properties of NaCl solution. Experimental Procedure Viscosity measurements were conducted using a high temperature tube designed to operate up to a temperature of 315 degrees C and a pressure of 14 MPa. Fig. 1 shows a schematic of the experimental apparatus. Flow in the capillary tube with a length of 30.48 cm was monitored by measuring the pressure differential across the tube using a Gould Statham differential pressure transducer (PC 2002-100-11) JPT P. 621^