morgan_warren Posted October 14, 2016 Share Posted October 14, 2016 Hi: Can you please provide the equations and reference that GWB uses to calculate the density of a solution from TDS and temperature? I'm trying to understand how GWB includes these variables in the calculation. Additionally, does the calculation change if the solution is not dominantly Na-Cl, but is, say, dominantly Ca-SO4 or Mg-SO4? Thanks! Morgan Quote Link to comment Share on other sites More sharing options...
Brian Farrell Posted October 17, 2016 Share Posted October 17, 2016 Hi Morgan, Density is calculated from a correlation developed by Phillips et al. 1981 for NaCl solutions. The correlation uses the temperature, pressure (which corresponds to the principal temperatures in the thermo dataset), and salinity of the fluid. The program normally figures density as that of an NaCl solution with the same TDS (which it calculates by adding the masses of solutes) as the fluid in question, at the temperature of interest. With the command “density = chlorinity” you can tell the program to instead use the density of an NaCl solution of equivalent chlorinity. Please see 5.15, density, in the GWB Reference Manual for more information. Phillips, S. L., A. Igbene, J. A. Fair and H. Ozbek, 1981, A technical databook for geothermal energy utilization. Lawrence Berkeley Laboratory Report LBL-12810, 46 p. Hope this helps, Brian Farrell Aqueous Solutions LLC Quote Link to comment Share on other sites More sharing options...
tiziano Posted May 28, 2020 Share Posted May 28, 2020 Hi, I am using GWB 12.0.6. These are the results obtained by Spec8: Temperature = 13.9 C Pressure = 1.013 bars pH = 7.997 Ionic strength = 0.005059 molal Charge imbalance = -0.000267 eq/kg (-3.873% error) Activity of water = 0.999995 Solvent mass = 1.0000 kg Solution mass = 1.0003 kg Mineral mass = 0.00000 kg Solution density = 1.021 g/cm3 Solution viscosity = 0.012 poise Chlorinity = 0.000132 molal Dissolved solids = 291 mg/kg sol'n Elect. conductivity = 326.08 uS/cm (or umho/cm) Hardness = 164.64 mg/kg sol'n as CaCO3 carbonate = 164.64 mg/kg sol'n as CaCO3 non-carbonate = 0.00 mg/kg sol'n as CaCO3 Carbonate alkalinity= 172.96 mg/kg sol'n as CaCO3 Water type = Ca-HCO3 Bulk volume = 980. cm3 Fluid volume = 980. cm3 Mineral volume = 0.000 cm3 Inert volume = 0.000 cm3 Porosity = 100. % Permeability = 98.7 cm2 Water is fresh (TDS = 291 mg/kg sol'n), so I am very surprised to obtain a calculated density of 1.021 g/cm3. This latter value is typical of seawater or an NaCl brine. Indeed, using PHREEQC and density calculation by Pizer.dat dataset, I have obtained Density (g/cm³) = 0.99950. Thank you. Quote Link to comment Share on other sites More sharing options...
Brian Farrell Posted May 28, 2020 Share Posted May 28, 2020 Hello, Density of electrolyte solutions is not calculated in most geochemical modeling programs. It's secondary in importance to species distributions, saturation indices, and so on, but it's our opinion that a simple calculation is better than nothing. The Phillips correlation described above, which is the default method for GWB12 and older releases, was designed for geothermal applications, so it works best at higher temperatures and salinities. The correlation is known to be valid for NaCl solutions from 10 < T < 350 °C, 0.25 < m < 5 molal, and P < 50 MPa and greater than the fluid’s vapor pressure. Your fluid is outside (below) the range of valid salinities and very close to the lower T limit. GWB14 by default uses the Batzle-Wang method (Batzle, M. and Z. Wang, 1992, Seismic properties of pore fluids. Geophysics 57, 1396–1408) for calculating density. It was fit over the range 20 < T < 350 °C and 0 < m < 8 molal, so it works better over a range of conditions. I checked a NaCl fluid of the same ionic strength and temperature as your fluid in GWB14.0.1 and calculated a density of 0.998 g/cm3. PHREEQC's method is quite new. It requires a large number of parameters, but it looks interesting. Regards, Brian Farrell Aqueous Solutions Quote Link to comment Share on other sites More sharing options...
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