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[OLD] Calculations at pressures in excess of steam saturatio


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From: Ross McCartney

Subject: Calculations at pressures in excess of steam saturation pressures

I have React release 3.1.2 and would like to undertake simple equilibrium calculations on some sedimentary formation water analyses (TDS < 20000 mg/l). Reservoir conditions are: T ~ 100oC and pressure ~ 400 bar. I only have analyses for major ions and I am only interested in determining whether equilibrium exists with a few minerals. Ideally, I'd like to modify 'thermo.com.v8.r6+.dat' so that (a) it contains a subset of constituents/species/minerals already present and (B) so that the thermodynamic data are appropriate for the reservoir conditions. Any tips/guidance that anyone can offer will be greatly appreciated. I already have log K values for the species/minerals of interest at 0, 25, 60, 100, 150, 200, 250 and 300oC and 400 bar from SUPCRT92.

 

From: Ross McCartney

Subject: Re: Calculations at pressures in excess of steam saturation pressures

I recently posted a message (see above). Based on the replies I received, the advice was....

1. Substitute the log K values for those I have obtained from SUPCRT (ensuring that the reaction conventions in thermo.com.v8.r6+.dat are followed).

2. Change the numbers in the headings in each section (xx basis species, etc).

I didn't receive any advice regarding the first section of the thermo.com.v8.r6+.dat data set and would be grateful for your further input:

a) I tried modifying the 'pressures' in thermo.com.v8.r6+.dat to my pressure of interest (400 atm) but the pressure indicated on the React output was still the steam saturation pressure. Where are the pressure data used in React and is React using my input pressure or not?

B) If I was to input data for 500 atm is there likely to be a problem? I ask this because 500.000 appears to be a flag to indicate no data in the dataset.

c) Are the debye-huckel A and B, b-dot, c CO2, and c H2O terms pressure dependent? If so, are there any references that indicate how I can calculate these values for pressures other than those on the steam saturation curve.

d) Are there any other bits of advice you can offer on the use of React for higher pressure calculations?

 

From: Craig Bethke

Subject: Re: Calculations at pressures in excess of steam saturation pressures

a) I tried modifying the 'pressures' in thermo.com.v8.r6+.dat to my pressure of interest (400 atm) but the pressure indicated on the React output was still the steam saturation pressure. Where are the pressure data used in React and is React using my input pressure or not? The value you set for pressure in the thermo dataset has no actual effect on the calculations, which use the log Ks as specified. For some reason React seems to indicate a pressure of 1 atm when temperature is 100°C or below, but you can ignore this. We will investigate.

B) If I was to input data for 500 atm is there likely to be a problem? I ask this because 500.000 appears to be a flag to indicate no data in the dataset. You can safely set a value of 500 for parameters in the header section.To check this yourself, set a value of 499.9999 and see if your results differ from when you set a value of 500.

c) Are the debye-huckel A and B, b-dot, c CO2, and c H2O terms pressure dependent? If so, are there any references that indicate how I can calculate these values for pressures other than those on the steam saturation curve. Pressure affects both the ideal and nonideal components of the free energy change of reaction, so to be complete you would need to adjust both log K values and activity coefficients. Lewis and Randall, Ch.20, for example, give the relationship for the pressure dependence of the activity coefficient.

Theory aside, however, I don't know of any practical advice for making this correction, which most people simply ignore.

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