AMSwift

This post contains what I currently know (actually, just *think* I know) about handling elevated pressure. That's not very much. I'd be very grateful indeed for corrections and additions on this topic. GWB doesn't adjust activity coefficients for pressure. All system are assumed to be either 1 bar or at the vapor pressure of water. If you're dealing with hydrothermal systems, say, this can be a bit of a problem. If I want to include pressure in my GWB models, then the choices I'm aware of are: 1. Read "K2GWB: Utility for generating thermodynamic data files for The Geochemist's Workbench® at 0–1000 °C and 1–5000 bar from UT2K and the UNITHERM database" by Cleverley and Bastrakov at http://www.sciencedirect.com/science/article/pii/S0098300405000105. Contact Dr. Evgeniy Bastrakov and request UT2K. He will supply UT2K, but you'll have to purchase HCh (which does the actual Gibbs free energy work required) and UNITERM (the database). Cost is Australian $737 Product page. The HCh manuals are here:at Dr. Yuri Shvarov's (the author) site.I'm currently struggling to make this work for all species and minerals relevant to my application, so will NOT claim that this is a complete fix. More if I get results. 2. Use one of several competing software packages, PHREEQC/PHAST, TOUGHREACT, EQ3/6, or several others. Do you have a better (or at least easier) idea?

Thanks a lot, Brian. I do believe that's problem solved. Now, on to bigger challenges!

A fugacity of 10 does indeed solve the pH problem, but (unless I'm mistaken), the paper gives fugacities in MPa and the software wants them in bars. To get from one to the other, you multiply by (roughly) 10. So, in order to reproduce 10 MPa, we have to input 100 bars into the software. Please advise if this is mistaken!

The paper is at: http://webpages.fc.ul.pt/~fbarriga/ZeroEm/Bibliografia_files/Zerai+al_2006_ApplGeochem.pdf (I hope this link will work for everyone) Title is Computer simulation of CO2 trapped through mineral precipitation in the Rose Run Sandstone, Ohio My current script is "Rose Run brine, react with CO2.rea". It yields a pH of just under 3 on my machine. An alternative script is "Rose Run brine, react with CO2 (alt f-f).rea". CO2(g) is swapped for HCO3-, and an fugacity of 8E-3 assigned (as per paper). It yields a pH of about 3.3 A second alternative is "Rose Run brine, react with CO2 (alt a-f, no balance).rea". CO2(aq) was swapped for HCO3- and an activity assigned. Change balance had to be turned off for the script to run - all other methods yielded residuals. It yields a pH of just under 4. Rose Run brine, react with CO2.rea Rose Run brine, react with CO2 (alt f-f).rea Rose Run brine, react with CO2 (alt a-f, no balance).rea

I'm attempting to reproduce in GWB 8.0.12, program React, the results of a paper (Zerai et. al., 2005) that used GWB 3.2.2. In the paper, a particular Na-Cl (Ca) brine of ionic strength ~7, initially at a pH of 6.4, is brought into contact with CO2(g) at a fugacity of 100 bars, at 54 degrees C. It absorbs 0.7 mol of CO2(aq) and reaches a pH of approximately 4.25. I get the same molality of CO2(aq), but a pH of 2.9 - 3.0. Having tried everything I know of within the software to reconcile the two pHs, I'm here to ask for help. Setup details: Initial brine composition and pH is input as the basis. CO2(g) is added as a reactant, fugacity sliding to 100 bars. Using the B-dot model and thermo.dat. Model limit on ionic strength (by default, 3) removed. Diameter of CO2(g) set to -0.5 A, as per instructions on simulating the salting-out effect. All other variables reset to default values. Any suggestions on where to look for differences?