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AMSwift

CO2 under pressure; problems with pH

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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?

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Hi, could you attach a script of yours and the paper you reference so I can compare?

Thanks,

Brian Farrell

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

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Hi,

I was able to get a pH of about 4.4 using a fucagity of 10, so it's a little closer. Scanning through the paper it looks like the equilibrium calculations were only simulated at discrete points (fugacity = 10, 8, 6, ...) and then the mineral assemblages were titrated in. So I constrained the CO2(g) fugacity directly at the beginning and then added in minerals. It seems to be closer to the paper results, except at very low fugacity. You might try contacting the authors to see if they could provide the exact script they used. Unfortunately I couldn't find any sort of data repository with such information. Best of luck.

Brian

React_CO2.txt.rea

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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!

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You are correct. Their values are in MPa, so a conversion of roughly 10 is needed. By sliding the f CO2(g) to 20, 40, 60, 80, and 100 I get pH values of 4.60, 4.45, 4.36, 4.29, and 4.25, respectively, which look to be in agreement with the results of the paper. Basically your basis is the water analysis reported in Table 3, including HCO3-. On the Reactants pane, you set a sliding f CO2(g) of 20, for example, and titrate in Dolomite, Calcite, and Siderite as simple minerals. Attached is a working script, all you need is to change the CO2(g) fugacity.

Hope this helps,

Brian

React_CO2.txt-1.rea

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You are correct. Their values are in MPa, so a conversion of roughly 10 is needed. By sliding the f CO2(g) to 20, 40, 60, 80, and 100 I get pH values of 4.60, 4.45, 4.36, 4.29, and 4.25, respectively, which look to be in agreement with the results of the paper. Basically your basis is the water analysis reported in Table 3, including HCO3-. On the Reactants pane, you set a sliding f CO2(g) of 20, for example, and titrate in Dolomite, Calcite, and Siderite as simple minerals. Attached is a working script, all you need is to change the CO2(g) fugacity.

Hope this helps,

Brian

Thanks a lot, Brian. I do believe that's problem solved.

Now, on to bigger challenges!

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