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Unable to Speciate a Water Sample at Reservoir Conditions

Bob C

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I would appreciate any guidance on the this.  

  1.  I have mixed two different waters (1/1) under atmospheric conditions; the first is highly saline and the second is less saline.  
  2.  I would then like to speciate the mixed water (above) under reservoir conditions - pressure of 3500kPa. Methane comprises more than 97% of the gas phase in the reservoir.
  3.  When I add CH4 under "Basis" at 3.5MPa (3500kPa), and run the speciation program, the program informs that O2 (aq) needs to be constrained in the basis entry for fluid "initial".
  4.   When I add 0.001 mg/L O2(aq), the program fails to converge after 999 iterations.
  5.   Any thoughts on what needs to be done to get the program to converge?




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Hello Bob,

Thanks for attaching your input file. I noticed a few things in your input file that might be the problem. Here are a couple of suggestions that might help.

Methane is not one of the basis species available in the thermo.tdat dataset. You can swap it into the basis pane as you have done so by swapping CH4(g) for H+. However, in doing so, you are setting the reaction H+ + H2O + HCO3- = CH4(g) + 2 O2(aq) to fix pH. I am not sure if this is what you want to do. If you wish to set both bicarbonate and methane gas concentrations in your initial system, I would suggest you engage disequilibrium by decoupling CH4(aq) and HCO3- in the Redox Couples dialog. By disengaging equilibrium between HCO3- and CH4(aq), you can enter the species of CH4(aq) as a basis species into your initial condition, which then you can swap for CH4(g). This would also eliminate the need to have to enter O2(aq) as part of your basis constraint. 

Another thing to consider is the value used for the partial pressure of the methane in your basis pane. The value entered should the partial pressure of CH4(g) in your reservoir and not the confining pressure. For more information, please refer to section 7.5 Gas Partial Pressure in the GWB Essentials Guide for more information. Currently, you cannot fix the confining pressure of the system in Speciate.

Hope this helps.

Best Regards,

Jia Wang

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