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total concentration of sulfate increasing over time

Hello,

I ran a chemical initialization simulation for 10 million years to achieve steady state. In order to figure out how much dissolution and precipitation I have in my system, I have to figure out what happened to the concentration of some of my species. 

Let's consider anhydrite (CaSO4). If I want to figure out how much dissolution/precipitation I have in my system, I do the following calculation: 

Mass of SO4(2-)   =   volume of the block * the porosity of the block * the water saturation in the block * the concentration of SO4(2-) in g/m3 in the block. 

Now, when I look at the total concentration of my SO4(2-) far away from my well (where CO2 has not reached), I expect that the concentration stays the same. But it increases! 

I was wondering why this is the case?

I have applied large volumes to the outermost (eastern) boundary. I am injecting CO2 into a formation that has dolomite, anhydrite, and calcite at the innermost (western) boundary. 

Would really appreciate the community's response.

Majid 

1 reply

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    • TOUGHREACT Developer
    • Eric_Sonnenthal
    • 6 mths ago
    • Reported - view

    Hi Majid,

    Quickly looking at your files, I don't see a flow steady-state simulation? First do a flow-steady state with the correct pressure and temperature gradients. I see that XCO2=0 in your INCON. XCO2 should be equivalent to that calculated from the total HCO3- molality in chemical.inp. After running the flow steady state, then you can add the chemical input files and run a THC "steady-state". It's usually not a true steady-state, just a solution that is changing only slightly over long time periods. However, for chemistry the timestep cannot be allowed to increase indefinitely. First, set RCOUR=0.5 in solute.inp. Then set the maximum timestep in flow.inp to something probably 1 year or less. That really depends on how reactive the system is. You may have to reduce the reactive surface areas by a few orders of magnitude, run it, and then restart with higher surface areas. Once you have a smooth chemical system and steady-state flow, then the injection can be done. Here the maximum timestep could be very small because injecting CO2 often causes dryout and faster reactions. For CO2 injection, ico2h20 should be set to 3 to enable the coupling of CO2 and H2O generated/sequestered in chemistry to feedback to the mass balance in flow, and vice-versa. These problems can be very challenging, so if the injection rate is high, the timestep may have to be quite small and even a relatively small problem may take several hours or days to run. But having a small timestep is usually necessary because even if it runs with a large timestep the results may have a lot of mass balance error which builds up over many time steps.

    good luck!

    Eric

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