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Mathew

rock volume% versus H2O volume% in X2t

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I am having trouble matching the rock volume% versus H2O volume% in X2t. I have tried various combos of putting minerals as 'Reactants' in vol% and as weights (kg) proportional to 1 kg H2O, or as swapped species in the 'Initial' tab. In the 'Initial' tab I tried to use 1.) mol/kg picked-up from 'React' 1D models and 2.) as kg proportional to kg H2O]. When I compare the final vol% H2O versus mineral vol% it is ~ 98:2 depending on how I input the minerals. I am trying to get 92% vol% rock + 8% porosity in the output. Also, the 'Rock Mass' in wt% around 3000% in all models but does decreaase around the injection well corresponding to mineral dissolution and ppt. I set the porosity 8% and that is responding realistically. Could the mineral vol% (or wt%) in the output be the amount of mineral precipitated or dissolved and not the total mineral vol% (or wt%)?

Many Thanks,

Mathew

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Hi Mathew,

Although you can set the mass of equilibrium minerals in X1t and X2t, it is generally easier to set the volume% instead. This way, if you change the dimensions or nodal block spacing of your domain you won't have to adjust the amount of minerals in your system. I have a feeling the problem may be that your input is not the same as what you are conceptualizing. Could you please post a script so I can take a closer look at your problem?

Hope to help,

Brian Farrell

Aqueous Solutions

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Hi Mathew,

It sounded from your original post that you want these minerals to make up the rock composition of your domain (ie the 92% that is not water). If this is the case, those minerals should be swapped into the Initial pane, not added in as Reactants. Give this a shot.

Make sure to take a look at the User's Guide for info on setting up your domain. See section 2.8 (Initial conditions) and 2.12 (Porosity evolution) from The Reactive Transport Modeling Guide (Release 8).

Hope this helps,

Brian

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Will do. I have to put them in as free wt% as there is no option to do vol%. I thought I tried this configuration because my initial understanding from teh GWB week short course a few years back was that the Reactants tab forces minerals to react hence the model crash. Good to confirm again.

Thanks! Model running.... will let you know.

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Hi Brian,

The model finished to completion but I still have the same discrepency where the Rock Mass = ~3000 wt% (30 wt fraction) under System Parameters in the output. In Minerals output I added up the total vol% of minerals which = 7.164 wt% max and the 'Mass H2O' (in System Parametrs) = ~93.2 wt% for background in the aquifer and down to ~90.4 wt% at the injection well. Some of the minerals I added to the Input tab are less than new minerals by an order of magnitude (eg. witherite (new min) = 0.6635 vol% versus quartz = 0.06702 vol% (I Input 4.8 wt%)). These result are nearly identical to previous model runs I have tried. Again the porosity, pH, species concentrations, temperature, etc all look reasonable.

...I tried to change the mineral wt% to 1000% to compensate but I get an error saying 'Mineral volume in node 0 is greater than node volume'.

...I calculated the volume of each mineral in cm3 per 10x10x10 meter node and got some reasonable mineral wt% for all the minerals. The Rock Mass still reports 3000% so not sure what is happening there. Everything else looks ok so far. Is there a vol% units option in the Initial tab as you suggested so that the porosity is directly tied to mineral volumes?

Thanks for your help,

Mathew

Mathew

14Bnov11_Debolt_0.1m-yr_10k-m3-d_Flow-min.x2t

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Hi Mathew,

I think there are some issues with the GUI in GWB8 that aren't allowing you to select volume% as a unit (GWB9 has a brand new GUI that is much improved). You can enter the volume% directly from the Command pane, however. Use the volume% or vol% commands (i.e. Quartz = 20 vol%). You can make the mineral volumes and porosity add up to 100, or less than 100 and the remainder will be taken up by "inert volume."

Rock mass includes minerals and the "inert volume." If you look at the variable mass H2O or Solution mass (set to wt %) you will find that this equals 100% I think. The rock mass, then, is 30 times the mass of the water. In your case, rock volume is 92% and solution volume (porosity) is 8%. This means that the rock's volume is 11.5 times greater than that of the water. Taking a density value of 2.65 g/cm3, the rock mass is approximately 30 times that of water. So I think that is where the rock mass = 3000% comes from.

You might be getting a lot of inert volume if the values you entered for each minerals wt% (or mass, volume, etc.) don't match up with the porosity and amount of water. Using mineral volume% should help to clear that up. The rest of your results are probably pretty close to where they should be, because the mass of equilibrium minerals only matters in the case of sorption, if a mineral is consumed by some bufering reaction, perhaps, or if the permeability is tied to the mineral volume.

Let me know how that works out,

Brian

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Hi Brian,

Yes using the command line with vol% worked great. I look forward to GWB9!

One last question: The formation I am modeling has some CH4, H2S and CO2 gas in it and as you saw in my script I have the H2S and the CO2 in the Initial tab as swapped gas species. Through trial and error I have put CH4(g) as a Reactant in at 0.1 mol/m3year which gives me reasonable gas fugacities in the output that are proportional to the mol% of gas sampled from the formation. Is there a better way to have CH4(g) at the initial state of the formation, perhaps using the command line? It does seem to slightly impact mineral equilibriums when I bring the CH4(g) up to observed proportions.

Thanks,

Mathew

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Hi Mathew,

Glad that worked out. You can keep it as a reactant if that works, or try adding a fixed CH4(g) fugacity from the Reactants pane. Alternatively, you could decouple the Methane(aq)/HCO3- redox pair and add Methane to the Initial pane. If you think that the Methane will not react with other carbon species, then just constrain Methane. If you think methane is reacting with other carbon (slow enough to not assume equilibrium, but fast enought that it matters), you might set a kinetic rate law for Methane generation or production on the Reactants pane, in addition to setting its initial value from the Initial pane.

Hope that helps,

Brian

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