Fix fugacity constraint in X1t

[rarthur]

Can I fix the fugacity of a gas node-by-node (e.g., above, but not below, the water table) in module X1t?

[Brian Farrell]

Hi rarthur,

 

You cannot currently fix fugacity on a node-by-node basis in X1t.

 

The most elegant solution would be to use ChemPlugin instead of X1t. ChemPlugin is a software object derived from The Geochemist’s Workbench. It’s a brand new product that we’re really excited about. It can be embedded within software programs, such as a simple flow code that you write, to handle the reactions and transport. An instance of ChemPlugin is spawned for each nodal block; the instances link together but each maintains its own memory space. As a result, you have a lot more flexibility in setting up a chemical model. For example, separate instances can load different thermo datasets, use completely different buffer minerals or kinetic rate laws, or maintain separate fugacity constraints, as you desire. We have a number of example programs to simulate everything from simple titrations and flow-through reactors to 1D reactive transport. If you’re interested in trying a demo, we could help you get up and running.

 

The faster workaround is to implement gas transfer kinetics in your X1t model. In a kinetic gas transfer model, you set the fugacity of a gas in an external reservoir along with a rate constant and a specific contact area between the fluid and that reservoir. These three parameters are field variables, so you can set them node-by-node. You could set up your model so that exchange is rapid enough to maintain your desired fugacity in the nodes above the water table. You should be careful to ensure that exchange is not so rapid that the program is bogged down by taking very small time steps. For the nodes in the saturated zone, you can prevent gas transfer by setting the rate constant or specific contact area to 0.

 

Hope this helps,

 

Brian Farrell

Aqueous Solutions LLC