Geochemist's Workbench Support Forum

# GWB10 and transient properties

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Hi Brian,

Thank you very much for your information in the previous topic. My scripts for rate laws work but transient properties. Specifically, I am trying to vary surface area (SA) during the reaction but it does not work. Could you please see the attached scripts and give me some advice?

Also, I am wondering how the porosity, permeability and inert volume work in React? I input porosity 28%, used the default values to calculate permeability and varied the inert volume (> 99.92%) but the results were similar. Could you please explain how the React calculate the volume/mass of minerals if we input porosity and weight percents?

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Hi hongvu,

1) Relative to the custom rate laws, the field variables and transient field variables are somewhat limited in their ability to evaluate expressions (Remember, your expressions can only evaluate the parameters in Table A.1 of the Transport Modeling Guide). I don’t think this should pose a significant obstacle to you, however. Instead of specifying an equation for the mineral’s specific surface area that the program will continuously evaluate outside your rate law equation, you can simply include an expression for surface area inside your custom rate law. You can assume some geometry for the mineral grain, perhaps a sphere, and from that geometry obtain a relationship between surface area and mass, which the program knows. What you’re trying to do is something like this:

Rate = rate constant * surface area * (1 – Q/K), where surface area = specific surface area * mass, with the equation for specific surface area evaluated transiently outside the custom rate law.

Instead, I think you can specify a custom rate law like this, ignoring the specific surface area:

Rate = rate constant * surface area as a function of mass * (1 – Q/K).

2) Permeability doesn’t affect your calculations at all in React. It’s merely a reported value. That being said, it helps to figure it out in React without worrying about all the additional complexities of setting up a reactive transport model. Section 2.13 Permeability Correlation in the GWB Transport Modeling Guide describes the manner in which permeability is calculated. This should be straightforward, once you understand porosity.

Try taking a look at 2.12 Porosity evolution in the Transport Modeling Guide for an explanation. You can also look at 6.47 inert and 6.72 porosity in the GWB Reference Manual.

Hope this helps,

Brian

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