Xt1 Permeability value

[jnabiyar]

How can you put in a direct value for permeability. For some reason the program provides an equation that incorporates porosity and some standard values ( permeability= A * porosity + B ), where A=15 and B=-5. It makes no sense. But when I do put in my porosity value and manipulate values A and/or B to equal my permeability, run the model, and plot it, the initial permeability value that I want is completely different from the one that is plotted. Any ideas?

[Brian Farrell]

Hi jnabiyar,

 

From the GWB Reactive Transport Modeling Guide:

 

There is no general relationship by which the permeability of actual sediments or rocks varies with porosity and mineralogic composition. For a specific suite of sediments or rocks, however, it is commonly possible to establish a statistical correlation among these variables. The programs use a correlation of the form

 

log k = (A x porosity) + B + Sum over m of (Am x Xm)

 

where A, B, and Am are empirical constants and Xm are the volume fraction of an arbitrary set of minerals indexed by m.

 

By default, the values for A and B in the correlation are set to 15 and -5, respectively, and no minerals are carried. The default describes a trend that has been observed in sandstone. The default settings, of course, are of no general significance. In constructing a model, it is important to remember that all such correlations are empirical, not functional constraints.

 

To set a permeability of 0.01 darcys (log permeability = -2) at any porosity, you could set A to 0 and B to -2. Thus, log 0.01 = -2 = (0 x porosity) + -2

 

If you would like permeability to change with porosity, you can enter a value for the A term (be sure you are calculating the porosity as a volume fraction instead of a percentage). If porosity is 50% (0.5 volume fraction), you could get the same permeability by setting A to 2, and B to -3. Thus, log 0.01 = -2 = (2 x 0.5) + -3

 

Does this make sense? You should read section 2.13 (Permeability correlation) in the Reactive Transport Modeling Guide for more information on setting permeability.

 

Regards,

 

Brian Farrell

Aqueous Solutions LLC

 

P.S. I moved your topic from the archive of old posts to the front GWB forum page.

[jnabiyar]

Hi Brian,

 

This makes perfect sense. I was actually getting confused with taking the log of 'k' but this makes a lot more sense.

 

Thank you.

 

Jay

 

Hi jnabiyar,

 

From the GWB Reactive Transport Modeling Guide:

 

There is no general relationship by which the permeability of actual sediments or rocks varies with porosity and mineralogic composition. For a specific suite of sediments or rocks, however, it is commonly possible to establish a statistical correlation among these variables. The programs use a correlation of the form

 

log k = (A x porosity) + B + Sum over m of (Am x Xm)

 

where A, B, and Am are empirical constants and Xm are the volume fraction of an arbitrary set of minerals indexed by m.

 

By default, the values for A and B in the correlation are set to 15 and -5, respectively, and no minerals are carried. The default describes a trend that has been observed in sandstone. The default settings, of course, are of no general significance. In constructing a model, it is important to remember that all such correlations are empirical, not functional constraints.

 

To set a permeability of 0.01 darcys (log permeability = -2) at any porosity, you could set A to 0 and B to -2. Thus, log 0.01 = -2 = (0 x porosity) + -2

 

If you would like permeability to change with porosity, you can enter a value for the A term (be sure you are calculating the porosity as a volume fraction instead of a percentage). If porosity is 50% (0.5 volume fraction), you could get the same permeability by setting A to 2, and B to -3. Thus, log 0.01 = -2 = (2 x 0.5) + -3

 

Does this make sense? You should read section 2.13 (Permeability correlation) in the Reactive Transport Modeling Guide for more information on setting permeability.

 

Regards,

 

Brian Farrell

Aqueous Solutions LLC

 

P.S. I moved your topic from the archive of old posts to the front GWB forum page.