jnabiyar Posted April 16, 2013 Share Posted April 16, 2013 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? Quote Link to comment Share on other sites More sharing options...

Brian Farrell Posted April 16, 2013 Share Posted April 16, 2013 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. Quote Link to comment Share on other sites More sharing options...

jnabiyar Posted April 17, 2013 Author Share Posted April 17, 2013 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. Quote Link to comment Share on other sites More sharing options...

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