Archie Cementation Factor m: 2005 #11 (similar)

Given a formation with known porosity (say 9%), Ro (say 25 ohmm), and Rw from a core test (say 0.25 ohmm): calculate the formation cementation factor (or exponent m; this may be called either).

Two equations for F are on 15 LOG 1, with 2 variables each. Given 3 of the variables (we are missing only m) this is easy to solve. The Guidebook actually gives the equation for m here so it's just plug-and-chug.

Problems like this may show many sample points; if so, it doesn't matter which you choose. But you can check your work using a second set if you have the time.

F = Ro/Rw = 25/.25 = 100
F = 1/por^m = 1/0.09^m
(m)log(por) = log(1/F)
m = log(1/F)/log(por)
m = log(1/100)/log(.09)
m = -2/-1.05 = 1.9

Be aware on other problems you may be asked for (or given) a tortuosity factor "a" (F=a/por^m) rather than a = 1 (the norm). It's all on the 15 LOG 1 page if this unlikely event arises.