Calculating water saturation -- Archie method
Sw = ((a/por^m)*Rw/Rt)^(1/n)
F=(a/por^m)
BVW=por*Sw
Rwa=Rt/F
where:
a=correlation coefficient
m=cementation factor
n=saturation exponent
por=porosity
F=formation factor
BVW=bulk volume water
Rwa=apparent water resistivity
Rw=formation water resistivity
Rt=formation resistivity
Optional shale correction:
Fertl & Hammack model:
Sw(corr)=Sw-(Vshale*Rw)/(0.4*Rshale*por)
where:
Vshale=volume fraction of shale
Rshale=shale resistivity
(Vshale is calculated from the gamma-ray log)
The program lets you put in your own model (a, m, & n) for the
saturation equation, if you have measured or derived data, from
a Pickett plot, for example. The trickiest parameter is really
'm,' the cementation factor, and it can bite you, hard, in
carbonates. The value of 'm' can vary by an order of magnitude
across a seemingly uniform pile of limestone. The caveat here
is to buttress your notion of a zone being productive by using
more than one method. There are resistivity ratio approaches to
guessing Sw, and there is always the inexact but surprisingly
helpful mudlog for the well, which for all of its failings
still directly indicates hydrocarbons. I'm also a big fan of
not confining yourself to one well's set of log surveys. Pull a
few offset wells, look at producers and dry holes, and get a
feel for what your formation looks like when it does, and
doesn't, contain oil. In a dewatering type of play, where you
expect to move lots of water with production, there may be no
oil saturation at all within the radius of investigation of
logging tools.
The program also gives you the option to correct for shale using Fertl & Hammack's model. While there are many ways to come up with the volume of shale, I prefer to use the gamma-ray curve. It typically responds to the high cation-exchange clays that are of interest to you as a log analyst (because they are the ones that can lower the formation's apparent resistivity), and because the gamma-ray curve has been run for a long time and is easy to calibrate. Here are Larionov's equations:
Gamma-ray index=I=(Grlog-GRclean)/(GRshale-GRclean)
Vshale=0.33*(2^(2*I)-1) [for old rocks]
Vshale=0.083*(2^(3.7)*I-1) [for Tertiary-age rocks]
The major assumption with this shale correction model is that
parameters you take from an adjacent shale bed will also apply
to shales embedded in your formation of interest. This may not
be the case... If you do correct for shale, be sure to flag the
"Tertiary age?" checkbox if dealing with very young formations.
Apparent water resistivity, Rwa, is a quick indicator of possible production if the value of Rwa is greater than 3 X Rw.
The Bulk Volume Water, BVW%, is a measure of how much water the rock might be able to hold by capillary attraction and not contribute to production, with rule-of-thumb cutoffs of BVW below 7% for water-free production in sandstones, and below 4% for limestones. Keep in mind that these are averages! You can evaluate whether water is going to flow with production by plotting porosities versus water saturations on a graph. If they tend to track along hyperbolic lines (that taper towards the ends of the X and Y axes), then your formation might be at irreducible water saturation, regardless of the value of BVW.
DATA LOGGING
Do you want to save your calculations? The input box at the very bottom of the screen records all the inputs and outputs for each calculation run. To save this information, select all the text in the box and copy it, then open a spreadsheet and paste it in as comma-separated values. Each data type will land in its own column, and each calculation run, or depth, will occupy a row. Format the spreadsheet to separate rows into different geologic formations, and you're done. Isn't that easier than writing everything down?
Don't have a spreadsheet handy? If you are working on a phone or a tablet, you can still copy the text and paste it into a note or an email.
The Recording box will reset if you press the "Help" or "Reset" buttons, or if you navigate to a different page.