# How to Calculate Water Saturation After Trimming a Plug? | Ask PERM

//How to Calculate Water Saturation After Trimming a Plug? | Ask PERM

## How to Calculate Water Saturation After Trimming a Plug? | Ask PERM

After re-trimming, is the water saturation just obtained by the subtraction of the volume of the initial plug to the re-trimmed plug?

What I mean is,
As, Sw=Vw/Vpore
Sw: water saturation
Vw: volume of water collected
Vpore: pore volume

Can we use this formula after retrimming:

Sw = (Vw * Vpa) / (Vp*Vpore)
Vpa: Volume of the retrimmed plug
Vp: Volume of the plug non-retrimmed

Best regards,

Julie

### Dr. Jonathan Bryan from PERM Answers:

No, this probably won’t work.  The equation you have assumes that the core volume got smaller, but the pore volume stayed the same.  But in reality when you trim a core plug, you trim both the core volume and the pore volume.

Assuming that porosity is homogeneous in the sample, if you make the core smaller, the pore volume would also get smaller by that same amount.  And by the way, so would the water present in the core (assuming that saturation is also uniform in the sample).  So in that case, the amount of water present and the pore volume present would both be smaller, but the Sw may be even the same!

We do this routinely in core analysis testing.  We do a Dean-Stark to measure water saturation on a small piece of core, and then use that as an estimate of water saturation in a larger core that is adjacent to the tested small piece.  It is an approximation only, but it is a good one, unless you have something that would make water saturation not uniform within the sample.

Best Regards,

Dr. Jonathan Bryan