Welcome to Petroleum Geology Forums

Hi,

I am analysing an oilzone from different wells and got some water samples sent to the lab. However, the water salinities differ, in fact the resistivity on th elogs differ.

Question: How does de resistivity react in an oilzone (oil and matrix has the same properties) in two different formation water salinities? For instance fresh water(3000ppm) vs Saline water (6000ppm).

Is then R(Oil+fresh water) > R(Oil+saline water)?

Regards

JS

Jeremy,

Indeed I would expect the HC zone in the fresh water formation to be higher (less conductive ions to transfer the current) than that of the saline water. How to quantify this is unknown to me though....

I also think so, but does this also affect the Sw of the HC zone?

Jeremy,

The resistivity of a clean (non-shaly) formation is a function of formation water saturation, porosity and the resistivity (in other words salinity) of the formation water.
The hydrocarbon (oil or gas) portion of the formation fluid does not contribute to the formation resistivity as hydrocarbons are non conductive.
Therefore, for a given formation porosity, the resistivity of this formation will depend on the water saturation (in other words hydrocarbon saturation) and the formation water resistivity.
If you have formation water with different salinities, you will have different formation resistivities depending on the water saturation in that formation.
This is the basis for computing water saturation in a hydrocarbon bearing reservoir.
In short, R(oil + fresh water) > R(oil + saline water)

As you already know the salinities of the water samples, you can estimate the Rw (water resistivity) using a "resistivity vs salinity" chart, which is available from service contractors or from a resistivity calculator (freely available on the web).
Then you can compute the water saturation Sw from the corresponding resistivity logs, using a suitable water saturation equation. If your formations are relatively clean, with not much clays in them, you can use Archie's equation to computer water saturation. You will have to assume the values of m (cementation exponent) and n (saturation exponent) if you do not have core analysis data.

I hope I do not sound like giving you a lecture.

Jeremy,

How about the porosity in the wells? is it similar? because resistivity will increase if the porosity decrease.. as Koko Kyi mentioned..

Because I encountered as well that I have two wet wells, both were sampled using DST, but the salinity are different.. but then i used the more representative water sample (thicker, more porous, hopefully cleaner) for the basis of saturation calculation across the field..

hope i can add some things to the discussion

So in short for every well I will have to use the corresponding Rw, but still I am not sure if this will represent the correct Sw (ps. I am using Archie) as the log is effected by varying salinity.
Any views on the behavior for Sw?

The porosity for all the wells are relatively the same.

All thank you for the replies and comments, very much appreciated.

JS

Jeremy, i think it would help if you have cores' water saturation measured or maybe capillary pressures to see if the logs' saturation model confirms with both data.

maybe compare Sw calculated with both Rw to the core sw/cap pres measurement..

just my 2 cents..